[jira] [Created] (FLINK-6394) GroupCombine reuses instances even though object reuse is disabled

["Jaromir Vanek (JIRA)" <ji...@apache.org>]

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#L550

When 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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