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

["Robert Metzger (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/FLINK-6394?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Robert Metzger updated FLINK-6394:
----------------------------------
    Component/s: DataSet API

> 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
>          Components: DataSet API
>    Affects Versions: 1.2.0
>            Reporter: Jaromir Vanek
>            Assignee: Kurt Young
>            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 of view it's highly critical as I am getting unpredictable results.



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