[DISCUSS] Long-term goal of making flink-table Scala-free

[Timo Walther-2]

Hi everyone,

as you all know, currently the Table & SQL API is implemented in Scala.
This decision was made a long-time ago when the initital code base was
created as part of a master's thesis. The community kept Scala because
of the nice language features that enable a fluent Table API like
table.select('field.trim()) and because Scala allows for quick
prototyping (e.g. multi-line comments for code generation). The
committers enforced not splitting the code-base into two programming
languages.

However, nowadays the flink-table module more and more becomes an
important part in the Flink ecosystem. Connectors, formats, and SQL
client are actually implemented in Java but need to interoperate with
flink-table which makes these modules dependent on Scala. As mentioned
in an earlier mail thread, using Scala for API classes also exposes
member variables and methods in Java that should not be exposed to users
[1]. Java is still the most important API language and right now we
treat it as a second-class citizen. I just noticed that you even need to
add Scala if you just want to implement a ScalarFunction because of
method clashes between `public String toString()` and `public
scala.Predef.String toString()`.

Given the size of the current code base, reimplementing the entire
flink-table code in Java is a goal that we might never reach. However,
we should at least treat the symptoms and have this as a long-term goal
in mind. My suggestion would be to convert user-facing and runtime
classes and split the code base into multiple modules:

 > flink-table-java {depends on flink-table-core}
Implemented in Java. Java users can use this. This would require to
convert classes like TableEnvironment, Table.

 > flink-table-scala {depends on flink-table-core}
Implemented in Scala. Scala users can use this.

 > flink-table-common
Implemented in Java. Connectors, formats, and UDFs can use this. It
contains interface classes such as descriptors, table sink, table source.

 > flink-table-core {depends on flink-table-common and flink-table-runtime}
Implemented in Scala. Contains the current main code base.

 > flink-table-runtime
Implemented in Java. This would require to convert classes in
o.a.f.table.runtime but would improve the runtime potentially.


What do you think?


Regards,

Timo

[1]
http://apache-flink-mailing-list-archive.1008284.n3.nabble.com/DISCUSS-Convert-main-Table-API-classes-into-traits-tp21335.html

[Piotr Nowojski]

Hi,

I do not have an experience with how scala and java interacts with each other, so I can not fully validate your proposal, but generally speaking +1 from me.

Does it also mean, that we should slowly migrate `flink-table-core` to Java? How would you envision it? It would be nice to be able to add new classes/features written in Java and so that they can coexist with old Scala code until we gradually switch from Scala to Java.

Piotrek

> On 13 Jun 2018, at 11:32, Timo Walther <[hidden email]> wrote:
>
> Hi everyone,
>
> as you all know, currently the Table & SQL API is implemented in Scala. This decision was made a long-time ago when the initital code base was created as part of a master's thesis. The community kept Scala because of the nice language features that enable a fluent Table API like table.select('field.trim()) and because Scala allows for quick prototyping (e.g. multi-line comments for code generation). The committers enforced not splitting the code-base into two programming languages.
>
> However, nowadays the flink-table module more and more becomes an important part in the Flink ecosystem. Connectors, formats, and SQL client are actually implemented in Java but need to interoperate with flink-table which makes these modules dependent on Scala. As mentioned in an earlier mail thread, using Scala for API classes also exposes member variables and methods in Java that should not be exposed to users [1]. Java is still the most important API language and right now we treat it as a second-class citizen. I just noticed that you even need to add Scala if you just want to implement a ScalarFunction because of method clashes between `public String toString()` and `public scala.Predef.String toString()`.
>
> Given the size of the current code base, reimplementing the entire flink-table code in Java is a goal that we might never reach. However, we should at least treat the symptoms and have this as a long-term goal in mind. My suggestion would be to convert user-facing and runtime classes and split the code base into multiple modules:
>
> > flink-table-java {depends on flink-table-core}
> Implemented in Java. Java users can use this. This would require to convert classes like TableEnvironment, Table.
>
> > flink-table-scala {depends on flink-table-core}
> Implemented in Scala. Scala users can use this.
>
> > flink-table-common
> Implemented in Java. Connectors, formats, and UDFs can use this. It contains interface classes such as descriptors, table sink, table source.
>
> > flink-table-core {depends on flink-table-common and flink-table-runtime}
> Implemented in Scala. Contains the current main code base.
>
> > flink-table-runtime
> Implemented in Java. This would require to convert classes in o.a.f.table.runtime but would improve the runtime potentially.
>
>
> What do you think?
>
>
> Regards,
>
> Timo
>
> [1] http://apache-flink-mailing-list-archive.1008284.n3.nabble.com/DISCUSS-Convert-main-Table-API-classes-into-traits-tp21335.html
>

[Till Rohrmann]

I think that is a noble and honorable goal and we should strive for it.
This, however, must be an iterative process given the sheer size of the
code base. I like the approach to define common Java modules which are used
by more specific Scala modules and slowly moving classes from Scala to
Java. Thus +1 for the proposal.

Cheers,
Till

On Wed, Jun 13, 2018 at 12:01 PM Piotr Nowojski <[hidden email]>
wrote:

> Hi,
>
> I do not have an experience with how scala and java interacts with each
> other, so I can not fully validate your proposal, but generally speaking +1
> from me.
>
> Does it also mean, that we should slowly migrate `flink-table-core` to
> Java? How would you envision it? It would be nice to be able to add new
> classes/features written in Java and so that they can coexist with old
> Scala code until we gradually switch from Scala to Java.
>
> Piotrek
>
> > On 13 Jun 2018, at 11:32, Timo Walther <[hidden email]> wrote:
> >
> > Hi everyone,
> >
> > as you all know, currently the Table & SQL API is implemented in Scala.
> This decision was made a long-time ago when the initital code base was
> created as part of a master's thesis. The community kept Scala because of
> the nice language features that enable a fluent Table API like
> table.select('field.trim()) and because Scala allows for quick prototyping
> (e.g. multi-line comments for code generation). The committers enforced not
> splitting the code-base into two programming languages.
> >
> > However, nowadays the flink-table module more and more becomes an
> important part in the Flink ecosystem. Connectors, formats, and SQL client
> are actually implemented in Java but need to interoperate with flink-table
> which makes these modules dependent on Scala. As mentioned in an earlier
> mail thread, using Scala for API classes also exposes member variables and
> methods in Java that should not be exposed to users [1]. Java is still the
> most important API language and right now we treat it as a second-class
> citizen. I just noticed that you even need to add Scala if you just want to
> implement a ScalarFunction because of method clashes between `public String
> toString()` and `public scala.Predef.String toString()`.
> >
> > Given the size of the current code base, reimplementing the entire
> flink-table code in Java is a goal that we might never reach. However, we
> should at least treat the symptoms and have this as a long-term goal in
> mind. My suggestion would be to convert user-facing and runtime classes and
> split the code base into multiple modules:
> >
> > > flink-table-java {depends on flink-table-core}
> > Implemented in Java. Java users can use this. This would require to
> convert classes like TableEnvironment, Table.
> >
> > > flink-table-scala {depends on flink-table-core}
> > Implemented in Scala. Scala users can use this.
> >
> > > flink-table-common
> > Implemented in Java. Connectors, formats, and UDFs can use this. It
> contains interface classes such as descriptors, table sink, table source.
> >
> > > flink-table-core {depends on flink-table-common and
> flink-table-runtime}
> > Implemented in Scala. Contains the current main code base.
> >
> > > flink-table-runtime
> > Implemented in Java. This would require to convert classes in
> o.a.f.table.runtime but would improve the runtime potentially.
> >
> >
> > What do you think?
> >
> >
> > Regards,
> >
> > Timo
> >
> > [1]
> http://apache-flink-mailing-list-archive.1008284.n3.nabble.com/DISCUSS-Convert-main-Table-API-classes-into-traits-tp21335.html
> >
>
>

[Fabian Hueske-2]

Hi,

In general, I think this is a good effort. However, it won't be easy and I
think we have to plan this well.
I don't like the idea of having the whole code base fragmented into Java
and Scala code for too long.

I think we should do this one step at a time and focus on migrating one
module at a time.
IMO, the easiest start would be to port the runtime to Java.
Extracting the API classes into an own module, porting them to Java, and
removing the Scala dependency won't be possible without breaking the API
since a few classes depend on the Scala Table API.

Best, Fabian


2018-06-14 10:33 GMT+02:00 Till Rohrmann <[hidden email]>:

> I think that is a noble and honorable goal and we should strive for it.
> This, however, must be an iterative process given the sheer size of the
> code base. I like the approach to define common Java modules which are used
> by more specific Scala modules and slowly moving classes from Scala to
> Java. Thus +1 for the proposal.
>
> Cheers,
> Till
>
> On Wed, Jun 13, 2018 at 12:01 PM Piotr Nowojski <[hidden email]>
> wrote:
>
> > Hi,
> >
> > I do not have an experience with how scala and java interacts with each
> > other, so I can not fully validate your proposal, but generally speaking
> +1
> > from me.
> >
> > Does it also mean, that we should slowly migrate `flink-table-core` to
> > Java? How would you envision it? It would be nice to be able to add new
> > classes/features written in Java and so that they can coexist with old
> > Scala code until we gradually switch from Scala to Java.
> >
> > Piotrek
> >
> > > On 13 Jun 2018, at 11:32, Timo Walther <[hidden email]> wrote:
> > >
> > > Hi everyone,
> > >
> > > as you all know, currently the Table & SQL API is implemented in Scala.
> > This decision was made a long-time ago when the initital code base was
> > created as part of a master's thesis. The community kept Scala because of
> > the nice language features that enable a fluent Table API like
> > table.select('field.trim()) and because Scala allows for quick
> prototyping
> > (e.g. multi-line comments for code generation). The committers enforced
> not
> > splitting the code-base into two programming languages.
> > >
> > > However, nowadays the flink-table module more and more becomes an
> > important part in the Flink ecosystem. Connectors, formats, and SQL
> client
> > are actually implemented in Java but need to interoperate with
> flink-table
> > which makes these modules dependent on Scala. As mentioned in an earlier
> > mail thread, using Scala for API classes also exposes member variables
> and
> > methods in Java that should not be exposed to users [1]. Java is still
> the
> > most important API language and right now we treat it as a second-class
> > citizen. I just noticed that you even need to add Scala if you just want
> to
> > implement a ScalarFunction because of method clashes between `public
> String
> > toString()` and `public scala.Predef.String toString()`.
> > >
> > > Given the size of the current code base, reimplementing the entire
> > flink-table code in Java is a goal that we might never reach. However, we
> > should at least treat the symptoms and have this as a long-term goal in
> > mind. My suggestion would be to convert user-facing and runtime classes
> and
> > split the code base into multiple modules:
> > >
> > > > flink-table-java {depends on flink-table-core}
> > > Implemented in Java. Java users can use this. This would require to
> > convert classes like TableEnvironment, Table.
> > >
> > > > flink-table-scala {depends on flink-table-core}
> > > Implemented in Scala. Scala users can use this.
> > >
> > > > flink-table-common
> > > Implemented in Java. Connectors, formats, and UDFs can use this. It
> > contains interface classes such as descriptors, table sink, table source.
> > >
> > > > flink-table-core {depends on flink-table-common and
> > flink-table-runtime}
> > > Implemented in Scala. Contains the current main code base.
> > >
> > > > flink-table-runtime
> > > Implemented in Java. This would require to convert classes in
> > o.a.f.table.runtime but would improve the runtime potentially.
> > >
> > >
> > > What do you think?
> > >
> > >
> > > Regards,
> > >
> > > Timo
> > >
> > > [1]
> > http://apache-flink-mailing-list-archive.1008284.n3.
> nabble.com/DISCUSS-Convert-main-Table-API-classes-into-traits-tp21335.html
> > >
> >
> >
>

[Piotr Nowojski]

Bumping the topic.

If we want to do this, the sooner we decide, the less code we will have to rewrite. I have some objections/counter proposals to Fabian's proposal of doing it module wise and one module at a time.

First, I do not see a problem of having java/scala code even within one module, especially not if there are clean boundaries. Like we could have API in Scala and optimizer rules/logical nodes written in Java in the same module. However I haven’t previously maintained mixed scala/java code bases before, so I might be missing something here.

Secondly this whole migration might and most like will take longer then expected, so that creates a problem for a new code that we will be creating. After making a decision to migrate to Java, almost any new Scala line of code will be immediately a technological debt and we will have to rewrite it to Java later.

Thus I would propose first to state our end goal - modules structure and which parts of modules we want to have eventually Scala-free. Secondly taking all steps necessary that will allow us to write new code complaint with our end goal. Only after that we should/could focus on incrementally rewriting the old code. Otherwise we could be stuck/blocked for years writing new code in Scala (and increasing technological debt), because nobody have found a time to rewrite some non important and not actively developed part of some module.

Piotrek

> On 14 Jun 2018, at 15:34, Fabian Hueske <[hidden email]> wrote:
>
> Hi,
>
> In general, I think this is a good effort. However, it won't be easy and I
> think we have to plan this well.
> I don't like the idea of having the whole code base fragmented into Java
> and Scala code for too long.
>
> I think we should do this one step at a time and focus on migrating one
> module at a time.
> IMO, the easiest start would be to port the runtime to Java.
> Extracting the API classes into an own module, porting them to Java, and
> removing the Scala dependency won't be possible without breaking the API
> since a few classes depend on the Scala Table API.
>
> Best, Fabian
>
>
> 2018-06-14 10:33 GMT+02:00 Till Rohrmann <[hidden email]>:
>
>> I think that is a noble and honorable goal and we should strive for it.
>> This, however, must be an iterative process given the sheer size of the
>> code base. I like the approach to define common Java modules which are used
>> by more specific Scala modules and slowly moving classes from Scala to
>> Java. Thus +1 for the proposal.
>>
>> Cheers,
>> Till
>>
>> On Wed, Jun 13, 2018 at 12:01 PM Piotr Nowojski <[hidden email]>
>> wrote:
>>
>>> Hi,
>>>
>>> I do not have an experience with how scala and java interacts with each
>>> other, so I can not fully validate your proposal, but generally speaking
>> +1
>>> from me.
>>>
>>> Does it also mean, that we should slowly migrate `flink-table-core` to
>>> Java? How would you envision it? It would be nice to be able to add new
>>> classes/features written in Java and so that they can coexist with old
>>> Scala code until we gradually switch from Scala to Java.
>>>
>>> Piotrek
>>>
>>>> On 13 Jun 2018, at 11:32, Timo Walther <[hidden email]> wrote:
>>>>
>>>> Hi everyone,
>>>>
>>>> as you all know, currently the Table & SQL API is implemented in Scala.
>>> This decision was made a long-time ago when the initital code base was
>>> created as part of a master's thesis. The community kept Scala because of
>>> the nice language features that enable a fluent Table API like
>>> table.select('field.trim()) and because Scala allows for quick
>> prototyping
>>> (e.g. multi-line comments for code generation). The committers enforced
>> not
>>> splitting the code-base into two programming languages.
>>>>
>>>> However, nowadays the flink-table module more and more becomes an
>>> important part in the Flink ecosystem. Connectors, formats, and SQL
>> client
>>> are actually implemented in Java but need to interoperate with
>> flink-table
>>> which makes these modules dependent on Scala. As mentioned in an earlier
>>> mail thread, using Scala for API classes also exposes member variables
>> and
>>> methods in Java that should not be exposed to users [1]. Java is still
>> the
>>> most important API language and right now we treat it as a second-class
>>> citizen. I just noticed that you even need to add Scala if you just want
>> to
>>> implement a ScalarFunction because of method clashes between `public
>> String
>>> toString()` and `public scala.Predef.String toString()`.
>>>>
>>>> Given the size of the current code base, reimplementing the entire
>>> flink-table code in Java is a goal that we might never reach. However, we
>>> should at least treat the symptoms and have this as a long-term goal in
>>> mind. My suggestion would be to convert user-facing and runtime classes
>> and
>>> split the code base into multiple modules:
>>>>
>>>>> flink-table-java {depends on flink-table-core}
>>>> Implemented in Java. Java users can use this. This would require to
>>> convert classes like TableEnvironment, Table.
>>>>
>>>>> flink-table-scala {depends on flink-table-core}
>>>> Implemented in Scala. Scala users can use this.
>>>>
>>>>> flink-table-common
>>>> Implemented in Java. Connectors, formats, and UDFs can use this. It
>>> contains interface classes such as descriptors, table sink, table source.
>>>>
>>>>> flink-table-core {depends on flink-table-common and
>>> flink-table-runtime}
>>>> Implemented in Scala. Contains the current main code base.
>>>>
>>>>> flink-table-runtime
>>>> Implemented in Java. This would require to convert classes in
>>> o.a.f.table.runtime but would improve the runtime potentially.
>>>>
>>>>
>>>> What do you think?
>>>>
>>>>
>>>> Regards,
>>>>
>>>> Timo
>>>>
>>>> [1]
>>> http://apache-flink-mailing-list-archive.1008284.n3.
>> nabble.com/DISCUSS-Convert-main-Table-API-classes-into-traits-tp21335.html
>>>>
>>>
>>>
>>

[xingcanc]

Hi all,

I also think about this problem these days and here are my thoughts.

1) We must admit that it’s really a tough task to interoperate with Java and Scala. E.g., they have different collection types (Scala collections v.s. java.util.*) and in Java, it's hard to implement a method which takes Scala functions as parameters. Considering the major part of the code base is implemented in Java, +1 for this goal from a long-term view.

2) The ideal solution would be to just expose a Scala API and make all the other parts Scala-free. But I am not sure if it could be achieved even in a long-term. Thus as Timo suggested, keep the Scala codes in "flink-table-core" would be a compromise solution.

3) If the community makes the final decision, maybe any new features should be added in Java (regardless of the modules), in order to prevent the Scala codes from growing.

Best,
Xingcan

> On Jul 2, 2018, at 9:30 PM, Piotr Nowojski <[hidden email]> wrote:
>
> Bumping the topic.
>
> If we want to do this, the sooner we decide, the less code we will have to rewrite. I have some objections/counter proposals to Fabian's proposal of doing it module wise and one module at a time.
>
> First, I do not see a problem of having java/scala code even within one module, especially not if there are clean boundaries. Like we could have API in Scala and optimizer rules/logical nodes written in Java in the same module. However I haven’t previously maintained mixed scala/java code bases before, so I might be missing something here.
>
> Secondly this whole migration might and most like will take longer then expected, so that creates a problem for a new code that we will be creating. After making a decision to migrate to Java, almost any new Scala line of code will be immediately a technological debt and we will have to rewrite it to Java later.
>
> Thus I would propose first to state our end goal - modules structure and which parts of modules we want to have eventually Scala-free. Secondly taking all steps necessary that will allow us to write new code complaint with our end goal. Only after that we should/could focus on incrementally rewriting the old code. Otherwise we could be stuck/blocked for years writing new code in Scala (and increasing technological debt), because nobody have found a time to rewrite some non important and not actively developed part of some module.
>
> Piotrek
>
>> On 14 Jun 2018, at 15:34, Fabian Hueske <[hidden email]> wrote:
>>
>> Hi,
>>
>> In general, I think this is a good effort. However, it won't be easy and I
>> think we have to plan this well.
>> I don't like the idea of having the whole code base fragmented into Java
>> and Scala code for too long.
>>
>> I think we should do this one step at a time and focus on migrating one
>> module at a time.
>> IMO, the easiest start would be to port the runtime to Java.
>> Extracting the API classes into an own module, porting them to Java, and
>> removing the Scala dependency won't be possible without breaking the API
>> since a few classes depend on the Scala Table API.
>>
>> Best, Fabian
>>
>>
>> 2018-06-14 10:33 GMT+02:00 Till Rohrmann <[hidden email]>:
>>
>>> I think that is a noble and honorable goal and we should strive for it.
>>> This, however, must be an iterative process given the sheer size of the
>>> code base. I like the approach to define common Java modules which are used
>>> by more specific Scala modules and slowly moving classes from Scala to
>>> Java. Thus +1 for the proposal.
>>>
>>> Cheers,
>>> Till
>>>
>>> On Wed, Jun 13, 2018 at 12:01 PM Piotr Nowojski <[hidden email]>
>>> wrote:
>>>
>>>> Hi,
>>>>
>>>> I do not have an experience with how scala and java interacts with each
>>>> other, so I can not fully validate your proposal, but generally speaking
>>> +1
>>>> from me.
>>>>
>>>> Does it also mean, that we should slowly migrate `flink-table-core` to
>>>> Java? How would you envision it? It would be nice to be able to add new
>>>> classes/features written in Java and so that they can coexist with old
>>>> Scala code until we gradually switch from Scala to Java.
>>>>
>>>> Piotrek
>>>>
>>>>> On 13 Jun 2018, at 11:32, Timo Walther <[hidden email]> wrote:
>>>>>
>>>>> Hi everyone,
>>>>>
>>>>> as you all know, currently the Table & SQL API is implemented in Scala.
>>>> This decision was made a long-time ago when the initital code base was
>>>> created as part of a master's thesis. The community kept Scala because of
>>>> the nice language features that enable a fluent Table API like
>>>> table.select('field.trim()) and because Scala allows for quick
>>> prototyping
>>>> (e.g. multi-line comments for code generation). The committers enforced
>>> not
>>>> splitting the code-base into two programming languages.
>>>>>
>>>>> However, nowadays the flink-table module more and more becomes an
>>>> important part in the Flink ecosystem. Connectors, formats, and SQL
>>> client
>>>> are actually implemented in Java but need to interoperate with
>>> flink-table
>>>> which makes these modules dependent on Scala. As mentioned in an earlier
>>>> mail thread, using Scala for API classes also exposes member variables
>>> and
>>>> methods in Java that should not be exposed to users [1]. Java is still
>>> the
>>>> most important API language and right now we treat it as a second-class
>>>> citizen. I just noticed that you even need to add Scala if you just want
>>> to
>>>> implement a ScalarFunction because of method clashes between `public
>>> String
>>>> toString()` and `public scala.Predef.String toString()`.
>>>>>
>>>>> Given the size of the current code base, reimplementing the entire
>>>> flink-table code in Java is a goal that we might never reach. However, we
>>>> should at least treat the symptoms and have this as a long-term goal in
>>>> mind. My suggestion would be to convert user-facing and runtime classes
>>> and
>>>> split the code base into multiple modules:
>>>>>
>>>>>> flink-table-java {depends on flink-table-core}
>>>>> Implemented in Java. Java users can use this. This would require to
>>>> convert classes like TableEnvironment, Table.
>>>>>
>>>>>> flink-table-scala {depends on flink-table-core}
>>>>> Implemented in Scala. Scala users can use this.
>>>>>
>>>>>> flink-table-common
>>>>> Implemented in Java. Connectors, formats, and UDFs can use this. It
>>>> contains interface classes such as descriptors, table sink, table source.
>>>>>
>>>>>> flink-table-core {depends on flink-table-common and
>>>> flink-table-runtime}
>>>>> Implemented in Scala. Contains the current main code base.
>>>>>
>>>>>> flink-table-runtime
>>>>> Implemented in Java. This would require to convert classes in
>>>> o.a.f.table.runtime but would improve the runtime potentially.
>>>>>
>>>>>
>>>>> What do you think?
>>>>>
>>>>>
>>>>> Regards,
>>>>>
>>>>> Timo
>>>>>
>>>>> [1]
>>>> http://apache-flink-mailing-list-archive.1008284.n3.
>>> nabble.com/DISCUSS-Convert-main-Table-API-classes-into-traits-tp21335.html
>>>>>
>>>>
>>>>
>>>
>

[Fabian Hueske-2]

Hi Piotr,

thanks for bumping this thread and thanks for Xingcan for the comments.

I think the first step would be to separate the flink-table module into
multiple sub modules. These could be:

- flink-table-api: All API facing classes. Can be later divided further
into Java/Scala Table API/SQL
- flink-table-planning: involves all planning (basically everything we do
with Calcite)
- flink-table-runtime: the runtime code

IMO, a realistic mid-term goal is to have the runtime module and certain
parts of the planning module ported to Java.
The api module will be much harder to port because of several dependencies
to Scala core classes (the parser framework, tree iterations, etc.). I'm
not saying we should not port this to Java, but it is not clear to me (yet)
how to do it.

I think flink-table-runtime should not be too hard to port. The code does
not make use of many Scala features, i.e., it's writing very Java-like.
Also, there are not many dependencies and operators can be individually
ported step-by-step.
For flink-table-planning, we can have certain packages that we port to Java
like planning rules or plan nodes. The related classes mostly extend
Calcite's Java interfaces/classes and would be natural choices for being
ported. The code generation classes will require more effort to port. There
are also some dependencies in planning on the api module that we would need
to resolve somehow.

For SQL most work when adding new features is done in the planning and
runtime modules. So, this separation should already reduce "technological
dept" quite a lot.
The Table API depends much more on Scala than SQL.

Cheers, Fabian



2018-07-02 16:26 GMT+02:00 Xingcan Cui <[hidden email]>:

> Hi all,
>
> I also think about this problem these days and here are my thoughts.
>
> 1) We must admit that it’s really a tough task to interoperate with Java
> and Scala. E.g., they have different collection types (Scala collections
> v.s. java.util.*) and in Java, it's hard to implement a method which takes
> Scala functions as parameters. Considering the major part of the code base
> is implemented in Java, +1 for this goal from a long-term view.
>
> 2) The ideal solution would be to just expose a Scala API and make all the
> other parts Scala-free. But I am not sure if it could be achieved even in a
> long-term. Thus as Timo suggested, keep the Scala codes in
> "flink-table-core" would be a compromise solution.
>
> 3) If the community makes the final decision, maybe any new features
> should be added in Java (regardless of the modules), in order to prevent
> the Scala codes from growing.
>
> Best,
> Xingcan
>
>
> > On Jul 2, 2018, at 9:30 PM, Piotr Nowojski <[hidden email]>
> wrote:
> >
> > Bumping the topic.
> >
> > If we want to do this, the sooner we decide, the less code we will have
> to rewrite. I have some objections/counter proposals to Fabian's proposal
> of doing it module wise and one module at a time.
> >
> > First, I do not see a problem of having java/scala code even within one
> module, especially not if there are clean boundaries. Like we could have
> API in Scala and optimizer rules/logical nodes written in Java in the same
> module. However I haven’t previously maintained mixed scala/java code bases
> before, so I might be missing something here.
> >
> > Secondly this whole migration might and most like will take longer then
> expected, so that creates a problem for a new code that we will be
> creating. After making a decision to migrate to Java, almost any new Scala
> line of code will be immediately a technological debt and we will have to
> rewrite it to Java later.
> >
> > Thus I would propose first to state our end goal - modules structure and
> which parts of modules we want to have eventually Scala-free. Secondly
> taking all steps necessary that will allow us to write new code complaint
> with our end goal. Only after that we should/could focus on incrementally
> rewriting the old code. Otherwise we could be stuck/blocked for years
> writing new code in Scala (and increasing technological debt), because
> nobody have found a time to rewrite some non important and not actively
> developed part of some module.
> >
> > Piotrek
> >
> >> On 14 Jun 2018, at 15:34, Fabian Hueske <[hidden email]> wrote:
> >>
> >> Hi,
> >>
> >> In general, I think this is a good effort. However, it won't be easy
> and I
> >> think we have to plan this well.
> >> I don't like the idea of having the whole code base fragmented into Java
> >> and Scala code for too long.
> >>
> >> I think we should do this one step at a time and focus on migrating one
> >> module at a time.
> >> IMO, the easiest start would be to port the runtime to Java.
> >> Extracting the API classes into an own module, porting them to Java, and
> >> removing the Scala dependency won't be possible without breaking the API
> >> since a few classes depend on the Scala Table API.
> >>
> >> Best, Fabian
> >>
> >>
> >> 2018-06-14 10:33 GMT+02:00 Till Rohrmann <[hidden email]>:
> >>
> >>> I think that is a noble and honorable goal and we should strive for it.
> >>> This, however, must be an iterative process given the sheer size of the
> >>> code base. I like the approach to define common Java modules which are
> used
> >>> by more specific Scala modules and slowly moving classes from Scala to
> >>> Java. Thus +1 for the proposal.
> >>>
> >>> Cheers,
> >>> Till
> >>>
> >>> On Wed, Jun 13, 2018 at 12:01 PM Piotr Nowojski <
> [hidden email]>
> >>> wrote:
> >>>
> >>>> Hi,
> >>>>
> >>>> I do not have an experience with how scala and java interacts with
> each
> >>>> other, so I can not fully validate your proposal, but generally
> speaking
> >>> +1
> >>>> from me.
> >>>>
> >>>> Does it also mean, that we should slowly migrate `flink-table-core` to
> >>>> Java? How would you envision it? It would be nice to be able to add
> new
> >>>> classes/features written in Java and so that they can coexist with old
> >>>> Scala code until we gradually switch from Scala to Java.
> >>>>
> >>>> Piotrek
> >>>>
> >>>>> On 13 Jun 2018, at 11:32, Timo Walther <[hidden email]> wrote:
> >>>>>
> >>>>> Hi everyone,
> >>>>>
> >>>>> as you all know, currently the Table & SQL API is implemented in
> Scala.
> >>>> This decision was made a long-time ago when the initital code base was
> >>>> created as part of a master's thesis. The community kept Scala
> because of
> >>>> the nice language features that enable a fluent Table API like
> >>>> table.select('field.trim()) and because Scala allows for quick
> >>> prototyping
> >>>> (e.g. multi-line comments for code generation). The committers
> enforced
> >>> not
> >>>> splitting the code-base into two programming languages.
> >>>>>
> >>>>> However, nowadays the flink-table module more and more becomes an
> >>>> important part in the Flink ecosystem. Connectors, formats, and SQL
> >>> client
> >>>> are actually implemented in Java but need to interoperate with
> >>> flink-table
> >>>> which makes these modules dependent on Scala. As mentioned in an
> earlier
> >>>> mail thread, using Scala for API classes also exposes member variables
> >>> and
> >>>> methods in Java that should not be exposed to users [1]. Java is still
> >>> the
> >>>> most important API language and right now we treat it as a
> second-class
> >>>> citizen. I just noticed that you even need to add Scala if you just
> want
> >>> to
> >>>> implement a ScalarFunction because of method clashes between `public
> >>> String
> >>>> toString()` and `public scala.Predef.String toString()`.
> >>>>>
> >>>>> Given the size of the current code base, reimplementing the entire
> >>>> flink-table code in Java is a goal that we might never reach.
> However, we
> >>>> should at least treat the symptoms and have this as a long-term goal
> in
> >>>> mind. My suggestion would be to convert user-facing and runtime
> classes
> >>> and
> >>>> split the code base into multiple modules:
> >>>>>
> >>>>>> flink-table-java {depends on flink-table-core}
> >>>>> Implemented in Java. Java users can use this. This would require to
> >>>> convert classes like TableEnvironment, Table.
> >>>>>
> >>>>>> flink-table-scala {depends on flink-table-core}
> >>>>> Implemented in Scala. Scala users can use this.
> >>>>>
> >>>>>> flink-table-common
> >>>>> Implemented in Java. Connectors, formats, and UDFs can use this. It
> >>>> contains interface classes such as descriptors, table sink, table
> source.
> >>>>>
> >>>>>> flink-table-core {depends on flink-table-common and
> >>>> flink-table-runtime}
> >>>>> Implemented in Scala. Contains the current main code base.
> >>>>>
> >>>>>> flink-table-runtime
> >>>>> Implemented in Java. This would require to convert classes in
> >>>> o.a.f.table.runtime but would improve the runtime potentially.
> >>>>>
> >>>>>
> >>>>> What do you think?
> >>>>>
> >>>>>
> >>>>> Regards,
> >>>>>
> >>>>> Timo
> >>>>>
> >>>>> [1]
> >>>> http://apache-flink-mailing-list-archive.1008284.n3.
> >>> nabble.com/DISCUSS-Convert-main-Table-API-classes-into-
> traits-tp21335.html
> >>>>>
> >>>>
> >>>>
> >>>
> >
>
>

[Timo Walther-2]

Hi everyone,

I would like to continue this discussion thread and convert the outcome
into a FLIP such that users and contributors know what to expect in the
upcoming releases.

I created a design document [1] that clarifies our motivation why we
want to do this, how a Maven module structure could look like, and a
suggestion for a migration plan.

It would be great to start with the efforts for the 1.8 release such
that new features can be developed in Java and major refactorings such
as improvements to the connectors and external catalog support are not
blocked.

Please let me know what you think.

Regards,
Timo

[1]
https://docs.google.com/document/d/1PPo6goW7tOwxmpFuvLSjFnx7BF8IVz0w3dcmPPyqvoY/edit?usp=sharing


Am 02.07.18 um 17:08 schrieb Fabian Hueske:

> Hi Piotr,
>
> thanks for bumping this thread and thanks for Xingcan for the comments.
>
> I think the first step would be to separate the flink-table module into
> multiple sub modules. These could be:
>
> - flink-table-api: All API facing classes. Can be later divided further
> into Java/Scala Table API/SQL
> - flink-table-planning: involves all planning (basically everything we do
> with Calcite)
> - flink-table-runtime: the runtime code
>
> IMO, a realistic mid-term goal is to have the runtime module and certain
> parts of the planning module ported to Java.
> The api module will be much harder to port because of several dependencies
> to Scala core classes (the parser framework, tree iterations, etc.). I'm
> not saying we should not port this to Java, but it is not clear to me (yet)
> how to do it.
>
> I think flink-table-runtime should not be too hard to port. The code does
> not make use of many Scala features, i.e., it's writing very Java-like.
> Also, there are not many dependencies and operators can be individually
> ported step-by-step.
> For flink-table-planning, we can have certain packages that we port to Java
> like planning rules or plan nodes. The related classes mostly extend
> Calcite's Java interfaces/classes and would be natural choices for being
> ported. The code generation classes will require more effort to port. There
> are also some dependencies in planning on the api module that we would need
> to resolve somehow.
>
> For SQL most work when adding new features is done in the planning and
> runtime modules. So, this separation should already reduce "technological
> dept" quite a lot.
> The Table API depends much more on Scala than SQL.
>
> Cheers, Fabian
>
>
>
> 2018-07-02 16:26 GMT+02:00 Xingcan Cui <[hidden email]>:
>
>> Hi all,
>>
>> I also think about this problem these days and here are my thoughts.
>>
>> 1) We must admit that it’s really a tough task to interoperate with Java
>> and Scala. E.g., they have different collection types (Scala collections
>> v.s. java.util.*) and in Java, it's hard to implement a method which takes
>> Scala functions as parameters. Considering the major part of the code base
>> is implemented in Java, +1 for this goal from a long-term view.
>>
>> 2) The ideal solution would be to just expose a Scala API and make all the
>> other parts Scala-free. But I am not sure if it could be achieved even in a
>> long-term. Thus as Timo suggested, keep the Scala codes in
>> "flink-table-core" would be a compromise solution.
>>
>> 3) If the community makes the final decision, maybe any new features
>> should be added in Java (regardless of the modules), in order to prevent
>> the Scala codes from growing.
>>
>> Best,
>> Xingcan
>>
>>
>>> On Jul 2, 2018, at 9:30 PM, Piotr Nowojski <[hidden email]>
>> wrote:
>>> Bumping the topic.
>>>
>>> If we want to do this, the sooner we decide, the less code we will have
>> to rewrite. I have some objections/counter proposals to Fabian's proposal
>> of doing it module wise and one module at a time.
>>> First, I do not see a problem of having java/scala code even within one
>> module, especially not if there are clean boundaries. Like we could have
>> API in Scala and optimizer rules/logical nodes written in Java in the same
>> module. However I haven’t previously maintained mixed scala/java code bases
>> before, so I might be missing something here.
>>> Secondly this whole migration might and most like will take longer then
>> expected, so that creates a problem for a new code that we will be
>> creating. After making a decision to migrate to Java, almost any new Scala
>> line of code will be immediately a technological debt and we will have to
>> rewrite it to Java later.
>>> Thus I would propose first to state our end goal - modules structure and
>> which parts of modules we want to have eventually Scala-free. Secondly
>> taking all steps necessary that will allow us to write new code complaint
>> with our end goal. Only after that we should/could focus on incrementally
>> rewriting the old code. Otherwise we could be stuck/blocked for years
>> writing new code in Scala (and increasing technological debt), because
>> nobody have found a time to rewrite some non important and not actively
>> developed part of some module.
>>> Piotrek
>>>
>>>> On 14 Jun 2018, at 15:34, Fabian Hueske <[hidden email]> wrote:
>>>>
>>>> Hi,
>>>>
>>>> In general, I think this is a good effort. However, it won't be easy
>> and I
>>>> think we have to plan this well.
>>>> I don't like the idea of having the whole code base fragmented into Java
>>>> and Scala code for too long.
>>>>
>>>> I think we should do this one step at a time and focus on migrating one
>>>> module at a time.
>>>> IMO, the easiest start would be to port the runtime to Java.
>>>> Extracting the API classes into an own module, porting them to Java, and
>>>> removing the Scala dependency won't be possible without breaking the API
>>>> since a few classes depend on the Scala Table API.
>>>>
>>>> Best, Fabian
>>>>
>>>>
>>>> 2018-06-14 10:33 GMT+02:00 Till Rohrmann <[hidden email]>:
>>>>
>>>>> I think that is a noble and honorable goal and we should strive for it.
>>>>> This, however, must be an iterative process given the sheer size of the
>>>>> code base. I like the approach to define common Java modules which are
>> used
>>>>> by more specific Scala modules and slowly moving classes from Scala to
>>>>> Java. Thus +1 for the proposal.
>>>>>
>>>>> Cheers,
>>>>> Till
>>>>>
>>>>> On Wed, Jun 13, 2018 at 12:01 PM Piotr Nowojski <
>> [hidden email]>
>>>>> wrote:
>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> I do not have an experience with how scala and java interacts with
>> each
>>>>>> other, so I can not fully validate your proposal, but generally
>> speaking
>>>>> +1
>>>>>> from me.
>>>>>>
>>>>>> Does it also mean, that we should slowly migrate `flink-table-core` to
>>>>>> Java? How would you envision it? It would be nice to be able to add
>> new
>>>>>> classes/features written in Java and so that they can coexist with old
>>>>>> Scala code until we gradually switch from Scala to Java.
>>>>>>
>>>>>> Piotrek
>>>>>>
>>>>>>> On 13 Jun 2018, at 11:32, Timo Walther <[hidden email]> wrote:
>>>>>>>
>>>>>>> Hi everyone,
>>>>>>>
>>>>>>> as you all know, currently the Table & SQL API is implemented in
>> Scala.
>>>>>> This decision was made a long-time ago when the initital code base was
>>>>>> created as part of a master's thesis. The community kept Scala
>> because of
>>>>>> the nice language features that enable a fluent Table API like
>>>>>> table.select('field.trim()) and because Scala allows for quick
>>>>> prototyping
>>>>>> (e.g. multi-line comments for code generation). The committers
>> enforced
>>>>> not
>>>>>> splitting the code-base into two programming languages.
>>>>>>> However, nowadays the flink-table module more and more becomes an
>>>>>> important part in the Flink ecosystem. Connectors, formats, and SQL
>>>>> client
>>>>>> are actually implemented in Java but need to interoperate with
>>>>> flink-table
>>>>>> which makes these modules dependent on Scala. As mentioned in an
>> earlier
>>>>>> mail thread, using Scala for API classes also exposes member variables
>>>>> and
>>>>>> methods in Java that should not be exposed to users [1]. Java is still
>>>>> the
>>>>>> most important API language and right now we treat it as a
>> second-class
>>>>>> citizen. I just noticed that you even need to add Scala if you just
>> want
>>>>> to
>>>>>> implement a ScalarFunction because of method clashes between `public
>>>>> String
>>>>>> toString()` and `public scala.Predef.String toString()`.
>>>>>>> Given the size of the current code base, reimplementing the entire
>>>>>> flink-table code in Java is a goal that we might never reach.
>> However, we
>>>>>> should at least treat the symptoms and have this as a long-term goal
>> in
>>>>>> mind. My suggestion would be to convert user-facing and runtime
>> classes
>>>>> and
>>>>>> split the code base into multiple modules:
>>>>>>>> flink-table-java {depends on flink-table-core}
>>>>>>> Implemented in Java. Java users can use this. This would require to
>>>>>> convert classes like TableEnvironment, Table.
>>>>>>>> flink-table-scala {depends on flink-table-core}
>>>>>>> Implemented in Scala. Scala users can use this.
>>>>>>>
>>>>>>>> flink-table-common
>>>>>>> Implemented in Java. Connectors, formats, and UDFs can use this. It
>>>>>> contains interface classes such as descriptors, table sink, table
>> source.
>>>>>>>> flink-table-core {depends on flink-table-common and
>>>>>> flink-table-runtime}
>>>>>>> Implemented in Scala. Contains the current main code base.
>>>>>>>
>>>>>>>> flink-table-runtime
>>>>>>> Implemented in Java. This would require to convert classes in
>>>>>> o.a.f.table.runtime but would improve the runtime potentially.
>>>>>>>
>>>>>>> What do you think?
>>>>>>>
>>>>>>>
>>>>>>> Regards,
>>>>>>>
>>>>>>> Timo
>>>>>>>
>>>>>>> [1]
>>>>>> http://apache-flink-mailing-list-archive.1008284.n3.
>>>>> nabble.com/DISCUSS-Convert-main-Table-API-classes-into-
>> traits-tp21335.html
>>>>>>
>>

[Fabian Hueske-2]

Hi Timo,

Thanks for writing up this document.
I like the new structure and agree to prioritize the porting of the
flink-table-common classes.
Since flink-table-runtime is (or should be) independent of the API and
planner modules, we could start porting these classes once the code is
split into the new module structure.
The benefits of a Scala-free flink-table-runtime would be a Scala-free
execution Jar.

Best, Fabian


Am Do., 22. Nov. 2018 um 10:54 Uhr schrieb Timo Walther <[hidden email]
>:

> Hi everyone,
>
> I would like to continue this discussion thread and convert the outcome
> into a FLIP such that users and contributors know what to expect in the
> upcoming releases.
>
> I created a design document [1] that clarifies our motivation why we
> want to do this, how a Maven module structure could look like, and a
> suggestion for a migration plan.
>
> It would be great to start with the efforts for the 1.8 release such
> that new features can be developed in Java and major refactorings such
> as improvements to the connectors and external catalog support are not
> blocked.
>
> Please let me know what you think.
>
> Regards,
> Timo
>
> [1]
>
> https://docs.google.com/document/d/1PPo6goW7tOwxmpFuvLSjFnx7BF8IVz0w3dcmPPyqvoY/edit?usp=sharing
>
>
> Am 02.07.18 um 17:08 schrieb Fabian Hueske:
> > Hi Piotr,
> >
> > thanks for bumping this thread and thanks for Xingcan for the comments.
> >
> > I think the first step would be to separate the flink-table module into
> > multiple sub modules. These could be:
> >
> > - flink-table-api: All API facing classes. Can be later divided further
> > into Java/Scala Table API/SQL
> > - flink-table-planning: involves all planning (basically everything we do
> > with Calcite)
> > - flink-table-runtime: the runtime code
> >
> > IMO, a realistic mid-term goal is to have the runtime module and certain
> > parts of the planning module ported to Java.
> > The api module will be much harder to port because of several
> dependencies
> > to Scala core classes (the parser framework, tree iterations, etc.). I'm
> > not saying we should not port this to Java, but it is not clear to me
> (yet)
> > how to do it.
> >
> > I think flink-table-runtime should not be too hard to port. The code does
> > not make use of many Scala features, i.e., it's writing very Java-like.
> > Also, there are not many dependencies and operators can be individually
> > ported step-by-step.
> > For flink-table-planning, we can have certain packages that we port to
> Java
> > like planning rules or plan nodes. The related classes mostly extend
> > Calcite's Java interfaces/classes and would be natural choices for being
> > ported. The code generation classes will require more effort to port.
> There
> > are also some dependencies in planning on the api module that we would
> need
> > to resolve somehow.
> >
> > For SQL most work when adding new features is done in the planning and
> > runtime modules. So, this separation should already reduce "technological
> > dept" quite a lot.
> > The Table API depends much more on Scala than SQL.
> >
> > Cheers, Fabian
> >
> >
> >
> > 2018-07-02 16:26 GMT+02:00 Xingcan Cui <[hidden email]>:
> >
> >> Hi all,
> >>
> >> I also think about this problem these days and here are my thoughts.
> >>
> >> 1) We must admit that it’s really a tough task to interoperate with Java
> >> and Scala. E.g., they have different collection types (Scala collections
> >> v.s. java.util.*) and in Java, it's hard to implement a method which
> takes
> >> Scala functions as parameters. Considering the major part of the code
> base
> >> is implemented in Java, +1 for this goal from a long-term view.
> >>
> >> 2) The ideal solution would be to just expose a Scala API and make all
> the
> >> other parts Scala-free. But I am not sure if it could be achieved even
> in a
> >> long-term. Thus as Timo suggested, keep the Scala codes in
> >> "flink-table-core" would be a compromise solution.
> >>
> >> 3) If the community makes the final decision, maybe any new features
> >> should be added in Java (regardless of the modules), in order to prevent
> >> the Scala codes from growing.
> >>
> >> Best,
> >> Xingcan
> >>
> >>
> >>> On Jul 2, 2018, at 9:30 PM, Piotr Nowojski <[hidden email]>
> >> wrote:
> >>> Bumping the topic.
> >>>
> >>> If we want to do this, the sooner we decide, the less code we will have
> >> to rewrite. I have some objections/counter proposals to Fabian's
> proposal
> >> of doing it module wise and one module at a time.
> >>> First, I do not see a problem of having java/scala code even within one
> >> module, especially not if there are clean boundaries. Like we could have
> >> API in Scala and optimizer rules/logical nodes written in Java in the
> same
> >> module. However I haven’t previously maintained mixed scala/java code
> bases
> >> before, so I might be missing something here.
> >>> Secondly this whole migration might and most like will take longer then
> >> expected, so that creates a problem for a new code that we will be
> >> creating. After making a decision to migrate to Java, almost any new
> Scala
> >> line of code will be immediately a technological debt and we will have
> to
> >> rewrite it to Java later.
> >>> Thus I would propose first to state our end goal - modules structure
> and
> >> which parts of modules we want to have eventually Scala-free. Secondly
> >> taking all steps necessary that will allow us to write new code
> complaint
> >> with our end goal. Only after that we should/could focus on
> incrementally
> >> rewriting the old code. Otherwise we could be stuck/blocked for years
> >> writing new code in Scala (and increasing technological debt), because
> >> nobody have found a time to rewrite some non important and not actively
> >> developed part of some module.
> >>> Piotrek
> >>>
> >>>> On 14 Jun 2018, at 15:34, Fabian Hueske <[hidden email]> wrote:
> >>>>
> >>>> Hi,
> >>>>
> >>>> In general, I think this is a good effort. However, it won't be easy
> >> and I
> >>>> think we have to plan this well.
> >>>> I don't like the idea of having the whole code base fragmented into
> Java
> >>>> and Scala code for too long.
> >>>>
> >>>> I think we should do this one step at a time and focus on migrating
> one
> >>>> module at a time.
> >>>> IMO, the easiest start would be to port the runtime to Java.
> >>>> Extracting the API classes into an own module, porting them to Java,
> and
> >>>> removing the Scala dependency won't be possible without breaking the
> API
> >>>> since a few classes depend on the Scala Table API.
> >>>>
> >>>> Best, Fabian
> >>>>
> >>>>
> >>>> 2018-06-14 10:33 GMT+02:00 Till Rohrmann <[hidden email]>:
> >>>>
> >>>>> I think that is a noble and honorable goal and we should strive for
> it.
> >>>>> This, however, must be an iterative process given the sheer size of
> the
> >>>>> code base. I like the approach to define common Java modules which
> are
> >> used
> >>>>> by more specific Scala modules and slowly moving classes from Scala
> to
> >>>>> Java. Thus +1 for the proposal.
> >>>>>
> >>>>> Cheers,
> >>>>> Till
> >>>>>
> >>>>> On Wed, Jun 13, 2018 at 12:01 PM Piotr Nowojski <
> >> [hidden email]>
> >>>>> wrote:
> >>>>>
> >>>>>> Hi,
> >>>>>>
> >>>>>> I do not have an experience with how scala and java interacts with
> >> each
> >>>>>> other, so I can not fully validate your proposal, but generally
> >> speaking
> >>>>> +1
> >>>>>> from me.
> >>>>>>
> >>>>>> Does it also mean, that we should slowly migrate `flink-table-core`
> to
> >>>>>> Java? How would you envision it? It would be nice to be able to add
> >> new
> >>>>>> classes/features written in Java and so that they can coexist with
> old
> >>>>>> Scala code until we gradually switch from Scala to Java.
> >>>>>>
> >>>>>> Piotrek
> >>>>>>
> >>>>>>> On 13 Jun 2018, at 11:32, Timo Walther <[hidden email]> wrote:
> >>>>>>>
> >>>>>>> Hi everyone,
> >>>>>>>
> >>>>>>> as you all know, currently the Table & SQL API is implemented in
> >> Scala.
> >>>>>> This decision was made a long-time ago when the initital code base
> was
> >>>>>> created as part of a master's thesis. The community kept Scala
> >> because of
> >>>>>> the nice language features that enable a fluent Table API like
> >>>>>> table.select('field.trim()) and because Scala allows for quick
> >>>>> prototyping
> >>>>>> (e.g. multi-line comments for code generation). The committers
> >> enforced
> >>>>> not
> >>>>>> splitting the code-base into two programming languages.
> >>>>>>> However, nowadays the flink-table module more and more becomes an
> >>>>>> important part in the Flink ecosystem. Connectors, formats, and SQL
> >>>>> client
> >>>>>> are actually implemented in Java but need to interoperate with
> >>>>> flink-table
> >>>>>> which makes these modules dependent on Scala. As mentioned in an
> >> earlier
> >>>>>> mail thread, using Scala for API classes also exposes member
> variables
> >>>>> and
> >>>>>> methods in Java that should not be exposed to users [1]. Java is
> still
> >>>>> the
> >>>>>> most important API language and right now we treat it as a
> >> second-class
> >>>>>> citizen. I just noticed that you even need to add Scala if you just
> >> want
> >>>>> to
> >>>>>> implement a ScalarFunction because of method clashes between `public
> >>>>> String
> >>>>>> toString()` and `public scala.Predef.String toString()`.
> >>>>>>> Given the size of the current code base, reimplementing the entire
> >>>>>> flink-table code in Java is a goal that we might never reach.
> >> However, we
> >>>>>> should at least treat the symptoms and have this as a long-term goal
> >> in
> >>>>>> mind. My suggestion would be to convert user-facing and runtime
> >> classes
> >>>>> and
> >>>>>> split the code base into multiple modules:
> >>>>>>>> flink-table-java {depends on flink-table-core}
> >>>>>>> Implemented in Java. Java users can use this. This would require to
> >>>>>> convert classes like TableEnvironment, Table.
> >>>>>>>> flink-table-scala {depends on flink-table-core}
> >>>>>>> Implemented in Scala. Scala users can use this.
> >>>>>>>
> >>>>>>>> flink-table-common
> >>>>>>> Implemented in Java. Connectors, formats, and UDFs can use this. It
> >>>>>> contains interface classes such as descriptors, table sink, table
> >> source.
> >>>>>>>> flink-table-core {depends on flink-table-common and
> >>>>>> flink-table-runtime}
> >>>>>>> Implemented in Scala. Contains the current main code base.
> >>>>>>>
> >>>>>>>> flink-table-runtime
> >>>>>>> Implemented in Java. This would require to convert classes in
> >>>>>> o.a.f.table.runtime but would improve the runtime potentially.
> >>>>>>>
> >>>>>>> What do you think?
> >>>>>>>
> >>>>>>>
> >>>>>>> Regards,
> >>>>>>>
> >>>>>>> Timo
> >>>>>>>
> >>>>>>> [1]
> >>>>>> http://apache-flink-mailing-list-archive.1008284.n3.
> >>>>> nabble.com/DISCUSS-Convert-main-Table-API-classes-into-
> >> traits-tp21335.html
> >>>>>>
> >>
>
>

[Xiaowei Jiang]

Hi Timo, thanks for driving this! I think that this is a nice thing to do.
While we are doing this, can we also keep in mind that we want to
eventually have a TableAPI interface only module which users can take
dependency on, but without including any implementation details?

Xiaowei

On Thu, Nov 22, 2018 at 6:37 PM Fabian Hueske <[hidden email]> wrote:

> Hi Timo,
>
> Thanks for writing up this document.
> I like the new structure and agree to prioritize the porting of the
> flink-table-common classes.
> Since flink-table-runtime is (or should be) independent of the API and
> planner modules, we could start porting these classes once the code is
> split into the new module structure.
> The benefits of a Scala-free flink-table-runtime would be a Scala-free
> execution Jar.
>
> Best, Fabian
>
>
> Am Do., 22. Nov. 2018 um 10:54 Uhr schrieb Timo Walther <
> [hidden email]
> >:
>
> > Hi everyone,
> >
> > I would like to continue this discussion thread and convert the outcome
> > into a FLIP such that users and contributors know what to expect in the
> > upcoming releases.
> >
> > I created a design document [1] that clarifies our motivation why we
> > want to do this, how a Maven module structure could look like, and a
> > suggestion for a migration plan.
> >
> > It would be great to start with the efforts for the 1.8 release such
> > that new features can be developed in Java and major refactorings such
> > as improvements to the connectors and external catalog support are not
> > blocked.
> >
> > Please let me know what you think.
> >
> > Regards,
> > Timo
> >
> > [1]
> >
> >
> https://docs.google.com/document/d/1PPo6goW7tOwxmpFuvLSjFnx7BF8IVz0w3dcmPPyqvoY/edit?usp=sharing
> >
> >
> > Am 02.07.18 um 17:08 schrieb Fabian Hueske:
> > > Hi Piotr,
> > >
> > > thanks for bumping this thread and thanks for Xingcan for the comments.
> > >
> > > I think the first step would be to separate the flink-table module into
> > > multiple sub modules. These could be:
> > >
> > > - flink-table-api: All API facing classes. Can be later divided further
> > > into Java/Scala Table API/SQL
> > > - flink-table-planning: involves all planning (basically everything we
> do
> > > with Calcite)
> > > - flink-table-runtime: the runtime code
> > >
> > > IMO, a realistic mid-term goal is to have the runtime module and
> certain
> > > parts of the planning module ported to Java.
> > > The api module will be much harder to port because of several
> > dependencies
> > > to Scala core classes (the parser framework, tree iterations, etc.).
> I'm
> > > not saying we should not port this to Java, but it is not clear to me
> > (yet)
> > > how to do it.
> > >
> > > I think flink-table-runtime should not be too hard to port. The code
> does
> > > not make use of many Scala features, i.e., it's writing very Java-like.
> > > Also, there are not many dependencies and operators can be individually
> > > ported step-by-step.
> > > For flink-table-planning, we can have certain packages that we port to
> > Java
> > > like planning rules or plan nodes. The related classes mostly extend
> > > Calcite's Java interfaces/classes and would be natural choices for
> being
> > > ported. The code generation classes will require more effort to port.
> > There
> > > are also some dependencies in planning on the api module that we would
> > need
> > > to resolve somehow.
> > >
> > > For SQL most work when adding new features is done in the planning and
> > > runtime modules. So, this separation should already reduce
> "technological
> > > dept" quite a lot.
> > > The Table API depends much more on Scala than SQL.
> > >
> > > Cheers, Fabian
> > >
> > >
> > >
> > > 2018-07-02 16:26 GMT+02:00 Xingcan Cui <[hidden email]>:
> > >
> > >> Hi all,
> > >>
> > >> I also think about this problem these days and here are my thoughts.
> > >>
> > >> 1) We must admit that it’s really a tough task to interoperate with
> Java
> > >> and Scala. E.g., they have different collection types (Scala
> collections
> > >> v.s. java.util.*) and in Java, it's hard to implement a method which
> > takes
> > >> Scala functions as parameters. Considering the major part of the code
> > base
> > >> is implemented in Java, +1 for this goal from a long-term view.
> > >>
> > >> 2) The ideal solution would be to just expose a Scala API and make all
> > the
> > >> other parts Scala-free. But I am not sure if it could be achieved even
> > in a
> > >> long-term. Thus as Timo suggested, keep the Scala codes in
> > >> "flink-table-core" would be a compromise solution.
> > >>
> > >> 3) If the community makes the final decision, maybe any new features
> > >> should be added in Java (regardless of the modules), in order to
> prevent
> > >> the Scala codes from growing.
> > >>
> > >> Best,
> > >> Xingcan
> > >>
> > >>
> > >>> On Jul 2, 2018, at 9:30 PM, Piotr Nowojski <[hidden email]>
> > >> wrote:
> > >>> Bumping the topic.
> > >>>
> > >>> If we want to do this, the sooner we decide, the less code we will
> have
> > >> to rewrite. I have some objections/counter proposals to Fabian's
> > proposal
> > >> of doing it module wise and one module at a time.
> > >>> First, I do not see a problem of having java/scala code even within
> one
> > >> module, especially not if there are clean boundaries. Like we could
> have
> > >> API in Scala and optimizer rules/logical nodes written in Java in the
> > same
> > >> module. However I haven’t previously maintained mixed scala/java code
> > bases
> > >> before, so I might be missing something here.
> > >>> Secondly this whole migration might and most like will take longer
> then
> > >> expected, so that creates a problem for a new code that we will be
> > >> creating. After making a decision to migrate to Java, almost any new
> > Scala
> > >> line of code will be immediately a technological debt and we will have
> > to
> > >> rewrite it to Java later.
> > >>> Thus I would propose first to state our end goal - modules structure
> > and
> > >> which parts of modules we want to have eventually Scala-free. Secondly
> > >> taking all steps necessary that will allow us to write new code
> > complaint
> > >> with our end goal. Only after that we should/could focus on
> > incrementally
> > >> rewriting the old code. Otherwise we could be stuck/blocked for years
> > >> writing new code in Scala (and increasing technological debt), because
> > >> nobody have found a time to rewrite some non important and not
> actively
> > >> developed part of some module.
> > >>> Piotrek
> > >>>
> > >>>> On 14 Jun 2018, at 15:34, Fabian Hueske <[hidden email]> wrote:
> > >>>>
> > >>>> Hi,
> > >>>>
> > >>>> In general, I think this is a good effort. However, it won't be easy
> > >> and I
> > >>>> think we have to plan this well.
> > >>>> I don't like the idea of having the whole code base fragmented into
> > Java
> > >>>> and Scala code for too long.
> > >>>>
> > >>>> I think we should do this one step at a time and focus on migrating
> > one
> > >>>> module at a time.
> > >>>> IMO, the easiest start would be to port the runtime to Java.
> > >>>> Extracting the API classes into an own module, porting them to Java,
> > and
> > >>>> removing the Scala dependency won't be possible without breaking the
> > API
> > >>>> since a few classes depend on the Scala Table API.
> > >>>>
> > >>>> Best, Fabian
> > >>>>
> > >>>>
> > >>>> 2018-06-14 10:33 GMT+02:00 Till Rohrmann <[hidden email]>:
> > >>>>
> > >>>>> I think that is a noble and honorable goal and we should strive for
> > it.
> > >>>>> This, however, must be an iterative process given the sheer size of
> > the
> > >>>>> code base. I like the approach to define common Java modules which
> > are
> > >> used
> > >>>>> by more specific Scala modules and slowly moving classes from Scala
> > to
> > >>>>> Java. Thus +1 for the proposal.
> > >>>>>
> > >>>>> Cheers,
> > >>>>> Till
> > >>>>>
> > >>>>> On Wed, Jun 13, 2018 at 12:01 PM Piotr Nowojski <
> > >> [hidden email]>
> > >>>>> wrote:
> > >>>>>
> > >>>>>> Hi,
> > >>>>>>
> > >>>>>> I do not have an experience with how scala and java interacts with
> > >> each
> > >>>>>> other, so I can not fully validate your proposal, but generally
> > >> speaking
> > >>>>> +1
> > >>>>>> from me.
> > >>>>>>
> > >>>>>> Does it also mean, that we should slowly migrate
> `flink-table-core`
> > to
> > >>>>>> Java? How would you envision it? It would be nice to be able to
> add
> > >> new
> > >>>>>> classes/features written in Java and so that they can coexist with
> > old
> > >>>>>> Scala code until we gradually switch from Scala to Java.
> > >>>>>>
> > >>>>>> Piotrek
> > >>>>>>
> > >>>>>>> On 13 Jun 2018, at 11:32, Timo Walther <[hidden email]>
> wrote:
> > >>>>>>>
> > >>>>>>> Hi everyone,
> > >>>>>>>
> > >>>>>>> as you all know, currently the Table & SQL API is implemented in
> > >> Scala.
> > >>>>>> This decision was made a long-time ago when the initital code base
> > was
> > >>>>>> created as part of a master's thesis. The community kept Scala
> > >> because of
> > >>>>>> the nice language features that enable a fluent Table API like
> > >>>>>> table.select('field.trim()) and because Scala allows for quick
> > >>>>> prototyping
> > >>>>>> (e.g. multi-line comments for code generation). The committers
> > >> enforced
> > >>>>> not
> > >>>>>> splitting the code-base into two programming languages.
> > >>>>>>> However, nowadays the flink-table module more and more becomes an
> > >>>>>> important part in the Flink ecosystem. Connectors, formats, and
> SQL
> > >>>>> client
> > >>>>>> are actually implemented in Java but need to interoperate with
> > >>>>> flink-table
> > >>>>>> which makes these modules dependent on Scala. As mentioned in an
> > >> earlier
> > >>>>>> mail thread, using Scala for API classes also exposes member
> > variables
> > >>>>> and
> > >>>>>> methods in Java that should not be exposed to users [1]. Java is
> > still
> > >>>>> the
> > >>>>>> most important API language and right now we treat it as a
> > >> second-class
> > >>>>>> citizen. I just noticed that you even need to add Scala if you
> just
> > >> want
> > >>>>> to
> > >>>>>> implement a ScalarFunction because of method clashes between
> `public
> > >>>>> String
> > >>>>>> toString()` and `public scala.Predef.String toString()`.
> > >>>>>>> Given the size of the current code base, reimplementing the
> entire
> > >>>>>> flink-table code in Java is a goal that we might never reach.
> > >> However, we
> > >>>>>> should at least treat the symptoms and have this as a long-term
> goal
> > >> in
> > >>>>>> mind. My suggestion would be to convert user-facing and runtime
> > >> classes
> > >>>>> and
> > >>>>>> split the code base into multiple modules:
> > >>>>>>>> flink-table-java {depends on flink-table-core}
> > >>>>>>> Implemented in Java. Java users can use this. This would require
> to
> > >>>>>> convert classes like TableEnvironment, Table.
> > >>>>>>>> flink-table-scala {depends on flink-table-core}
> > >>>>>>> Implemented in Scala. Scala users can use this.
> > >>>>>>>
> > >>>>>>>> flink-table-common
> > >>>>>>> Implemented in Java. Connectors, formats, and UDFs can use this.
> It
> > >>>>>> contains interface classes such as descriptors, table sink, table
> > >> source.
> > >>>>>>>> flink-table-core {depends on flink-table-common and
> > >>>>>> flink-table-runtime}
> > >>>>>>> Implemented in Scala. Contains the current main code base.
> > >>>>>>>
> > >>>>>>>> flink-table-runtime
> > >>>>>>> Implemented in Java. This would require to convert classes in
> > >>>>>> o.a.f.table.runtime but would improve the runtime potentially.
> > >>>>>>>
> > >>>>>>> What do you think?
> > >>>>>>>
> > >>>>>>>
> > >>>>>>> Regards,
> > >>>>>>>
> > >>>>>>> Timo
> > >>>>>>>
> > >>>>>>> [1]
> > >>>>>> http://apache-flink-mailing-list-archive.1008284.n3.
> > >>>>> nabble.com/DISCUSS-Convert-main-Table-API-classes-into-
> > >> traits-tp21335.html
> > >>>>>>
> > >>
> >
> >
>

[jincheng sun]

Hi Timo,
Thanks for initiating this great discussion.

Currently when using SQL/TableAPI should include many dependence. In
particular, it is not necessary to introduce the specific implementation
dependencies which users do not care about. So I am glad to see your
proposal, and hope when we consider splitting the API interface into a
separate module, so that the user can introduce minimum of dependencies.

So, +1 to [separation of interface and implementation; e.g. `Table` &
`TableImpl`] which you mentioned in the google doc.
Best,
Jincheng

Xiaowei Jiang <[hidden email]> 于2018年11月22日周四 下午10:50写道：

> Hi Timo, thanks for driving this! I think that this is a nice thing to do.
> While we are doing this, can we also keep in mind that we want to
> eventually have a TableAPI interface only module which users can take
> dependency on, but without including any implementation details?
>
> Xiaowei
>
> On Thu, Nov 22, 2018 at 6:37 PM Fabian Hueske <[hidden email]> wrote:
>
> > Hi Timo,
> >
> > Thanks for writing up this document.
> > I like the new structure and agree to prioritize the porting of the
> > flink-table-common classes.
> > Since flink-table-runtime is (or should be) independent of the API and
> > planner modules, we could start porting these classes once the code is
> > split into the new module structure.
> > The benefits of a Scala-free flink-table-runtime would be a Scala-free
> > execution Jar.
> >
> > Best, Fabian
> >
> >
> > Am Do., 22. Nov. 2018 um 10:54 Uhr schrieb Timo Walther <
> > [hidden email]
> > >:
> >
> > > Hi everyone,
> > >
> > > I would like to continue this discussion thread and convert the outcome
> > > into a FLIP such that users and contributors know what to expect in the
> > > upcoming releases.
> > >
> > > I created a design document [1] that clarifies our motivation why we
> > > want to do this, how a Maven module structure could look like, and a
> > > suggestion for a migration plan.
> > >
> > > It would be great to start with the efforts for the 1.8 release such
> > > that new features can be developed in Java and major refactorings such
> > > as improvements to the connectors and external catalog support are not
> > > blocked.
> > >
> > > Please let me know what you think.
> > >
> > > Regards,
> > > Timo
> > >
> > > [1]
> > >
> > >
> >
> https://docs.google.com/document/d/1PPo6goW7tOwxmpFuvLSjFnx7BF8IVz0w3dcmPPyqvoY/edit?usp=sharing
> > >
> > >
> > > Am 02.07.18 um 17:08 schrieb Fabian Hueske:
> > > > Hi Piotr,
> > > >
> > > > thanks for bumping this thread and thanks for Xingcan for the
> comments.
> > > >
> > > > I think the first step would be to separate the flink-table module
> into
> > > > multiple sub modules. These could be:
> > > >
> > > > - flink-table-api: All API facing classes. Can be later divided
> further
> > > > into Java/Scala Table API/SQL
> > > > - flink-table-planning: involves all planning (basically everything
> we
> > do
> > > > with Calcite)
> > > > - flink-table-runtime: the runtime code
> > > >
> > > > IMO, a realistic mid-term goal is to have the runtime module and
> > certain
> > > > parts of the planning module ported to Java.
> > > > The api module will be much harder to port because of several
> > > dependencies
> > > > to Scala core classes (the parser framework, tree iterations, etc.).
> > I'm
> > > > not saying we should not port this to Java, but it is not clear to me
> > > (yet)
> > > > how to do it.
> > > >
> > > > I think flink-table-runtime should not be too hard to port. The code
> > does
> > > > not make use of many Scala features, i.e., it's writing very
> Java-like.
> > > > Also, there are not many dependencies and operators can be
> individually
> > > > ported step-by-step.
> > > > For flink-table-planning, we can have certain packages that we port
> to
> > > Java
> > > > like planning rules or plan nodes. The related classes mostly extend
> > > > Calcite's Java interfaces/classes and would be natural choices for
> > being
> > > > ported. The code generation classes will require more effort to port.
> > > There
> > > > are also some dependencies in planning on the api module that we
> would
> > > need
> > > > to resolve somehow.
> > > >
> > > > For SQL most work when adding new features is done in the planning
> and
> > > > runtime modules. So, this separation should already reduce
> > "technological
> > > > dept" quite a lot.
> > > > The Table API depends much more on Scala than SQL.
> > > >
> > > > Cheers, Fabian
> > > >
> > > >
> > > >
> > > > 2018-07-02 16:26 GMT+02:00 Xingcan Cui <[hidden email]>:
> > > >
> > > >> Hi all,
> > > >>
> > > >> I also think about this problem these days and here are my thoughts.
> > > >>
> > > >> 1) We must admit that it’s really a tough task to interoperate with
> > Java
> > > >> and Scala. E.g., they have different collection types (Scala
> > collections
> > > >> v.s. java.util.*) and in Java, it's hard to implement a method which
> > > takes
> > > >> Scala functions as parameters. Considering the major part of the
> code
> > > base
> > > >> is implemented in Java, +1 for this goal from a long-term view.
> > > >>
> > > >> 2) The ideal solution would be to just expose a Scala API and make
> all
> > > the
> > > >> other parts Scala-free. But I am not sure if it could be achieved
> even
> > > in a
> > > >> long-term. Thus as Timo suggested, keep the Scala codes in
> > > >> "flink-table-core" would be a compromise solution.
> > > >>
> > > >> 3) If the community makes the final decision, maybe any new features
> > > >> should be added in Java (regardless of the modules), in order to
> > prevent
> > > >> the Scala codes from growing.
> > > >>
> > > >> Best,
> > > >> Xingcan
> > > >>
> > > >>
> > > >>> On Jul 2, 2018, at 9:30 PM, Piotr Nowojski <
> [hidden email]>
> > > >> wrote:
> > > >>> Bumping the topic.
> > > >>>
> > > >>> If we want to do this, the sooner we decide, the less code we will
> > have
> > > >> to rewrite. I have some objections/counter proposals to Fabian's
> > > proposal
> > > >> of doing it module wise and one module at a time.
> > > >>> First, I do not see a problem of having java/scala code even within
> > one
> > > >> module, especially not if there are clean boundaries. Like we could
> > have
> > > >> API in Scala and optimizer rules/logical nodes written in Java in
> the
> > > same
> > > >> module. However I haven’t previously maintained mixed scala/java
> code
> > > bases
> > > >> before, so I might be missing something here.
> > > >>> Secondly this whole migration might and most like will take longer
> > then
> > > >> expected, so that creates a problem for a new code that we will be
> > > >> creating. After making a decision to migrate to Java, almost any new
> > > Scala
> > > >> line of code will be immediately a technological debt and we will
> have
> > > to
> > > >> rewrite it to Java later.
> > > >>> Thus I would propose first to state our end goal - modules
> structure
> > > and
> > > >> which parts of modules we want to have eventually Scala-free.
> Secondly
> > > >> taking all steps necessary that will allow us to write new code
> > > complaint
> > > >> with our end goal. Only after that we should/could focus on
> > > incrementally
> > > >> rewriting the old code. Otherwise we could be stuck/blocked for
> years
> > > >> writing new code in Scala (and increasing technological debt),
> because
> > > >> nobody have found a time to rewrite some non important and not
> > actively
> > > >> developed part of some module.
> > > >>> Piotrek
> > > >>>
> > > >>>> On 14 Jun 2018, at 15:34, Fabian Hueske <[hidden email]>
> wrote:
> > > >>>>
> > > >>>> Hi,
> > > >>>>
> > > >>>> In general, I think this is a good effort. However, it won't be
> easy
> > > >> and I
> > > >>>> think we have to plan this well.
> > > >>>> I don't like the idea of having the whole code base fragmented
> into
> > > Java
> > > >>>> and Scala code for too long.
> > > >>>>
> > > >>>> I think we should do this one step at a time and focus on
> migrating
> > > one
> > > >>>> module at a time.
> > > >>>> IMO, the easiest start would be to port the runtime to Java.
> > > >>>> Extracting the API classes into an own module, porting them to
> Java,
> > > and
> > > >>>> removing the Scala dependency won't be possible without breaking
> the
> > > API
> > > >>>> since a few classes depend on the Scala Table API.
> > > >>>>
> > > >>>> Best, Fabian
> > > >>>>
> > > >>>>
> > > >>>> 2018-06-14 10:33 GMT+02:00 Till Rohrmann <[hidden email]>:
> > > >>>>
> > > >>>>> I think that is a noble and honorable goal and we should strive
> for
> > > it.
> > > >>>>> This, however, must be an iterative process given the sheer size
> of
> > > the
> > > >>>>> code base. I like the approach to define common Java modules
> which
> > > are
> > > >> used
> > > >>>>> by more specific Scala modules and slowly moving classes from
> Scala
> > > to
> > > >>>>> Java. Thus +1 for the proposal.
> > > >>>>>
> > > >>>>> Cheers,
> > > >>>>> Till
> > > >>>>>
> > > >>>>> On Wed, Jun 13, 2018 at 12:01 PM Piotr Nowojski <
> > > >> [hidden email]>
> > > >>>>> wrote:
> > > >>>>>
> > > >>>>>> Hi,
> > > >>>>>>
> > > >>>>>> I do not have an experience with how scala and java interacts
> with
> > > >> each
> > > >>>>>> other, so I can not fully validate your proposal, but generally
> > > >> speaking
> > > >>>>> +1
> > > >>>>>> from me.
> > > >>>>>>
> > > >>>>>> Does it also mean, that we should slowly migrate
> > `flink-table-core`
> > > to
> > > >>>>>> Java? How would you envision it? It would be nice to be able to
> > add
> > > >> new
> > > >>>>>> classes/features written in Java and so that they can coexist
> with
> > > old
> > > >>>>>> Scala code until we gradually switch from Scala to Java.
> > > >>>>>>
> > > >>>>>> Piotrek
> > > >>>>>>
> > > >>>>>>> On 13 Jun 2018, at 11:32, Timo Walther <[hidden email]>
> > wrote:
> > > >>>>>>>
> > > >>>>>>> Hi everyone,
> > > >>>>>>>
> > > >>>>>>> as you all know, currently the Table & SQL API is implemented
> in
> > > >> Scala.
> > > >>>>>> This decision was made a long-time ago when the initital code
> base
> > > was
> > > >>>>>> created as part of a master's thesis. The community kept Scala
> > > >> because of
> > > >>>>>> the nice language features that enable a fluent Table API like
> > > >>>>>> table.select('field.trim()) and because Scala allows for quick
> > > >>>>> prototyping
> > > >>>>>> (e.g. multi-line comments for code generation). The committers
> > > >> enforced
> > > >>>>> not
> > > >>>>>> splitting the code-base into two programming languages.
> > > >>>>>>> However, nowadays the flink-table module more and more becomes
> an
> > > >>>>>> important part in the Flink ecosystem. Connectors, formats, and
> > SQL
> > > >>>>> client
> > > >>>>>> are actually implemented in Java but need to interoperate with
> > > >>>>> flink-table
> > > >>>>>> which makes these modules dependent on Scala. As mentioned in an
> > > >> earlier
> > > >>>>>> mail thread, using Scala for API classes also exposes member
> > > variables
> > > >>>>> and
> > > >>>>>> methods in Java that should not be exposed to users [1]. Java is
> > > still
> > > >>>>> the
> > > >>>>>> most important API language and right now we treat it as a
> > > >> second-class
> > > >>>>>> citizen. I just noticed that you even need to add Scala if you
> > just
> > > >> want
> > > >>>>> to
> > > >>>>>> implement a ScalarFunction because of method clashes between
> > `public
> > > >>>>> String
> > > >>>>>> toString()` and `public scala.Predef.String toString()`.
> > > >>>>>>> Given the size of the current code base, reimplementing the
> > entire
> > > >>>>>> flink-table code in Java is a goal that we might never reach.
> > > >> However, we
> > > >>>>>> should at least treat the symptoms and have this as a long-term
> > goal
> > > >> in
> > > >>>>>> mind. My suggestion would be to convert user-facing and runtime
> > > >> classes
> > > >>>>> and
> > > >>>>>> split the code base into multiple modules:
> > > >>>>>>>> flink-table-java {depends on flink-table-core}
> > > >>>>>>> Implemented in Java. Java users can use this. This would
> require
> > to
> > > >>>>>> convert classes like TableEnvironment, Table.
> > > >>>>>>>> flink-table-scala {depends on flink-table-core}
> > > >>>>>>> Implemented in Scala. Scala users can use this.
> > > >>>>>>>
> > > >>>>>>>> flink-table-common
> > > >>>>>>> Implemented in Java. Connectors, formats, and UDFs can use
> this.
> > It
> > > >>>>>> contains interface classes such as descriptors, table sink,
> table
> > > >> source.
> > > >>>>>>>> flink-table-core {depends on flink-table-common and
> > > >>>>>> flink-table-runtime}
> > > >>>>>>> Implemented in Scala. Contains the current main code base.
> > > >>>>>>>
> > > >>>>>>>> flink-table-runtime
> > > >>>>>>> Implemented in Java. This would require to convert classes in
> > > >>>>>> o.a.f.table.runtime but would improve the runtime potentially.
> > > >>>>>>>
> > > >>>>>>> What do you think?
> > > >>>>>>>
> > > >>>>>>>
> > > >>>>>>> Regards,
> > > >>>>>>>
> > > >>>>>>> Timo
> > > >>>>>>>
> > > >>>>>>> [1]
> > > >>>>>> http://apache-flink-mailing-list-archive.1008284.n3.
> > > >>>>> nabble.com/DISCUSS-Convert-main-Table-API-classes-into-
> > > >> traits-tp21335.html
> > > >>>>>>
> > > >>
> > >
> > >
> >
>

[Zhang, Xuefu]

Hi Timo,

Thanks for the effort and the Google writeup. During our external catalog rework, we found much confusion between Java and Scala, and this Scala-free roadmap should greatly mitigate that.

I'm wondering that whether we can have rule in the interim when Java and Scala coexist that dependency can only be one-way. I found that in the current code base there are cases where a Scala class extends Java and vise versa. This is quite painful. I'm thinking if we could say that extension can only be from Java to Scala, which will help the situation. However, I'm not sure if this is practical.

Thanks,
Xuefu


------------------------------------------------------------------
Sender:jincheng sun <[hidden email]>
Sent at:2018 Nov 23 (Fri) 09:49
Recipient:dev <[hidden email]>
Subject:Re: [DISCUSS] Long-term goal of making flink-table Scala-free

Hi Timo,
Thanks for initiating this great discussion.

Currently when using SQL/TableAPI should include many dependence. In
particular, it is not necessary to introduce the specific implementation
dependencies which users do not care about. So I am glad to see your
proposal, and hope when we consider splitting the API interface into a
separate module, so that the user can introduce minimum of dependencies.

So, +1 to [separation of interface and implementation; e.g. `Table` &
`TableImpl`] which you mentioned in the google doc.
Best,
Jincheng

Xiaowei Jiang <[hidden email]> 于2018年11月22日周四 下午10:50写道：

> Hi Timo, thanks for driving this! I think that this is a nice thing to do.
> While we are doing this, can we also keep in mind that we want to
> eventually have a TableAPI interface only module which users can take
> dependency on, but without including any implementation details?
>
> Xiaowei
>
> On Thu, Nov 22, 2018 at 6:37 PM Fabian Hueske <[hidden email]> wrote:
>
> > Hi Timo,
> >
> > Thanks for writing up this document.
> > I like the new structure and agree to prioritize the porting of the
> > flink-table-common classes.
> > Since flink-table-runtime is (or should be) independent of the API and
> > planner modules, we could start porting these classes once the code is
> > split into the new module structure.
> > The benefits of a Scala-free flink-table-runtime would be a Scala-free
> > execution Jar.
> >
> > Best, Fabian
> >
> >
> > Am Do., 22. Nov. 2018 um 10:54 Uhr schrieb Timo Walther <
> > [hidden email]
> > >:
> >
> > > Hi everyone,
> > >
> > > I would like to continue this discussion thread and convert the outcome
> > > into a FLIP such that users and contributors know what to expect in the
> > > upcoming releases.
> > >
> > > I created a design document [1] that clarifies our motivation why we
> > > want to do this, how a Maven module structure could look like, and a
> > > suggestion for a migration plan.
> > >
> > > It would be great to start with the efforts for the 1.8 release such
> > > that new features can be developed in Java and major refactorings such
> > > as improvements to the connectors and external catalog support are not
> > > blocked.
> > >
> > > Please let me know what you think.
> > >
> > > Regards,
> > > Timo
> > >
> > > [1]
> > >
> > >
> >
> https://docs.google.com/document/d/1PPo6goW7tOwxmpFuvLSjFnx7BF8IVz0w3dcmPPyqvoY/edit?usp=sharing
> > >
> > >
> > > Am 02.07.18 um 17:08 schrieb Fabian Hueske:
> > > > Hi Piotr,
> > > >
> > > > thanks for bumping this thread and thanks for Xingcan for the
> comments.
> > > >
> > > > I think the first step would be to separate the flink-table module
> into
> > > > multiple sub modules. These could be:
> > > >
> > > > - flink-table-api: All API facing classes. Can be later divided
> further
> > > > into Java/Scala Table API/SQL
> > > > - flink-table-planning: involves all planning (basically everything
> we
> > do
> > > > with Calcite)
> > > > - flink-table-runtime: the runtime code
> > > >
> > > > IMO, a realistic mid-term goal is to have the runtime module and
> > certain
> > > > parts of the planning module ported to Java.
> > > > The api module will be much harder to port because of several
> > > dependencies
> > > > to Scala core classes (the parser framework, tree iterations, etc.).
> > I'm
> > > > not saying we should not port this to Java, but it is not clear to me
> > > (yet)
> > > > how to do it.
> > > >
> > > > I think flink-table-runtime should not be too hard to port. The code
> > does
> > > > not make use of many Scala features, i.e., it's writing very
> Java-like.
> > > > Also, there are not many dependencies and operators can be
> individually
> > > > ported step-by-step.
> > > > For flink-table-planning, we can have certain packages that we port
> to
> > > Java
> > > > like planning rules or plan nodes. The related classes mostly extend
> > > > Calcite's Java interfaces/classes and would be natural choices for
> > being
> > > > ported. The code generation classes will require more effort to port.
> > > There
> > > > are also some dependencies in planning on the api module that we
> would
> > > need
> > > > to resolve somehow.
> > > >
> > > > For SQL most work when adding new features is done in the planning
> and
> > > > runtime modules. So, this separation should already reduce
> > "technological
> > > > dept" quite a lot.
> > > > The Table API depends much more on Scala than SQL.
> > > >
> > > > Cheers, Fabian
> > > >
> > > >
> > > >
> > > > 2018-07-02 16:26 GMT+02:00 Xingcan Cui <[hidden email]>:
> > > >
> > > >> Hi all,
> > > >>
> > > >> I also think about this problem these days and here are my thoughts.
> > > >>
> > > >> 1) We must admit that it’s really a tough task to interoperate with
> > Java
> > > >> and Scala. E.g., they have different collection types (Scala
> > collections
> > > >> v.s. java.util.*) and in Java, it's hard to implement a method which
> > > takes
> > > >> Scala functions as parameters. Considering the major part of the
> code
> > > base
> > > >> is implemented in Java, +1 for this goal from a long-term view.
> > > >>
> > > >> 2) The ideal solution would be to just expose a Scala API and make
> all
> > > the
> > > >> other parts Scala-free. But I am not sure if it could be achieved
> even
> > > in a
> > > >> long-term. Thus as Timo suggested, keep the Scala codes in
> > > >> "flink-table-core" would be a compromise solution.
> > > >>
> > > >> 3) If the community makes the final decision, maybe any new features
> > > >> should be added in Java (regardless of the modules), in order to
> > prevent
> > > >> the Scala codes from growing.
> > > >>
> > > >> Best,
> > > >> Xingcan
> > > >>
> > > >>
> > > >>> On Jul 2, 2018, at 9:30 PM, Piotr Nowojski <
> [hidden email]>
> > > >> wrote:
> > > >>> Bumping the topic.
> > > >>>
> > > >>> If we want to do this, the sooner we decide, the less code we will
> > have
> > > >> to rewrite. I have some objections/counter proposals to Fabian's
> > > proposal
> > > >> of doing it module wise and one module at a time.
> > > >>> First, I do not see a problem of having java/scala code even within
> > one
> > > >> module, especially not if there are clean boundaries. Like we could
> > have
> > > >> API in Scala and optimizer rules/logical nodes written in Java in
> the
> > > same
> > > >> module. However I haven’t previously maintained mixed scala/java
> code
> > > bases
> > > >> before, so I might be missing something here.
> > > >>> Secondly this whole migration might and most like will take longer
> > then
> > > >> expected, so that creates a problem for a new code that we will be
> > > >> creating. After making a decision to migrate to Java, almost any new
> > > Scala
> > > >> line of code will be immediately a technological debt and we will
> have
> > > to
> > > >> rewrite it to Java later.
> > > >>> Thus I would propose first to state our end goal - modules
> structure
> > > and
> > > >> which parts of modules we want to have eventually Scala-free.
> Secondly
> > > >> taking all steps necessary that will allow us to write new code
> > > complaint
> > > >> with our end goal. Only after that we should/could focus on
> > > incrementally
> > > >> rewriting the old code. Otherwise we could be stuck/blocked for
> years
> > > >> writing new code in Scala (and increasing technological debt),
> because
> > > >> nobody have found a time to rewrite some non important and not
> > actively
> > > >> developed part of some module.
> > > >>> Piotrek
> > > >>>
> > > >>>> On 14 Jun 2018, at 15:34, Fabian Hueske <[hidden email]>
> wrote:
> > > >>>>
> > > >>>> Hi,
> > > >>>>
> > > >>>> In general, I think this is a good effort. However, it won't be
> easy
> > > >> and I
> > > >>>> think we have to plan this well.
> > > >>>> I don't like the idea of having the whole code base fragmented
> into
> > > Java
> > > >>>> and Scala code for too long.
> > > >>>>
> > > >>>> I think we should do this one step at a time and focus on
> migrating
> > > one
> > > >>>> module at a time.
> > > >>>> IMO, the easiest start would be to port the runtime to Java.
> > > >>>> Extracting the API classes into an own module, porting them to
> Java,
> > > and
> > > >>>> removing the Scala dependency won't be possible without breaking
> the
> > > API
> > > >>>> since a few classes depend on the Scala Table API.
> > > >>>>
> > > >>>> Best, Fabian
> > > >>>>
> > > >>>>
> > > >>>> 2018-06-14 10:33 GMT+02:00 Till Rohrmann <[hidden email]>:
> > > >>>>
> > > >>>>> I think that is a noble and honorable goal and we should strive
> for
> > > it.
> > > >>>>> This, however, must be an iterative process given the sheer size
> of
> > > the
> > > >>>>> code base. I like the approach to define common Java modules
> which
> > > are
> > > >> used
> > > >>>>> by more specific Scala modules and slowly moving classes from
> Scala
> > > to
> > > >>>>> Java. Thus +1 for the proposal.
> > > >>>>>
> > > >>>>> Cheers,
> > > >>>>> Till
> > > >>>>>
> > > >>>>> On Wed, Jun 13, 2018 at 12:01 PM Piotr Nowojski <
> > > >> [hidden email]>
> > > >>>>> wrote:
> > > >>>>>
> > > >>>>>> Hi,
> > > >>>>>>
> > > >>>>>> I do not have an experience with how scala and java interacts
> with
> > > >> each
> > > >>>>>> other, so I can not fully validate your proposal, but generally
> > > >> speaking
> > > >>>>> +1
> > > >>>>>> from me.
> > > >>>>>>
> > > >>>>>> Does it also mean, that we should slowly migrate
> > `flink-table-core`
> > > to
> > > >>>>>> Java? How would you envision it? It would be nice to be able to
> > add
> > > >> new
> > > >>>>>> classes/features written in Java and so that they can coexist
> with
> > > old
> > > >>>>>> Scala code until we gradually switch from Scala to Java.
> > > >>>>>>
> > > >>>>>> Piotrek
> > > >>>>>>
> > > >>>>>>> On 13 Jun 2018, at 11:32, Timo Walther <[hidden email]>
> > wrote:
> > > >>>>>>>
> > > >>>>>>> Hi everyone,
> > > >>>>>>>
> > > >>>>>>> as you all know, currently the Table & SQL API is implemented
> in
> > > >> Scala.
> > > >>>>>> This decision was made a long-time ago when the initital code
> base
> > > was
> > > >>>>>> created as part of a master's thesis. The community kept Scala
> > > >> because of
> > > >>>>>> the nice language features that enable a fluent Table API like
> > > >>>>>> table.select('field.trim()) and because Scala allows for quick
> > > >>>>> prototyping
> > > >>>>>> (e.g. multi-line comments for code generation). The committers
> > > >> enforced
> > > >>>>> not
> > > >>>>>> splitting the code-base into two programming languages.
> > > >>>>>>> However, nowadays the flink-table module more and more becomes
> an
> > > >>>>>> important part in the Flink ecosystem. Connectors, formats, and
> > SQL
> > > >>>>> client
> > > >>>>>> are actually implemented in Java but need to interoperate with
> > > >>>>> flink-table
> > > >>>>>> which makes these modules dependent on Scala. As mentioned in an
> > > >> earlier
> > > >>>>>> mail thread, using Scala for API classes also exposes member
> > > variables
> > > >>>>> and
> > > >>>>>> methods in Java that should not be exposed to users [1]. Java is
> > > still
> > > >>>>> the
> > > >>>>>> most important API language and right now we treat it as a
> > > >> second-class
> > > >>>>>> citizen. I just noticed that you even need to add Scala if you
> > just
> > > >> want
> > > >>>>> to
> > > >>>>>> implement a ScalarFunction because of method clashes between
> > `public
> > > >>>>> String
> > > >>>>>> toString()` and `public scala.Predef.String toString()`.
> > > >>>>>>> Given the size of the current code base, reimplementing the
> > entire
> > > >>>>>> flink-table code in Java is a goal that we might never reach.
> > > >> However, we
> > > >>>>>> should at least treat the symptoms and have this as a long-term
> > goal
> > > >> in
> > > >>>>>> mind. My suggestion would be to convert user-facing and runtime
> > > >> classes
> > > >>>>> and
> > > >>>>>> split the code base into multiple modules:
> > > >>>>>>>> flink-table-java {depends on flink-table-core}
> > > >>>>>>> Implemented in Java. Java users can use this. This would
> require
> > to
> > > >>>>>> convert classes like TableEnvironment, Table.
> > > >>>>>>>> flink-table-scala {depends on flink-table-core}
> > > >>>>>>> Implemented in Scala. Scala users can use this.
> > > >>>>>>>
> > > >>>>>>>> flink-table-common
> > > >>>>>>> Implemented in Java. Connectors, formats, and UDFs can use
> this.
> > It
> > > >>>>>> contains interface classes such as descriptors, table sink,
> table
> > > >> source.
> > > >>>>>>>> flink-table-core {depends on flink-table-common and
> > > >>>>>> flink-table-runtime}
> > > >>>>>>> Implemented in Scala. Contains the current main code base.
> > > >>>>>>>
> > > >>>>>>>> flink-table-runtime
> > > >>>>>>> Implemented in Java. This would require to convert classes in
> > > >>>>>> o.a.f.table.runtime but would improve the runtime potentially.
> > > >>>>>>>
> > > >>>>>>> What do you think?
> > > >>>>>>>
> > > >>>>>>>
> > > >>>>>>> Regards,
> > > >>>>>>>
> > > >>>>>>> Timo
> > > >>>>>>>
> > > >>>>>>> [1]
> > > >>>>>> http://apache-flink-mailing-list-archive.1008284.n3.
> > > >>>>> nabble.com/DISCUSS-Convert-main-Table-API-classes-into-
> > > >> traits-tp21335.html
> > > >>>>>>
> > > >>
> > >
> > >
> >
>

[Piotr Nowojski]

Hi Timo,

Thanks for writing this down +1 from my side :)

> I'm wondering that whether we can have rule in the interim when Java and Scala coexist that dependency can only be one-way. I found that in the current code base there are cases where a Scala class extends Java and vise versa. This is quite painful. I'm thinking if we could say that extension can only be from Java to Scala, which will help the situation. However, I'm not sure if this is practical.

Xuefu: I’m also not sure what’s the best approach here, probably we will have to work it out as we go. One thing to consider is that from now on, every single new code line written in Scala anywhere in Flink-table (except of Flink-table-api-scala) is an instant technological debt. From this perspective I would be in favour of tolerating quite big inchonvieneces just to avoid any new Scala code.

Piotrek

> On 23 Nov 2018, at 03:25, Zhang, Xuefu <[hidden email]> wrote:
>
> Hi Timo,
>
> Thanks for the effort and the Google writeup. During our external catalog rework, we found much confusion between Java and Scala, and this Scala-free roadmap should greatly mitigate that.
>
> I'm wondering that whether we can have rule in the interim when Java and Scala coexist that dependency can only be one-way. I found that in the current code base there are cases where a Scala class extends Java and vise versa. This is quite painful. I'm thinking if we could say that extension can only be from Java to Scala, which will help the situation. However, I'm not sure if this is practical.
>
> Thanks,
> Xuefu
>
>
> ------------------------------------------------------------------
> Sender:jincheng sun <[hidden email]>
> Sent at:2018 Nov 23 (Fri) 09:49
> Recipient:dev <[hidden email]>
> Subject:Re: [DISCUSS] Long-term goal of making flink-table Scala-free
>
> Hi Timo,
> Thanks for initiating this great discussion.
>
> Currently when using SQL/TableAPI should include many dependence. In
> particular, it is not necessary to introduce the specific implementation
> dependencies which users do not care about. So I am glad to see your
> proposal, and hope when we consider splitting the API interface into a
> separate module, so that the user can introduce minimum of dependencies.
>
> So, +1 to [separation of interface and implementation; e.g. `Table` &
> `TableImpl`] which you mentioned in the google doc.
> Best,
> Jincheng
>
> Xiaowei Jiang <[hidden email]> 于2018年11月22日周四 下午10:50写道：
>
>> Hi Timo, thanks for driving this! I think that this is a nice thing to do.
>> While we are doing this, can we also keep in mind that we want to
>> eventually have a TableAPI interface only module which users can take
>> dependency on, but without including any implementation details?
>>
>> Xiaowei
>>
>> On Thu, Nov 22, 2018 at 6:37 PM Fabian Hueske <[hidden email]> wrote:
>>
>>> Hi Timo,
>>>
>>> Thanks for writing up this document.
>>> I like the new structure and agree to prioritize the porting of the
>>> flink-table-common classes.
>>> Since flink-table-runtime is (or should be) independent of the API and
>>> planner modules, we could start porting these classes once the code is
>>> split into the new module structure.
>>> The benefits of a Scala-free flink-table-runtime would be a Scala-free
>>> execution Jar.
>>>
>>> Best, Fabian
>>>
>>>
>>> Am Do., 22. Nov. 2018 um 10:54 Uhr schrieb Timo Walther <
>>> [hidden email]
>>>> :
>>>
>>>> Hi everyone,
>>>>
>>>> I would like to continue this discussion thread and convert the outcome
>>>> into a FLIP such that users and contributors know what to expect in the
>>>> upcoming releases.
>>>>
>>>> I created a design document [1] that clarifies our motivation why we
>>>> want to do this, how a Maven module structure could look like, and a
>>>> suggestion for a migration plan.
>>>>
>>>> It would be great to start with the efforts for the 1.8 release such
>>>> that new features can be developed in Java and major refactorings such
>>>> as improvements to the connectors and external catalog support are not
>>>> blocked.
>>>>
>>>> Please let me know what you think.
>>>>
>>>> Regards,
>>>> Timo
>>>>
>>>> [1]
>>>>
>>>>
>>>
>> https://docs.google.com/document/d/1PPo6goW7tOwxmpFuvLSjFnx7BF8IVz0w3dcmPPyqvoY/edit?usp=sharing
>>>>
>>>>
>>>> Am 02.07.18 um 17:08 schrieb Fabian Hueske:
>>>>> Hi Piotr,
>>>>>
>>>>> thanks for bumping this thread and thanks for Xingcan for the
>> comments.
>>>>>
>>>>> I think the first step would be to separate the flink-table module
>> into
>>>>> multiple sub modules. These could be:
>>>>>
>>>>> - flink-table-api: All API facing classes. Can be later divided
>> further
>>>>> into Java/Scala Table API/SQL
>>>>> - flink-table-planning: involves all planning (basically everything
>> we
>>> do
>>>>> with Calcite)
>>>>> - flink-table-runtime: the runtime code
>>>>>
>>>>> IMO, a realistic mid-term goal is to have the runtime module and
>>> certain
>>>>> parts of the planning module ported to Java.
>>>>> The api module will be much harder to port because of several
>>>> dependencies
>>>>> to Scala core classes (the parser framework, tree iterations, etc.).
>>> I'm
>>>>> not saying we should not port this to Java, but it is not clear to me
>>>> (yet)
>>>>> how to do it.
>>>>>
>>>>> I think flink-table-runtime should not be too hard to port. The code
>>> does
>>>>> not make use of many Scala features, i.e., it's writing very
>> Java-like.
>>>>> Also, there are not many dependencies and operators can be
>> individually
>>>>> ported step-by-step.
>>>>> For flink-table-planning, we can have certain packages that we port
>> to
>>>> Java
>>>>> like planning rules or plan nodes. The related classes mostly extend
>>>>> Calcite's Java interfaces/classes and would be natural choices for
>>> being
>>>>> ported. The code generation classes will require more effort to port.
>>>> There
>>>>> are also some dependencies in planning on the api module that we
>> would
>>>> need
>>>>> to resolve somehow.
>>>>>
>>>>> For SQL most work when adding new features is done in the planning
>> and
>>>>> runtime modules. So, this separation should already reduce
>>> "technological
>>>>> dept" quite a lot.
>>>>> The Table API depends much more on Scala than SQL.
>>>>>
>>>>> Cheers, Fabian
>>>>>
>>>>>
>>>>>
>>>>> 2018-07-02 16:26 GMT+02:00 Xingcan Cui <[hidden email]>:
>>>>>
>>>>>> Hi all,
>>>>>>
>>>>>> I also think about this problem these days and here are my thoughts.
>>>>>>
>>>>>> 1) We must admit that it’s really a tough task to interoperate with
>>> Java
>>>>>> and Scala. E.g., they have different collection types (Scala
>>> collections
>>>>>> v.s. java.util.*) and in Java, it's hard to implement a method which
>>>> takes
>>>>>> Scala functions as parameters. Considering the major part of the
>> code
>>>> base
>>>>>> is implemented in Java, +1 for this goal from a long-term view.
>>>>>>
>>>>>> 2) The ideal solution would be to just expose a Scala API and make
>> all
>>>> the
>>>>>> other parts Scala-free. But I am not sure if it could be achieved
>> even
>>>> in a
>>>>>> long-term. Thus as Timo suggested, keep the Scala codes in
>>>>>> "flink-table-core" would be a compromise solution.
>>>>>>
>>>>>> 3) If the community makes the final decision, maybe any new features
>>>>>> should be added in Java (regardless of the modules), in order to
>>> prevent
>>>>>> the Scala codes from growing.
>>>>>>
>>>>>> Best,
>>>>>> Xingcan
>>>>>>
>>>>>>
>>>>>>> On Jul 2, 2018, at 9:30 PM, Piotr Nowojski <
>> [hidden email]>
>>>>>> wrote:
>>>>>>> Bumping the topic.
>>>>>>>
>>>>>>> If we want to do this, the sooner we decide, the less code we will
>>> have
>>>>>> to rewrite. I have some objections/counter proposals to Fabian's
>>>> proposal
>>>>>> of doing it module wise and one module at a time.
>>>>>>> First, I do not see a problem of having java/scala code even within
>>> one
>>>>>> module, especially not if there are clean boundaries. Like we could
>>> have
>>>>>> API in Scala and optimizer rules/logical nodes written in Java in
>> the
>>>> same
>>>>>> module. However I haven’t previously maintained mixed scala/java
>> code
>>>> bases
>>>>>> before, so I might be missing something here.
>>>>>>> Secondly this whole migration might and most like will take longer
>>> then
>>>>>> expected, so that creates a problem for a new code that we will be
>>>>>> creating. After making a decision to migrate to Java, almost any new
>>>> Scala
>>>>>> line of code will be immediately a technological debt and we will
>> have
>>>> to
>>>>>> rewrite it to Java later.
>>>>>>> Thus I would propose first to state our end goal - modules
>> structure
>>>> and
>>>>>> which parts of modules we want to have eventually Scala-free.
>> Secondly
>>>>>> taking all steps necessary that will allow us to write new code
>>>> complaint
>>>>>> with our end goal. Only after that we should/could focus on
>>>> incrementally
>>>>>> rewriting the old code. Otherwise we could be stuck/blocked for
>> years
>>>>>> writing new code in Scala (and increasing technological debt),
>> because
>>>>>> nobody have found a time to rewrite some non important and not
>>> actively
>>>>>> developed part of some module.
>>>>>>> Piotrek
>>>>>>>
>>>>>>>> On 14 Jun 2018, at 15:34, Fabian Hueske <[hidden email]>
>> wrote:
>>>>>>>>
>>>>>>>> Hi,
>>>>>>>>
>>>>>>>> In general, I think this is a good effort. However, it won't be
>> easy
>>>>>> and I
>>>>>>>> think we have to plan this well.
>>>>>>>> I don't like the idea of having the whole code base fragmented
>> into
>>>> Java
>>>>>>>> and Scala code for too long.
>>>>>>>>
>>>>>>>> I think we should do this one step at a time and focus on
>> migrating
>>>> one
>>>>>>>> module at a time.
>>>>>>>> IMO, the easiest start would be to port the runtime to Java.
>>>>>>>> Extracting the API classes into an own module, porting them to
>> Java,
>>>> and
>>>>>>>> removing the Scala dependency won't be possible without breaking
>> the
>>>> API
>>>>>>>> since a few classes depend on the Scala Table API.
>>>>>>>>
>>>>>>>> Best, Fabian
>>>>>>>>
>>>>>>>>
>>>>>>>> 2018-06-14 10:33 GMT+02:00 Till Rohrmann <[hidden email]>:
>>>>>>>>
>>>>>>>>> I think that is a noble and honorable goal and we should strive
>> for
>>>> it.
>>>>>>>>> This, however, must be an iterative process given the sheer size
>> of
>>>> the
>>>>>>>>> code base. I like the approach to define common Java modules
>> which
>>>> are
>>>>>> used
>>>>>>>>> by more specific Scala modules and slowly moving classes from
>> Scala
>>>> to
>>>>>>>>> Java. Thus +1 for the proposal.
>>>>>>>>>
>>>>>>>>> Cheers,
>>>>>>>>> Till
>>>>>>>>>
>>>>>>>>> On Wed, Jun 13, 2018 at 12:01 PM Piotr Nowojski <
>>>>>> [hidden email]>
>>>>>>>>> wrote:
>>>>>>>>>
>>>>>>>>>> Hi,
>>>>>>>>>>
>>>>>>>>>> I do not have an experience with how scala and java interacts
>> with
>>>>>> each
>>>>>>>>>> other, so I can not fully validate your proposal, but generally
>>>>>> speaking
>>>>>>>>> +1
>>>>>>>>>> from me.
>>>>>>>>>>
>>>>>>>>>> Does it also mean, that we should slowly migrate
>>> `flink-table-core`
>>>> to
>>>>>>>>>> Java? How would you envision it? It would be nice to be able to
>>> add
>>>>>> new
>>>>>>>>>> classes/features written in Java and so that they can coexist
>> with
>>>> old
>>>>>>>>>> Scala code until we gradually switch from Scala to Java.
>>>>>>>>>>
>>>>>>>>>> Piotrek
>>>>>>>>>>
>>>>>>>>>>> On 13 Jun 2018, at 11:32, Timo Walther <[hidden email]>
>>> wrote:
>>>>>>>>>>>
>>>>>>>>>>> Hi everyone,
>>>>>>>>>>>
>>>>>>>>>>> as you all know, currently the Table & SQL API is implemented
>> in
>>>>>> Scala.
>>>>>>>>>> This decision was made a long-time ago when the initital code
>> base
>>>> was
>>>>>>>>>> created as part of a master's thesis. The community kept Scala
>>>>>> because of
>>>>>>>>>> the nice language features that enable a fluent Table API like
>>>>>>>>>> table.select('field.trim()) and because Scala allows for quick
>>>>>>>>> prototyping
>>>>>>>>>> (e.g. multi-line comments for code generation). The committers
>>>>>> enforced
>>>>>>>>> not
>>>>>>>>>> splitting the code-base into two programming languages.
>>>>>>>>>>> However, nowadays the flink-table module more and more becomes
>> an
>>>>>>>>>> important part in the Flink ecosystem. Connectors, formats, and
>>> SQL
>>>>>>>>> client
>>>>>>>>>> are actually implemented in Java but need to interoperate with
>>>>>>>>> flink-table
>>>>>>>>>> which makes these modules dependent on Scala. As mentioned in an
>>>>>> earlier
>>>>>>>>>> mail thread, using Scala for API classes also exposes member
>>>> variables
>>>>>>>>> and
>>>>>>>>>> methods in Java that should not be exposed to users [1]. Java is
>>>> still
>>>>>>>>> the
>>>>>>>>>> most important API language and right now we treat it as a
>>>>>> second-class
>>>>>>>>>> citizen. I just noticed that you even need to add Scala if you
>>> just
>>>>>> want
>>>>>>>>> to
>>>>>>>>>> implement a ScalarFunction because of method clashes between
>>> `public
>>>>>>>>> String
>>>>>>>>>> toString()` and `public scala.Predef.String toString()`.
>>>>>>>>>>> Given the size of the current code base, reimplementing the
>>> entire
>>>>>>>>>> flink-table code in Java is a goal that we might never reach.
>>>>>> However, we
>>>>>>>>>> should at least treat the symptoms and have this as a long-term
>>> goal
>>>>>> in
>>>>>>>>>> mind. My suggestion would be to convert user-facing and runtime
>>>>>> classes
>>>>>>>>> and
>>>>>>>>>> split the code base into multiple modules:
>>>>>>>>>>>> flink-table-java {depends on flink-table-core}
>>>>>>>>>>> Implemented in Java. Java users can use this. This would
>> require
>>> to
>>>>>>>>>> convert classes like TableEnvironment, Table.
>>>>>>>>>>>> flink-table-scala {depends on flink-table-core}
>>>>>>>>>>> Implemented in Scala. Scala users can use this.
>>>>>>>>>>>
>>>>>>>>>>>> flink-table-common
>>>>>>>>>>> Implemented in Java. Connectors, formats, and UDFs can use
>> this.
>>> It
>>>>>>>>>> contains interface classes such as descriptors, table sink,
>> table
>>>>>> source.
>>>>>>>>>>>> flink-table-core {depends on flink-table-common and
>>>>>>>>>> flink-table-runtime}
>>>>>>>>>>> Implemented in Scala. Contains the current main code base.
>>>>>>>>>>>
>>>>>>>>>>>> flink-table-runtime
>>>>>>>>>>> Implemented in Java. This would require to convert classes in
>>>>>>>>>> o.a.f.table.runtime but would improve the runtime potentially.
>>>>>>>>>>>
>>>>>>>>>>> What do you think?
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> Regards,
>>>>>>>>>>>
>>>>>>>>>>> Timo
>>>>>>>>>>>
>>>>>>>>>>> [1]
>>>>>>>>>> http://apache-flink-mailing-list-archive.1008284.n3.
>>>>>>>>> nabble.com/DISCUSS-Convert-main-Table-API-classes-into-
>>>>>> traits-tp21335.html
>>>>>>>>>>
>>>>>>
>>>>
>>>>
>>>
>>

[Timo Walther-2]

Hi everyone,

thanks for the great feedback so far. I updated the document with the
input I got so far

@Fabian: I moved the porting of flink-table-runtime classes up in the list.

@Xiaowei: Could you elaborate what "interface only" means to you? Do you
mean a module containing pure Java `interface`s? Or is the validation
logic also part of the API module? Are 50+ expression classes part of
the API interface or already too implementation-specific?

@Xuefu: I extended the document by almost a page to clarify when we
should develop in Scala and when in Java. As Piotr said, every new Scala
line is instant technical debt.

Thanks,
Timo


Am 23.11.18 um 10:29 schrieb Piotr Nowojski:

> Hi Timo,
>
> Thanks for writing this down +1 from my side :)
>
>> I'm wondering that whether we can have rule in the interim when Java and Scala coexist that dependency can only be one-way. I found that in the current code base there are cases where a Scala class extends Java and vise versa. This is quite painful. I'm thinking if we could say that extension can only be from Java to Scala, which will help the situation. However, I'm not sure if this is practical.
> Xuefu: I’m also not sure what’s the best approach here, probably we will have to work it out as we go. One thing to consider is that from now on, every single new code line written in Scala anywhere in Flink-table (except of Flink-table-api-scala) is an instant technological debt. From this perspective I would be in favour of tolerating quite big inchonvieneces just to avoid any new Scala code.
>
> Piotrek
>
>> On 23 Nov 2018, at 03:25, Zhang, Xuefu <[hidden email]> wrote:
>>
>> Hi Timo,
>>
>> Thanks for the effort and the Google writeup. During our external catalog rework, we found much confusion between Java and Scala, and this Scala-free roadmap should greatly mitigate that.
>>
>> I'm wondering that whether we can have rule in the interim when Java and Scala coexist that dependency can only be one-way. I found that in the current code base there are cases where a Scala class extends Java and vise versa. This is quite painful. I'm thinking if we could say that extension can only be from Java to Scala, which will help the situation. However, I'm not sure if this is practical.
>>
>> Thanks,
>> Xuefu
>>
>>
>> ------------------------------------------------------------------
>> Sender:jincheng sun <[hidden email]>
>> Sent at:2018 Nov 23 (Fri) 09:49
>> Recipient:dev <[hidden email]>
>> Subject:Re: [DISCUSS] Long-term goal of making flink-table Scala-free
>>
>> Hi Timo,
>> Thanks for initiating this great discussion.
>>
>> Currently when using SQL/TableAPI should include many dependence. In
>> particular, it is not necessary to introduce the specific implementation
>> dependencies which users do not care about. So I am glad to see your
>> proposal, and hope when we consider splitting the API interface into a
>> separate module, so that the user can introduce minimum of dependencies.
>>
>> So, +1 to [separation of interface and implementation; e.g. `Table` &
>> `TableImpl`] which you mentioned in the google doc.
>> Best,
>> Jincheng
>>
>> Xiaowei Jiang <[hidden email]> 于2018年11月22日周四 下午10:50写道：
>>
>>> Hi Timo, thanks for driving this! I think that this is a nice thing to do.
>>> While we are doing this, can we also keep in mind that we want to
>>> eventually have a TableAPI interface only module which users can take
>>> dependency on, but without including any implementation details?
>>>
>>> Xiaowei
>>>
>>> On Thu, Nov 22, 2018 at 6:37 PM Fabian Hueske <[hidden email]> wrote:
>>>
>>>> Hi Timo,
>>>>
>>>> Thanks for writing up this document.
>>>> I like the new structure and agree to prioritize the porting of the
>>>> flink-table-common classes.
>>>> Since flink-table-runtime is (or should be) independent of the API and
>>>> planner modules, we could start porting these classes once the code is
>>>> split into the new module structure.
>>>> The benefits of a Scala-free flink-table-runtime would be a Scala-free
>>>> execution Jar.
>>>>
>>>> Best, Fabian
>>>>
>>>>
>>>> Am Do., 22. Nov. 2018 um 10:54 Uhr schrieb Timo Walther <
>>>> [hidden email]
>>>>> :
>>>>> Hi everyone,
>>>>>
>>>>> I would like to continue this discussion thread and convert the outcome
>>>>> into a FLIP such that users and contributors know what to expect in the
>>>>> upcoming releases.
>>>>>
>>>>> I created a design document [1] that clarifies our motivation why we
>>>>> want to do this, how a Maven module structure could look like, and a
>>>>> suggestion for a migration plan.
>>>>>
>>>>> It would be great to start with the efforts for the 1.8 release such
>>>>> that new features can be developed in Java and major refactorings such
>>>>> as improvements to the connectors and external catalog support are not
>>>>> blocked.
>>>>>
>>>>> Please let me know what you think.
>>>>>
>>>>> Regards,
>>>>> Timo
>>>>>
>>>>> [1]
>>>>>
>>>>>
>>> https://docs.google.com/document/d/1PPo6goW7tOwxmpFuvLSjFnx7BF8IVz0w3dcmPPyqvoY/edit?usp=sharing
>>>>>
>>>>> Am 02.07.18 um 17:08 schrieb Fabian Hueske:
>>>>>> Hi Piotr,
>>>>>>
>>>>>> thanks for bumping this thread and thanks for Xingcan for the
>>> comments.
>>>>>> I think the first step would be to separate the flink-table module
>>> into
>>>>>> multiple sub modules. These could be:
>>>>>>
>>>>>> - flink-table-api: All API facing classes. Can be later divided
>>> further
>>>>>> into Java/Scala Table API/SQL
>>>>>> - flink-table-planning: involves all planning (basically everything
>>> we
>>>> do
>>>>>> with Calcite)
>>>>>> - flink-table-runtime: the runtime code
>>>>>>
>>>>>> IMO, a realistic mid-term goal is to have the runtime module and
>>>> certain
>>>>>> parts of the planning module ported to Java.
>>>>>> The api module will be much harder to port because of several
>>>>> dependencies
>>>>>> to Scala core classes (the parser framework, tree iterations, etc.).
>>>> I'm
>>>>>> not saying we should not port this to Java, but it is not clear to me
>>>>> (yet)
>>>>>> how to do it.
>>>>>>
>>>>>> I think flink-table-runtime should not be too hard to port. The code
>>>> does
>>>>>> not make use of many Scala features, i.e., it's writing very
>>> Java-like.
>>>>>> Also, there are not many dependencies and operators can be
>>> individually
>>>>>> ported step-by-step.
>>>>>> For flink-table-planning, we can have certain packages that we port
>>> to
>>>>> Java
>>>>>> like planning rules or plan nodes. The related classes mostly extend
>>>>>> Calcite's Java interfaces/classes and would be natural choices for
>>>> being
>>>>>> ported. The code generation classes will require more effort to port.
>>>>> There
>>>>>> are also some dependencies in planning on the api module that we
>>> would
>>>>> need
>>>>>> to resolve somehow.
>>>>>>
>>>>>> For SQL most work when adding new features is done in the planning
>>> and
>>>>>> runtime modules. So, this separation should already reduce
>>>> "technological
>>>>>> dept" quite a lot.
>>>>>> The Table API depends much more on Scala than SQL.
>>>>>>
>>>>>> Cheers, Fabian
>>>>>>
>>>>>>
>>>>>>
>>>>>> 2018-07-02 16:26 GMT+02:00 Xingcan Cui <[hidden email]>:
>>>>>>
>>>>>>> Hi all,
>>>>>>>
>>>>>>> I also think about this problem these days and here are my thoughts.
>>>>>>>
>>>>>>> 1) We must admit that it’s really a tough task to interoperate with
>>>> Java
>>>>>>> and Scala. E.g., they have different collection types (Scala
>>>> collections
>>>>>>> v.s. java.util.*) and in Java, it's hard to implement a method which
>>>>> takes
>>>>>>> Scala functions as parameters. Considering the major part of the
>>> code
>>>>> base
>>>>>>> is implemented in Java, +1 for this goal from a long-term view.
>>>>>>>
>>>>>>> 2) The ideal solution would be to just expose a Scala API and make
>>> all
>>>>> the
>>>>>>> other parts Scala-free. But I am not sure if it could be achieved
>>> even
>>>>> in a
>>>>>>> long-term. Thus as Timo suggested, keep the Scala codes in
>>>>>>> "flink-table-core" would be a compromise solution.
>>>>>>>
>>>>>>> 3) If the community makes the final decision, maybe any new features
>>>>>>> should be added in Java (regardless of the modules), in order to
>>>> prevent
>>>>>>> the Scala codes from growing.
>>>>>>>
>>>>>>> Best,
>>>>>>> Xingcan
>>>>>>>
>>>>>>>
>>>>>>>> On Jul 2, 2018, at 9:30 PM, Piotr Nowojski <
>>> [hidden email]>
>>>>>>> wrote:
>>>>>>>> Bumping the topic.
>>>>>>>>
>>>>>>>> If we want to do this, the sooner we decide, the less code we will
>>>> have
>>>>>>> to rewrite. I have some objections/counter proposals to Fabian's
>>>>> proposal
>>>>>>> of doing it module wise and one module at a time.
>>>>>>>> First, I do not see a problem of having java/scala code even within
>>>> one
>>>>>>> module, especially not if there are clean boundaries. Like we could
>>>> have
>>>>>>> API in Scala and optimizer rules/logical nodes written in Java in
>>> the
>>>>> same
>>>>>>> module. However I haven’t previously maintained mixed scala/java
>>> code
>>>>> bases
>>>>>>> before, so I might be missing something here.
>>>>>>>> Secondly this whole migration might and most like will take longer
>>>> then
>>>>>>> expected, so that creates a problem for a new code that we will be
>>>>>>> creating. After making a decision to migrate to Java, almost any new
>>>>> Scala
>>>>>>> line of code will be immediately a technological debt and we will
>>> have
>>>>> to
>>>>>>> rewrite it to Java later.
>>>>>>>> Thus I would propose first to state our end goal - modules
>>> structure
>>>>> and
>>>>>>> which parts of modules we want to have eventually Scala-free.
>>> Secondly
>>>>>>> taking all steps necessary that will allow us to write new code
>>>>> complaint
>>>>>>> with our end goal. Only after that we should/could focus on
>>>>> incrementally
>>>>>>> rewriting the old code. Otherwise we could be stuck/blocked for
>>> years
>>>>>>> writing new code in Scala (and increasing technological debt),
>>> because
>>>>>>> nobody have found a time to rewrite some non important and not
>>>> actively
>>>>>>> developed part of some module.
>>>>>>>> Piotrek
>>>>>>>>
>>>>>>>>> On 14 Jun 2018, at 15:34, Fabian Hueske <[hidden email]>
>>> wrote:
>>>>>>>>> Hi,
>>>>>>>>>
>>>>>>>>> In general, I think this is a good effort. However, it won't be
>>> easy
>>>>>>> and I
>>>>>>>>> think we have to plan this well.
>>>>>>>>> I don't like the idea of having the whole code base fragmented
>>> into
>>>>> Java
>>>>>>>>> and Scala code for too long.
>>>>>>>>>
>>>>>>>>> I think we should do this one step at a time and focus on
>>> migrating
>>>>> one
>>>>>>>>> module at a time.
>>>>>>>>> IMO, the easiest start would be to port the runtime to Java.
>>>>>>>>> Extracting the API classes into an own module, porting them to
>>> Java,
>>>>> and
>>>>>>>>> removing the Scala dependency won't be possible without breaking
>>> the
>>>>> API
>>>>>>>>> since a few classes depend on the Scala Table API.
>>>>>>>>>
>>>>>>>>> Best, Fabian
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> 2018-06-14 10:33 GMT+02:00 Till Rohrmann <[hidden email]>:
>>>>>>>>>
>>>>>>>>>> I think that is a noble and honorable goal and we should strive
>>> for
>>>>> it.
>>>>>>>>>> This, however, must be an iterative process given the sheer size
>>> of
>>>>> the
>>>>>>>>>> code base. I like the approach to define common Java modules
>>> which
>>>>> are
>>>>>>> used
>>>>>>>>>> by more specific Scala modules and slowly moving classes from
>>> Scala
>>>>> to
>>>>>>>>>> Java. Thus +1 for the proposal.
>>>>>>>>>>
>>>>>>>>>> Cheers,
>>>>>>>>>> Till
>>>>>>>>>>
>>>>>>>>>> On Wed, Jun 13, 2018 at 12:01 PM Piotr Nowojski <
>>>>>>> [hidden email]>
>>>>>>>>>> wrote:
>>>>>>>>>>
>>>>>>>>>>> Hi,
>>>>>>>>>>>
>>>>>>>>>>> I do not have an experience with how scala and java interacts
>>> with
>>>>>>> each
>>>>>>>>>>> other, so I can not fully validate your proposal, but generally
>>>>>>> speaking
>>>>>>>>>> +1
>>>>>>>>>>> from me.
>>>>>>>>>>>
>>>>>>>>>>> Does it also mean, that we should slowly migrate
>>>> `flink-table-core`
>>>>> to
>>>>>>>>>>> Java? How would you envision it? It would be nice to be able to
>>>> add
>>>>>>> new
>>>>>>>>>>> classes/features written in Java and so that they can coexist
>>> with
>>>>> old
>>>>>>>>>>> Scala code until we gradually switch from Scala to Java.
>>>>>>>>>>>
>>>>>>>>>>> Piotrek
>>>>>>>>>>>
>>>>>>>>>>>> On 13 Jun 2018, at 11:32, Timo Walther <[hidden email]>
>>>> wrote:
>>>>>>>>>>>> Hi everyone,
>>>>>>>>>>>>
>>>>>>>>>>>> as you all know, currently the Table & SQL API is implemented
>>> in
>>>>>>> Scala.
>>>>>>>>>>> This decision was made a long-time ago when the initital code
>>> base
>>>>> was
>>>>>>>>>>> created as part of a master's thesis. The community kept Scala
>>>>>>> because of
>>>>>>>>>>> the nice language features that enable a fluent Table API like
>>>>>>>>>>> table.select('field.trim()) and because Scala allows for quick
>>>>>>>>>> prototyping
>>>>>>>>>>> (e.g. multi-line comments for code generation). The committers
>>>>>>> enforced
>>>>>>>>>> not
>>>>>>>>>>> splitting the code-base into two programming languages.
>>>>>>>>>>>> However, nowadays the flink-table module more and more becomes
>>> an
>>>>>>>>>>> important part in the Flink ecosystem. Connectors, formats, and
>>>> SQL
>>>>>>>>>> client
>>>>>>>>>>> are actually implemented in Java but need to interoperate with
>>>>>>>>>> flink-table
>>>>>>>>>>> which makes these modules dependent on Scala. As mentioned in an
>>>>>>> earlier
>>>>>>>>>>> mail thread, using Scala for API classes also exposes member
>>>>> variables
>>>>>>>>>> and
>>>>>>>>>>> methods in Java that should not be exposed to users [1]. Java is
>>>>> still
>>>>>>>>>> the
>>>>>>>>>>> most important API language and right now we treat it as a
>>>>>>> second-class
>>>>>>>>>>> citizen. I just noticed that you even need to add Scala if you
>>>> just
>>>>>>> want
>>>>>>>>>> to
>>>>>>>>>>> implement a ScalarFunction because of method clashes between
>>>> `public
>>>>>>>>>> String
>>>>>>>>>>> toString()` and `public scala.Predef.String toString()`.
>>>>>>>>>>>> Given the size of the current code base, reimplementing the
>>>> entire
>>>>>>>>>>> flink-table code in Java is a goal that we might never reach.
>>>>>>> However, we
>>>>>>>>>>> should at least treat the symptoms and have this as a long-term
>>>> goal
>>>>>>> in
>>>>>>>>>>> mind. My suggestion would be to convert user-facing and runtime
>>>>>>> classes
>>>>>>>>>> and
>>>>>>>>>>> split the code base into multiple modules:
>>>>>>>>>>>>> flink-table-java {depends on flink-table-core}
>>>>>>>>>>>> Implemented in Java. Java users can use this. This would
>>> require
>>>> to
>>>>>>>>>>> convert classes like TableEnvironment, Table.
>>>>>>>>>>>>> flink-table-scala {depends on flink-table-core}
>>>>>>>>>>>> Implemented in Scala. Scala users can use this.
>>>>>>>>>>>>
>>>>>>>>>>>>> flink-table-common
>>>>>>>>>>>> Implemented in Java. Connectors, formats, and UDFs can use
>>> this.
>>>> It
>>>>>>>>>>> contains interface classes such as descriptors, table sink,
>>> table
>>>>>>> source.
>>>>>>>>>>>>> flink-table-core {depends on flink-table-common and
>>>>>>>>>>> flink-table-runtime}
>>>>>>>>>>>> Implemented in Scala. Contains the current main code base.
>>>>>>>>>>>>
>>>>>>>>>>>>> flink-table-runtime
>>>>>>>>>>>> Implemented in Java. This would require to convert classes in
>>>>>>>>>>> o.a.f.table.runtime but would improve the runtime potentially.
>>>>>>>>>>>> What do you think?
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> Regards,
>>>>>>>>>>>>
>>>>>>>>>>>> Timo
>>>>>>>>>>>>
>>>>>>>>>>>> [1]
>>>>>>>>>>> http://apache-flink-mailing-list-archive.1008284.n3.
>>>>>>>>>> nabble.com/DISCUSS-Convert-main-Table-API-classes-into-
>>>>>>> traits-tp21335.html
>>>>>

[Hequn Cheng]

Hi Timo,

Thanks for the effort and writing up this document. I like the idea to make
flink-table scala free, so +1 for the proposal!

It's good to make Java the first-class citizen. For a long time, we have
neglected java so that many features in Table are missed in Java Test
cases, such as this one[1] I found recently. And I think we may also need
to migrate our test cases, i.e, add java tests.

This definitely is a big change and will break API compatible. In order to
bring a smaller impact on users, I think we should go fast when we migrate
APIs targeted to users. It's better to introduce the user sensitive changes
within a release. However, it may be not that easy. I can help to
contribute.

Separation of interface and implementation is a good idea. This may
introduce a minimum of dependencies or even no dependencies. I saw your
reply in the google doc. Java8 has already supported static method for
interfaces, I think we can make use of it?

Best,
Hequn

[1] https://issues.apache.org/jira/browse/FLINK-11001


On Fri, Nov 23, 2018 at 5:36 PM Timo Walther <[hidden email]> wrote:

> Hi everyone,
>
> thanks for the great feedback so far. I updated the document with the
> input I got so far
>
> @Fabian: I moved the porting of flink-table-runtime classes up in the list.
>
> @Xiaowei: Could you elaborate what "interface only" means to you? Do you
> mean a module containing pure Java `interface`s? Or is the validation
> logic also part of the API module? Are 50+ expression classes part of
> the API interface or already too implementation-specific?
>
> @Xuefu: I extended the document by almost a page to clarify when we
> should develop in Scala and when in Java. As Piotr said, every new Scala
> line is instant technical debt.
>
> Thanks,
> Timo
>
>
> Am 23.11.18 um 10:29 schrieb Piotr Nowojski:
> > Hi Timo,
> >
> > Thanks for writing this down +1 from my side :)
> >
> >> I'm wondering that whether we can have rule in the interim when Java
> and Scala coexist that dependency can only be one-way. I found that in the
> current code base there are cases where a Scala class extends Java and vise
> versa. This is quite painful. I'm thinking if we could say that extension
> can only be from Java to Scala, which will help the situation. However, I'm
> not sure if this is practical.
> > Xuefu: I’m also not sure what’s the best approach here, probably we will
> have to work it out as we go. One thing to consider is that from now on,
> every single new code line written in Scala anywhere in Flink-table (except
> of Flink-table-api-scala) is an instant technological debt. From this
> perspective I would be in favour of tolerating quite big inchonvieneces
> just to avoid any new Scala code.
> >
> > Piotrek
> >
> >> On 23 Nov 2018, at 03:25, Zhang, Xuefu <[hidden email]> wrote:
> >>
> >> Hi Timo,
> >>
> >> Thanks for the effort and the Google writeup. During our external
> catalog rework, we found much confusion between Java and Scala, and this
> Scala-free roadmap should greatly mitigate that.
> >>
> >> I'm wondering that whether we can have rule in the interim when Java
> and Scala coexist that dependency can only be one-way. I found that in the
> current code base there are cases where a Scala class extends Java and vise
> versa. This is quite painful. I'm thinking if we could say that extension
> can only be from Java to Scala, which will help the situation. However, I'm
> not sure if this is practical.
> >>
> >> Thanks,
> >> Xuefu
> >>
> >>
> >> ------------------------------------------------------------------
> >> Sender:jincheng sun <[hidden email]>
> >> Sent at:2018 Nov 23 (Fri) 09:49
> >> Recipient:dev <[hidden email]>
> >> Subject:Re: [DISCUSS] Long-term goal of making flink-table Scala-free
> >>
> >> Hi Timo,
> >> Thanks for initiating this great discussion.
> >>
> >> Currently when using SQL/TableAPI should include many dependence. In
> >> particular, it is not necessary to introduce the specific implementation
> >> dependencies which users do not care about. So I am glad to see your
> >> proposal, and hope when we consider splitting the API interface into a
> >> separate module, so that the user can introduce minimum of dependencies.
> >>
> >> So, +1 to [separation of interface and implementation; e.g. `Table` &
> >> `TableImpl`] which you mentioned in the google doc.
> >> Best,
> >> Jincheng
> >>
> >> Xiaowei Jiang <[hidden email]> 于2018年11月22日周四 下午10:50写道：
> >>
> >>> Hi Timo, thanks for driving this! I think that this is a nice thing to
> do.
> >>> While we are doing this, can we also keep in mind that we want to
> >>> eventually have a TableAPI interface only module which users can take
> >>> dependency on, but without including any implementation details?
> >>>
> >>> Xiaowei
> >>>
> >>> On Thu, Nov 22, 2018 at 6:37 PM Fabian Hueske <[hidden email]>
> wrote:
> >>>
> >>>> Hi Timo,
> >>>>
> >>>> Thanks for writing up this document.
> >>>> I like the new structure and agree to prioritize the porting of the
> >>>> flink-table-common classes.
> >>>> Since flink-table-runtime is (or should be) independent of the API and
> >>>> planner modules, we could start porting these classes once the code is
> >>>> split into the new module structure.
> >>>> The benefits of a Scala-free flink-table-runtime would be a Scala-free
> >>>> execution Jar.
> >>>>
> >>>> Best, Fabian
> >>>>
> >>>>
> >>>> Am Do., 22. Nov. 2018 um 10:54 Uhr schrieb Timo Walther <
> >>>> [hidden email]
> >>>>> :
> >>>>> Hi everyone,
> >>>>>
> >>>>> I would like to continue this discussion thread and convert the
> outcome
> >>>>> into a FLIP such that users and contributors know what to expect in
> the
> >>>>> upcoming releases.
> >>>>>
> >>>>> I created a design document [1] that clarifies our motivation why we
> >>>>> want to do this, how a Maven module structure could look like, and a
> >>>>> suggestion for a migration plan.
> >>>>>
> >>>>> It would be great to start with the efforts for the 1.8 release such
> >>>>> that new features can be developed in Java and major refactorings
> such
> >>>>> as improvements to the connectors and external catalog support are
> not
> >>>>> blocked.
> >>>>>
> >>>>> Please let me know what you think.
> >>>>>
> >>>>> Regards,
> >>>>> Timo
> >>>>>
> >>>>> [1]
> >>>>>
> >>>>>
> >>>
> https://docs.google.com/document/d/1PPo6goW7tOwxmpFuvLSjFnx7BF8IVz0w3dcmPPyqvoY/edit?usp=sharing
> >>>>>
> >>>>> Am 02.07.18 um 17:08 schrieb Fabian Hueske:
> >>>>>> Hi Piotr,
> >>>>>>
> >>>>>> thanks for bumping this thread and thanks for Xingcan for the
> >>> comments.
> >>>>>> I think the first step would be to separate the flink-table module
> >>> into
> >>>>>> multiple sub modules. These could be:
> >>>>>>
> >>>>>> - flink-table-api: All API facing classes. Can be later divided
> >>> further
> >>>>>> into Java/Scala Table API/SQL
> >>>>>> - flink-table-planning: involves all planning (basically everything
> >>> we
> >>>> do
> >>>>>> with Calcite)
> >>>>>> - flink-table-runtime: the runtime code
> >>>>>>
> >>>>>> IMO, a realistic mid-term goal is to have the runtime module and
> >>>> certain
> >>>>>> parts of the planning module ported to Java.
> >>>>>> The api module will be much harder to port because of several
> >>>>> dependencies
> >>>>>> to Scala core classes (the parser framework, tree iterations, etc.).
> >>>> I'm
> >>>>>> not saying we should not port this to Java, but it is not clear to
> me
> >>>>> (yet)
> >>>>>> how to do it.
> >>>>>>
> >>>>>> I think flink-table-runtime should not be too hard to port. The code
> >>>> does
> >>>>>> not make use of many Scala features, i.e., it's writing very
> >>> Java-like.
> >>>>>> Also, there are not many dependencies and operators can be
> >>> individually
> >>>>>> ported step-by-step.
> >>>>>> For flink-table-planning, we can have certain packages that we port
> >>> to
> >>>>> Java
> >>>>>> like planning rules or plan nodes. The related classes mostly extend
> >>>>>> Calcite's Java interfaces/classes and would be natural choices for
> >>>> being
> >>>>>> ported. The code generation classes will require more effort to
> port.
> >>>>> There
> >>>>>> are also some dependencies in planning on the api module that we
> >>> would
> >>>>> need
> >>>>>> to resolve somehow.
> >>>>>>
> >>>>>> For SQL most work when adding new features is done in the planning
> >>> and
> >>>>>> runtime modules. So, this separation should already reduce
> >>>> "technological
> >>>>>> dept" quite a lot.
> >>>>>> The Table API depends much more on Scala than SQL.
> >>>>>>
> >>>>>> Cheers, Fabian
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>> 2018-07-02 16:26 GMT+02:00 Xingcan Cui <[hidden email]>:
> >>>>>>
> >>>>>>> Hi all,
> >>>>>>>
> >>>>>>> I also think about this problem these days and here are my
> thoughts.
> >>>>>>>
> >>>>>>> 1) We must admit that it’s really a tough task to interoperate with
> >>>> Java
> >>>>>>> and Scala. E.g., they have different collection types (Scala
> >>>> collections
> >>>>>>> v.s. java.util.*) and in Java, it's hard to implement a method
> which
> >>>>> takes
> >>>>>>> Scala functions as parameters. Considering the major part of the
> >>> code
> >>>>> base
> >>>>>>> is implemented in Java, +1 for this goal from a long-term view.
> >>>>>>>
> >>>>>>> 2) The ideal solution would be to just expose a Scala API and make
> >>> all
> >>>>> the
> >>>>>>> other parts Scala-free. But I am not sure if it could be achieved
> >>> even
> >>>>> in a
> >>>>>>> long-term. Thus as Timo suggested, keep the Scala codes in
> >>>>>>> "flink-table-core" would be a compromise solution.
> >>>>>>>
> >>>>>>> 3) If the community makes the final decision, maybe any new
> features
> >>>>>>> should be added in Java (regardless of the modules), in order to
> >>>> prevent
> >>>>>>> the Scala codes from growing.
> >>>>>>>
> >>>>>>> Best,
> >>>>>>> Xingcan
> >>>>>>>
> >>>>>>>
> >>>>>>>> On Jul 2, 2018, at 9:30 PM, Piotr Nowojski <
> >>> [hidden email]>
> >>>>>>> wrote:
> >>>>>>>> Bumping the topic.
> >>>>>>>>
> >>>>>>>> If we want to do this, the sooner we decide, the less code we will
> >>>> have
> >>>>>>> to rewrite. I have some objections/counter proposals to Fabian's
> >>>>> proposal
> >>>>>>> of doing it module wise and one module at a time.
> >>>>>>>> First, I do not see a problem of having java/scala code even
> within
> >>>> one
> >>>>>>> module, especially not if there are clean boundaries. Like we could
> >>>> have
> >>>>>>> API in Scala and optimizer rules/logical nodes written in Java in
> >>> the
> >>>>> same
> >>>>>>> module. However I haven’t previously maintained mixed scala/java
> >>> code
> >>>>> bases
> >>>>>>> before, so I might be missing something here.
> >>>>>>>> Secondly this whole migration might and most like will take longer
> >>>> then
> >>>>>>> expected, so that creates a problem for a new code that we will be
> >>>>>>> creating. After making a decision to migrate to Java, almost any
> new
> >>>>> Scala
> >>>>>>> line of code will be immediately a technological debt and we will
> >>> have
> >>>>> to
> >>>>>>> rewrite it to Java later.
> >>>>>>>> Thus I would propose first to state our end goal - modules
> >>> structure
> >>>>> and
> >>>>>>> which parts of modules we want to have eventually Scala-free.
> >>> Secondly
> >>>>>>> taking all steps necessary that will allow us to write new code
> >>>>> complaint
> >>>>>>> with our end goal. Only after that we should/could focus on
> >>>>> incrementally
> >>>>>>> rewriting the old code. Otherwise we could be stuck/blocked for
> >>> years
> >>>>>>> writing new code in Scala (and increasing technological debt),
> >>> because
> >>>>>>> nobody have found a time to rewrite some non important and not
> >>>> actively
> >>>>>>> developed part of some module.
> >>>>>>>> Piotrek
> >>>>>>>>
> >>>>>>>>> On 14 Jun 2018, at 15:34, Fabian Hueske <[hidden email]>
> >>> wrote:
> >>>>>>>>> Hi,
> >>>>>>>>>
> >>>>>>>>> In general, I think this is a good effort. However, it won't be
> >>> easy
> >>>>>>> and I
> >>>>>>>>> think we have to plan this well.
> >>>>>>>>> I don't like the idea of having the whole code base fragmented
> >>> into
> >>>>> Java
> >>>>>>>>> and Scala code for too long.
> >>>>>>>>>
> >>>>>>>>> I think we should do this one step at a time and focus on
> >>> migrating
> >>>>> one
> >>>>>>>>> module at a time.
> >>>>>>>>> IMO, the easiest start would be to port the runtime to Java.
> >>>>>>>>> Extracting the API classes into an own module, porting them to
> >>> Java,
> >>>>> and
> >>>>>>>>> removing the Scala dependency won't be possible without breaking
> >>> the
> >>>>> API
> >>>>>>>>> since a few classes depend on the Scala Table API.
> >>>>>>>>>
> >>>>>>>>> Best, Fabian
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> 2018-06-14 10:33 GMT+02:00 Till Rohrmann <[hidden email]>:
> >>>>>>>>>
> >>>>>>>>>> I think that is a noble and honorable goal and we should strive
> >>> for
> >>>>> it.
> >>>>>>>>>> This, however, must be an iterative process given the sheer size
> >>> of
> >>>>> the
> >>>>>>>>>> code base. I like the approach to define common Java modules
> >>> which
> >>>>> are
> >>>>>>> used
> >>>>>>>>>> by more specific Scala modules and slowly moving classes from
> >>> Scala
> >>>>> to
> >>>>>>>>>> Java. Thus +1 for the proposal.
> >>>>>>>>>>
> >>>>>>>>>> Cheers,
> >>>>>>>>>> Till
> >>>>>>>>>>
> >>>>>>>>>> On Wed, Jun 13, 2018 at 12:01 PM Piotr Nowojski <
> >>>>>>> [hidden email]>
> >>>>>>>>>> wrote:
> >>>>>>>>>>
> >>>>>>>>>>> Hi,
> >>>>>>>>>>>
> >>>>>>>>>>> I do not have an experience with how scala and java interacts
> >>> with
> >>>>>>> each
> >>>>>>>>>>> other, so I can not fully validate your proposal, but generally
> >>>>>>> speaking
> >>>>>>>>>> +1
> >>>>>>>>>>> from me.
> >>>>>>>>>>>
> >>>>>>>>>>> Does it also mean, that we should slowly migrate
> >>>> `flink-table-core`
> >>>>> to
> >>>>>>>>>>> Java? How would you envision it? It would be nice to be able to
> >>>> add
> >>>>>>> new
> >>>>>>>>>>> classes/features written in Java and so that they can coexist
> >>> with
> >>>>> old
> >>>>>>>>>>> Scala code until we gradually switch from Scala to Java.
> >>>>>>>>>>>
> >>>>>>>>>>> Piotrek
> >>>>>>>>>>>
> >>>>>>>>>>>> On 13 Jun 2018, at 11:32, Timo Walther <[hidden email]>
> >>>> wrote:
> >>>>>>>>>>>> Hi everyone,
> >>>>>>>>>>>>
> >>>>>>>>>>>> as you all know, currently the Table & SQL API is implemented
> >>> in
> >>>>>>> Scala.
> >>>>>>>>>>> This decision was made a long-time ago when the initital code
> >>> base
> >>>>> was
> >>>>>>>>>>> created as part of a master's thesis. The community kept Scala
> >>>>>>> because of
> >>>>>>>>>>> the nice language features that enable a fluent Table API like
> >>>>>>>>>>> table.select('field.trim()) and because Scala allows for quick
> >>>>>>>>>> prototyping
> >>>>>>>>>>> (e.g. multi-line comments for code generation). The committers
> >>>>>>> enforced
> >>>>>>>>>> not
> >>>>>>>>>>> splitting the code-base into two programming languages.
> >>>>>>>>>>>> However, nowadays the flink-table module more and more becomes
> >>> an
> >>>>>>>>>>> important part in the Flink ecosystem. Connectors, formats, and
> >>>> SQL
> >>>>>>>>>> client
> >>>>>>>>>>> are actually implemented in Java but need to interoperate with
> >>>>>>>>>> flink-table
> >>>>>>>>>>> which makes these modules dependent on Scala. As mentioned in
> an
> >>>>>>> earlier
> >>>>>>>>>>> mail thread, using Scala for API classes also exposes member
> >>>>> variables
> >>>>>>>>>> and
> >>>>>>>>>>> methods in Java that should not be exposed to users [1]. Java
> is
> >>>>> still
> >>>>>>>>>> the
> >>>>>>>>>>> most important API language and right now we treat it as a
> >>>>>>> second-class
> >>>>>>>>>>> citizen. I just noticed that you even need to add Scala if you
> >>>> just
> >>>>>>> want
> >>>>>>>>>> to
> >>>>>>>>>>> implement a ScalarFunction because of method clashes between
> >>>> `public
> >>>>>>>>>> String
> >>>>>>>>>>> toString()` and `public scala.Predef.String toString()`.
> >>>>>>>>>>>> Given the size of the current code base, reimplementing the
> >>>> entire
> >>>>>>>>>>> flink-table code in Java is a goal that we might never reach.
> >>>>>>> However, we
> >>>>>>>>>>> should at least treat the symptoms and have this as a long-term
> >>>> goal
> >>>>>>> in
> >>>>>>>>>>> mind. My suggestion would be to convert user-facing and runtime
> >>>>>>> classes
> >>>>>>>>>> and
> >>>>>>>>>>> split the code base into multiple modules:
> >>>>>>>>>>>>> flink-table-java {depends on flink-table-core}
> >>>>>>>>>>>> Implemented in Java. Java users can use this. This would
> >>> require
> >>>> to
> >>>>>>>>>>> convert classes like TableEnvironment, Table.
> >>>>>>>>>>>>> flink-table-scala {depends on flink-table-core}
> >>>>>>>>>>>> Implemented in Scala. Scala users can use this.
> >>>>>>>>>>>>
> >>>>>>>>>>>>> flink-table-common
> >>>>>>>>>>>> Implemented in Java. Connectors, formats, and UDFs can use
> >>> this.
> >>>> It
> >>>>>>>>>>> contains interface classes such as descriptors, table sink,
> >>> table
> >>>>>>> source.
> >>>>>>>>>>>>> flink-table-core {depends on flink-table-common and
> >>>>>>>>>>> flink-table-runtime}
> >>>>>>>>>>>> Implemented in Scala. Contains the current main code base.
> >>>>>>>>>>>>
> >>>>>>>>>>>>> flink-table-runtime
> >>>>>>>>>>>> Implemented in Java. This would require to convert classes in
> >>>>>>>>>>> o.a.f.table.runtime but would improve the runtime potentially.
> >>>>>>>>>>>> What do you think?
> >>>>>>>>>>>>
> >>>>>>>>>>>>
> >>>>>>>>>>>> Regards,
> >>>>>>>>>>>>
> >>>>>>>>>>>> Timo
> >>>>>>>>>>>>
> >>>>>>>>>>>> [1]
> >>>>>>>>>>> http://apache-flink-mailing-list-archive.1008284.n3.
> >>>>>>>>>> nabble.com/DISCUSS-Convert-main-Table-API-classes-into-
> >>>>>>> traits-tp21335.html
> >>>>>
>
>

[vino yang]

Hi hequn,

I am very glad to hear that you are interested in this work.
As we all know, this process involves a lot.
Currently, the migration work has begun. I started with the
Kafka connector's dependency on flink-table and moved the
related dependencies to flink-table-common.
This work is tracked by FLINK-9461.  [1]
I don't know if it will conflict with what you expect to do, but from the
impact I have observed,
it will involve many classes that are currently in flink-table.

*Just a statement to prevent unnecessary conflicts.*

Thanks, vino.

[1]: https://issues.apache.org/jira/browse/FLINK-9461

Hequn Cheng <[hidden email]> 于2018年11月24日周六 下午7:20写道：

> Hi Timo,
>
> Thanks for the effort and writing up this document. I like the idea to make
> flink-table scala free, so +1 for the proposal!
>
> It's good to make Java the first-class citizen. For a long time, we have
> neglected java so that many features in Table are missed in Java Test
> cases, such as this one[1] I found recently. And I think we may also need
> to migrate our test cases, i.e, add java tests.
>
> This definitely is a big change and will break API compatible. In order to
> bring a smaller impact on users, I think we should go fast when we migrate
> APIs targeted to users. It's better to introduce the user sensitive changes
> within a release. However, it may be not that easy. I can help to
> contribute.
>
> Separation of interface and implementation is a good idea. This may
> introduce a minimum of dependencies or even no dependencies. I saw your
> reply in the google doc. Java8 has already supported static method for
> interfaces, I think we can make use of it?
>
> Best,
> Hequn
>
> [1] https://issues.apache.org/jira/browse/FLINK-11001
>
>
> On Fri, Nov 23, 2018 at 5:36 PM Timo Walther <[hidden email]> wrote:
>
> > Hi everyone,
> >
> > thanks for the great feedback so far. I updated the document with the
> > input I got so far
> >
> > @Fabian: I moved the porting of flink-table-runtime classes up in the
> list.
> >
> > @Xiaowei: Could you elaborate what "interface only" means to you? Do you
> > mean a module containing pure Java `interface`s? Or is the validation
> > logic also part of the API module? Are 50+ expression classes part of
> > the API interface or already too implementation-specific?
> >
> > @Xuefu: I extended the document by almost a page to clarify when we
> > should develop in Scala and when in Java. As Piotr said, every new Scala
> > line is instant technical debt.
> >
> > Thanks,
> > Timo
> >
> >
> > Am 23.11.18 um 10:29 schrieb Piotr Nowojski:
> > > Hi Timo,
> > >
> > > Thanks for writing this down +1 from my side :)
> > >
> > >> I'm wondering that whether we can have rule in the interim when Java
> > and Scala coexist that dependency can only be one-way. I found that in
> the
> > current code base there are cases where a Scala class extends Java and
> vise
> > versa. This is quite painful. I'm thinking if we could say that extension
> > can only be from Java to Scala, which will help the situation. However,
> I'm
> > not sure if this is practical.
> > > Xuefu: I’m also not sure what’s the best approach here, probably we
> will
> > have to work it out as we go. One thing to consider is that from now on,
> > every single new code line written in Scala anywhere in Flink-table
> (except
> > of Flink-table-api-scala) is an instant technological debt. From this
> > perspective I would be in favour of tolerating quite big inchonvieneces
> > just to avoid any new Scala code.
> > >
> > > Piotrek
> > >
> > >> On 23 Nov 2018, at 03:25, Zhang, Xuefu <[hidden email]>
> wrote:
> > >>
> > >> Hi Timo,
> > >>
> > >> Thanks for the effort and the Google writeup. During our external
> > catalog rework, we found much confusion between Java and Scala, and this
> > Scala-free roadmap should greatly mitigate that.
> > >>
> > >> I'm wondering that whether we can have rule in the interim when Java
> > and Scala coexist that dependency can only be one-way. I found that in
> the
> > current code base there are cases where a Scala class extends Java and
> vise
> > versa. This is quite painful. I'm thinking if we could say that extension
> > can only be from Java to Scala, which will help the situation. However,
> I'm
> > not sure if this is practical.
> > >>
> > >> Thanks,
> > >> Xuefu
> > >>
> > >>
> > >> ------------------------------------------------------------------
> > >> Sender:jincheng sun <[hidden email]>
> > >> Sent at:2018 Nov 23 (Fri) 09:49
> > >> Recipient:dev <[hidden email]>
> > >> Subject:Re: [DISCUSS] Long-term goal of making flink-table Scala-free
> > >>
> > >> Hi Timo,
> > >> Thanks for initiating this great discussion.
> > >>
> > >> Currently when using SQL/TableAPI should include many dependence. In
> > >> particular, it is not necessary to introduce the specific
> implementation
> > >> dependencies which users do not care about. So I am glad to see your
> > >> proposal, and hope when we consider splitting the API interface into a
> > >> separate module, so that the user can introduce minimum of
> dependencies.
> > >>
> > >> So, +1 to [separation of interface and implementation; e.g. `Table` &
> > >> `TableImpl`] which you mentioned in the google doc.
> > >> Best,
> > >> Jincheng
> > >>
> > >> Xiaowei Jiang <[hidden email]> 于2018年11月22日周四 下午10:50写道：
> > >>
> > >>> Hi Timo, thanks for driving this! I think that this is a nice thing
> to
> > do.
> > >>> While we are doing this, can we also keep in mind that we want to
> > >>> eventually have a TableAPI interface only module which users can take
> > >>> dependency on, but without including any implementation details?
> > >>>
> > >>> Xiaowei
> > >>>
> > >>> On Thu, Nov 22, 2018 at 6:37 PM Fabian Hueske <[hidden email]>
> > wrote:
> > >>>
> > >>>> Hi Timo,
> > >>>>
> > >>>> Thanks for writing up this document.
> > >>>> I like the new structure and agree to prioritize the porting of the
> > >>>> flink-table-common classes.
> > >>>> Since flink-table-runtime is (or should be) independent of the API
> and
> > >>>> planner modules, we could start porting these classes once the code
> is
> > >>>> split into the new module structure.
> > >>>> The benefits of a Scala-free flink-table-runtime would be a
> Scala-free
> > >>>> execution Jar.
> > >>>>
> > >>>> Best, Fabian
> > >>>>
> > >>>>
> > >>>> Am Do., 22. Nov. 2018 um 10:54 Uhr schrieb Timo Walther <
> > >>>> [hidden email]
> > >>>>> :
> > >>>>> Hi everyone,
> > >>>>>
> > >>>>> I would like to continue this discussion thread and convert the
> > outcome
> > >>>>> into a FLIP such that users and contributors know what to expect in
> > the
> > >>>>> upcoming releases.
> > >>>>>
> > >>>>> I created a design document [1] that clarifies our motivation why
> we
> > >>>>> want to do this, how a Maven module structure could look like, and
> a
> > >>>>> suggestion for a migration plan.
> > >>>>>
> > >>>>> It would be great to start with the efforts for the 1.8 release
> such
> > >>>>> that new features can be developed in Java and major refactorings
> > such
> > >>>>> as improvements to the connectors and external catalog support are
> > not
> > >>>>> blocked.
> > >>>>>
> > >>>>> Please let me know what you think.
> > >>>>>
> > >>>>> Regards,
> > >>>>> Timo
> > >>>>>
> > >>>>> [1]
> > >>>>>
> > >>>>>
> > >>>
> >
> https://docs.google.com/document/d/1PPo6goW7tOwxmpFuvLSjFnx7BF8IVz0w3dcmPPyqvoY/edit?usp=sharing
> > >>>>>
> > >>>>> Am 02.07.18 um 17:08 schrieb Fabian Hueske:
> > >>>>>> Hi Piotr,
> > >>>>>>
> > >>>>>> thanks for bumping this thread and thanks for Xingcan for the
> > >>> comments.
> > >>>>>> I think the first step would be to separate the flink-table module
> > >>> into
> > >>>>>> multiple sub modules. These could be:
> > >>>>>>
> > >>>>>> - flink-table-api: All API facing classes. Can be later divided
> > >>> further
> > >>>>>> into Java/Scala Table API/SQL
> > >>>>>> - flink-table-planning: involves all planning (basically
> everything
> > >>> we
> > >>>> do
> > >>>>>> with Calcite)
> > >>>>>> - flink-table-runtime: the runtime code
> > >>>>>>
> > >>>>>> IMO, a realistic mid-term goal is to have the runtime module and
> > >>>> certain
> > >>>>>> parts of the planning module ported to Java.
> > >>>>>> The api module will be much harder to port because of several
> > >>>>> dependencies
> > >>>>>> to Scala core classes (the parser framework, tree iterations,
> etc.).
> > >>>> I'm
> > >>>>>> not saying we should not port this to Java, but it is not clear to
> > me
> > >>>>> (yet)
> > >>>>>> how to do it.
> > >>>>>>
> > >>>>>> I think flink-table-runtime should not be too hard to port. The
> code
> > >>>> does
> > >>>>>> not make use of many Scala features, i.e., it's writing very
> > >>> Java-like.
> > >>>>>> Also, there are not many dependencies and operators can be
> > >>> individually
> > >>>>>> ported step-by-step.
> > >>>>>> For flink-table-planning, we can have certain packages that we
> port
> > >>> to
> > >>>>> Java
> > >>>>>> like planning rules or plan nodes. The related classes mostly
> extend
> > >>>>>> Calcite's Java interfaces/classes and would be natural choices for
> > >>>> being
> > >>>>>> ported. The code generation classes will require more effort to
> > port.
> > >>>>> There
> > >>>>>> are also some dependencies in planning on the api module that we
> > >>> would
> > >>>>> need
> > >>>>>> to resolve somehow.
> > >>>>>>
> > >>>>>> For SQL most work when adding new features is done in the planning
> > >>> and
> > >>>>>> runtime modules. So, this separation should already reduce
> > >>>> "technological
> > >>>>>> dept" quite a lot.
> > >>>>>> The Table API depends much more on Scala than SQL.
> > >>>>>>
> > >>>>>> Cheers, Fabian
> > >>>>>>
> > >>>>>>
> > >>>>>>
> > >>>>>> 2018-07-02 16:26 GMT+02:00 Xingcan Cui <[hidden email]>:
> > >>>>>>
> > >>>>>>> Hi all,
> > >>>>>>>
> > >>>>>>> I also think about this problem these days and here are my
> > thoughts.
> > >>>>>>>
> > >>>>>>> 1) We must admit that it’s really a tough task to interoperate
> with
> > >>>> Java
> > >>>>>>> and Scala. E.g., they have different collection types (Scala
> > >>>> collections
> > >>>>>>> v.s. java.util.*) and in Java, it's hard to implement a method
> > which
> > >>>>> takes
> > >>>>>>> Scala functions as parameters. Considering the major part of the
> > >>> code
> > >>>>> base
> > >>>>>>> is implemented in Java, +1 for this goal from a long-term view.
> > >>>>>>>
> > >>>>>>> 2) The ideal solution would be to just expose a Scala API and
> make
> > >>> all
> > >>>>> the
> > >>>>>>> other parts Scala-free. But I am not sure if it could be achieved
> > >>> even
> > >>>>> in a
> > >>>>>>> long-term. Thus as Timo suggested, keep the Scala codes in
> > >>>>>>> "flink-table-core" would be a compromise solution.
> > >>>>>>>
> > >>>>>>> 3) If the community makes the final decision, maybe any new
> > features
> > >>>>>>> should be added in Java (regardless of the modules), in order to
> > >>>> prevent
> > >>>>>>> the Scala codes from growing.
> > >>>>>>>
> > >>>>>>> Best,
> > >>>>>>> Xingcan
> > >>>>>>>
> > >>>>>>>
> > >>>>>>>> On Jul 2, 2018, at 9:30 PM, Piotr Nowojski <
> > >>> [hidden email]>
> > >>>>>>> wrote:
> > >>>>>>>> Bumping the topic.
> > >>>>>>>>
> > >>>>>>>> If we want to do this, the sooner we decide, the less code we
> will
> > >>>> have
> > >>>>>>> to rewrite. I have some objections/counter proposals to Fabian's
> > >>>>> proposal
> > >>>>>>> of doing it module wise and one module at a time.
> > >>>>>>>> First, I do not see a problem of having java/scala code even
> > within
> > >>>> one
> > >>>>>>> module, especially not if there are clean boundaries. Like we
> could
> > >>>> have
> > >>>>>>> API in Scala and optimizer rules/logical nodes written in Java in
> > >>> the
> > >>>>> same
> > >>>>>>> module. However I haven’t previously maintained mixed scala/java
> > >>> code
> > >>>>> bases
> > >>>>>>> before, so I might be missing something here.
> > >>>>>>>> Secondly this whole migration might and most like will take
> longer
> > >>>> then
> > >>>>>>> expected, so that creates a problem for a new code that we will
> be
> > >>>>>>> creating. After making a decision to migrate to Java, almost any
> > new
> > >>>>> Scala
> > >>>>>>> line of code will be immediately a technological debt and we will
> > >>> have
> > >>>>> to
> > >>>>>>> rewrite it to Java later.
> > >>>>>>>> Thus I would propose first to state our end goal - modules
> > >>> structure
> > >>>>> and
> > >>>>>>> which parts of modules we want to have eventually Scala-free.
> > >>> Secondly
> > >>>>>>> taking all steps necessary that will allow us to write new code
> > >>>>> complaint
> > >>>>>>> with our end goal. Only after that we should/could focus on
> > >>>>> incrementally
> > >>>>>>> rewriting the old code. Otherwise we could be stuck/blocked for
> > >>> years
> > >>>>>>> writing new code in Scala (and increasing technological debt),
> > >>> because
> > >>>>>>> nobody have found a time to rewrite some non important and not
> > >>>> actively
> > >>>>>>> developed part of some module.
> > >>>>>>>> Piotrek
> > >>>>>>>>
> > >>>>>>>>> On 14 Jun 2018, at 15:34, Fabian Hueske <[hidden email]>
> > >>> wrote:
> > >>>>>>>>> Hi,
> > >>>>>>>>>
> > >>>>>>>>> In general, I think this is a good effort. However, it won't be
> > >>> easy
> > >>>>>>> and I
> > >>>>>>>>> think we have to plan this well.
> > >>>>>>>>> I don't like the idea of having the whole code base fragmented
> > >>> into
> > >>>>> Java
> > >>>>>>>>> and Scala code for too long.
> > >>>>>>>>>
> > >>>>>>>>> I think we should do this one step at a time and focus on
> > >>> migrating
> > >>>>> one
> > >>>>>>>>> module at a time.
> > >>>>>>>>> IMO, the easiest start would be to port the runtime to Java.
> > >>>>>>>>> Extracting the API classes into an own module, porting them to
> > >>> Java,
> > >>>>> and
> > >>>>>>>>> removing the Scala dependency won't be possible without
> breaking
> > >>> the
> > >>>>> API
> > >>>>>>>>> since a few classes depend on the Scala Table API.
> > >>>>>>>>>
> > >>>>>>>>> Best, Fabian
> > >>>>>>>>>
> > >>>>>>>>>
> > >>>>>>>>> 2018-06-14 10:33 GMT+02:00 Till Rohrmann <[hidden email]
> >:
> > >>>>>>>>>
> > >>>>>>>>>> I think that is a noble and honorable goal and we should
> strive
> > >>> for
> > >>>>> it.
> > >>>>>>>>>> This, however, must be an iterative process given the sheer
> size
> > >>> of
> > >>>>> the
> > >>>>>>>>>> code base. I like the approach to define common Java modules
> > >>> which
> > >>>>> are
> > >>>>>>> used
> > >>>>>>>>>> by more specific Scala modules and slowly moving classes from
> > >>> Scala
> > >>>>> to
> > >>>>>>>>>> Java. Thus +1 for the proposal.
> > >>>>>>>>>>
> > >>>>>>>>>> Cheers,
> > >>>>>>>>>> Till
> > >>>>>>>>>>
> > >>>>>>>>>> On Wed, Jun 13, 2018 at 12:01 PM Piotr Nowojski <
> > >>>>>>> [hidden email]>
> > >>>>>>>>>> wrote:
> > >>>>>>>>>>
> > >>>>>>>>>>> Hi,
> > >>>>>>>>>>>
> > >>>>>>>>>>> I do not have an experience with how scala and java interacts
> > >>> with
> > >>>>>>> each
> > >>>>>>>>>>> other, so I can not fully validate your proposal, but
> generally
> > >>>>>>> speaking
> > >>>>>>>>>> +1
> > >>>>>>>>>>> from me.
> > >>>>>>>>>>>
> > >>>>>>>>>>> Does it also mean, that we should slowly migrate
> > >>>> `flink-table-core`
> > >>>>> to
> > >>>>>>>>>>> Java? How would you envision it? It would be nice to be able
> to
> > >>>> add
> > >>>>>>> new
> > >>>>>>>>>>> classes/features written in Java and so that they can coexist
> > >>> with
> > >>>>> old
> > >>>>>>>>>>> Scala code until we gradually switch from Scala to Java.
> > >>>>>>>>>>>
> > >>>>>>>>>>> Piotrek
> > >>>>>>>>>>>
> > >>>>>>>>>>>> On 13 Jun 2018, at 11:32, Timo Walther <[hidden email]>
> > >>>> wrote:
> > >>>>>>>>>>>> Hi everyone,
> > >>>>>>>>>>>>
> > >>>>>>>>>>>> as you all know, currently the Table & SQL API is
> implemented
> > >>> in
> > >>>>>>> Scala.
> > >>>>>>>>>>> This decision was made a long-time ago when the initital code
> > >>> base
> > >>>>> was
> > >>>>>>>>>>> created as part of a master's thesis. The community kept
> Scala
> > >>>>>>> because of
> > >>>>>>>>>>> the nice language features that enable a fluent Table API
> like
> > >>>>>>>>>>> table.select('field.trim()) and because Scala allows for
> quick
> > >>>>>>>>>> prototyping
> > >>>>>>>>>>> (e.g. multi-line comments for code generation). The
> committers
> > >>>>>>> enforced
> > >>>>>>>>>> not
> > >>>>>>>>>>> splitting the code-base into two programming languages.
> > >>>>>>>>>>>> However, nowadays the flink-table module more and more
> becomes
> > >>> an
> > >>>>>>>>>>> important part in the Flink ecosystem. Connectors, formats,
> and
> > >>>> SQL
> > >>>>>>>>>> client
> > >>>>>>>>>>> are actually implemented in Java but need to interoperate
> with
> > >>>>>>>>>> flink-table
> > >>>>>>>>>>> which makes these modules dependent on Scala. As mentioned in
> > an
> > >>>>>>> earlier
> > >>>>>>>>>>> mail thread, using Scala for API classes also exposes member
> > >>>>> variables
> > >>>>>>>>>> and
> > >>>>>>>>>>> methods in Java that should not be exposed to users [1]. Java
> > is
> > >>>>> still
> > >>>>>>>>>> the
> > >>>>>>>>>>> most important API language and right now we treat it as a
> > >>>>>>> second-class
> > >>>>>>>>>>> citizen. I just noticed that you even need to add Scala if
> you
> > >>>> just
> > >>>>>>> want
> > >>>>>>>>>> to
> > >>>>>>>>>>> implement a ScalarFunction because of method clashes between
> > >>>> `public
> > >>>>>>>>>> String
> > >>>>>>>>>>> toString()` and `public scala.Predef.String toString()`.
> > >>>>>>>>>>>> Given the size of the current code base, reimplementing the
> > >>>> entire
> > >>>>>>>>>>> flink-table code in Java is a goal that we might never reach.
> > >>>>>>> However, we
> > >>>>>>>>>>> should at least treat the symptoms and have this as a
> long-term
> > >>>> goal
> > >>>>>>> in
> > >>>>>>>>>>> mind. My suggestion would be to convert user-facing and
> runtime
> > >>>>>>> classes
> > >>>>>>>>>> and
> > >>>>>>>>>>> split the code base into multiple modules:
> > >>>>>>>>>>>>> flink-table-java {depends on flink-table-core}
> > >>>>>>>>>>>> Implemented in Java. Java users can use this. This would
> > >>> require
> > >>>> to
> > >>>>>>>>>>> convert classes like TableEnvironment, Table.
> > >>>>>>>>>>>>> flink-table-scala {depends on flink-table-core}
> > >>>>>>>>>>>> Implemented in Scala. Scala users can use this.
> > >>>>>>>>>>>>
> > >>>>>>>>>>>>> flink-table-common
> > >>>>>>>>>>>> Implemented in Java. Connectors, formats, and UDFs can use
> > >>> this.
> > >>>> It
> > >>>>>>>>>>> contains interface classes such as descriptors, table sink,
> > >>> table
> > >>>>>>> source.
> > >>>>>>>>>>>>> flink-table-core {depends on flink-table-common and
> > >>>>>>>>>>> flink-table-runtime}
> > >>>>>>>>>>>> Implemented in Scala. Contains the current main code base.
> > >>>>>>>>>>>>
> > >>>>>>>>>>>>> flink-table-runtime
> > >>>>>>>>>>>> Implemented in Java. This would require to convert classes
> in
> > >>>>>>>>>>> o.a.f.table.runtime but would improve the runtime
> potentially.
> > >>>>>>>>>>>> What do you think?
> > >>>>>>>>>>>>
> > >>>>>>>>>>>>
> > >>>>>>>>>>>> Regards,
> > >>>>>>>>>>>>
> > >>>>>>>>>>>> Timo
> > >>>>>>>>>>>>
> > >>>>>>>>>>>> [1]
> > >>>>>>>>>>> http://apache-flink-mailing-list-archive.1008284.n3.
> > >>>>>>>>>> nabble.com/DISCUSS-Convert-main-Table-API-classes-into-
> > >>>>>>> traits-tp21335.html
> > >>>>>
> >
> >
>

[Timo Walther-2]

Hi Hequn,

thanks for your feedback. Yes, migrating the test cases is another issue
that is not represented in the document but should naturally go along
with the migration.

I agree that we should migrate the main API classes quickly within this
1.8 release after the module split has been performed. Help here is
highly appreciated!

I forgot that Java supports static methods in interfaces now, but
actually I don't like the design of calling `TableEnvironment.get(env)`.
Because people often use `TableEnvironment tEnd =
TableEnvironment.get(env)` and then wonder why there is no
`toAppendStream` or `toDataSet` because they are using the base class.
However, things like that can be discussed in the corresponding issue
when it comes to implementation.

@Vino: I think your work fits nicely to these efforts.

@everyone: I will wait for more feedback until end of this week. Then I
will convert the design document into a FLIP and open subtasks in Jira,
if there are no objections?

Regards,
Timo

Am 24.11.18 um 13:45 schrieb vino yang:

> Hi hequn,
>
> I am very glad to hear that you are interested in this work.
> As we all know, this process involves a lot.
> Currently, the migration work has begun. I started with the
> Kafka connector's dependency on flink-table and moved the
> related dependencies to flink-table-common.
> This work is tracked by FLINK-9461.  [1]
> I don't know if it will conflict with what you expect to do, but from the
> impact I have observed,
> it will involve many classes that are currently in flink-table.
>
> *Just a statement to prevent unnecessary conflicts.*
>
> Thanks, vino.
>
> [1]: https://issues.apache.org/jira/browse/FLINK-9461
>
> Hequn Cheng <[hidden email]> 于2018年11月24日周六 下午7:20写道：
>
>> Hi Timo,
>>
>> Thanks for the effort and writing up this document. I like the idea to make
>> flink-table scala free, so +1 for the proposal!
>>
>> It's good to make Java the first-class citizen. For a long time, we have
>> neglected java so that many features in Table are missed in Java Test
>> cases, such as this one[1] I found recently. And I think we may also need
>> to migrate our test cases, i.e, add java tests.
>>
>> This definitely is a big change and will break API compatible. In order to
>> bring a smaller impact on users, I think we should go fast when we migrate
>> APIs targeted to users. It's better to introduce the user sensitive changes
>> within a release. However, it may be not that easy. I can help to
>> contribute.
>>
>> Separation of interface and implementation is a good idea. This may
>> introduce a minimum of dependencies or even no dependencies. I saw your
>> reply in the google doc. Java8 has already supported static method for
>> interfaces, I think we can make use of it?
>>
>> Best,
>> Hequn
>>
>> [1] https://issues.apache.org/jira/browse/FLINK-11001
>>
>>
>> On Fri, Nov 23, 2018 at 5:36 PM Timo Walther <[hidden email]> wrote:
>>
>>> Hi everyone,
>>>
>>> thanks for the great feedback so far. I updated the document with the
>>> input I got so far
>>>
>>> @Fabian: I moved the porting of flink-table-runtime classes up in the
>> list.
>>> @Xiaowei: Could you elaborate what "interface only" means to you? Do you
>>> mean a module containing pure Java `interface`s? Or is the validation
>>> logic also part of the API module? Are 50+ expression classes part of
>>> the API interface or already too implementation-specific?
>>>
>>> @Xuefu: I extended the document by almost a page to clarify when we
>>> should develop in Scala and when in Java. As Piotr said, every new Scala
>>> line is instant technical debt.
>>>
>>> Thanks,
>>> Timo
>>>
>>>
>>> Am 23.11.18 um 10:29 schrieb Piotr Nowojski:
>>>> Hi Timo,
>>>>
>>>> Thanks for writing this down +1 from my side :)
>>>>
>>>>> I'm wondering that whether we can have rule in the interim when Java
>>> and Scala coexist that dependency can only be one-way. I found that in
>> the
>>> current code base there are cases where a Scala class extends Java and
>> vise
>>> versa. This is quite painful. I'm thinking if we could say that extension
>>> can only be from Java to Scala, which will help the situation. However,
>> I'm
>>> not sure if this is practical.
>>>> Xuefu: I’m also not sure what’s the best approach here, probably we
>> will
>>> have to work it out as we go. One thing to consider is that from now on,
>>> every single new code line written in Scala anywhere in Flink-table
>> (except
>>> of Flink-table-api-scala) is an instant technological debt. From this
>>> perspective I would be in favour of tolerating quite big inchonvieneces
>>> just to avoid any new Scala code.
>>>> Piotrek
>>>>
>>>>> On 23 Nov 2018, at 03:25, Zhang, Xuefu <[hidden email]>
>> wrote:
>>>>> Hi Timo,
>>>>>
>>>>> Thanks for the effort and the Google writeup. During our external
>>> catalog rework, we found much confusion between Java and Scala, and this
>>> Scala-free roadmap should greatly mitigate that.
>>>>> I'm wondering that whether we can have rule in the interim when Java
>>> and Scala coexist that dependency can only be one-way. I found that in
>> the
>>> current code base there are cases where a Scala class extends Java and
>> vise
>>> versa. This is quite painful. I'm thinking if we could say that extension
>>> can only be from Java to Scala, which will help the situation. However,
>> I'm
>>> not sure if this is practical.
>>>>> Thanks,
>>>>> Xuefu
>>>>>
>>>>>
>>>>> ------------------------------------------------------------------
>>>>> Sender:jincheng sun <[hidden email]>
>>>>> Sent at:2018 Nov 23 (Fri) 09:49
>>>>> Recipient:dev <[hidden email]>
>>>>> Subject:Re: [DISCUSS] Long-term goal of making flink-table Scala-free
>>>>>
>>>>> Hi Timo,
>>>>> Thanks for initiating this great discussion.
>>>>>
>>>>> Currently when using SQL/TableAPI should include many dependence. In
>>>>> particular, it is not necessary to introduce the specific
>> implementation
>>>>> dependencies which users do not care about. So I am glad to see your
>>>>> proposal, and hope when we consider splitting the API interface into a
>>>>> separate module, so that the user can introduce minimum of
>> dependencies.
>>>>> So, +1 to [separation of interface and implementation; e.g. `Table` &
>>>>> `TableImpl`] which you mentioned in the google doc.
>>>>> Best,
>>>>> Jincheng
>>>>>
>>>>> Xiaowei Jiang <[hidden email]> 于2018年11月22日周四 下午10:50写道：
>>>>>
>>>>>> Hi Timo, thanks for driving this! I think that this is a nice thing
>> to
>>> do.
>>>>>> While we are doing this, can we also keep in mind that we want to
>>>>>> eventually have a TableAPI interface only module which users can take
>>>>>> dependency on, but without including any implementation details?
>>>>>>
>>>>>> Xiaowei
>>>>>>
>>>>>> On Thu, Nov 22, 2018 at 6:37 PM Fabian Hueske <[hidden email]>
>>> wrote:
>>>>>>> Hi Timo,
>>>>>>>
>>>>>>> Thanks for writing up this document.
>>>>>>> I like the new structure and agree to prioritize the porting of the
>>>>>>> flink-table-common classes.
>>>>>>> Since flink-table-runtime is (or should be) independent of the API
>> and
>>>>>>> planner modules, we could start porting these classes once the code
>> is
>>>>>>> split into the new module structure.
>>>>>>> The benefits of a Scala-free flink-table-runtime would be a
>> Scala-free
>>>>>>> execution Jar.
>>>>>>>
>>>>>>> Best, Fabian
>>>>>>>
>>>>>>>
>>>>>>> Am Do., 22. Nov. 2018 um 10:54 Uhr schrieb Timo Walther <
>>>>>>> [hidden email]
>>>>>>>> :
>>>>>>>> Hi everyone,
>>>>>>>>
>>>>>>>> I would like to continue this discussion thread and convert the
>>> outcome
>>>>>>>> into a FLIP such that users and contributors know what to expect in
>>> the
>>>>>>>> upcoming releases.
>>>>>>>>
>>>>>>>> I created a design document [1] that clarifies our motivation why
>> we
>>>>>>>> want to do this, how a Maven module structure could look like, and
>> a
>>>>>>>> suggestion for a migration plan.
>>>>>>>>
>>>>>>>> It would be great to start with the efforts for the 1.8 release
>> such
>>>>>>>> that new features can be developed in Java and major refactorings
>>> such
>>>>>>>> as improvements to the connectors and external catalog support are
>>> not
>>>>>>>> blocked.
>>>>>>>>
>>>>>>>> Please let me know what you think.
>>>>>>>>
>>>>>>>> Regards,
>>>>>>>> Timo
>>>>>>>>
>>>>>>>> [1]
>>>>>>>>
>>>>>>>>
>> https://docs.google.com/document/d/1PPo6goW7tOwxmpFuvLSjFnx7BF8IVz0w3dcmPPyqvoY/edit?usp=sharing
>>>>>>>> Am 02.07.18 um 17:08 schrieb Fabian Hueske:
>>>>>>>>> Hi Piotr,
>>>>>>>>>
>>>>>>>>> thanks for bumping this thread and thanks for Xingcan for the
>>>>>> comments.
>>>>>>>>> I think the first step would be to separate the flink-table module
>>>>>> into
>>>>>>>>> multiple sub modules. These could be:
>>>>>>>>>
>>>>>>>>> - flink-table-api: All API facing classes. Can be later divided
>>>>>> further
>>>>>>>>> into Java/Scala Table API/SQL
>>>>>>>>> - flink-table-planning: involves all planning (basically
>> everything
>>>>>> we
>>>>>>> do
>>>>>>>>> with Calcite)
>>>>>>>>> - flink-table-runtime: the runtime code
>>>>>>>>>
>>>>>>>>> IMO, a realistic mid-term goal is to have the runtime module and
>>>>>>> certain
>>>>>>>>> parts of the planning module ported to Java.
>>>>>>>>> The api module will be much harder to port because of several
>>>>>>>> dependencies
>>>>>>>>> to Scala core classes (the parser framework, tree iterations,
>> etc.).
>>>>>>> I'm
>>>>>>>>> not saying we should not port this to Java, but it is not clear to
>>> me
>>>>>>>> (yet)
>>>>>>>>> how to do it.
>>>>>>>>>
>>>>>>>>> I think flink-table-runtime should not be too hard to port. The
>> code
>>>>>>> does
>>>>>>>>> not make use of many Scala features, i.e., it's writing very
>>>>>> Java-like.
>>>>>>>>> Also, there are not many dependencies and operators can be
>>>>>> individually
>>>>>>>>> ported step-by-step.
>>>>>>>>> For flink-table-planning, we can have certain packages that we
>> port
>>>>>> to
>>>>>>>> Java
>>>>>>>>> like planning rules or plan nodes. The related classes mostly
>> extend
>>>>>>>>> Calcite's Java interfaces/classes and would be natural choices for
>>>>>>> being
>>>>>>>>> ported. The code generation classes will require more effort to
>>> port.
>>>>>>>> There
>>>>>>>>> are also some dependencies in planning on the api module that we
>>>>>> would
>>>>>>>> need
>>>>>>>>> to resolve somehow.
>>>>>>>>>
>>>>>>>>> For SQL most work when adding new features is done in the planning
>>>>>> and
>>>>>>>>> runtime modules. So, this separation should already reduce
>>>>>>> "technological
>>>>>>>>> dept" quite a lot.
>>>>>>>>> The Table API depends much more on Scala than SQL.
>>>>>>>>>
>>>>>>>>> Cheers, Fabian
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> 2018-07-02 16:26 GMT+02:00 Xingcan Cui <[hidden email]>:
>>>>>>>>>
>>>>>>>>>> Hi all,
>>>>>>>>>>
>>>>>>>>>> I also think about this problem these days and here are my
>>> thoughts.
>>>>>>>>>> 1) We must admit that it’s really a tough task to interoperate
>> with
>>>>>>> Java
>>>>>>>>>> and Scala. E.g., they have different collection types (Scala
>>>>>>> collections
>>>>>>>>>> v.s. java.util.*) and in Java, it's hard to implement a method
>>> which
>>>>>>>> takes
>>>>>>>>>> Scala functions as parameters. Considering the major part of the
>>>>>> code
>>>>>>>> base
>>>>>>>>>> is implemented in Java, +1 for this goal from a long-term view.
>>>>>>>>>>
>>>>>>>>>> 2) The ideal solution would be to just expose a Scala API and
>> make
>>>>>> all
>>>>>>>> the
>>>>>>>>>> other parts Scala-free. But I am not sure if it could be achieved
>>>>>> even
>>>>>>>> in a
>>>>>>>>>> long-term. Thus as Timo suggested, keep the Scala codes in
>>>>>>>>>> "flink-table-core" would be a compromise solution.
>>>>>>>>>>
>>>>>>>>>> 3) If the community makes the final decision, maybe any new
>>> features
>>>>>>>>>> should be added in Java (regardless of the modules), in order to
>>>>>>> prevent
>>>>>>>>>> the Scala codes from growing.
>>>>>>>>>>
>>>>>>>>>> Best,
>>>>>>>>>> Xingcan
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>> On Jul 2, 2018, at 9:30 PM, Piotr Nowojski <
>>>>>> [hidden email]>
>>>>>>>>>> wrote:
>>>>>>>>>>> Bumping the topic.
>>>>>>>>>>>
>>>>>>>>>>> If we want to do this, the sooner we decide, the less code we
>> will
>>>>>>> have
>>>>>>>>>> to rewrite. I have some objections/counter proposals to Fabian's
>>>>>>>> proposal
>>>>>>>>>> of doing it module wise and one module at a time.
>>>>>>>>>>> First, I do not see a problem of having java/scala code even
>>> within
>>>>>>> one
>>>>>>>>>> module, especially not if there are clean boundaries. Like we
>> could
>>>>>>> have
>>>>>>>>>> API in Scala and optimizer rules/logical nodes written in Java in
>>>>>> the
>>>>>>>> same
>>>>>>>>>> module. However I haven’t previously maintained mixed scala/java
>>>>>> code
>>>>>>>> bases
>>>>>>>>>> before, so I might be missing something here.
>>>>>>>>>>> Secondly this whole migration might and most like will take
>> longer
>>>>>>> then
>>>>>>>>>> expected, so that creates a problem for a new code that we will
>> be
>>>>>>>>>> creating. After making a decision to migrate to Java, almost any
>>> new
>>>>>>>> Scala
>>>>>>>>>> line of code will be immediately a technological debt and we will
>>>>>> have
>>>>>>>> to
>>>>>>>>>> rewrite it to Java later.
>>>>>>>>>>> Thus I would propose first to state our end goal - modules
>>>>>> structure
>>>>>>>> and
>>>>>>>>>> which parts of modules we want to have eventually Scala-free.
>>>>>> Secondly
>>>>>>>>>> taking all steps necessary that will allow us to write new code
>>>>>>>> complaint
>>>>>>>>>> with our end goal. Only after that we should/could focus on
>>>>>>>> incrementally
>>>>>>>>>> rewriting the old code. Otherwise we could be stuck/blocked for
>>>>>> years
>>>>>>>>>> writing new code in Scala (and increasing technological debt),
>>>>>> because
>>>>>>>>>> nobody have found a time to rewrite some non important and not
>>>>>>> actively
>>>>>>>>>> developed part of some module.
>>>>>>>>>>> Piotrek
>>>>>>>>>>>
>>>>>>>>>>>> On 14 Jun 2018, at 15:34, Fabian Hueske <[hidden email]>
>>>>>> wrote:
>>>>>>>>>>>> Hi,
>>>>>>>>>>>>
>>>>>>>>>>>> In general, I think this is a good effort. However, it won't be
>>>>>> easy
>>>>>>>>>> and I
>>>>>>>>>>>> think we have to plan this well.
>>>>>>>>>>>> I don't like the idea of having the whole code base fragmented
>>>>>> into
>>>>>>>> Java
>>>>>>>>>>>> and Scala code for too long.
>>>>>>>>>>>>
>>>>>>>>>>>> I think we should do this one step at a time and focus on
>>>>>> migrating
>>>>>>>> one
>>>>>>>>>>>> module at a time.
>>>>>>>>>>>> IMO, the easiest start would be to port the runtime to Java.
>>>>>>>>>>>> Extracting the API classes into an own module, porting them to
>>>>>> Java,
>>>>>>>> and
>>>>>>>>>>>> removing the Scala dependency won't be possible without
>> breaking
>>>>>> the
>>>>>>>> API
>>>>>>>>>>>> since a few classes depend on the Scala Table API.
>>>>>>>>>>>>
>>>>>>>>>>>> Best, Fabian
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> 2018-06-14 10:33 GMT+02:00 Till Rohrmann <[hidden email]
>>> :
>>>>>>>>>>>>> I think that is a noble and honorable goal and we should
>> strive
>>>>>> for
>>>>>>>> it.
>>>>>>>>>>>>> This, however, must be an iterative process given the sheer
>> size
>>>>>> of
>>>>>>>> the
>>>>>>>>>>>>> code base. I like the approach to define common Java modules
>>>>>> which
>>>>>>>> are
>>>>>>>>>> used
>>>>>>>>>>>>> by more specific Scala modules and slowly moving classes from
>>>>>> Scala
>>>>>>>> to
>>>>>>>>>>>>> Java. Thus +1 for the proposal.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Cheers,
>>>>>>>>>>>>> Till
>>>>>>>>>>>>>
>>>>>>>>>>>>> On Wed, Jun 13, 2018 at 12:01 PM Piotr Nowojski <
>>>>>>>>>> [hidden email]>
>>>>>>>>>>>>> wrote:
>>>>>>>>>>>>>
>>>>>>>>>>>>>> Hi,
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> I do not have an experience with how scala and java interacts
>>>>>> with
>>>>>>>>>> each
>>>>>>>>>>>>>> other, so I can not fully validate your proposal, but
>> generally
>>>>>>>>>> speaking
>>>>>>>>>>>>> +1
>>>>>>>>>>>>>> from me.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Does it also mean, that we should slowly migrate
>>>>>>> `flink-table-core`
>>>>>>>> to
>>>>>>>>>>>>>> Java? How would you envision it? It would be nice to be able
>> to
>>>>>>> add
>>>>>>>>>> new
>>>>>>>>>>>>>> classes/features written in Java and so that they can coexist
>>>>>> with
>>>>>>>> old
>>>>>>>>>>>>>> Scala code until we gradually switch from Scala to Java.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Piotrek
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> On 13 Jun 2018, at 11:32, Timo Walther <[hidden email]>
>>>>>>> wrote:
>>>>>>>>>>>>>>> Hi everyone,
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> as you all know, currently the Table & SQL API is
>> implemented
>>>>>> in
>>>>>>>>>> Scala.
>>>>>>>>>>>>>> This decision was made a long-time ago when the initital code
>>>>>> base
>>>>>>>> was
>>>>>>>>>>>>>> created as part of a master's thesis. The community kept
>> Scala
>>>>>>>>>> because of
>>>>>>>>>>>>>> the nice language features that enable a fluent Table API
>> like
>>>>>>>>>>>>>> table.select('field.trim()) and because Scala allows for
>> quick
>>>>>>>>>>>>> prototyping
>>>>>>>>>>>>>> (e.g. multi-line comments for code generation). The
>> committers
>>>>>>>>>> enforced
>>>>>>>>>>>>> not
>>>>>>>>>>>>>> splitting the code-base into two programming languages.
>>>>>>>>>>>>>>> However, nowadays the flink-table module more and more
>> becomes
>>>>>> an
>>>>>>>>>>>>>> important part in the Flink ecosystem. Connectors, formats,
>> and
>>>>>>> SQL
>>>>>>>>>>>>> client
>>>>>>>>>>>>>> are actually implemented in Java but need to interoperate
>> with
>>>>>>>>>>>>> flink-table
>>>>>>>>>>>>>> which makes these modules dependent on Scala. As mentioned in
>>> an
>>>>>>>>>> earlier
>>>>>>>>>>>>>> mail thread, using Scala for API classes also exposes member
>>>>>>>> variables
>>>>>>>>>>>>> and
>>>>>>>>>>>>>> methods in Java that should not be exposed to users [1]. Java
>>> is
>>>>>>>> still
>>>>>>>>>>>>> the
>>>>>>>>>>>>>> most important API language and right now we treat it as a
>>>>>>>>>> second-class
>>>>>>>>>>>>>> citizen. I just noticed that you even need to add Scala if
>> you
>>>>>>> just
>>>>>>>>>> want
>>>>>>>>>>>>> to
>>>>>>>>>>>>>> implement a ScalarFunction because of method clashes between
>>>>>>> `public
>>>>>>>>>>>>> String
>>>>>>>>>>>>>> toString()` and `public scala.Predef.String toString()`.
>>>>>>>>>>>>>>> Given the size of the current code base, reimplementing the
>>>>>>> entire
>>>>>>>>>>>>>> flink-table code in Java is a goal that we might never reach.
>>>>>>>>>> However, we
>>>>>>>>>>>>>> should at least treat the symptoms and have this as a
>> long-term
>>>>>>> goal
>>>>>>>>>> in
>>>>>>>>>>>>>> mind. My suggestion would be to convert user-facing and
>> runtime
>>>>>>>>>> classes
>>>>>>>>>>>>> and
>>>>>>>>>>>>>> split the code base into multiple modules:
>>>>>>>>>>>>>>>> flink-table-java {depends on flink-table-core}
>>>>>>>>>>>>>>> Implemented in Java. Java users can use this. This would
>>>>>> require
>>>>>>> to
>>>>>>>>>>>>>> convert classes like TableEnvironment, Table.
>>>>>>>>>>>>>>>> flink-table-scala {depends on flink-table-core}
>>>>>>>>>>>>>>> Implemented in Scala. Scala users can use this.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> flink-table-common
>>>>>>>>>>>>>>> Implemented in Java. Connectors, formats, and UDFs can use
>>>>>> this.
>>>>>>> It
>>>>>>>>>>>>>> contains interface classes such as descriptors, table sink,
>>>>>> table
>>>>>>>>>> source.
>>>>>>>>>>>>>>>> flink-table-core {depends on flink-table-common and
>>>>>>>>>>>>>> flink-table-runtime}
>>>>>>>>>>>>>>> Implemented in Scala. Contains the current main code base.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> flink-table-runtime
>>>>>>>>>>>>>>> Implemented in Java. This would require to convert classes
>> in
>>>>>>>>>>>>>> o.a.f.table.runtime but would improve the runtime
>> potentially.
>>>>>>>>>>>>>>> What do you think?
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> Regards,
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> Timo
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> [1]
>>>>>>>>>>>>>> http://apache-flink-mailing-list-archive.1008284.n3.
>>>>>>>>>>>>> nabble.com/DISCUSS-Convert-main-Table-API-classes-into-
>>>>>>>>>> traits-tp21335.html
>>>

[Kurt Young]

Hi Timo,

Thanks for writing up the document. I'm +1 for reorganizing the module
structure and make table scala free. But I have
a little concern abount the timing. Is it more appropriate to get this done
when Flink decide to bump to next big version, like 2.x.
It's true you can keep all the class's package path as it is, and will not
introduce API change. But if some company are developing their own
Flink, and sync with community version by rebasing, may face a lot of
conflicts. Although you can avoid conflicts by always moving source codes
between packages, but I assume you still need to delete the original scala
file and add a new java file when you want to change program language.

Best,
Kurt


On Tue, Nov 27, 2018 at 5:57 PM Timo Walther <[hidden email]> wrote:

> Hi Hequn,
>
> thanks for your feedback. Yes, migrating the test cases is another issue
> that is not represented in the document but should naturally go along
> with the migration.
>
> I agree that we should migrate the main API classes quickly within this
> 1.8 release after the module split has been performed. Help here is
> highly appreciated!
>
> I forgot that Java supports static methods in interfaces now, but
> actually I don't like the design of calling `TableEnvironment.get(env)`.
> Because people often use `TableEnvironment tEnd =
> TableEnvironment.get(env)` and then wonder why there is no
> `toAppendStream` or `toDataSet` because they are using the base class.
> However, things like that can be discussed in the corresponding issue
> when it comes to implementation.
>
> @Vino: I think your work fits nicely to these efforts.
>
> @everyone: I will wait for more feedback until end of this week. Then I
> will convert the design document into a FLIP and open subtasks in Jira,
> if there are no objections?
>
> Regards,
> Timo
>
> Am 24.11.18 um 13:45 schrieb vino yang:
> > Hi hequn,
> >
> > I am very glad to hear that you are interested in this work.
> > As we all know, this process involves a lot.
> > Currently, the migration work has begun. I started with the
> > Kafka connector's dependency on flink-table and moved the
> > related dependencies to flink-table-common.
> > This work is tracked by FLINK-9461.  [1]
> > I don't know if it will conflict with what you expect to do, but from the
> > impact I have observed,
> > it will involve many classes that are currently in flink-table.
> >
> > *Just a statement to prevent unnecessary conflicts.*
> >
> > Thanks, vino.
> >
> > [1]: https://issues.apache.org/jira/browse/FLINK-9461
> >
> > Hequn Cheng <[hidden email]> 于2018年11月24日周六 下午7:20写道：
> >
> >> Hi Timo,
> >>
> >> Thanks for the effort and writing up this document. I like the idea to
> make
> >> flink-table scala free, so +1 for the proposal!
> >>
> >> It's good to make Java the first-class citizen. For a long time, we have
> >> neglected java so that many features in Table are missed in Java Test
> >> cases, such as this one[1] I found recently. And I think we may also
> need
> >> to migrate our test cases, i.e, add java tests.
> >>
> >> This definitely is a big change and will break API compatible. In order
> to
> >> bring a smaller impact on users, I think we should go fast when we
> migrate
> >> APIs targeted to users. It's better to introduce the user sensitive
> changes
> >> within a release. However, it may be not that easy. I can help to
> >> contribute.
> >>
> >> Separation of interface and implementation is a good idea. This may
> >> introduce a minimum of dependencies or even no dependencies. I saw your
> >> reply in the google doc. Java8 has already supported static method for
> >> interfaces, I think we can make use of it?
> >>
> >> Best,
> >> Hequn
> >>
> >> [1] https://issues.apache.org/jira/browse/FLINK-11001
> >>
> >>
> >> On Fri, Nov 23, 2018 at 5:36 PM Timo Walther <[hidden email]>
> wrote:
> >>
> >>> Hi everyone,
> >>>
> >>> thanks for the great feedback so far. I updated the document with the
> >>> input I got so far
> >>>
> >>> @Fabian: I moved the porting of flink-table-runtime classes up in the
> >> list.
> >>> @Xiaowei: Could you elaborate what "interface only" means to you? Do
> you
> >>> mean a module containing pure Java `interface`s? Or is the validation
> >>> logic also part of the API module? Are 50+ expression classes part of
> >>> the API interface or already too implementation-specific?
> >>>
> >>> @Xuefu: I extended the document by almost a page to clarify when we
> >>> should develop in Scala and when in Java. As Piotr said, every new
> Scala
> >>> line is instant technical debt.
> >>>
> >>> Thanks,
> >>> Timo
> >>>
> >>>
> >>> Am 23.11.18 um 10:29 schrieb Piotr Nowojski:
> >>>> Hi Timo,
> >>>>
> >>>> Thanks for writing this down +1 from my side :)
> >>>>
> >>>>> I'm wondering that whether we can have rule in the interim when Java
> >>> and Scala coexist that dependency can only be one-way. I found that in
> >> the
> >>> current code base there are cases where a Scala class extends Java and
> >> vise
> >>> versa. This is quite painful. I'm thinking if we could say that
> extension
> >>> can only be from Java to Scala, which will help the situation. However,
> >> I'm
> >>> not sure if this is practical.
> >>>> Xuefu: I’m also not sure what’s the best approach here, probably we
> >> will
> >>> have to work it out as we go. One thing to consider is that from now
> on,
> >>> every single new code line written in Scala anywhere in Flink-table
> >> (except
> >>> of Flink-table-api-scala) is an instant technological debt. From this
> >>> perspective I would be in favour of tolerating quite big inchonvieneces
> >>> just to avoid any new Scala code.
> >>>> Piotrek
> >>>>
> >>>>> On 23 Nov 2018, at 03:25, Zhang, Xuefu <[hidden email]>
> >> wrote:
> >>>>> Hi Timo,
> >>>>>
> >>>>> Thanks for the effort and the Google writeup. During our external
> >>> catalog rework, we found much confusion between Java and Scala, and
> this
> >>> Scala-free roadmap should greatly mitigate that.
> >>>>> I'm wondering that whether we can have rule in the interim when Java
> >>> and Scala coexist that dependency can only be one-way. I found that in
> >> the
> >>> current code base there are cases where a Scala class extends Java and
> >> vise
> >>> versa. This is quite painful. I'm thinking if we could say that
> extension
> >>> can only be from Java to Scala, which will help the situation. However,
> >> I'm
> >>> not sure if this is practical.
> >>>>> Thanks,
> >>>>> Xuefu
> >>>>>
> >>>>>
> >>>>> ------------------------------------------------------------------
> >>>>> Sender:jincheng sun <[hidden email]>
> >>>>> Sent at:2018 Nov 23 (Fri) 09:49
> >>>>> Recipient:dev <[hidden email]>
> >>>>> Subject:Re: [DISCUSS] Long-term goal of making flink-table Scala-free
> >>>>>
> >>>>> Hi Timo,
> >>>>> Thanks for initiating this great discussion.
> >>>>>
> >>>>> Currently when using SQL/TableAPI should include many dependence. In
> >>>>> particular, it is not necessary to introduce the specific
> >> implementation
> >>>>> dependencies which users do not care about. So I am glad to see your
> >>>>> proposal, and hope when we consider splitting the API interface into
> a
> >>>>> separate module, so that the user can introduce minimum of
> >> dependencies.
> >>>>> So, +1 to [separation of interface and implementation; e.g. `Table` &
> >>>>> `TableImpl`] which you mentioned in the google doc.
> >>>>> Best,
> >>>>> Jincheng
> >>>>>
> >>>>> Xiaowei Jiang <[hidden email]> 于2018年11月22日周四 下午10:50写道：
> >>>>>
> >>>>>> Hi Timo, thanks for driving this! I think that this is a nice thing
> >> to
> >>> do.
> >>>>>> While we are doing this, can we also keep in mind that we want to
> >>>>>> eventually have a TableAPI interface only module which users can
> take
> >>>>>> dependency on, but without including any implementation details?
> >>>>>>
> >>>>>> Xiaowei
> >>>>>>
> >>>>>> On Thu, Nov 22, 2018 at 6:37 PM Fabian Hueske <[hidden email]>
> >>> wrote:
> >>>>>>> Hi Timo,
> >>>>>>>
> >>>>>>> Thanks for writing up this document.
> >>>>>>> I like the new structure and agree to prioritize the porting of the
> >>>>>>> flink-table-common classes.
> >>>>>>> Since flink-table-runtime is (or should be) independent of the API
> >> and
> >>>>>>> planner modules, we could start porting these classes once the code
> >> is
> >>>>>>> split into the new module structure.
> >>>>>>> The benefits of a Scala-free flink-table-runtime would be a
> >> Scala-free
> >>>>>>> execution Jar.
> >>>>>>>
> >>>>>>> Best, Fabian
> >>>>>>>
> >>>>>>>
> >>>>>>> Am Do., 22. Nov. 2018 um 10:54 Uhr schrieb Timo Walther <
> >>>>>>> [hidden email]
> >>>>>>>> :
> >>>>>>>> Hi everyone,
> >>>>>>>>
> >>>>>>>> I would like to continue this discussion thread and convert the
> >>> outcome
> >>>>>>>> into a FLIP such that users and contributors know what to expect
> in
> >>> the
> >>>>>>>> upcoming releases.
> >>>>>>>>
> >>>>>>>> I created a design document [1] that clarifies our motivation why
> >> we
> >>>>>>>> want to do this, how a Maven module structure could look like, and
> >> a
> >>>>>>>> suggestion for a migration plan.
> >>>>>>>>
> >>>>>>>> It would be great to start with the efforts for the 1.8 release
> >> such
> >>>>>>>> that new features can be developed in Java and major refactorings
> >>> such
> >>>>>>>> as improvements to the connectors and external catalog support are
> >>> not
> >>>>>>>> blocked.
> >>>>>>>>
> >>>>>>>> Please let me know what you think.
> >>>>>>>>
> >>>>>>>> Regards,
> >>>>>>>> Timo
> >>>>>>>>
> >>>>>>>> [1]
> >>>>>>>>
> >>>>>>>>
> >>
> https://docs.google.com/document/d/1PPo6goW7tOwxmpFuvLSjFnx7BF8IVz0w3dcmPPyqvoY/edit?usp=sharing
> >>>>>>>> Am 02.07.18 um 17:08 schrieb Fabian Hueske:
> >>>>>>>>> Hi Piotr,
> >>>>>>>>>
> >>>>>>>>> thanks for bumping this thread and thanks for Xingcan for the
> >>>>>> comments.
> >>>>>>>>> I think the first step would be to separate the flink-table
> module
> >>>>>> into
> >>>>>>>>> multiple sub modules. These could be:
> >>>>>>>>>
> >>>>>>>>> - flink-table-api: All API facing classes. Can be later divided
> >>>>>> further
> >>>>>>>>> into Java/Scala Table API/SQL
> >>>>>>>>> - flink-table-planning: involves all planning (basically
> >> everything
> >>>>>> we
> >>>>>>> do
> >>>>>>>>> with Calcite)
> >>>>>>>>> - flink-table-runtime: the runtime code
> >>>>>>>>>
> >>>>>>>>> IMO, a realistic mid-term goal is to have the runtime module and
> >>>>>>> certain
> >>>>>>>>> parts of the planning module ported to Java.
> >>>>>>>>> The api module will be much harder to port because of several
> >>>>>>>> dependencies
> >>>>>>>>> to Scala core classes (the parser framework, tree iterations,
> >> etc.).
> >>>>>>> I'm
> >>>>>>>>> not saying we should not port this to Java, but it is not clear
> to
> >>> me
> >>>>>>>> (yet)
> >>>>>>>>> how to do it.
> >>>>>>>>>
> >>>>>>>>> I think flink-table-runtime should not be too hard to port. The
> >> code
> >>>>>>> does
> >>>>>>>>> not make use of many Scala features, i.e., it's writing very
> >>>>>> Java-like.
> >>>>>>>>> Also, there are not many dependencies and operators can be
> >>>>>> individually
> >>>>>>>>> ported step-by-step.
> >>>>>>>>> For flink-table-planning, we can have certain packages that we
> >> port
> >>>>>> to
> >>>>>>>> Java
> >>>>>>>>> like planning rules or plan nodes. The related classes mostly
> >> extend
> >>>>>>>>> Calcite's Java interfaces/classes and would be natural choices
> for
> >>>>>>> being
> >>>>>>>>> ported. The code generation classes will require more effort to
> >>> port.
> >>>>>>>> There
> >>>>>>>>> are also some dependencies in planning on the api module that we
> >>>>>> would
> >>>>>>>> need
> >>>>>>>>> to resolve somehow.
> >>>>>>>>>
> >>>>>>>>> For SQL most work when adding new features is done in the
> planning
> >>>>>> and
> >>>>>>>>> runtime modules. So, this separation should already reduce
> >>>>>>> "technological
> >>>>>>>>> dept" quite a lot.
> >>>>>>>>> The Table API depends much more on Scala than SQL.
> >>>>>>>>>
> >>>>>>>>> Cheers, Fabian
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> 2018-07-02 16:26 GMT+02:00 Xingcan Cui <[hidden email]>:
> >>>>>>>>>
> >>>>>>>>>> Hi all,
> >>>>>>>>>>
> >>>>>>>>>> I also think about this problem these days and here are my
> >>> thoughts.
> >>>>>>>>>> 1) We must admit that it’s really a tough task to interoperate
> >> with
> >>>>>>> Java
> >>>>>>>>>> and Scala. E.g., they have different collection types (Scala
> >>>>>>> collections
> >>>>>>>>>> v.s. java.util.*) and in Java, it's hard to implement a method
> >>> which
> >>>>>>>> takes
> >>>>>>>>>> Scala functions as parameters. Considering the major part of the
> >>>>>> code
> >>>>>>>> base
> >>>>>>>>>> is implemented in Java, +1 for this goal from a long-term view.
> >>>>>>>>>>
> >>>>>>>>>> 2) The ideal solution would be to just expose a Scala API and
> >> make
> >>>>>> all
> >>>>>>>> the
> >>>>>>>>>> other parts Scala-free. But I am not sure if it could be
> achieved
> >>>>>> even
> >>>>>>>> in a
> >>>>>>>>>> long-term. Thus as Timo suggested, keep the Scala codes in
> >>>>>>>>>> "flink-table-core" would be a compromise solution.
> >>>>>>>>>>
> >>>>>>>>>> 3) If the community makes the final decision, maybe any new
> >>> features
> >>>>>>>>>> should be added in Java (regardless of the modules), in order to
> >>>>>>> prevent
> >>>>>>>>>> the Scala codes from growing.
> >>>>>>>>>>
> >>>>>>>>>> Best,
> >>>>>>>>>> Xingcan
> >>>>>>>>>>
> >>>>>>>>>>
> >>>>>>>>>>> On Jul 2, 2018, at 9:30 PM, Piotr Nowojski <
> >>>>>> [hidden email]>
> >>>>>>>>>> wrote:
> >>>>>>>>>>> Bumping the topic.
> >>>>>>>>>>>
> >>>>>>>>>>> If we want to do this, the sooner we decide, the less code we
> >> will
> >>>>>>> have
> >>>>>>>>>> to rewrite. I have some objections/counter proposals to Fabian's
> >>>>>>>> proposal
> >>>>>>>>>> of doing it module wise and one module at a time.
> >>>>>>>>>>> First, I do not see a problem of having java/scala code even
> >>> within
> >>>>>>> one
> >>>>>>>>>> module, especially not if there are clean boundaries. Like we
> >> could
> >>>>>>> have
> >>>>>>>>>> API in Scala and optimizer rules/logical nodes written in Java
> in
> >>>>>> the
> >>>>>>>> same
> >>>>>>>>>> module. However I haven’t previously maintained mixed scala/java
> >>>>>> code
> >>>>>>>> bases
> >>>>>>>>>> before, so I might be missing something here.
> >>>>>>>>>>> Secondly this whole migration might and most like will take
> >> longer
> >>>>>>> then
> >>>>>>>>>> expected, so that creates a problem for a new code that we will
> >> be
> >>>>>>>>>> creating. After making a decision to migrate to Java, almost any
> >>> new
> >>>>>>>> Scala
> >>>>>>>>>> line of code will be immediately a technological debt and we
> will
> >>>>>> have
> >>>>>>>> to
> >>>>>>>>>> rewrite it to Java later.
> >>>>>>>>>>> Thus I would propose first to state our end goal - modules
> >>>>>> structure
> >>>>>>>> and
> >>>>>>>>>> which parts of modules we want to have eventually Scala-free.
> >>>>>> Secondly
> >>>>>>>>>> taking all steps necessary that will allow us to write new code
> >>>>>>>> complaint
> >>>>>>>>>> with our end goal. Only after that we should/could focus on
> >>>>>>>> incrementally
> >>>>>>>>>> rewriting the old code. Otherwise we could be stuck/blocked for
> >>>>>> years
> >>>>>>>>>> writing new code in Scala (and increasing technological debt),
> >>>>>> because
> >>>>>>>>>> nobody have found a time to rewrite some non important and not
> >>>>>>> actively
> >>>>>>>>>> developed part of some module.
> >>>>>>>>>>> Piotrek
> >>>>>>>>>>>
> >>>>>>>>>>>> On 14 Jun 2018, at 15:34, Fabian Hueske <[hidden email]>
> >>>>>> wrote:
> >>>>>>>>>>>> Hi,
> >>>>>>>>>>>>
> >>>>>>>>>>>> In general, I think this is a good effort. However, it won't
> be
> >>>>>> easy
> >>>>>>>>>> and I
> >>>>>>>>>>>> think we have to plan this well.
> >>>>>>>>>>>> I don't like the idea of having the whole code base fragmented
> >>>>>> into
> >>>>>>>> Java
> >>>>>>>>>>>> and Scala code for too long.
> >>>>>>>>>>>>
> >>>>>>>>>>>> I think we should do this one step at a time and focus on
> >>>>>> migrating
> >>>>>>>> one
> >>>>>>>>>>>> module at a time.
> >>>>>>>>>>>> IMO, the easiest start would be to port the runtime to Java.
> >>>>>>>>>>>> Extracting the API classes into an own module, porting them to
> >>>>>> Java,
> >>>>>>>> and
> >>>>>>>>>>>> removing the Scala dependency won't be possible without
> >> breaking
> >>>>>> the
> >>>>>>>> API
> >>>>>>>>>>>> since a few classes depend on the Scala Table API.
> >>>>>>>>>>>>
> >>>>>>>>>>>> Best, Fabian
> >>>>>>>>>>>>
> >>>>>>>>>>>>
> >>>>>>>>>>>> 2018-06-14 10:33 GMT+02:00 Till Rohrmann <
> [hidden email]
> >>> :
> >>>>>>>>>>>>> I think that is a noble and honorable goal and we should
> >> strive
> >>>>>> for
> >>>>>>>> it.
> >>>>>>>>>>>>> This, however, must be an iterative process given the sheer
> >> size
> >>>>>> of
> >>>>>>>> the
> >>>>>>>>>>>>> code base. I like the approach to define common Java modules
> >>>>>> which
> >>>>>>>> are
> >>>>>>>>>> used
> >>>>>>>>>>>>> by more specific Scala modules and slowly moving classes from
> >>>>>> Scala
> >>>>>>>> to
> >>>>>>>>>>>>> Java. Thus +1 for the proposal.
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> Cheers,
> >>>>>>>>>>>>> Till
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> On Wed, Jun 13, 2018 at 12:01 PM Piotr Nowojski <
> >>>>>>>>>> [hidden email]>
> >>>>>>>>>>>>> wrote:
> >>>>>>>>>>>>>
> >>>>>>>>>>>>>> Hi,
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>> I do not have an experience with how scala and java
> interacts
> >>>>>> with
> >>>>>>>>>> each
> >>>>>>>>>>>>>> other, so I can not fully validate your proposal, but
> >> generally
> >>>>>>>>>> speaking
> >>>>>>>>>>>>> +1
> >>>>>>>>>>>>>> from me.
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>> Does it also mean, that we should slowly migrate
> >>>>>>> `flink-table-core`
> >>>>>>>> to
> >>>>>>>>>>>>>> Java? How would you envision it? It would be nice to be able
> >> to
> >>>>>>> add
> >>>>>>>>>> new
> >>>>>>>>>>>>>> classes/features written in Java and so that they can
> coexist
> >>>>>> with
> >>>>>>>> old
> >>>>>>>>>>>>>> Scala code until we gradually switch from Scala to Java.
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>> Piotrek
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>>> On 13 Jun 2018, at 11:32, Timo Walther <[hidden email]
> >
> >>>>>>> wrote:
> >>>>>>>>>>>>>>> Hi everyone,
> >>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>> as you all know, currently the Table & SQL API is
> >> implemented
> >>>>>> in
> >>>>>>>>>> Scala.
> >>>>>>>>>>>>>> This decision was made a long-time ago when the initital
> code
> >>>>>> base
> >>>>>>>> was
> >>>>>>>>>>>>>> created as part of a master's thesis. The community kept
> >> Scala
> >>>>>>>>>> because of
> >>>>>>>>>>>>>> the nice language features that enable a fluent Table API
> >> like
> >>>>>>>>>>>>>> table.select('field.trim()) and because Scala allows for
> >> quick
> >>>>>>>>>>>>> prototyping
> >>>>>>>>>>>>>> (e.g. multi-line comments for code generation). The
> >> committers
> >>>>>>>>>> enforced
> >>>>>>>>>>>>> not
> >>>>>>>>>>>>>> splitting the code-base into two programming languages.
> >>>>>>>>>>>>>>> However, nowadays the flink-table module more and more
> >> becomes
> >>>>>> an
> >>>>>>>>>>>>>> important part in the Flink ecosystem. Connectors, formats,
> >> and
> >>>>>>> SQL
> >>>>>>>>>>>>> client
> >>>>>>>>>>>>>> are actually implemented in Java but need to interoperate
> >> with
> >>>>>>>>>>>>> flink-table
> >>>>>>>>>>>>>> which makes these modules dependent on Scala. As mentioned
> in
> >>> an
> >>>>>>>>>> earlier
> >>>>>>>>>>>>>> mail thread, using Scala for API classes also exposes member
> >>>>>>>> variables
> >>>>>>>>>>>>> and
> >>>>>>>>>>>>>> methods in Java that should not be exposed to users [1].
> Java
> >>> is
> >>>>>>>> still
> >>>>>>>>>>>>> the
> >>>>>>>>>>>>>> most important API language and right now we treat it as a
> >>>>>>>>>> second-class
> >>>>>>>>>>>>>> citizen. I just noticed that you even need to add Scala if
> >> you
> >>>>>>> just
> >>>>>>>>>> want
> >>>>>>>>>>>>> to
> >>>>>>>>>>>>>> implement a ScalarFunction because of method clashes between
> >>>>>>> `public
> >>>>>>>>>>>>> String
> >>>>>>>>>>>>>> toString()` and `public scala.Predef.String toString()`.
> >>>>>>>>>>>>>>> Given the size of the current code base, reimplementing the
> >>>>>>> entire
> >>>>>>>>>>>>>> flink-table code in Java is a goal that we might never
> reach.
> >>>>>>>>>> However, we
> >>>>>>>>>>>>>> should at least treat the symptoms and have this as a
> >> long-term
> >>>>>>> goal
> >>>>>>>>>> in
> >>>>>>>>>>>>>> mind. My suggestion would be to convert user-facing and
> >> runtime
> >>>>>>>>>> classes
> >>>>>>>>>>>>> and
> >>>>>>>>>>>>>> split the code base into multiple modules:
> >>>>>>>>>>>>>>>> flink-table-java {depends on flink-table-core}
> >>>>>>>>>>>>>>> Implemented in Java. Java users can use this. This would
> >>>>>> require
> >>>>>>> to
> >>>>>>>>>>>>>> convert classes like TableEnvironment, Table.
> >>>>>>>>>>>>>>>> flink-table-scala {depends on flink-table-core}
> >>>>>>>>>>>>>>> Implemented in Scala. Scala users can use this.
> >>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>> flink-table-common
> >>>>>>>>>>>>>>> Implemented in Java. Connectors, formats, and UDFs can use
> >>>>>> this.
> >>>>>>> It
> >>>>>>>>>>>>>> contains interface classes such as descriptors, table sink,
> >>>>>> table
> >>>>>>>>>> source.
> >>>>>>>>>>>>>>>> flink-table-core {depends on flink-table-common and
> >>>>>>>>>>>>>> flink-table-runtime}
> >>>>>>>>>>>>>>> Implemented in Scala. Contains the current main code base.
> >>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>> flink-table-runtime
> >>>>>>>>>>>>>>> Implemented in Java. This would require to convert classes
> >> in
> >>>>>>>>>>>>>> o.a.f.table.runtime but would improve the runtime
> >> potentially.
> >>>>>>>>>>>>>>> What do you think?
> >>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>> Regards,
> >>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>> Timo
> >>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>> [1]
> >>>>>>>>>>>>>> http://apache-flink-mailing-list-archive.1008284.n3.
> >>>>>>>>>>>>> nabble.com/DISCUSS-Convert-main-Table-API-classes-into-
> >>>>>>>>>> traits-tp21335.html
> >>>
>
>

[vino yang]

Hi Kurt,

Currently, there is still a long time to go from flink 2.0. Considering
that the flink-table
is one of the most active modules in the current flink project, each
version has
a number of changes and features added. I think that refactoring faster
will reduce subsequent
complexity and workload. And this may be a gradual and long process. We
should be able to
 regard it as a "technical debt", and if it does not change it, it will
also affect the decision-making of other issues.

Thanks, vino.

Kurt Young <[hidden email]> 于2018年11月27日周二 下午7:34写道：

> Hi Timo,
>
> Thanks for writing up the document. I'm +1 for reorganizing the module
> structure and make table scala free. But I have
> a little concern abount the timing. Is it more appropriate to get this done
> when Flink decide to bump to next big version, like 2.x.
> It's true you can keep all the class's package path as it is, and will not
> introduce API change. But if some company are developing their own
> Flink, and sync with community version by rebasing, may face a lot of
> conflicts. Although you can avoid conflicts by always moving source codes
> between packages, but I assume you still need to delete the original scala
> file and add a new java file when you want to change program language.
>
> Best,
> Kurt
>
>
> On Tue, Nov 27, 2018 at 5:57 PM Timo Walther <[hidden email]> wrote:
>
> > Hi Hequn,
> >
> > thanks for your feedback. Yes, migrating the test cases is another issue
> > that is not represented in the document but should naturally go along
> > with the migration.
> >
> > I agree that we should migrate the main API classes quickly within this
> > 1.8 release after the module split has been performed. Help here is
> > highly appreciated!
> >
> > I forgot that Java supports static methods in interfaces now, but
> > actually I don't like the design of calling `TableEnvironment.get(env)`.
> > Because people often use `TableEnvironment tEnd =
> > TableEnvironment.get(env)` and then wonder why there is no
> > `toAppendStream` or `toDataSet` because they are using the base class.
> > However, things like that can be discussed in the corresponding issue
> > when it comes to implementation.
> >
> > @Vino: I think your work fits nicely to these efforts.
> >
> > @everyone: I will wait for more feedback until end of this week. Then I
> > will convert the design document into a FLIP and open subtasks in Jira,
> > if there are no objections?
> >
> > Regards,
> > Timo
> >
> > Am 24.11.18 um 13:45 schrieb vino yang:
> > > Hi hequn,
> > >
> > > I am very glad to hear that you are interested in this work.
> > > As we all know, this process involves a lot.
> > > Currently, the migration work has begun. I started with the
> > > Kafka connector's dependency on flink-table and moved the
> > > related dependencies to flink-table-common.
> > > This work is tracked by FLINK-9461.  [1]
> > > I don't know if it will conflict with what you expect to do, but from
> the
> > > impact I have observed,
> > > it will involve many classes that are currently in flink-table.
> > >
> > > *Just a statement to prevent unnecessary conflicts.*
> > >
> > > Thanks, vino.
> > >
> > > [1]: https://issues.apache.org/jira/browse/FLINK-9461
> > >
> > > Hequn Cheng <[hidden email]> 于2018年11月24日周六 下午7:20写道：
> > >
> > >> Hi Timo,
> > >>
> > >> Thanks for the effort and writing up this document. I like the idea to
> > make
> > >> flink-table scala free, so +1 for the proposal!
> > >>
> > >> It's good to make Java the first-class citizen. For a long time, we
> have
> > >> neglected java so that many features in Table are missed in Java Test
> > >> cases, such as this one[1] I found recently. And I think we may also
> > need
> > >> to migrate our test cases, i.e, add java tests.
> > >>
> > >> This definitely is a big change and will break API compatible. In
> order
> > to
> > >> bring a smaller impact on users, I think we should go fast when we
> > migrate
> > >> APIs targeted to users. It's better to introduce the user sensitive
> > changes
> > >> within a release. However, it may be not that easy. I can help to
> > >> contribute.
> > >>
> > >> Separation of interface and implementation is a good idea. This may
> > >> introduce a minimum of dependencies or even no dependencies. I saw
> your
> > >> reply in the google doc. Java8 has already supported static method for
> > >> interfaces, I think we can make use of it?
> > >>
> > >> Best,
> > >> Hequn
> > >>
> > >> [1] https://issues.apache.org/jira/browse/FLINK-11001
> > >>
> > >>
> > >> On Fri, Nov 23, 2018 at 5:36 PM Timo Walther <[hidden email]>
> > wrote:
> > >>
> > >>> Hi everyone,
> > >>>
> > >>> thanks for the great feedback so far. I updated the document with the
> > >>> input I got so far
> > >>>
> > >>> @Fabian: I moved the porting of flink-table-runtime classes up in the
> > >> list.
> > >>> @Xiaowei: Could you elaborate what "interface only" means to you? Do
> > you
> > >>> mean a module containing pure Java `interface`s? Or is the validation
> > >>> logic also part of the API module? Are 50+ expression classes part of
> > >>> the API interface or already too implementation-specific?
> > >>>
> > >>> @Xuefu: I extended the document by almost a page to clarify when we
> > >>> should develop in Scala and when in Java. As Piotr said, every new
> > Scala
> > >>> line is instant technical debt.
> > >>>
> > >>> Thanks,
> > >>> Timo
> > >>>
> > >>>
> > >>> Am 23.11.18 um 10:29 schrieb Piotr Nowojski:
> > >>>> Hi Timo,
> > >>>>
> > >>>> Thanks for writing this down +1 from my side :)
> > >>>>
> > >>>>> I'm wondering that whether we can have rule in the interim when
> Java
> > >>> and Scala coexist that dependency can only be one-way. I found that
> in
> > >> the
> > >>> current code base there are cases where a Scala class extends Java
> and
> > >> vise
> > >>> versa. This is quite painful. I'm thinking if we could say that
> > extension
> > >>> can only be from Java to Scala, which will help the situation.
> However,
> > >> I'm
> > >>> not sure if this is practical.
> > >>>> Xuefu: I’m also not sure what’s the best approach here, probably we
> > >> will
> > >>> have to work it out as we go. One thing to consider is that from now
> > on,
> > >>> every single new code line written in Scala anywhere in Flink-table
> > >> (except
> > >>> of Flink-table-api-scala) is an instant technological debt. From this
> > >>> perspective I would be in favour of tolerating quite big
> inchonvieneces
> > >>> just to avoid any new Scala code.
> > >>>> Piotrek
> > >>>>
> > >>>>> On 23 Nov 2018, at 03:25, Zhang, Xuefu <[hidden email]>
> > >> wrote:
> > >>>>> Hi Timo,
> > >>>>>
> > >>>>> Thanks for the effort and the Google writeup. During our external
> > >>> catalog rework, we found much confusion between Java and Scala, and
> > this
> > >>> Scala-free roadmap should greatly mitigate that.
> > >>>>> I'm wondering that whether we can have rule in the interim when
> Java
> > >>> and Scala coexist that dependency can only be one-way. I found that
> in
> > >> the
> > >>> current code base there are cases where a Scala class extends Java
> and
> > >> vise
> > >>> versa. This is quite painful. I'm thinking if we could say that
> > extension
> > >>> can only be from Java to Scala, which will help the situation.
> However,
> > >> I'm
> > >>> not sure if this is practical.
> > >>>>> Thanks,
> > >>>>> Xuefu
> > >>>>>
> > >>>>>
> > >>>>> ------------------------------------------------------------------
> > >>>>> Sender:jincheng sun <[hidden email]>
> > >>>>> Sent at:2018 Nov 23 (Fri) 09:49
> > >>>>> Recipient:dev <[hidden email]>
> > >>>>> Subject:Re: [DISCUSS] Long-term goal of making flink-table
> Scala-free
> > >>>>>
> > >>>>> Hi Timo,
> > >>>>> Thanks for initiating this great discussion.
> > >>>>>
> > >>>>> Currently when using SQL/TableAPI should include many dependence.
> In
> > >>>>> particular, it is not necessary to introduce the specific
> > >> implementation
> > >>>>> dependencies which users do not care about. So I am glad to see
> your
> > >>>>> proposal, and hope when we consider splitting the API interface
> into
> > a
> > >>>>> separate module, so that the user can introduce minimum of
> > >> dependencies.
> > >>>>> So, +1 to [separation of interface and implementation; e.g.
> `Table` &
> > >>>>> `TableImpl`] which you mentioned in the google doc.
> > >>>>> Best,
> > >>>>> Jincheng
> > >>>>>
> > >>>>> Xiaowei Jiang <[hidden email]> 于2018年11月22日周四 下午10:50写道：
> > >>>>>
> > >>>>>> Hi Timo, thanks for driving this! I think that this is a nice
> thing
> > >> to
> > >>> do.
> > >>>>>> While we are doing this, can we also keep in mind that we want to
> > >>>>>> eventually have a TableAPI interface only module which users can
> > take
> > >>>>>> dependency on, but without including any implementation details?
> > >>>>>>
> > >>>>>> Xiaowei
> > >>>>>>
> > >>>>>> On Thu, Nov 22, 2018 at 6:37 PM Fabian Hueske <[hidden email]>
> > >>> wrote:
> > >>>>>>> Hi Timo,
> > >>>>>>>
> > >>>>>>> Thanks for writing up this document.
> > >>>>>>> I like the new structure and agree to prioritize the porting of
> the
> > >>>>>>> flink-table-common classes.
> > >>>>>>> Since flink-table-runtime is (or should be) independent of the
> API
> > >> and
> > >>>>>>> planner modules, we could start porting these classes once the
> code
> > >> is
> > >>>>>>> split into the new module structure.
> > >>>>>>> The benefits of a Scala-free flink-table-runtime would be a
> > >> Scala-free
> > >>>>>>> execution Jar.
> > >>>>>>>
> > >>>>>>> Best, Fabian
> > >>>>>>>
> > >>>>>>>
> > >>>>>>> Am Do., 22. Nov. 2018 um 10:54 Uhr schrieb Timo Walther <
> > >>>>>>> [hidden email]
> > >>>>>>>> :
> > >>>>>>>> Hi everyone,
> > >>>>>>>>
> > >>>>>>>> I would like to continue this discussion thread and convert the
> > >>> outcome
> > >>>>>>>> into a FLIP such that users and contributors know what to expect
> > in
> > >>> the
> > >>>>>>>> upcoming releases.
> > >>>>>>>>
> > >>>>>>>> I created a design document [1] that clarifies our motivation
> why
> > >> we
> > >>>>>>>> want to do this, how a Maven module structure could look like,
> and
> > >> a
> > >>>>>>>> suggestion for a migration plan.
> > >>>>>>>>
> > >>>>>>>> It would be great to start with the efforts for the 1.8 release
> > >> such
> > >>>>>>>> that new features can be developed in Java and major
> refactorings
> > >>> such
> > >>>>>>>> as improvements to the connectors and external catalog support
> are
> > >>> not
> > >>>>>>>> blocked.
> > >>>>>>>>
> > >>>>>>>> Please let me know what you think.
> > >>>>>>>>
> > >>>>>>>> Regards,
> > >>>>>>>> Timo
> > >>>>>>>>
> > >>>>>>>> [1]
> > >>>>>>>>
> > >>>>>>>>
> > >>
> >
> https://docs.google.com/document/d/1PPo6goW7tOwxmpFuvLSjFnx7BF8IVz0w3dcmPPyqvoY/edit?usp=sharing
> > >>>>>>>> Am 02.07.18 um 17:08 schrieb Fabian Hueske:
> > >>>>>>>>> Hi Piotr,
> > >>>>>>>>>
> > >>>>>>>>> thanks for bumping this thread and thanks for Xingcan for the
> > >>>>>> comments.
> > >>>>>>>>> I think the first step would be to separate the flink-table
> > module
> > >>>>>> into
> > >>>>>>>>> multiple sub modules. These could be:
> > >>>>>>>>>
> > >>>>>>>>> - flink-table-api: All API facing classes. Can be later divided
> > >>>>>> further
> > >>>>>>>>> into Java/Scala Table API/SQL
> > >>>>>>>>> - flink-table-planning: involves all planning (basically
> > >> everything
> > >>>>>> we
> > >>>>>>> do
> > >>>>>>>>> with Calcite)
> > >>>>>>>>> - flink-table-runtime: the runtime code
> > >>>>>>>>>
> > >>>>>>>>> IMO, a realistic mid-term goal is to have the runtime module
> and
> > >>>>>>> certain
> > >>>>>>>>> parts of the planning module ported to Java.
> > >>>>>>>>> The api module will be much harder to port because of several
> > >>>>>>>> dependencies
> > >>>>>>>>> to Scala core classes (the parser framework, tree iterations,
> > >> etc.).
> > >>>>>>> I'm
> > >>>>>>>>> not saying we should not port this to Java, but it is not clear
> > to
> > >>> me
> > >>>>>>>> (yet)
> > >>>>>>>>> how to do it.
> > >>>>>>>>>
> > >>>>>>>>> I think flink-table-runtime should not be too hard to port. The
> > >> code
> > >>>>>>> does
> > >>>>>>>>> not make use of many Scala features, i.e., it's writing very
> > >>>>>> Java-like.
> > >>>>>>>>> Also, there are not many dependencies and operators can be
> > >>>>>> individually
> > >>>>>>>>> ported step-by-step.
> > >>>>>>>>> For flink-table-planning, we can have certain packages that we
> > >> port
> > >>>>>> to
> > >>>>>>>> Java
> > >>>>>>>>> like planning rules or plan nodes. The related classes mostly
> > >> extend
> > >>>>>>>>> Calcite's Java interfaces/classes and would be natural choices
> > for
> > >>>>>>> being
> > >>>>>>>>> ported. The code generation classes will require more effort to
> > >>> port.
> > >>>>>>>> There
> > >>>>>>>>> are also some dependencies in planning on the api module that
> we
> > >>>>>> would
> > >>>>>>>> need
> > >>>>>>>>> to resolve somehow.
> > >>>>>>>>>
> > >>>>>>>>> For SQL most work when adding new features is done in the
> > planning
> > >>>>>> and
> > >>>>>>>>> runtime modules. So, this separation should already reduce
> > >>>>>>> "technological
> > >>>>>>>>> dept" quite a lot.
> > >>>>>>>>> The Table API depends much more on Scala than SQL.
> > >>>>>>>>>
> > >>>>>>>>> Cheers, Fabian
> > >>>>>>>>>
> > >>>>>>>>>
> > >>>>>>>>>
> > >>>>>>>>> 2018-07-02 16:26 GMT+02:00 Xingcan Cui <[hidden email]>:
> > >>>>>>>>>
> > >>>>>>>>>> Hi all,
> > >>>>>>>>>>
> > >>>>>>>>>> I also think about this problem these days and here are my
> > >>> thoughts.
> > >>>>>>>>>> 1) We must admit that it’s really a tough task to interoperate
> > >> with
> > >>>>>>> Java
> > >>>>>>>>>> and Scala. E.g., they have different collection types (Scala
> > >>>>>>> collections
> > >>>>>>>>>> v.s. java.util.*) and in Java, it's hard to implement a method
> > >>> which
> > >>>>>>>> takes
> > >>>>>>>>>> Scala functions as parameters. Considering the major part of
> the
> > >>>>>> code
> > >>>>>>>> base
> > >>>>>>>>>> is implemented in Java, +1 for this goal from a long-term
> view.
> > >>>>>>>>>>
> > >>>>>>>>>> 2) The ideal solution would be to just expose a Scala API and
> > >> make
> > >>>>>> all
> > >>>>>>>> the
> > >>>>>>>>>> other parts Scala-free. But I am not sure if it could be
> > achieved
> > >>>>>> even
> > >>>>>>>> in a
> > >>>>>>>>>> long-term. Thus as Timo suggested, keep the Scala codes in
> > >>>>>>>>>> "flink-table-core" would be a compromise solution.
> > >>>>>>>>>>
> > >>>>>>>>>> 3) If the community makes the final decision, maybe any new
> > >>> features
> > >>>>>>>>>> should be added in Java (regardless of the modules), in order
> to
> > >>>>>>> prevent
> > >>>>>>>>>> the Scala codes from growing.
> > >>>>>>>>>>
> > >>>>>>>>>> Best,
> > >>>>>>>>>> Xingcan
> > >>>>>>>>>>
> > >>>>>>>>>>
> > >>>>>>>>>>> On Jul 2, 2018, at 9:30 PM, Piotr Nowojski <
> > >>>>>> [hidden email]>
> > >>>>>>>>>> wrote:
> > >>>>>>>>>>> Bumping the topic.
> > >>>>>>>>>>>
> > >>>>>>>>>>> If we want to do this, the sooner we decide, the less code we
> > >> will
> > >>>>>>> have
> > >>>>>>>>>> to rewrite. I have some objections/counter proposals to
> Fabian's
> > >>>>>>>> proposal
> > >>>>>>>>>> of doing it module wise and one module at a time.
> > >>>>>>>>>>> First, I do not see a problem of having java/scala code even
> > >>> within
> > >>>>>>> one
> > >>>>>>>>>> module, especially not if there are clean boundaries. Like we
> > >> could
> > >>>>>>> have
> > >>>>>>>>>> API in Scala and optimizer rules/logical nodes written in Java
> > in
> > >>>>>> the
> > >>>>>>>> same
> > >>>>>>>>>> module. However I haven’t previously maintained mixed
> scala/java
> > >>>>>> code
> > >>>>>>>> bases
> > >>>>>>>>>> before, so I might be missing something here.
> > >>>>>>>>>>> Secondly this whole migration might and most like will take
> > >> longer
> > >>>>>>> then
> > >>>>>>>>>> expected, so that creates a problem for a new code that we
> will
> > >> be
> > >>>>>>>>>> creating. After making a decision to migrate to Java, almost
> any
> > >>> new
> > >>>>>>>> Scala
> > >>>>>>>>>> line of code will be immediately a technological debt and we
> > will
> > >>>>>> have
> > >>>>>>>> to
> > >>>>>>>>>> rewrite it to Java later.
> > >>>>>>>>>>> Thus I would propose first to state our end goal - modules
> > >>>>>> structure
> > >>>>>>>> and
> > >>>>>>>>>> which parts of modules we want to have eventually Scala-free.
> > >>>>>> Secondly
> > >>>>>>>>>> taking all steps necessary that will allow us to write new
> code
> > >>>>>>>> complaint
> > >>>>>>>>>> with our end goal. Only after that we should/could focus on
> > >>>>>>>> incrementally
> > >>>>>>>>>> rewriting the old code. Otherwise we could be stuck/blocked
> for
> > >>>>>> years
> > >>>>>>>>>> writing new code in Scala (and increasing technological debt),
> > >>>>>> because
> > >>>>>>>>>> nobody have found a time to rewrite some non important and not
> > >>>>>>> actively
> > >>>>>>>>>> developed part of some module.
> > >>>>>>>>>>> Piotrek
> > >>>>>>>>>>>
> > >>>>>>>>>>>> On 14 Jun 2018, at 15:34, Fabian Hueske <[hidden email]>
> > >>>>>> wrote:
> > >>>>>>>>>>>> Hi,
> > >>>>>>>>>>>>
> > >>>>>>>>>>>> In general, I think this is a good effort. However, it won't
> > be
> > >>>>>> easy
> > >>>>>>>>>> and I
> > >>>>>>>>>>>> think we have to plan this well.
> > >>>>>>>>>>>> I don't like the idea of having the whole code base
> fragmented
> > >>>>>> into
> > >>>>>>>> Java
> > >>>>>>>>>>>> and Scala code for too long.
> > >>>>>>>>>>>>
> > >>>>>>>>>>>> I think we should do this one step at a time and focus on
> > >>>>>> migrating
> > >>>>>>>> one
> > >>>>>>>>>>>> module at a time.
> > >>>>>>>>>>>> IMO, the easiest start would be to port the runtime to Java.
> > >>>>>>>>>>>> Extracting the API classes into an own module, porting them
> to
> > >>>>>> Java,
> > >>>>>>>> and
> > >>>>>>>>>>>> removing the Scala dependency won't be possible without
> > >> breaking
> > >>>>>> the
> > >>>>>>>> API
> > >>>>>>>>>>>> since a few classes depend on the Scala Table API.
> > >>>>>>>>>>>>
> > >>>>>>>>>>>> Best, Fabian
> > >>>>>>>>>>>>
> > >>>>>>>>>>>>
> > >>>>>>>>>>>> 2018-06-14 10:33 GMT+02:00 Till Rohrmann <
> > [hidden email]
> > >>> :
> > >>>>>>>>>>>>> I think that is a noble and honorable goal and we should
> > >> strive
> > >>>>>> for
> > >>>>>>>> it.
> > >>>>>>>>>>>>> This, however, must be an iterative process given the sheer
> > >> size
> > >>>>>> of
> > >>>>>>>> the
> > >>>>>>>>>>>>> code base. I like the approach to define common Java
> modules
> > >>>>>> which
> > >>>>>>>> are
> > >>>>>>>>>> used
> > >>>>>>>>>>>>> by more specific Scala modules and slowly moving classes
> from
> > >>>>>> Scala
> > >>>>>>>> to
> > >>>>>>>>>>>>> Java. Thus +1 for the proposal.
> > >>>>>>>>>>>>>
> > >>>>>>>>>>>>> Cheers,
> > >>>>>>>>>>>>> Till
> > >>>>>>>>>>>>>
> > >>>>>>>>>>>>> On Wed, Jun 13, 2018 at 12:01 PM Piotr Nowojski <
> > >>>>>>>>>> [hidden email]>
> > >>>>>>>>>>>>> wrote:
> > >>>>>>>>>>>>>
> > >>>>>>>>>>>>>> Hi,
> > >>>>>>>>>>>>>>
> > >>>>>>>>>>>>>> I do not have an experience with how scala and java
> > interacts
> > >>>>>> with
> > >>>>>>>>>> each
> > >>>>>>>>>>>>>> other, so I can not fully validate your proposal, but
> > >> generally
> > >>>>>>>>>> speaking
> > >>>>>>>>>>>>> +1
> > >>>>>>>>>>>>>> from me.
> > >>>>>>>>>>>>>>
> > >>>>>>>>>>>>>> Does it also mean, that we should slowly migrate
> > >>>>>>> `flink-table-core`
> > >>>>>>>> to
> > >>>>>>>>>>>>>> Java? How would you envision it? It would be nice to be
> able
> > >> to
> > >>>>>>> add
> > >>>>>>>>>> new
> > >>>>>>>>>>>>>> classes/features written in Java and so that they can
> > coexist
> > >>>>>> with
> > >>>>>>>> old
> > >>>>>>>>>>>>>> Scala code until we gradually switch from Scala to Java.
> > >>>>>>>>>>>>>>
> > >>>>>>>>>>>>>> Piotrek
> > >>>>>>>>>>>>>>
> > >>>>>>>>>>>>>>> On 13 Jun 2018, at 11:32, Timo Walther <
> [hidden email]
> > >
> > >>>>>>> wrote:
> > >>>>>>>>>>>>>>> Hi everyone,
> > >>>>>>>>>>>>>>>
> > >>>>>>>>>>>>>>> as you all know, currently the Table & SQL API is
> > >> implemented
> > >>>>>> in
> > >>>>>>>>>> Scala.
> > >>>>>>>>>>>>>> This decision was made a long-time ago when the initital
> > code
> > >>>>>> base
> > >>>>>>>> was
> > >>>>>>>>>>>>>> created as part of a master's thesis. The community kept
> > >> Scala
> > >>>>>>>>>> because of
> > >>>>>>>>>>>>>> the nice language features that enable a fluent Table API
> > >> like
> > >>>>>>>>>>>>>> table.select('field.trim()) and because Scala allows for
> > >> quick
> > >>>>>>>>>>>>> prototyping
> > >>>>>>>>>>>>>> (e.g. multi-line comments for code generation). The
> > >> committers
> > >>>>>>>>>> enforced
> > >>>>>>>>>>>>> not
> > >>>>>>>>>>>>>> splitting the code-base into two programming languages.
> > >>>>>>>>>>>>>>> However, nowadays the flink-table module more and more
> > >> becomes
> > >>>>>> an
> > >>>>>>>>>>>>>> important part in the Flink ecosystem. Connectors,
> formats,
> > >> and
> > >>>>>>> SQL
> > >>>>>>>>>>>>> client
> > >>>>>>>>>>>>>> are actually implemented in Java but need to interoperate
> > >> with
> > >>>>>>>>>>>>> flink-table
> > >>>>>>>>>>>>>> which makes these modules dependent on Scala. As mentioned
> > in
> > >>> an
> > >>>>>>>>>> earlier
> > >>>>>>>>>>>>>> mail thread, using Scala for API classes also exposes
> member
> > >>>>>>>> variables
> > >>>>>>>>>>>>> and
> > >>>>>>>>>>>>>> methods in Java that should not be exposed to users [1].
> > Java
> > >>> is
> > >>>>>>>> still
> > >>>>>>>>>>>>> the
> > >>>>>>>>>>>>>> most important API language and right now we treat it as a
> > >>>>>>>>>> second-class
> > >>>>>>>>>>>>>> citizen. I just noticed that you even need to add Scala if
> > >> you
> > >>>>>>> just
> > >>>>>>>>>> want
> > >>>>>>>>>>>>> to
> > >>>>>>>>>>>>>> implement a ScalarFunction because of method clashes
> between
> > >>>>>>> `public
> > >>>>>>>>>>>>> String
> > >>>>>>>>>>>>>> toString()` and `public scala.Predef.String toString()`.
> > >>>>>>>>>>>>>>> Given the size of the current code base, reimplementing
> the
> > >>>>>>> entire
> > >>>>>>>>>>>>>> flink-table code in Java is a goal that we might never
> > reach.
> > >>>>>>>>>> However, we
> > >>>>>>>>>>>>>> should at least treat the symptoms and have this as a
> > >> long-term
> > >>>>>>> goal
> > >>>>>>>>>> in
> > >>>>>>>>>>>>>> mind. My suggestion would be to convert user-facing and
> > >> runtime
> > >>>>>>>>>> classes
> > >>>>>>>>>>>>> and
> > >>>>>>>>>>>>>> split the code base into multiple modules:
> > >>>>>>>>>>>>>>>> flink-table-java {depends on flink-table-core}
> > >>>>>>>>>>>>>>> Implemented in Java. Java users can use this. This would
> > >>>>>> require
> > >>>>>>> to
> > >>>>>>>>>>>>>> convert classes like TableEnvironment, Table.
> > >>>>>>>>>>>>>>>> flink-table-scala {depends on flink-table-core}
> > >>>>>>>>>>>>>>> Implemented in Scala. Scala users can use this.
> > >>>>>>>>>>>>>>>
> > >>>>>>>>>>>>>>>> flink-table-common
> > >>>>>>>>>>>>>>> Implemented in Java. Connectors, formats, and UDFs can
> use
> > >>>>>> this.
> > >>>>>>> It
> > >>>>>>>>>>>>>> contains interface classes such as descriptors, table
> sink,
> > >>>>>> table
> > >>>>>>>>>> source.
> > >>>>>>>>>>>>>>>> flink-table-core {depends on flink-table-common and
> > >>>>>>>>>>>>>> flink-table-runtime}
> > >>>>>>>>>>>>>>> Implemented in Scala. Contains the current main code
> base.
> > >>>>>>>>>>>>>>>
> > >>>>>>>>>>>>>>>> flink-table-runtime
> > >>>>>>>>>>>>>>> Implemented in Java. This would require to convert
> classes
> > >> in
> > >>>>>>>>>>>>>> o.a.f.table.runtime but would improve the runtime
> > >> potentially.
> > >>>>>>>>>>>>>>> What do you think?
> > >>>>>>>>>>>>>>>
> > >>>>>>>>>>>>>>>
> > >>>>>>>>>>>>>>> Regards,
> > >>>>>>>>>>>>>>>
> > >>>>>>>>>>>>>>> Timo
> > >>>>>>>>>>>>>>>
> > >>>>>>>>>>>>>>> [1]
> > >>>>>>>>>>>>>> http://apache-flink-mailing-list-archive.1008284.n3.
> > >>>>>>>>>>>>> nabble.com/DISCUSS-Convert-main-Table-API-classes-into-
> > >>>>>>>>>> traits-tp21335.html
> > >>>
> >
> >
>

[Timo Walther-2]

Hi Kurt,

I understand your concerns. However, there is no concrete roadmap for
Flink 2.0 and (as Vino said) the flink-table is developed very actively.
Major refactorings happened in the past and will also happen with or
without Scala migration. A good example, is the proper catalog support
which will refactor big parts of the TableEnvironment class. Or the
introduction of "retractions" which needed a big refactoring of the
planning phase. Stability is only guaranteed for the API and the general
behavior, however, currently flink-table is not using @Public or
@PublicEvolving annotations for a reason.

I think the migration will still happen slowly because it needs people
that allocate time for that. Therefore, even Flink forks can slowly
adapt to the evolving Scala-to-Java code base.

Regards,
Timo


Am 27.11.18 um 13:16 schrieb vino yang:

> Hi Kurt,
>
> Currently, there is still a long time to go from flink 2.0. Considering
> that the flink-table
> is one of the most active modules in the current flink project, each
> version has
> a number of changes and features added. I think that refactoring faster
> will reduce subsequent
> complexity and workload. And this may be a gradual and long process. We
> should be able to
>   regard it as a "technical debt", and if it does not change it, it will
> also affect the decision-making of other issues.
>
> Thanks, vino.
>
> Kurt Young <[hidden email]> 于2018年11月27日周二 下午7:34写道：
>
>> Hi Timo,
>>
>> Thanks for writing up the document. I'm +1 for reorganizing the module
>> structure and make table scala free. But I have
>> a little concern abount the timing. Is it more appropriate to get this done
>> when Flink decide to bump to next big version, like 2.x.
>> It's true you can keep all the class's package path as it is, and will not
>> introduce API change. But if some company are developing their own
>> Flink, and sync with community version by rebasing, may face a lot of
>> conflicts. Although you can avoid conflicts by always moving source codes
>> between packages, but I assume you still need to delete the original scala
>> file and add a new java file when you want to change program language.
>>
>> Best,
>> Kurt
>>
>>
>> On Tue, Nov 27, 2018 at 5:57 PM Timo Walther <[hidden email]> wrote:
>>
>>> Hi Hequn,
>>>
>>> thanks for your feedback. Yes, migrating the test cases is another issue
>>> that is not represented in the document but should naturally go along
>>> with the migration.
>>>
>>> I agree that we should migrate the main API classes quickly within this
>>> 1.8 release after the module split has been performed. Help here is
>>> highly appreciated!
>>>
>>> I forgot that Java supports static methods in interfaces now, but
>>> actually I don't like the design of calling `TableEnvironment.get(env)`.
>>> Because people often use `TableEnvironment tEnd =
>>> TableEnvironment.get(env)` and then wonder why there is no
>>> `toAppendStream` or `toDataSet` because they are using the base class.
>>> However, things like that can be discussed in the corresponding issue
>>> when it comes to implementation.
>>>
>>> @Vino: I think your work fits nicely to these efforts.
>>>
>>> @everyone: I will wait for more feedback until end of this week. Then I
>>> will convert the design document into a FLIP and open subtasks in Jira,
>>> if there are no objections?
>>>
>>> Regards,
>>> Timo
>>>
>>> Am 24.11.18 um 13:45 schrieb vino yang:
>>>> Hi hequn,
>>>>
>>>> I am very glad to hear that you are interested in this work.
>>>> As we all know, this process involves a lot.
>>>> Currently, the migration work has begun. I started with the
>>>> Kafka connector's dependency on flink-table and moved the
>>>> related dependencies to flink-table-common.
>>>> This work is tracked by FLINK-9461.  [1]
>>>> I don't know if it will conflict with what you expect to do, but from
>> the
>>>> impact I have observed,
>>>> it will involve many classes that are currently in flink-table.
>>>>
>>>> *Just a statement to prevent unnecessary conflicts.*
>>>>
>>>> Thanks, vino.
>>>>
>>>> [1]: https://issues.apache.org/jira/browse/FLINK-9461
>>>>
>>>> Hequn Cheng <[hidden email]> 于2018年11月24日周六 下午7:20写道：
>>>>
>>>>> Hi Timo,
>>>>>
>>>>> Thanks for the effort and writing up this document. I like the idea to
>>> make
>>>>> flink-table scala free, so +1 for the proposal!
>>>>>
>>>>> It's good to make Java the first-class citizen. For a long time, we
>> have
>>>>> neglected java so that many features in Table are missed in Java Test
>>>>> cases, such as this one[1] I found recently. And I think we may also
>>> need
>>>>> to migrate our test cases, i.e, add java tests.
>>>>>
>>>>> This definitely is a big change and will break API compatible. In
>> order
>>> to
>>>>> bring a smaller impact on users, I think we should go fast when we
>>> migrate
>>>>> APIs targeted to users. It's better to introduce the user sensitive
>>> changes
>>>>> within a release. However, it may be not that easy. I can help to
>>>>> contribute.
>>>>>
>>>>> Separation of interface and implementation is a good idea. This may
>>>>> introduce a minimum of dependencies or even no dependencies. I saw
>> your
>>>>> reply in the google doc. Java8 has already supported static method for
>>>>> interfaces, I think we can make use of it?
>>>>>
>>>>> Best,
>>>>> Hequn
>>>>>
>>>>> [1] https://issues.apache.org/jira/browse/FLINK-11001
>>>>>
>>>>>
>>>>> On Fri, Nov 23, 2018 at 5:36 PM Timo Walther <[hidden email]>
>>> wrote:
>>>>>> Hi everyone,
>>>>>>
>>>>>> thanks for the great feedback so far. I updated the document with the
>>>>>> input I got so far
>>>>>>
>>>>>> @Fabian: I moved the porting of flink-table-runtime classes up in the
>>>>> list.
>>>>>> @Xiaowei: Could you elaborate what "interface only" means to you? Do
>>> you
>>>>>> mean a module containing pure Java `interface`s? Or is the validation
>>>>>> logic also part of the API module? Are 50+ expression classes part of
>>>>>> the API interface or already too implementation-specific?
>>>>>>
>>>>>> @Xuefu: I extended the document by almost a page to clarify when we
>>>>>> should develop in Scala and when in Java. As Piotr said, every new
>>> Scala
>>>>>> line is instant technical debt.
>>>>>>
>>>>>> Thanks,
>>>>>> Timo
>>>>>>
>>>>>>
>>>>>> Am 23.11.18 um 10:29 schrieb Piotr Nowojski:
>>>>>>> Hi Timo,
>>>>>>>
>>>>>>> Thanks for writing this down +1 from my side :)
>>>>>>>
>>>>>>>> I'm wondering that whether we can have rule in the interim when
>> Java
>>>>>> and Scala coexist that dependency can only be one-way. I found that
>> in
>>>>> the
>>>>>> current code base there are cases where a Scala class extends Java
>> and
>>>>> vise
>>>>>> versa. This is quite painful. I'm thinking if we could say that
>>> extension
>>>>>> can only be from Java to Scala, which will help the situation.
>> However,
>>>>> I'm
>>>>>> not sure if this is practical.
>>>>>>> Xuefu: I’m also not sure what’s the best approach here, probably we
>>>>> will
>>>>>> have to work it out as we go. One thing to consider is that from now
>>> on,
>>>>>> every single new code line written in Scala anywhere in Flink-table
>>>>> (except
>>>>>> of Flink-table-api-scala) is an instant technological debt. From this
>>>>>> perspective I would be in favour of tolerating quite big
>> inchonvieneces
>>>>>> just to avoid any new Scala code.
>>>>>>> Piotrek
>>>>>>>
>>>>>>>> On 23 Nov 2018, at 03:25, Zhang, Xuefu <[hidden email]>
>>>>> wrote:
>>>>>>>> Hi Timo,
>>>>>>>>
>>>>>>>> Thanks for the effort and the Google writeup. During our external
>>>>>> catalog rework, we found much confusion between Java and Scala, and
>>> this
>>>>>> Scala-free roadmap should greatly mitigate that.
>>>>>>>> I'm wondering that whether we can have rule in the interim when
>> Java
>>>>>> and Scala coexist that dependency can only be one-way. I found that
>> in
>>>>> the
>>>>>> current code base there are cases where a Scala class extends Java
>> and
>>>>> vise
>>>>>> versa. This is quite painful. I'm thinking if we could say that
>>> extension
>>>>>> can only be from Java to Scala, which will help the situation.
>> However,
>>>>> I'm
>>>>>> not sure if this is practical.
>>>>>>>> Thanks,
>>>>>>>> Xuefu
>>>>>>>>
>>>>>>>>
>>>>>>>> ------------------------------------------------------------------
>>>>>>>> Sender:jincheng sun <[hidden email]>
>>>>>>>> Sent at:2018 Nov 23 (Fri) 09:49
>>>>>>>> Recipient:dev <[hidden email]>
>>>>>>>> Subject:Re: [DISCUSS] Long-term goal of making flink-table
>> Scala-free
>>>>>>>> Hi Timo,
>>>>>>>> Thanks for initiating this great discussion.
>>>>>>>>
>>>>>>>> Currently when using SQL/TableAPI should include many dependence.
>> In
>>>>>>>> particular, it is not necessary to introduce the specific
>>>>> implementation
>>>>>>>> dependencies which users do not care about. So I am glad to see
>> your
>>>>>>>> proposal, and hope when we consider splitting the API interface
>> into
>>> a
>>>>>>>> separate module, so that the user can introduce minimum of
>>>>> dependencies.
>>>>>>>> So, +1 to [separation of interface and implementation; e.g.
>> `Table` &
>>>>>>>> `TableImpl`] which you mentioned in the google doc.
>>>>>>>> Best,
>>>>>>>> Jincheng
>>>>>>>>
>>>>>>>> Xiaowei Jiang <[hidden email]> 于2018年11月22日周四 下午10:50写道：
>>>>>>>>
>>>>>>>>> Hi Timo, thanks for driving this! I think that this is a nice
>> thing
>>>>> to
>>>>>> do.
>>>>>>>>> While we are doing this, can we also keep in mind that we want to
>>>>>>>>> eventually have a TableAPI interface only module which users can
>>> take
>>>>>>>>> dependency on, but without including any implementation details?
>>>>>>>>>
>>>>>>>>> Xiaowei
>>>>>>>>>
>>>>>>>>> On Thu, Nov 22, 2018 at 6:37 PM Fabian Hueske <[hidden email]>
>>>>>> wrote:
>>>>>>>>>> Hi Timo,
>>>>>>>>>>
>>>>>>>>>> Thanks for writing up this document.
>>>>>>>>>> I like the new structure and agree to prioritize the porting of
>> the
>>>>>>>>>> flink-table-common classes.
>>>>>>>>>> Since flink-table-runtime is (or should be) independent of the
>> API
>>>>> and
>>>>>>>>>> planner modules, we could start porting these classes once the
>> code
>>>>> is
>>>>>>>>>> split into the new module structure.
>>>>>>>>>> The benefits of a Scala-free flink-table-runtime would be a
>>>>> Scala-free
>>>>>>>>>> execution Jar.
>>>>>>>>>>
>>>>>>>>>> Best, Fabian
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Am Do., 22. Nov. 2018 um 10:54 Uhr schrieb Timo Walther <
>>>>>>>>>> [hidden email]
>>>>>>>>>>> :
>>>>>>>>>>> Hi everyone,
>>>>>>>>>>>
>>>>>>>>>>> I would like to continue this discussion thread and convert the
>>>>>> outcome
>>>>>>>>>>> into a FLIP such that users and contributors know what to expect
>>> in
>>>>>> the
>>>>>>>>>>> upcoming releases.
>>>>>>>>>>>
>>>>>>>>>>> I created a design document [1] that clarifies our motivation
>> why
>>>>> we
>>>>>>>>>>> want to do this, how a Maven module structure could look like,
>> and
>>>>> a
>>>>>>>>>>> suggestion for a migration plan.
>>>>>>>>>>>
>>>>>>>>>>> It would be great to start with the efforts for the 1.8 release
>>>>> such
>>>>>>>>>>> that new features can be developed in Java and major
>> refactorings
>>>>>> such
>>>>>>>>>>> as improvements to the connectors and external catalog support
>> are
>>>>>> not
>>>>>>>>>>> blocked.
>>>>>>>>>>>
>>>>>>>>>>> Please let me know what you think.
>>>>>>>>>>>
>>>>>>>>>>> Regards,
>>>>>>>>>>> Timo
>>>>>>>>>>>
>>>>>>>>>>> [1]
>>>>>>>>>>>
>>>>>>>>>>>
>> https://docs.google.com/document/d/1PPo6goW7tOwxmpFuvLSjFnx7BF8IVz0w3dcmPPyqvoY/edit?usp=sharing
>>>>>>>>>>> Am 02.07.18 um 17:08 schrieb Fabian Hueske:
>>>>>>>>>>>> Hi Piotr,
>>>>>>>>>>>>
>>>>>>>>>>>> thanks for bumping this thread and thanks for Xingcan for the
>>>>>>>>> comments.
>>>>>>>>>>>> I think the first step would be to separate the flink-table
>>> module
>>>>>>>>> into
>>>>>>>>>>>> multiple sub modules. These could be:
>>>>>>>>>>>>
>>>>>>>>>>>> - flink-table-api: All API facing classes. Can be later divided
>>>>>>>>> further
>>>>>>>>>>>> into Java/Scala Table API/SQL
>>>>>>>>>>>> - flink-table-planning: involves all planning (basically
>>>>> everything
>>>>>>>>> we
>>>>>>>>>> do
>>>>>>>>>>>> with Calcite)
>>>>>>>>>>>> - flink-table-runtime: the runtime code
>>>>>>>>>>>>
>>>>>>>>>>>> IMO, a realistic mid-term goal is to have the runtime module
>> and
>>>>>>>>>> certain
>>>>>>>>>>>> parts of the planning module ported to Java.
>>>>>>>>>>>> The api module will be much harder to port because of several
>>>>>>>>>>> dependencies
>>>>>>>>>>>> to Scala core classes (the parser framework, tree iterations,
>>>>> etc.).
>>>>>>>>>> I'm
>>>>>>>>>>>> not saying we should not port this to Java, but it is not clear
>>> to
>>>>>> me
>>>>>>>>>>> (yet)
>>>>>>>>>>>> how to do it.
>>>>>>>>>>>>
>>>>>>>>>>>> I think flink-table-runtime should not be too hard to port. The
>>>>> code
>>>>>>>>>> does
>>>>>>>>>>>> not make use of many Scala features, i.e., it's writing very
>>>>>>>>> Java-like.
>>>>>>>>>>>> Also, there are not many dependencies and operators can be
>>>>>>>>> individually
>>>>>>>>>>>> ported step-by-step.
>>>>>>>>>>>> For flink-table-planning, we can have certain packages that we
>>>>> port
>>>>>>>>> to
>>>>>>>>>>> Java
>>>>>>>>>>>> like planning rules or plan nodes. The related classes mostly
>>>>> extend
>>>>>>>>>>>> Calcite's Java interfaces/classes and would be natural choices
>>> for
>>>>>>>>>> being
>>>>>>>>>>>> ported. The code generation classes will require more effort to
>>>>>> port.
>>>>>>>>>>> There
>>>>>>>>>>>> are also some dependencies in planning on the api module that
>> we
>>>>>>>>> would
>>>>>>>>>>> need
>>>>>>>>>>>> to resolve somehow.
>>>>>>>>>>>>
>>>>>>>>>>>> For SQL most work when adding new features is done in the
>>> planning
>>>>>>>>> and
>>>>>>>>>>>> runtime modules. So, this separation should already reduce
>>>>>>>>>> "technological
>>>>>>>>>>>> dept" quite a lot.
>>>>>>>>>>>> The Table API depends much more on Scala than SQL.
>>>>>>>>>>>>
>>>>>>>>>>>> Cheers, Fabian
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> 2018-07-02 16:26 GMT+02:00 Xingcan Cui <[hidden email]>:
>>>>>>>>>>>>
>>>>>>>>>>>>> Hi all,
>>>>>>>>>>>>>
>>>>>>>>>>>>> I also think about this problem these days and here are my
>>>>>> thoughts.
>>>>>>>>>>>>> 1) We must admit that it’s really a tough task to interoperate
>>>>> with
>>>>>>>>>> Java
>>>>>>>>>>>>> and Scala. E.g., they have different collection types (Scala
>>>>>>>>>> collections
>>>>>>>>>>>>> v.s. java.util.*) and in Java, it's hard to implement a method
>>>>>> which
>>>>>>>>>>> takes
>>>>>>>>>>>>> Scala functions as parameters. Considering the major part of
>> the
>>>>>>>>> code
>>>>>>>>>>> base
>>>>>>>>>>>>> is implemented in Java, +1 for this goal from a long-term
>> view.
>>>>>>>>>>>>> 2) The ideal solution would be to just expose a Scala API and
>>>>> make
>>>>>>>>> all
>>>>>>>>>>> the
>>>>>>>>>>>>> other parts Scala-free. But I am not sure if it could be
>>> achieved
>>>>>>>>> even
>>>>>>>>>>> in a
>>>>>>>>>>>>> long-term. Thus as Timo suggested, keep the Scala codes in
>>>>>>>>>>>>> "flink-table-core" would be a compromise solution.
>>>>>>>>>>>>>
>>>>>>>>>>>>> 3) If the community makes the final decision, maybe any new
>>>>>> features
>>>>>>>>>>>>> should be added in Java (regardless of the modules), in order
>> to
>>>>>>>>>> prevent
>>>>>>>>>>>>> the Scala codes from growing.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Best,
>>>>>>>>>>>>> Xingcan
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>> On Jul 2, 2018, at 9:30 PM, Piotr Nowojski <
>>>>>>>>> [hidden email]>
>>>>>>>>>>>>> wrote:
>>>>>>>>>>>>>> Bumping the topic.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> If we want to do this, the sooner we decide, the less code we
>>>>> will
>>>>>>>>>> have
>>>>>>>>>>>>> to rewrite. I have some objections/counter proposals to
>> Fabian's
>>>>>>>>>>> proposal
>>>>>>>>>>>>> of doing it module wise and one module at a time.
>>>>>>>>>>>>>> First, I do not see a problem of having java/scala code even
>>>>>> within
>>>>>>>>>> one
>>>>>>>>>>>>> module, especially not if there are clean boundaries. Like we
>>>>> could
>>>>>>>>>> have
>>>>>>>>>>>>> API in Scala and optimizer rules/logical nodes written in Java
>>> in
>>>>>>>>> the
>>>>>>>>>>> same
>>>>>>>>>>>>> module. However I haven’t previously maintained mixed
>> scala/java
>>>>>>>>> code
>>>>>>>>>>> bases
>>>>>>>>>>>>> before, so I might be missing something here.
>>>>>>>>>>>>>> Secondly this whole migration might and most like will take
>>>>> longer
>>>>>>>>>> then
>>>>>>>>>>>>> expected, so that creates a problem for a new code that we
>> will
>>>>> be
>>>>>>>>>>>>> creating. After making a decision to migrate to Java, almost
>> any
>>>>>> new
>>>>>>>>>>> Scala
>>>>>>>>>>>>> line of code will be immediately a technological debt and we
>>> will
>>>>>>>>> have
>>>>>>>>>>> to
>>>>>>>>>>>>> rewrite it to Java later.
>>>>>>>>>>>>>> Thus I would propose first to state our end goal - modules
>>>>>>>>> structure
>>>>>>>>>>> and
>>>>>>>>>>>>> which parts of modules we want to have eventually Scala-free.
>>>>>>>>> Secondly
>>>>>>>>>>>>> taking all steps necessary that will allow us to write new
>> code
>>>>>>>>>>> complaint
>>>>>>>>>>>>> with our end goal. Only after that we should/could focus on
>>>>>>>>>>> incrementally
>>>>>>>>>>>>> rewriting the old code. Otherwise we could be stuck/blocked
>> for
>>>>>>>>> years
>>>>>>>>>>>>> writing new code in Scala (and increasing technological debt),
>>>>>>>>> because
>>>>>>>>>>>>> nobody have found a time to rewrite some non important and not
>>>>>>>>>> actively
>>>>>>>>>>>>> developed part of some module.
>>>>>>>>>>>>>> Piotrek
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> On 14 Jun 2018, at 15:34, Fabian Hueske <[hidden email]>
>>>>>>>>> wrote:
>>>>>>>>>>>>>>> Hi,
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> In general, I think this is a good effort. However, it won't
>>> be
>>>>>>>>> easy
>>>>>>>>>>>>> and I
>>>>>>>>>>>>>>> think we have to plan this well.
>>>>>>>>>>>>>>> I don't like the idea of having the whole code base
>> fragmented
>>>>>>>>> into
>>>>>>>>>>> Java
>>>>>>>>>>>>>>> and Scala code for too long.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> I think we should do this one step at a time and focus on
>>>>>>>>> migrating
>>>>>>>>>>> one
>>>>>>>>>>>>>>> module at a time.
>>>>>>>>>>>>>>> IMO, the easiest start would be to port the runtime to Java.
>>>>>>>>>>>>>>> Extracting the API classes into an own module, porting them
>> to
>>>>>>>>> Java,
>>>>>>>>>>> and
>>>>>>>>>>>>>>> removing the Scala dependency won't be possible without
>>>>> breaking
>>>>>>>>> the
>>>>>>>>>>> API
>>>>>>>>>>>>>>> since a few classes depend on the Scala Table API.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> Best, Fabian
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> 2018-06-14 10:33 GMT+02:00 Till Rohrmann <
>>> [hidden email]
>>>>>> :
>>>>>>>>>>>>>>>> I think that is a noble and honorable goal and we should
>>>>> strive
>>>>>>>>> for
>>>>>>>>>>> it.
>>>>>>>>>>>>>>>> This, however, must be an iterative process given the sheer
>>>>> size
>>>>>>>>> of
>>>>>>>>>>> the
>>>>>>>>>>>>>>>> code base. I like the approach to define common Java
>> modules
>>>>>>>>> which
>>>>>>>>>>> are
>>>>>>>>>>>>> used
>>>>>>>>>>>>>>>> by more specific Scala modules and slowly moving classes
>> from
>>>>>>>>> Scala
>>>>>>>>>>> to
>>>>>>>>>>>>>>>> Java. Thus +1 for the proposal.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> Cheers,
>>>>>>>>>>>>>>>> Till
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> On Wed, Jun 13, 2018 at 12:01 PM Piotr Nowojski <
>>>>>>>>>>>>> [hidden email]>
>>>>>>>>>>>>>>>> wrote:
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> Hi,
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> I do not have an experience with how scala and java
>>> interacts
>>>>>>>>> with
>>>>>>>>>>>>> each
>>>>>>>>>>>>>>>>> other, so I can not fully validate your proposal, but
>>>>> generally
>>>>>>>>>>>>> speaking
>>>>>>>>>>>>>>>> +1
>>>>>>>>>>>>>>>>> from me.
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> Does it also mean, that we should slowly migrate
>>>>>>>>>> `flink-table-core`
>>>>>>>>>>> to
>>>>>>>>>>>>>>>>> Java? How would you envision it? It would be nice to be
>> able
>>>>> to
>>>>>>>>>> add
>>>>>>>>>>>>> new
>>>>>>>>>>>>>>>>> classes/features written in Java and so that they can
>>> coexist
>>>>>>>>> with
>>>>>>>>>>> old
>>>>>>>>>>>>>>>>> Scala code until we gradually switch from Scala to Java.
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> Piotrek
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> On 13 Jun 2018, at 11:32, Timo Walther <
>> [hidden email]
>>>>>>>>>> wrote:
>>>>>>>>>>>>>>>>>> Hi everyone,
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> as you all know, currently the Table & SQL API is
>>>>> implemented
>>>>>>>>> in
>>>>>>>>>>>>> Scala.
>>>>>>>>>>>>>>>>> This decision was made a long-time ago when the initital
>>> code
>>>>>>>>> base
>>>>>>>>>>> was
>>>>>>>>>>>>>>>>> created as part of a master's thesis. The community kept
>>>>> Scala
>>>>>>>>>>>>> because of
>>>>>>>>>>>>>>>>> the nice language features that enable a fluent Table API
>>>>> like
>>>>>>>>>>>>>>>>> table.select('field.trim()) and because Scala allows for
>>>>> quick
>>>>>>>>>>>>>>>> prototyping
>>>>>>>>>>>>>>>>> (e.g. multi-line comments for code generation). The
>>>>> committers
>>>>>>>>>>>>> enforced
>>>>>>>>>>>>>>>> not
>>>>>>>>>>>>>>>>> splitting the code-base into two programming languages.
>>>>>>>>>>>>>>>>>> However, nowadays the flink-table module more and more
>>>>> becomes
>>>>>>>>> an
>>>>>>>>>>>>>>>>> important part in the Flink ecosystem. Connectors,
>> formats,
>>>>> and
>>>>>>>>>> SQL
>>>>>>>>>>>>>>>> client
>>>>>>>>>>>>>>>>> are actually implemented in Java but need to interoperate
>>>>> with
>>>>>>>>>>>>>>>> flink-table
>>>>>>>>>>>>>>>>> which makes these modules dependent on Scala. As mentioned
>>> in
>>>>>> an
>>>>>>>>>>>>> earlier
>>>>>>>>>>>>>>>>> mail thread, using Scala for API classes also exposes
>> member
>>>>>>>>>>> variables
>>>>>>>>>>>>>>>> and
>>>>>>>>>>>>>>>>> methods in Java that should not be exposed to users [1].
>>> Java
>>>>>> is
>>>>>>>>>>> still
>>>>>>>>>>>>>>>> the
>>>>>>>>>>>>>>>>> most important API language and right now we treat it as a
>>>>>>>>>>>>> second-class
>>>>>>>>>>>>>>>>> citizen. I just noticed that you even need to add Scala if
>>>>> you
>>>>>>>>>> just
>>>>>>>>>>>>> want
>>>>>>>>>>>>>>>> to
>>>>>>>>>>>>>>>>> implement a ScalarFunction because of method clashes
>> between
>>>>>>>>>> `public
>>>>>>>>>>>>>>>> String
>>>>>>>>>>>>>>>>> toString()` and `public scala.Predef.String toString()`.
>>>>>>>>>>>>>>>>>> Given the size of the current code base, reimplementing
>> the
>>>>>>>>>> entire
>>>>>>>>>>>>>>>>> flink-table code in Java is a goal that we might never
>>> reach.
>>>>>>>>>>>>> However, we
>>>>>>>>>>>>>>>>> should at least treat the symptoms and have this as a
>>>>> long-term
>>>>>>>>>> goal
>>>>>>>>>>>>> in
>>>>>>>>>>>>>>>>> mind. My suggestion would be to convert user-facing and
>>>>> runtime
>>>>>>>>>>>>> classes
>>>>>>>>>>>>>>>> and
>>>>>>>>>>>>>>>>> split the code base into multiple modules:
>>>>>>>>>>>>>>>>>>> flink-table-java {depends on flink-table-core}
>>>>>>>>>>>>>>>>>> Implemented in Java. Java users can use this. This would
>>>>>>>>> require
>>>>>>>>>> to
>>>>>>>>>>>>>>>>> convert classes like TableEnvironment, Table.
>>>>>>>>>>>>>>>>>>> flink-table-scala {depends on flink-table-core}
>>>>>>>>>>>>>>>>>> Implemented in Scala. Scala users can use this.
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> flink-table-common
>>>>>>>>>>>>>>>>>> Implemented in Java. Connectors, formats, and UDFs can
>> use
>>>>>>>>> this.
>>>>>>>>>> It
>>>>>>>>>>>>>>>>> contains interface classes such as descriptors, table
>> sink,
>>>>>>>>> table
>>>>>>>>>>>>> source.
>>>>>>>>>>>>>>>>>>> flink-table-core {depends on flink-table-common and
>>>>>>>>>>>>>>>>> flink-table-runtime}
>>>>>>>>>>>>>>>>>> Implemented in Scala. Contains the current main code
>> base.
>>>>>>>>>>>>>>>>>>> flink-table-runtime
>>>>>>>>>>>>>>>>>> Implemented in Java. This would require to convert
>> classes
>>>>> in
>>>>>>>>>>>>>>>>> o.a.f.table.runtime but would improve the runtime
>>>>> potentially.
>>>>>>>>>>>>>>>>>> What do you think?
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> Regards,
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> Timo
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> [1]
>>>>>>>>>>>>>>>>> http://apache-flink-mailing-list-archive.1008284.n3.
>>>>>>>>>>>>>>>> nabble.com/DISCUSS-Convert-main-Table-API-classes-into-
>>>>>>>>>>>>> traits-tp21335.html
>>>