[DISCUSS] FLIP-126: Unify (and separate) Watermark Assigners
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[DISCUSS] FLIP-126: Unify (and separate) Watermark Assigners
 
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Re: [DISCUSS] FLIP-126: Unify (and separate) Watermark Assigners
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Re: [DISCUSS] FLIP-126: Unify (and separate) Watermark Assigners
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Re: [DISCUSS] FLIP-126: Unify (and separate) Watermark Assigners
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Re: [DISCUSS] FLIP-126: Unify (and separate) Watermark Assigners
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Re: [DISCUSS] FLIP-126: Unify (and separate) Watermark Assigners
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Re: [DISCUSS] FLIP-126: Unify (and separate) Watermark Assigners
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Re: [DISCUSS] FLIP-126: Unify (and separate) Watermark Assigners
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Re: [DISCUSS] FLIP-126: Unify (and separate) Watermark Assigners
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Re: [DISCUSS] FLIP-126: Unify (and separate) Watermark Assigners
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Re: [DISCUSS] FLIP-126: Unify (and separate) Watermark Assigners
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Re: [DISCUSS] FLIP-126: Unify (and separate) Watermark Assigners
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Hi Aljoscha,
Sorry for adding comments during the vote, but I have some really minor suggestions that should not influence the voting thread imo. 1) Does it make sense to have the TimestampAssigner extend from Flink's Function? This implies it has to be serializable which with the factory pattern is not strictly necessary, right? BTW I really like that you suggested the FunctionInterface annotation there. 2) Could we rename the IdentityTimestampAssigner to e.g. RecordTimestampAssigner/SystemTimestampAssigner/NativeTimestampAssigner... Personally I found the IdentityTimestampAssigner a bit misleading as it usually mean a no-op. Which did not click for me, as I assumed it somehow returns the incoming record itself. 3) Could we rename the second parameter of TimestampAssigner#extract to e.g. recordTimestamp/nativeTimestamp? This is similar to the point above. This parameter was also a bit confusing for me as I thought at times its somehow related to TimerService#currentProcessingTimestamp()/currentWatermark() as the whole system currentTimestamp. Other than those three points I like the proposal and I was about to vote +1 if it was not for those three points. Best, Dawid On 11/05/2020 16:57, Jark Wu wrote: > Thanks for the explanation. I like the fatory pattern to make the member > variables immutable and final. > > So +1 to the proposal. > > Best, > Jark > > On Mon, 11 May 2020 at 22:01, Stephan Ewen <[hidden email]> wrote: > >> I am fine with that. >> >> Much of the principles seem agreed upon. I understand the need to support >> code-generated extractors and we should support most of it already (as >> Aljoscha mentioned via the factories) can extend this if needed. >> >> I think that the factory approach supports code-generated extractors in a >> cleaner way even than an extractor with an open/init method. >> >> >> On Mon, May 11, 2020 at 3:38 PM Aljoscha Krettek <[hidden email]> >> wrote: >> >>> We're slightly running out of time. I would propose we vote on the basic >>> principle and remain open to later additions. This feature is quite >>> important to make the new Kafka Source that is developed as part of >>> FLIP-27 useful. Otherwise we would have to use the legacy interfaces in >>> the newly added connector. >>> >>> I know that's a bit unorthodox but would everyone be OK with what's >>> currently there and then we iterate? >>> >>> Best, >>> Aljoscha >>> >>> On 11.05.20 13:57, Aljoscha Krettek wrote: >>>> Ah, I meant to write this in my previous email, sorry about that. >>>> >>>> The WatermarkStrategy, which is basically a factory for a >>>> WatermarkGenerator is the replacement for the open() method. This is >> the >>>> same strategy that was followed for StreamOperatorFactory, which was >>>> introduced to allow code generation in the Table API [1]. If we need >>>> metrics or other things we would add that as a parameter to the factory >>>> method. What do you think? >>>> >>>> Best, >>>> Aljoscha >>>> >>>> [1] https://issues.apache.org/jira/browse/FLINK-11974 >>>> >>>> On 10.05.20 05:07, Jark Wu wrote: >>>>> Hi, >>>>> >>>>> Regarding to the `open()/close()`, I think it's necessary for >>>>> Table&SQL to >>>>> compile the generated code. >>>>> In Table&SQL, the watermark strategy and event-timestamp is defined >>> using >>>>> SQL expressions, we will >>>>> translate and generate Java code for the expressions. If we have >>>>> `open()/close()`, we don't need lazy initialization. >>>>> Besides that, I can see a need to report some metrics, e.g. the >> current >>>>> watermark, the dirty timestamps (null value), etc. >>>>> So I think a simple `open()/close()` with a context which can get >>>>> MetricGroup is nice and not complex for the first version. >>>>> >>>>> Best, >>>>> Jark >>>>> >>>>> >>>>> >>>>> On Sun, 10 May 2020 at 00:50, Stephan Ewen <[hidden email]> wrote: >>>>> >>>>>> Thanks, Aljoscha, for picking this up. >>>>>> >>>>>> I agree with the approach of doing the here proposed set of changes >> for >>>>>> now. It already makes things simpler and adds idleness support >>>>>> everywhere. >>>>>> >>>>>> Rich functions and state always add complexity, let's do this in a >> next >>>>>> step, if we have a really compelling case. >>>>>> >>>>>> >>>>>> On Wed, Apr 29, 2020 at 7:24 PM Aljoscha Krettek < >> [hidden email]> >>>>>> wrote: >>>>>> >>>>>>> Regarding the WatermarkGenerator (WG) interface itself. The proposal >>> is >>>>>>> basically to turn emitting into a "flatMap", we give the >>>>>>> WatermarkGenerator a "collector" (the WatermarkOutput) and the WG >> can >>>>>>> decide whether to output a watermark or not and can also mark the >>>>>>> output >>>>>>> as idle. Changing the interface to return a Watermark (as the >> previous >>>>>>> watermark assigner interface did) would not allow that flexibility. >>>>>>> >>>>>>> Regarding checkpointing the watermark and keeping track of the >> minimum >>>>>>> watermark, this would be the responsibility of the framework (or the >>>>>>> KafkaConsumer in the current implementation). The user-supplied WG >>> does >>>>>>> not need to make sure the watermark doesn't regress. >>>>>>> >>>>>>> Regarding making the WG a "rich function", I can see the potential >>>>>>> benefit but I also see a lot of pitfalls. For example, how should >> the >>>>>>> watermark state be handled in the case of scale-in? It could be made >>> to >>>>>>> work in the Kafka case by attaching the state to the partition state >>>>>>> that we keep, but then we have potential backwards compatibility >>>>>>> problems also for the WM state. Does the WG usually need to keep the >>>>>>> state or might it be enough if the state is transient, i.e. if you >>> have >>>>>>> a restart the WG would loose its histogram but it would rebuild it >>>>>>> quickly and you would get back to the same steady state as before. >>>>>>> >>>>>>> Best, >>>>>>> Aljoscha >>>>>>> >>>>>>> On 27.04.20 12:12, David Anderson wrote: >>>>>>>> Overall I like this proposal; thanks for bringing it forward, >>>>>>>> Aljoscha. >>>>>>>> >>>>>>>> I also like the idea of making the Watermark generator a rich >>> function >>>>>> -- >>>>>>>> this should make it more straightforward to implement smarter >>>>>>>> watermark >>>>>>>> generators. Eg, one that uses state to keep statistics about the >>>>>>>> actual >>>>>>>> out-of-orderness, and uses those statistics to implement a variable >>>>>>> delay. >>>>>>>> David >>>>>>>> >>>>>>>> On Mon, Apr 27, 2020 at 11:44 AM Kostas Kloudas < >> [hidden email]> >>>>>>> wrote: >>>>>>>>> Hi Aljoscha, >>>>>>>>> >>>>>>>>> Thanks for opening the discussion! >>>>>>>>> >>>>>>>>> I have two comments on the FLIP: >>>>>>>>> 1) we could add lifecycle methods to the Generator, i.e. open()/ >>>>>>>>> close(), probably with a Context as argument: I have not fully >>>>>>>>> thought >>>>>>>>> this through but I think that this is more aligned with the rest >> of >>>>>>>>> our rich functions. In addition, it will allow, for example, to >>>>>>>>> initialize the Watermark value, if we decide to checkpoint the >>>>>>>>> watermark (see [1]) (I also do not know if Table/SQL needs to do >>>>>>>>> anything in the open()). >>>>>>>>> 2) aligned with the above, and with the case where we want to >>>>>>>>> checkpoint the watermark in mind, I am wondering about how we >> could >>>>>>>>> implement this in the future. In the FLIP, it is proposed to >> expose >>>>>>>>> the WatermarkOutput in the methods of the WatermarkGenerator. >> Given >>>>>>>>> that there is the implicit contract that watermarks are >>>>>>>>> non-decreasing, the WatermarkOutput#emitWatermark() will have (I >>>>>>>>> assume) a check that will compare the last emitted WM against the >>>>>>>>> provided one, and emit it only if it is >=. If not, then we risk >>>>>>>>> having the user shooting himself on the foot if he/she >> accidentally >>>>>>>>> forgets the check. Given that the WatermarkGenerator and its >>>>>>>>> caller do >>>>>>>>> not know if the watermark was finally emitted or not (the >>>>>>>>> WatermarkOutput#emitWatermark returns void), who will be >> responsible >>>>>>>>> for checkpointing the WM? >>>>>>>>> >>>>>>>>> Given this, why not having the methods as: >>>>>>>>> >>>>>>>>> public interface WatermarkGenerator<T> { >>>>>>>>> >>>>>>>>> Watermark onEvent(T event, long eventTimestamp, >>> WatermarkOutput >>>>>>>>> output); >>>>>>>>> >>>>>>>>> Watermark onPeriodicEmit(WatermarkOutput output); >>>>>>>>> } >>>>>>>>> >>>>>>>>> and the caller will be the one enforcing any invariants, such as >>>>>>>>> non-decreasing watermarks. In this way, the caller can checkpoint >>>>>>>>> anything that is needed as it will have complete knowledge as to >> if >>>>>>>>> the WM was emitted or not. >>>>>>>>> >>>>>>>>> What do you think? >>>>>>>>> >>>>>>>>> Cheers, >>>>>>>>> Kostas >>>>>>>>> >>>>>>>>> [1] https://issues.apache.org/jira/browse/FLINK-5601 >>>>>>>>> >>>>>>>>> On Tue, Apr 21, 2020 at 2:25 PM Timo Walther <[hidden email]> >>>>>>> wrote: >>>>>>>>>> Thanks for the proposal Aljoscha. This is a very useful >>> unification. >>>>>> We >>>>>>>>>> have considered this FLIP already in the interfaces for FLIP-95 >> [1] >>>>>> and >>>>>>>>>> look forward to update to the new unified watermark generators >> once >>>>>>>>>> FLIP-126 has been accepted. >>>>>>>>>> >>>>>>>>>> Regards, >>>>>>>>>> Timo >>>>>>>>>> >>>>>>>>>> [1] https://github.com/apache/flink/pull/11692 >>>>>>>>>> >>>>>>>>>> On 20.04.20 18:10, Aljoscha Krettek wrote: >>>>>>>>>>> Hi Everyone! >>>>>>>>>>> >>>>>>>>>>> We would like to start a discussion on "FLIP-126: Unify (and >>>>>> separate) >>>>>>>>>>> Watermark Assigners" [1]. This work was started by Stephan in an >>>>>>>>>>> experimental branch. I expanded on that work to provide a PoC >> for >>>>>> the >>>>>>>>>>> changes proposed in this FLIP: [2]. >>>>>>>>>>> >>>>>>>>>>> Currently, we have two different flavours of Watermark >>>>>>>>>>> Assigners: AssignerWithPunctuatedWatermarks >>>>>>>>>>> and AssignerWithPeriodicWatermarks. Both of them extend >>>>>>>>>>> from TimestampAssigner. This means that sources that want to >>>>>>>>>>> support >>>>>>>>>>> watermark assignment/extraction in the source need to support >> two >>>>>>>>>>> separate interfaces, we have two operator implementations for >> the >>>>>>>>>>> different flavours. Also, this makes features such as generic >>>>>> support >>>>>>>>>>> for idleness detection more complicated to implemented because >> we >>>>>>> again >>>>>>>>>>> have to support two types of watermark assigners. >>>>>>>>>>> >>>>>>>>>>> In this FLIP we propose two things: >>>>>>>>>>> >>>>>>>>>>> Unify the Watermark Assigners into one Interface >>> WatermarkGenerator >>>>>>>>>>> Separate this new interface from the TimestampAssigner >>>>>>>>>>> The motivation for the first is to simplify future >> implementations >>>>>> and >>>>>>>>>>> code duplication. The motivation for the second point is again >>> code >>>>>>>>>>> deduplication, most assigners currently have to extend from some >>>>>> base >>>>>>>>>>> timestamp extractor or duplicate the extraction logic, or users >>>>>>>>>>> have >>>>>>> to >>>>>>>>>>> override an abstract method of the watermark assigner to provide >>>>>>>>>>> the >>>>>>>>>>> timestamp extraction logic. >>>>>>>>>>> >>>>>>>>>>> Additionally, we propose to add a generic wrapping >>>>>> WatermarkGenerator >>>>>>>>>>> that provides idleness detection, i.e. it can mark a >>>>>> stream/partition >>>>>>>>> as >>>>>>>>>>> idle if no data arrives after a configured timeout. >>>>>>>>>>> >>>>>>>>>>> The "unify and separate" part refers to the fact that we want to >>>>>> unify >>>>>>>>>>> punctuated and periodic assigners but at the same time split the >>>>>>>>>>> timestamp assigner from the watermark generator. >>>>>>>>>>> >>>>>>>>>>> Please find more details in the FLIP [1]. Looking forward to >>>>>>>>>>> your feedback. >>>>>>>>>>> >>>>>>>>>>> Best, >>>>>>>>>>> Aljoscha >>>>>>>>>>> >>>>>>>>>>> [1] >>>>>>>>>>> >> https://cwiki.apache.org/confluence/display/FLINK/FLIP-126%3A+Unify+%28and+separate%29+Watermark+Assigners >>>>>>>>>>> >>>>>>>>>>> [2] https://github.com/aljoscha/flink/tree/stephan-event-time >>>>>>> >>> |
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Re: [DISCUSS] FLIP-126: Unify (and separate) Watermark Assigners
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Re: [DISCUSS] FLIP-126: Unify (and separate) Watermark Assigners
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Re: [DISCUSS] FLIP-126: Unify (and separate) Watermark Assigners
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Re: [DISCUSS] FLIP-126: Unify (and separate) Watermark Assigners
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I have similar thoughts to @Stephan
Ad. 1 I tried something like this on your branch: /** * Adds the given {@link TimestampAssigner} to this {@link WatermarkStrategies}. For top-level classes that implement both Serializable and TimestampAssigner */ public <TA extends TimestampAssigner<T> & Serializable> WatermarkStrategies<T> withTimestampAssigner(TA timestampAssigner) { checkNotNull(timestampAssigner, "timestampAssigner"); this.timestampAssigner = timestampAssigner; return this; } @FunctionalInterface public interface SerializableTimestampAssigner<T> extends TimestampAssigner<T>, Serializable { } /** * Adds the given {@link TimestampAssigner} to this {@link WatermarkStrategies}. * Helper method for serializable lambdas. */ public WatermarkStrategies<T> withTimestampAssigner(SerializableTimestampAssigner<T> timestampAssigner) { checkNotNull(timestampAssigner, "timestampAssigner"); this.timestampAssigner = timestampAssigner; return this; } But I understand if that's too hacky. It's just a pity that we must enforce limitations on an interface that are not strictly necessary. Ad 2/3 I am aware the watermark assigner/timestamp extractor can be applied further down the graph. Originally I also wanted to suggest sourceTimestamp and SourceTimestampAssigner, but then I realized it can be used also after the sources as you correctly pointed out. Even if the TimestampAssigner is used after the source there might be some native/record timestamp in the StreamRecord, that could've been extracted by previous assigner. Best, Dawid On 12/05/2020 16:47, Stephan Ewen wrote: > @Aljoscha > > About (1) could we have an interface SerializableTimestampAssigner that > simply mixes in the java.io.Serializable interface? Or will this be too > clumsy? > > About (3) RecordTimeStamp seems to fit both cases (in-source-record > timestamp, in stream-record timestamp). > > On Tue, May 12, 2020 at 4:12 PM Aljoscha Krettek <[hidden email]> > wrote: > >> Definitely +1 to point 2) raised by Dawid. I'm not sure on points 1) and >> 3). >> >> 1) I can see the benefit of that but in reality most timestamp assigners >> will probably need to be Serializable. If you look at my (updated) POC >> branch [1] you can see how a TimestampAssigner would be specified on the >> WatermarkStrategies helper class: [2]. The signature of this would have >> to be changed to something like: >> >> public <TA extends TimestampAssigner<T> & Serializable> >> WatermarkStrategies<T> withTimestampAssigner(TA timestampAssigner) >> >> Then, however, it would not be possible for users to specify a lambda or >> anonymous inner function for the TimestampAssigner like this: >> >> WatermarkStrategy<Long> testWmStrategy = WatermarkStrategies >> .forGenerator(new PeriodicTestWatermarkGenerator()) >> .withTimestampAssigner((event, timestamp) -> event) >> .build(); >> >> 3) This makes sense if we only allow WatermarkStrategies on sources, >> where the previous timestamp really is the "native" timestamp. >> Currently, we also allow setting watermark strategies at arbitrary >> points in the graph. I'm thinking we probably should only allow that in >> sources but it's not the reality currently. I'm not against renaming it, >> just voicing those thoughts. >> >> Best, >> Aljoscha >> >> >> [1] https://github.com/aljoscha/flink/tree/flink-xxx-wm-generators-rebased >> [2] >> >> https://github.com/aljoscha/flink/blob/flink-xxx-wm-generators-rebased/flink-core/src/main/java/org/apache/flink/api/common/eventtime/WatermarkStrategies.java#L81 >> >> On 12.05.20 15:48, Stephan Ewen wrote: >>> +1 to all of Dawid's suggestions, makes a lot of sense to me >>> >>> On Tue, May 12, 2020 at 2:32 PM Dawid Wysakowicz <[hidden email] >>> >>> wrote: >>> >>>> Hi Aljoscha, >>>> >>>> Sorry for adding comments during the vote, but I have some really minor >>>> suggestions that should not influence the voting thread imo. >>>> >>>> 1) Does it make sense to have the TimestampAssigner extend from Flink's >>>> Function? This implies it has to be serializable which with the factory >>>> pattern is not strictly necessary, right? BTW I really like that you >>>> suggested the FunctionInterface annotation there. >>>> >>>> 2) Could we rename the IdentityTimestampAssigner to e.g. >>>> >> RecordTimestampAssigner/SystemTimestampAssigner/NativeTimestampAssigner... >>>> Personally I found the IdentityTimestampAssigner a bit misleading as it >>>> usually mean a no-op. Which did not click for me, as I assumed it >>>> somehow returns the incoming record itself. >>>> >>>> 3) Could we rename the second parameter of TimestampAssigner#extract to >>>> e.g. recordTimestamp/nativeTimestamp? This is similar to the point >>>> above. This parameter was also a bit confusing for me as I thought at >>>> times its somehow related to >>>> TimerService#currentProcessingTimestamp()/currentWatermark() as the >>>> whole system currentTimestamp. >>>> >>>> Other than those three points I like the proposal and I was about to >>>> vote +1 if it was not for those three points. >>>> >>>> Best, >>>> >>>> Dawid >>>> >>>> On 11/05/2020 16:57, Jark Wu wrote: >>>>> Thanks for the explanation. I like the fatory pattern to make the >> member >>>>> variables immutable and final. >>>>> >>>>> So +1 to the proposal. >>>>> >>>>> Best, >>>>> Jark >>>>> >>>>> On Mon, 11 May 2020 at 22:01, Stephan Ewen <[hidden email]> wrote: >>>>> >>>>>> I am fine with that. >>>>>> >>>>>> Much of the principles seem agreed upon. I understand the need to >>>> support >>>>>> code-generated extractors and we should support most of it already (as >>>>>> Aljoscha mentioned via the factories) can extend this if needed. >>>>>> >>>>>> I think that the factory approach supports code-generated extractors >> in >>>> a >>>>>> cleaner way even than an extractor with an open/init method. >>>>>> >>>>>> >>>>>> On Mon, May 11, 2020 at 3:38 PM Aljoscha Krettek <[hidden email] >>>>>> wrote: >>>>>> >>>>>>> We're slightly running out of time. I would propose we vote on the >>>> basic >>>>>>> principle and remain open to later additions. This feature is quite >>>>>>> important to make the new Kafka Source that is developed as part of >>>>>>> FLIP-27 useful. Otherwise we would have to use the legacy interfaces >> in >>>>>>> the newly added connector. >>>>>>> >>>>>>> I know that's a bit unorthodox but would everyone be OK with what's >>>>>>> currently there and then we iterate? >>>>>>> >>>>>>> Best, >>>>>>> Aljoscha >>>>>>> >>>>>>> On 11.05.20 13:57, Aljoscha Krettek wrote: >>>>>>>> Ah, I meant to write this in my previous email, sorry about that. >>>>>>>> >>>>>>>> The WatermarkStrategy, which is basically a factory for a >>>>>>>> WatermarkGenerator is the replacement for the open() method. This is >>>>>> the >>>>>>>> same strategy that was followed for StreamOperatorFactory, which was >>>>>>>> introduced to allow code generation in the Table API [1]. If we need >>>>>>>> metrics or other things we would add that as a parameter to the >>>> factory >>>>>>>> method. What do you think? >>>>>>>> >>>>>>>> Best, >>>>>>>> Aljoscha >>>>>>>> >>>>>>>> [1] https://issues.apache.org/jira/browse/FLINK-11974 >>>>>>>> >>>>>>>> On 10.05.20 05:07, Jark Wu wrote: >>>>>>>>> Hi, >>>>>>>>> >>>>>>>>> Regarding to the `open()/close()`, I think it's necessary for >>>>>>>>> Table&SQL to >>>>>>>>> compile the generated code. >>>>>>>>> In Table&SQL, the watermark strategy and event-timestamp is defined >>>>>>> using >>>>>>>>> SQL expressions, we will >>>>>>>>> translate and generate Java code for the expressions. If we have >>>>>>>>> `open()/close()`, we don't need lazy initialization. >>>>>>>>> Besides that, I can see a need to report some metrics, e.g. the >>>>>> current >>>>>>>>> watermark, the dirty timestamps (null value), etc. >>>>>>>>> So I think a simple `open()/close()` with a context which can get >>>>>>>>> MetricGroup is nice and not complex for the first version. >>>>>>>>> >>>>>>>>> Best, >>>>>>>>> Jark >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>> On Sun, 10 May 2020 at 00:50, Stephan Ewen <[hidden email]> >> wrote: >>>>>>>>>> Thanks, Aljoscha, for picking this up. >>>>>>>>>> >>>>>>>>>> I agree with the approach of doing the here proposed set of >> changes >>>>>> for >>>>>>>>>> now. It already makes things simpler and adds idleness support >>>>>>>>>> everywhere. >>>>>>>>>> >>>>>>>>>> Rich functions and state always add complexity, let's do this in a >>>>>> next >>>>>>>>>> step, if we have a really compelling case. >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> On Wed, Apr 29, 2020 at 7:24 PM Aljoscha Krettek < >>>>>> [hidden email]> >>>>>>>>>> wrote: >>>>>>>>>> >>>>>>>>>>> Regarding the WatermarkGenerator (WG) interface itself. The >>>> proposal >>>>>>> is >>>>>>>>>>> basically to turn emitting into a "flatMap", we give the >>>>>>>>>>> WatermarkGenerator a "collector" (the WatermarkOutput) and the WG >>>>>> can >>>>>>>>>>> decide whether to output a watermark or not and can also mark the >>>>>>>>>>> output >>>>>>>>>>> as idle. Changing the interface to return a Watermark (as the >>>>>> previous >>>>>>>>>>> watermark assigner interface did) would not allow that >> flexibility. >>>>>>>>>>> Regarding checkpointing the watermark and keeping track of the >>>>>> minimum >>>>>>>>>>> watermark, this would be the responsibility of the framework (or >>>> the >>>>>>>>>>> KafkaConsumer in the current implementation). The user-supplied >> WG >>>>>>> does >>>>>>>>>>> not need to make sure the watermark doesn't regress. >>>>>>>>>>> >>>>>>>>>>> Regarding making the WG a "rich function", I can see the >> potential >>>>>>>>>>> benefit but I also see a lot of pitfalls. For example, how should >>>>>> the >>>>>>>>>>> watermark state be handled in the case of scale-in? It could be >>>> made >>>>>>> to >>>>>>>>>>> work in the Kafka case by attaching the state to the partition >>>> state >>>>>>>>>>> that we keep, but then we have potential backwards compatibility >>>>>>>>>>> problems also for the WM state. Does the WG usually need to keep >>>> the >>>>>>>>>>> state or might it be enough if the state is transient, i.e. if >> you >>>>>>> have >>>>>>>>>>> a restart the WG would loose its histogram but it would rebuild >> it >>>>>>>>>>> quickly and you would get back to the same steady state as >> before. >>>>>>>>>>> Best, >>>>>>>>>>> Aljoscha >>>>>>>>>>> >>>>>>>>>>> On 27.04.20 12:12, David Anderson wrote: >>>>>>>>>>>> Overall I like this proposal; thanks for bringing it forward, >>>>>>>>>>>> Aljoscha. >>>>>>>>>>>> >>>>>>>>>>>> I also like the idea of making the Watermark generator a rich >>>>>>> function >>>>>>>>>> -- >>>>>>>>>>>> this should make it more straightforward to implement smarter >>>>>>>>>>>> watermark >>>>>>>>>>>> generators. Eg, one that uses state to keep statistics about the >>>>>>>>>>>> actual >>>>>>>>>>>> out-of-orderness, and uses those statistics to implement a >>>> variable >>>>>>>>>>> delay. >>>>>>>>>>>> David >>>>>>>>>>>> >>>>>>>>>>>> On Mon, Apr 27, 2020 at 11:44 AM Kostas Kloudas < >>>>>> [hidden email]> >>>>>>>>>>> wrote: >>>>>>>>>>>>> Hi Aljoscha, >>>>>>>>>>>>> >>>>>>>>>>>>> Thanks for opening the discussion! >>>>>>>>>>>>> >>>>>>>>>>>>> I have two comments on the FLIP: >>>>>>>>>>>>> 1) we could add lifecycle methods to the Generator, i.e. >> open()/ >>>>>>>>>>>>> close(), probably with a Context as argument: I have not fully >>>>>>>>>>>>> thought >>>>>>>>>>>>> this through but I think that this is more aligned with the >> rest >>>>>> of >>>>>>>>>>>>> our rich functions. In addition, it will allow, for example, to >>>>>>>>>>>>> initialize the Watermark value, if we decide to checkpoint the >>>>>>>>>>>>> watermark (see [1]) (I also do not know if Table/SQL needs to >> do >>>>>>>>>>>>> anything in the open()). >>>>>>>>>>>>> 2) aligned with the above, and with the case where we want to >>>>>>>>>>>>> checkpoint the watermark in mind, I am wondering about how we >>>>>> could >>>>>>>>>>>>> implement this in the future. In the FLIP, it is proposed to >>>>>> expose >>>>>>>>>>>>> the WatermarkOutput in the methods of the WatermarkGenerator. >>>>>> Given >>>>>>>>>>>>> that there is the implicit contract that watermarks are >>>>>>>>>>>>> non-decreasing, the WatermarkOutput#emitWatermark() will have >> (I >>>>>>>>>>>>> assume) a check that will compare the last emitted WM against >> the >>>>>>>>>>>>> provided one, and emit it only if it is >=. If not, then we >> risk >>>>>>>>>>>>> having the user shooting himself on the foot if he/she >>>>>> accidentally >>>>>>>>>>>>> forgets the check. Given that the WatermarkGenerator and its >>>>>>>>>>>>> caller do >>>>>>>>>>>>> not know if the watermark was finally emitted or not (the >>>>>>>>>>>>> WatermarkOutput#emitWatermark returns void), who will be >>>>>> responsible >>>>>>>>>>>>> for checkpointing the WM? >>>>>>>>>>>>> >>>>>>>>>>>>> Given this, why not having the methods as: >>>>>>>>>>>>> >>>>>>>>>>>>> public interface WatermarkGenerator<T> { >>>>>>>>>>>>> >>>>>>>>>>>>> Watermark onEvent(T event, long eventTimestamp, >>>>>>> WatermarkOutput >>>>>>>>>>>>> output); >>>>>>>>>>>>> >>>>>>>>>>>>> Watermark onPeriodicEmit(WatermarkOutput output); >>>>>>>>>>>>> } >>>>>>>>>>>>> >>>>>>>>>>>>> and the caller will be the one enforcing any invariants, such >> as >>>>>>>>>>>>> non-decreasing watermarks. In this way, the caller can >> checkpoint >>>>>>>>>>>>> anything that is needed as it will have complete knowledge as >> to >>>>>> if >>>>>>>>>>>>> the WM was emitted or not. >>>>>>>>>>>>> >>>>>>>>>>>>> What do you think? >>>>>>>>>>>>> >>>>>>>>>>>>> Cheers, >>>>>>>>>>>>> Kostas >>>>>>>>>>>>> >>>>>>>>>>>>> [1] https://issues.apache.org/jira/browse/FLINK-5601 >>>>>>>>>>>>> >>>>>>>>>>>>> On Tue, Apr 21, 2020 at 2:25 PM Timo Walther < >> [hidden email] >>>>>>>>>>> wrote: >>>>>>>>>>>>>> Thanks for the proposal Aljoscha. This is a very useful >>>>>>> unification. >>>>>>>>>> We >>>>>>>>>>>>>> have considered this FLIP already in the interfaces for >> FLIP-95 >>>>>> [1] >>>>>>>>>> and >>>>>>>>>>>>>> look forward to update to the new unified watermark generators >>>>>> once >>>>>>>>>>>>>> FLIP-126 has been accepted. >>>>>>>>>>>>>> >>>>>>>>>>>>>> Regards, >>>>>>>>>>>>>> Timo >>>>>>>>>>>>>> >>>>>>>>>>>>>> [1] https://github.com/apache/flink/pull/11692 >>>>>>>>>>>>>> >>>>>>>>>>>>>> On 20.04.20 18:10, Aljoscha Krettek wrote: >>>>>>>>>>>>>>> Hi Everyone! >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> We would like to start a discussion on "FLIP-126: Unify (and >>>>>>>>>> separate) >>>>>>>>>>>>>>> Watermark Assigners" [1]. This work was started by Stephan in >>>> an >>>>>>>>>>>>>>> experimental branch. I expanded on that work to provide a PoC >>>>>> for >>>>>>>>>> the >>>>>>>>>>>>>>> changes proposed in this FLIP: [2]. >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Currently, we have two different flavours of Watermark >>>>>>>>>>>>>>> Assigners: AssignerWithPunctuatedWatermarks >>>>>>>>>>>>>>> and AssignerWithPeriodicWatermarks. Both of them extend >>>>>>>>>>>>>>> from TimestampAssigner. This means that sources that want to >>>>>>>>>>>>>>> support >>>>>>>>>>>>>>> watermark assignment/extraction in the source need to support >>>>>> two >>>>>>>>>>>>>>> separate interfaces, we have two operator implementations for >>>>>> the >>>>>>>>>>>>>>> different flavours. Also, this makes features such as generic >>>>>>>>>> support >>>>>>>>>>>>>>> for idleness detection more complicated to implemented >> because >>>>>> we >>>>>>>>>>> again >>>>>>>>>>>>>>> have to support two types of watermark assigners. >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> In this FLIP we propose two things: >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Unify the Watermark Assigners into one Interface >>>>>>> WatermarkGenerator >>>>>>>>>>>>>>> Separate this new interface from the TimestampAssigner >>>>>>>>>>>>>>> The motivation for the first is to simplify future >>>>>> implementations >>>>>>>>>> and >>>>>>>>>>>>>>> code duplication. The motivation for the second point is >> again >>>>>>> code >>>>>>>>>>>>>>> deduplication, most assigners currently have to extend from >>>> some >>>>>>>>>> base >>>>>>>>>>>>>>> timestamp extractor or duplicate the extraction logic, or >> users >>>>>>>>>>>>>>> have >>>>>>>>>>> to >>>>>>>>>>>>>>> override an abstract method of the watermark assigner to >>>> provide >>>>>>>>>>>>>>> the >>>>>>>>>>>>>>> timestamp extraction logic. >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Additionally, we propose to add a generic wrapping >>>>>>>>>> WatermarkGenerator >>>>>>>>>>>>>>> that provides idleness detection, i.e. it can mark a >>>>>>>>>> stream/partition >>>>>>>>>>>>> as >>>>>>>>>>>>>>> idle if no data arrives after a configured timeout. >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> The "unify and separate" part refers to the fact that we want >>>> to >>>>>>>>>> unify >>>>>>>>>>>>>>> punctuated and periodic assigners but at the same time split >>>> the >>>>>>>>>>>>>>> timestamp assigner from the watermark generator. >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Please find more details in the FLIP [1]. Looking forward to >>>>>>>>>>>>>>> your feedback. >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Best, >>>>>>>>>>>>>>> Aljoscha >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> [1] >>>>>>>>>>>>>>> >> https://cwiki.apache.org/confluence/display/FLINK/FLIP-126%3A+Unify+%28and+separate%29+Watermark+Assigners >>>>>>>>>>>>>>> [2] >> https://github.com/aljoscha/flink/tree/stephan-event-time >>>> >> |
Re: [DISCUSS] FLIP-126: Unify (and separate) Watermark Assigners
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Re: [DISCUSS] FLIP-126: Unify (and separate) Watermark Assigners
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Thank you for the update and sorry again for chiming in so late...
Best, Dawid On 12/05/2020 18:21, Aljoscha Krettek wrote: > Yes, I am also ok with a SerializableTimestampAssigner. This only > looks a bit clumsy in the API but as a user (that uses lambdas) you > should not see this. I pushed changes for this to my branch: > https://github.com/aljoscha/flink/tree/flink-xxx-wm-generators-rebased > > And yes, recordTimestamp sounds good for the TimestampAssigner. I > admit I didn't read this well enough and only saw nativeTimestamp. > > Best, > Aljoscha > > On 12.05.20 17:16, Dawid Wysakowicz wrote: >> I have similar thoughts to @Stephan >> >> Ad. 1 I tried something like this on your branch: >> >> /** >> * Adds the given {@link TimestampAssigner} to this {@link >> WatermarkStrategies}. For top-level classes that implement both >> Serializable and TimestampAssigner >> */ >> public <TA extends TimestampAssigner<T> & Serializable> >> WatermarkStrategies<T> withTimestampAssigner(TA timestampAssigner) { >> checkNotNull(timestampAssigner, "timestampAssigner"); >> this.timestampAssigner = timestampAssigner; >> return this; >> } >> >> @FunctionalInterface >> public interface SerializableTimestampAssigner<T> extends >> TimestampAssigner<T>, Serializable { >> } >> >> /** >> * Adds the given {@link TimestampAssigner} to this {@link >> WatermarkStrategies}. >> * Helper method for serializable lambdas. >> */ >> public WatermarkStrategies<T> >> withTimestampAssigner(SerializableTimestampAssigner<T> >> timestampAssigner) { >> checkNotNull(timestampAssigner, "timestampAssigner"); >> this.timestampAssigner = timestampAssigner; >> return this; >> } >> >> But I understand if that's too hacky. It's just a pity that we must >> enforce limitations on an interface that are not strictly necessary. >> >> Ad 2/3 >> >> I am aware the watermark assigner/timestamp extractor can be applied >> further down the graph. Originally I also wanted to suggest >> sourceTimestamp and SourceTimestampAssigner, but then I realized it can >> be used also after the sources as you correctly pointed out. Even if the >> TimestampAssigner is used after the source there might be some >> native/record timestamp in the StreamRecord, that could've been >> extracted by previous assigner. >> >> Best, >> >> Dawid >> >> On 12/05/2020 16:47, Stephan Ewen wrote: >>> @Aljoscha >>> >>> About (1) could we have an interface SerializableTimestampAssigner that >>> simply mixes in the java.io.Serializable interface? Or will this be too >>> clumsy? >>> >>> About (3) RecordTimeStamp seems to fit both cases (in-source-record >>> timestamp, in stream-record timestamp). >>> >>> On Tue, May 12, 2020 at 4:12 PM Aljoscha Krettek <[hidden email]> >>> wrote: >>> >>>> Definitely +1 to point 2) raised by Dawid. I'm not sure on points >>>> 1) and >>>> 3). >>>> >>>> 1) I can see the benefit of that but in reality most timestamp >>>> assigners >>>> will probably need to be Serializable. If you look at my (updated) POC >>>> branch [1] you can see how a TimestampAssigner would be specified >>>> on the >>>> WatermarkStrategies helper class: [2]. The signature of this would >>>> have >>>> to be changed to something like: >>>> >>>> public <TA extends TimestampAssigner<T> & Serializable> >>>> WatermarkStrategies<T> withTimestampAssigner(TA timestampAssigner) >>>> >>>> Then, however, it would not be possible for users to specify a >>>> lambda or >>>> anonymous inner function for the TimestampAssigner like this: >>>> >>>> WatermarkStrategy<Long> testWmStrategy = WatermarkStrategies >>>> .forGenerator(new PeriodicTestWatermarkGenerator()) >>>> .withTimestampAssigner((event, timestamp) -> event) >>>> .build(); >>>> >>>> 3) This makes sense if we only allow WatermarkStrategies on sources, >>>> where the previous timestamp really is the "native" timestamp. >>>> Currently, we also allow setting watermark strategies at arbitrary >>>> points in the graph. I'm thinking we probably should only allow >>>> that in >>>> sources but it's not the reality currently. I'm not against >>>> renaming it, >>>> just voicing those thoughts. >>>> >>>> Best, >>>> Aljoscha >>>> >>>> >>>> [1] >>>> https://github.com/aljoscha/flink/tree/flink-xxx-wm-generators-rebased >>>> [2] >>>> >>>> https://github.com/aljoscha/flink/blob/flink-xxx-wm-generators-rebased/flink-core/src/main/java/org/apache/flink/api/common/eventtime/WatermarkStrategies.java#L81 >>>> >>>> >>>> On 12.05.20 15:48, Stephan Ewen wrote: >>>>> +1 to all of Dawid's suggestions, makes a lot of sense to me >>>>> >>>>> On Tue, May 12, 2020 at 2:32 PM Dawid Wysakowicz >>>>> <[hidden email] >>>>> >>>>> wrote: >>>>> >>>>>> Hi Aljoscha, >>>>>> >>>>>> Sorry for adding comments during the vote, but I have some really >>>>>> minor >>>>>> suggestions that should not influence the voting thread imo. >>>>>> >>>>>> 1) Does it make sense to have the TimestampAssigner extend from >>>>>> Flink's >>>>>> Function? This implies it has to be serializable which with the >>>>>> factory >>>>>> pattern is not strictly necessary, right? BTW I really like that you >>>>>> suggested the FunctionInterface annotation there. >>>>>> >>>>>> 2) Could we rename the IdentityTimestampAssigner to e.g. >>>>>> >>>> RecordTimestampAssigner/SystemTimestampAssigner/NativeTimestampAssigner... >>>> >>>>>> Personally I found the IdentityTimestampAssigner a bit misleading >>>>>> as it >>>>>> usually mean a no-op. Which did not click for me, as I assumed it >>>>>> somehow returns the incoming record itself. >>>>>> >>>>>> 3) Could we rename the second parameter of >>>>>> TimestampAssigner#extract to >>>>>> e.g. recordTimestamp/nativeTimestamp? This is similar to the point >>>>>> above. This parameter was also a bit confusing for me as I >>>>>> thought at >>>>>> times its somehow related to >>>>>> TimerService#currentProcessingTimestamp()/currentWatermark() as the >>>>>> whole system currentTimestamp. >>>>>> >>>>>> Other than those three points I like the proposal and I was about to >>>>>> vote +1 if it was not for those three points. >>>>>> >>>>>> Best, >>>>>> >>>>>> Dawid >>>>>> >>>>>> On 11/05/2020 16:57, Jark Wu wrote: >>>>>>> Thanks for the explanation. I like the fatory pattern to make the >>>> member >>>>>>> variables immutable and final. >>>>>>> >>>>>>> So +1 to the proposal. >>>>>>> >>>>>>> Best, >>>>>>> Jark >>>>>>> >>>>>>> On Mon, 11 May 2020 at 22:01, Stephan Ewen <[hidden email]> >>>>>>> wrote: >>>>>>> >>>>>>>> I am fine with that. >>>>>>>> >>>>>>>> Much of the principles seem agreed upon. I understand the need to >>>>>> support >>>>>>>> code-generated extractors and we should support most of it >>>>>>>> already (as >>>>>>>> Aljoscha mentioned via the factories) can extend this if needed. >>>>>>>> >>>>>>>> I think that the factory approach supports code-generated >>>>>>>> extractors >>>> in >>>>>> a >>>>>>>> cleaner way even than an extractor with an open/init method. >>>>>>>> >>>>>>>> >>>>>>>> On Mon, May 11, 2020 at 3:38 PM Aljoscha Krettek >>>>>>>> <[hidden email] >>>>>>>> wrote: >>>>>>>> >>>>>>>>> We're slightly running out of time. I would propose we vote on >>>>>>>>> the >>>>>> basic >>>>>>>>> principle and remain open to later additions. This feature is >>>>>>>>> quite >>>>>>>>> important to make the new Kafka Source that is developed as >>>>>>>>> part of >>>>>>>>> FLIP-27 useful. Otherwise we would have to use the legacy >>>>>>>>> interfaces >>>> in >>>>>>>>> the newly added connector. >>>>>>>>> >>>>>>>>> I know that's a bit unorthodox but would everyone be OK with >>>>>>>>> what's >>>>>>>>> currently there and then we iterate? >>>>>>>>> >>>>>>>>> Best, >>>>>>>>> Aljoscha >>>>>>>>> >>>>>>>>> On 11.05.20 13:57, Aljoscha Krettek wrote: >>>>>>>>>> Ah, I meant to write this in my previous email, sorry about >>>>>>>>>> that. >>>>>>>>>> >>>>>>>>>> The WatermarkStrategy, which is basically a factory for a >>>>>>>>>> WatermarkGenerator is the replacement for the open() method. >>>>>>>>>> This is >>>>>>>> the >>>>>>>>>> same strategy that was followed for StreamOperatorFactory, >>>>>>>>>> which was >>>>>>>>>> introduced to allow code generation in the Table API [1]. If >>>>>>>>>> we need >>>>>>>>>> metrics or other things we would add that as a parameter to the >>>>>> factory >>>>>>>>>> method. What do you think? >>>>>>>>>> >>>>>>>>>> Best, >>>>>>>>>> Aljoscha >>>>>>>>>> >>>>>>>>>> [1] https://issues.apache.org/jira/browse/FLINK-11974 >>>>>>>>>> >>>>>>>>>> On 10.05.20 05:07, Jark Wu wrote: >>>>>>>>>>> Hi, >>>>>>>>>>> >>>>>>>>>>> Regarding to the `open()/close()`, I think it's necessary for >>>>>>>>>>> Table&SQL to >>>>>>>>>>> compile the generated code. >>>>>>>>>>> In Table&SQL, the watermark strategy and event-timestamp is >>>>>>>>>>> defined >>>>>>>>> using >>>>>>>>>>> SQL expressions, we will >>>>>>>>>>> translate and generate Java code for the expressions. If we >>>>>>>>>>> have >>>>>>>>>>> `open()/close()`, we don't need lazy initialization. >>>>>>>>>>> Besides that, I can see a need to report some metrics, e.g. the >>>>>>>> current >>>>>>>>>>> watermark, the dirty timestamps (null value), etc. >>>>>>>>>>> So I think a simple `open()/close()` with a context which >>>>>>>>>>> can get >>>>>>>>>>> MetricGroup is nice and not complex for the first version. >>>>>>>>>>> >>>>>>>>>>> Best, >>>>>>>>>>> Jark >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> On Sun, 10 May 2020 at 00:50, Stephan Ewen <[hidden email]> >>>> wrote: >>>>>>>>>>>> Thanks, Aljoscha, for picking this up. >>>>>>>>>>>> >>>>>>>>>>>> I agree with the approach of doing the here proposed set of >>>> changes >>>>>>>> for >>>>>>>>>>>> now. It already makes things simpler and adds idleness support >>>>>>>>>>>> everywhere. >>>>>>>>>>>> >>>>>>>>>>>> Rich functions and state always add complexity, let's do >>>>>>>>>>>> this in a >>>>>>>> next >>>>>>>>>>>> step, if we have a really compelling case. >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> On Wed, Apr 29, 2020 at 7:24 PM Aljoscha Krettek < >>>>>>>> [hidden email]> >>>>>>>>>>>> wrote: >>>>>>>>>>>> >>>>>>>>>>>>> Regarding the WatermarkGenerator (WG) interface itself. The >>>>>> proposal >>>>>>>>> is >>>>>>>>>>>>> basically to turn emitting into a "flatMap", we give the >>>>>>>>>>>>> WatermarkGenerator a "collector" (the WatermarkOutput) and >>>>>>>>>>>>> the WG >>>>>>>> can >>>>>>>>>>>>> decide whether to output a watermark or not and can also >>>>>>>>>>>>> mark the >>>>>>>>>>>>> output >>>>>>>>>>>>> as idle. Changing the interface to return a Watermark (as the >>>>>>>> previous >>>>>>>>>>>>> watermark assigner interface did) would not allow that >>>> flexibility. >>>>>>>>>>>>> Regarding checkpointing the watermark and keeping track of >>>>>>>>>>>>> the >>>>>>>> minimum >>>>>>>>>>>>> watermark, this would be the responsibility of the >>>>>>>>>>>>> framework (or >>>>>> the >>>>>>>>>>>>> KafkaConsumer in the current implementation). The >>>>>>>>>>>>> user-supplied >>>> WG >>>>>>>>> does >>>>>>>>>>>>> not need to make sure the watermark doesn't regress. >>>>>>>>>>>>> >>>>>>>>>>>>> Regarding making the WG a "rich function", I can see the >>>> potential >>>>>>>>>>>>> benefit but I also see a lot of pitfalls. For example, how >>>>>>>>>>>>> should >>>>>>>> the >>>>>>>>>>>>> watermark state be handled in the case of scale-in? It >>>>>>>>>>>>> could be >>>>>> made >>>>>>>>> to >>>>>>>>>>>>> work in the Kafka case by attaching the state to the >>>>>>>>>>>>> partition >>>>>> state >>>>>>>>>>>>> that we keep, but then we have potential backwards >>>>>>>>>>>>> compatibility >>>>>>>>>>>>> problems also for the WM state. Does the WG usually need >>>>>>>>>>>>> to keep >>>>>> the >>>>>>>>>>>>> state or might it be enough if the state is transient, >>>>>>>>>>>>> i.e. if >>>> you >>>>>>>>> have >>>>>>>>>>>>> a restart the WG would loose its histogram but it would >>>>>>>>>>>>> rebuild >>>> it >>>>>>>>>>>>> quickly and you would get back to the same steady state as >>>> before. >>>>>>>>>>>>> Best, >>>>>>>>>>>>> Aljoscha >>>>>>>>>>>>> >>>>>>>>>>>>> On 27.04.20 12:12, David Anderson wrote: >>>>>>>>>>>>>> Overall I like this proposal; thanks for bringing it >>>>>>>>>>>>>> forward, >>>>>>>>>>>>>> Aljoscha. >>>>>>>>>>>>>> >>>>>>>>>>>>>> I also like the idea of making the Watermark generator a >>>>>>>>>>>>>> rich >>>>>>>>> function >>>>>>>>>>>> -- >>>>>>>>>>>>>> this should make it more straightforward to implement >>>>>>>>>>>>>> smarter >>>>>>>>>>>>>> watermark >>>>>>>>>>>>>> generators. Eg, one that uses state to keep statistics >>>>>>>>>>>>>> about the >>>>>>>>>>>>>> actual >>>>>>>>>>>>>> out-of-orderness, and uses those statistics to implement a >>>>>> variable >>>>>>>>>>>>> delay. >>>>>>>>>>>>>> David >>>>>>>>>>>>>> >>>>>>>>>>>>>> On Mon, Apr 27, 2020 at 11:44 AM Kostas Kloudas < >>>>>>>> [hidden email]> >>>>>>>>>>>>> wrote: >>>>>>>>>>>>>>> Hi Aljoscha, >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Thanks for opening the discussion! >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> I have two comments on the FLIP: >>>>>>>>>>>>>>> 1) we could add lifecycle methods to the Generator, i.e. >>>> open()/ >>>>>>>>>>>>>>> close(), probably with a Context as argument: I have not >>>>>>>>>>>>>>> fully >>>>>>>>>>>>>>> thought >>>>>>>>>>>>>>> this through but I think that this is more aligned with the >>>> rest >>>>>>>> of >>>>>>>>>>>>>>> our rich functions. In addition, it will allow, for >>>>>>>>>>>>>>> example, to >>>>>>>>>>>>>>> initialize the Watermark value, if we decide to >>>>>>>>>>>>>>> checkpoint the >>>>>>>>>>>>>>> watermark (see [1]) (I also do not know if Table/SQL >>>>>>>>>>>>>>> needs to >>>> do >>>>>>>>>>>>>>> anything in the open()). >>>>>>>>>>>>>>> 2) aligned with the above, and with the case where we >>>>>>>>>>>>>>> want to >>>>>>>>>>>>>>> checkpoint the watermark in mind, I am wondering about >>>>>>>>>>>>>>> how we >>>>>>>> could >>>>>>>>>>>>>>> implement this in the future. In the FLIP, it is >>>>>>>>>>>>>>> proposed to >>>>>>>> expose >>>>>>>>>>>>>>> the WatermarkOutput in the methods of the >>>>>>>>>>>>>>> WatermarkGenerator. >>>>>>>> Given >>>>>>>>>>>>>>> that there is the implicit contract that watermarks are >>>>>>>>>>>>>>> non-decreasing, the WatermarkOutput#emitWatermark() will >>>>>>>>>>>>>>> have >>>> (I >>>>>>>>>>>>>>> assume) a check that will compare the last emitted WM >>>>>>>>>>>>>>> against >>>> the >>>>>>>>>>>>>>> provided one, and emit it only if it is >=. If not, then we >>>> risk >>>>>>>>>>>>>>> having the user shooting himself on the foot if he/she >>>>>>>> accidentally >>>>>>>>>>>>>>> forgets the check. Given that the WatermarkGenerator and >>>>>>>>>>>>>>> its >>>>>>>>>>>>>>> caller do >>>>>>>>>>>>>>> not know if the watermark was finally emitted or not (the >>>>>>>>>>>>>>> WatermarkOutput#emitWatermark returns void), who will be >>>>>>>> responsible >>>>>>>>>>>>>>> for checkpointing the WM? >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Given this, why not having the methods as: >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> public interface WatermarkGenerator<T> { >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Watermark onEvent(T event, long eventTimestamp, >>>>>>>>> WatermarkOutput >>>>>>>>>>>>>>> output); >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Watermark onPeriodicEmit(WatermarkOutput output); >>>>>>>>>>>>>>> } >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> and the caller will be the one enforcing any invariants, >>>>>>>>>>>>>>> such >>>> as >>>>>>>>>>>>>>> non-decreasing watermarks. In this way, the caller can >>>> checkpoint >>>>>>>>>>>>>>> anything that is needed as it will have complete >>>>>>>>>>>>>>> knowledge as >>>> to >>>>>>>> if >>>>>>>>>>>>>>> the WM was emitted or not. >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> What do you think? >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Cheers, >>>>>>>>>>>>>>> Kostas >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> [1] https://issues.apache.org/jira/browse/FLINK-5601 >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> On Tue, Apr 21, 2020 at 2:25 PM Timo Walther < >>>> [hidden email] >>>>>>>>>>>>> wrote: >>>>>>>>>>>>>>>> Thanks for the proposal Aljoscha. This is a very useful >>>>>>>>> unification. >>>>>>>>>>>> We >>>>>>>>>>>>>>>> have considered this FLIP already in the interfaces for >>>> FLIP-95 >>>>>>>> [1] >>>>>>>>>>>> and >>>>>>>>>>>>>>>> look forward to update to the new unified watermark >>>>>>>>>>>>>>>> generators >>>>>>>> once >>>>>>>>>>>>>>>> FLIP-126 has been accepted. >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> Regards, >>>>>>>>>>>>>>>> Timo >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> [1] https://github.com/apache/flink/pull/11692 >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> On 20.04.20 18:10, Aljoscha Krettek wrote: >>>>>>>>>>>>>>>>> Hi Everyone! >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> We would like to start a discussion on "FLIP-126: >>>>>>>>>>>>>>>>> Unify (and >>>>>>>>>>>> separate) >>>>>>>>>>>>>>>>> Watermark Assigners" [1]. This work was started by >>>>>>>>>>>>>>>>> Stephan in >>>>>> an >>>>>>>>>>>>>>>>> experimental branch. I expanded on that work to >>>>>>>>>>>>>>>>> provide a PoC >>>>>>>> for >>>>>>>>>>>> the >>>>>>>>>>>>>>>>> changes proposed in this FLIP: [2]. >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> Currently, we have two different flavours of Watermark >>>>>>>>>>>>>>>>> Assigners: AssignerWithPunctuatedWatermarks >>>>>>>>>>>>>>>>> and AssignerWithPeriodicWatermarks. Both of them extend >>>>>>>>>>>>>>>>> from TimestampAssigner. This means that sources that >>>>>>>>>>>>>>>>> want to >>>>>>>>>>>>>>>>> support >>>>>>>>>>>>>>>>> watermark assignment/extraction in the source need to >>>>>>>>>>>>>>>>> support >>>>>>>> two >>>>>>>>>>>>>>>>> separate interfaces, we have two operator >>>>>>>>>>>>>>>>> implementations for >>>>>>>> the >>>>>>>>>>>>>>>>> different flavours. Also, this makes features such as >>>>>>>>>>>>>>>>> generic >>>>>>>>>>>> support >>>>>>>>>>>>>>>>> for idleness detection more complicated to implemented >>>> because >>>>>>>> we >>>>>>>>>>>>> again >>>>>>>>>>>>>>>>> have to support two types of watermark assigners. >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> In this FLIP we propose two things: >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> Unify the Watermark Assigners into one Interface >>>>>>>>> WatermarkGenerator >>>>>>>>>>>>>>>>> Separate this new interface from the TimestampAssigner >>>>>>>>>>>>>>>>> The motivation for the first is to simplify future >>>>>>>> implementations >>>>>>>>>>>> and >>>>>>>>>>>>>>>>> code duplication. The motivation for the second point is >>>> again >>>>>>>>> code >>>>>>>>>>>>>>>>> deduplication, most assigners currently have to extend >>>>>>>>>>>>>>>>> from >>>>>> some >>>>>>>>>>>> base >>>>>>>>>>>>>>>>> timestamp extractor or duplicate the extraction logic, or >>>> users >>>>>>>>>>>>>>>>> have >>>>>>>>>>>>> to >>>>>>>>>>>>>>>>> override an abstract method of the watermark assigner to >>>>>> provide >>>>>>>>>>>>>>>>> the >>>>>>>>>>>>>>>>> timestamp extraction logic. >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> Additionally, we propose to add a generic wrapping >>>>>>>>>>>> WatermarkGenerator >>>>>>>>>>>>>>>>> that provides idleness detection, i.e. it can mark a >>>>>>>>>>>> stream/partition >>>>>>>>>>>>>>> as >>>>>>>>>>>>>>>>> idle if no data arrives after a configured timeout. >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> The "unify and separate" part refers to the fact that >>>>>>>>>>>>>>>>> we want >>>>>> to >>>>>>>>>>>> unify >>>>>>>>>>>>>>>>> punctuated and periodic assigners but at the same time >>>>>>>>>>>>>>>>> split >>>>>> the >>>>>>>>>>>>>>>>> timestamp assigner from the watermark generator. >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> Please find more details in the FLIP [1]. Looking >>>>>>>>>>>>>>>>> forward to >>>>>>>>>>>>>>>>> your feedback. >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> Best, >>>>>>>>>>>>>>>>> Aljoscha >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> [1] >>>>>>>>>>>>>>>>> >>>> https://cwiki.apache.org/confluence/display/FLINK/FLIP-126%3A+Unify+%28and+separate%29+Watermark+Assigners >>>> >>>>>>>>>>>>>>>>> [2] >>>> https://github.com/aljoscha/flink/tree/stephan-event-time >>>>>> >>>> >> > |
Re: [DISCUSS] FLIP-126: Unify (and separate) Watermark Assigners
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Re: [DISCUSS] FLIP-126: Unify (and separate) Watermark Assigners
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