Jaromir Vanek created FLINK-6394:
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Summary: GroupCombine reuses instances even though object reuse is disabled
Key: FLINK-6394
URL:
https://issues.apache.org/jira/browse/FLINK-6394 Project: Flink
Issue Type: Bug
Affects Versions: 1.2.0
Reporter: Jaromir Vanek
Priority: Critical
I am using group combiner in DataSet API with disabled object reuse.
In code it may be expressed as follows:
{code:java}
tuples.groupBy(1)
.combineGroup((it, collector) -> {
// store first item for future use
Pojo stored = it.next();
while (it.hasNext()) {
....
}
})
{code}
It seems even the object reuse feature is disabled, my instance is actually replaced when {{.next()}} is called on the iterator. It leads to very confusing and wrong results.
I checked the Flink codebase and it seems {{CombiningUnilateralSortMerger}} is actually reusing object instances even though object reuse is explicitly disabled.
In spilling phase user's combiner is called with instance of {{CombineValueIterator}} that actually reuses instances without any warning.
See
https://github.com/apache/flink/blob/d7b59d761601baba6765bb4fc407bcd9fd6a9387/flink-runtime/src/main/java/org/apache/flink/runtime/operators/sort/CombiningUnilateralSortMerger.java#L550When I disable combiner and use {{groupReduce}} only with the same reduce function results are fine.
Please let me know if you can confirm this as a bug. From my point it's very critical as I am getting unpredictable results.
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