[jira] [Closed] (FLINK-2883) Add documentation to forbid key-modifying ReduceFunction

["Greg Hogan (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/FLINK-2883?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Greg Hogan closed FLINK-2883.
-----------------------------
    Resolution: Implemented

1.2: 37d6f8196f12714491c95adb73319bb36e7a0d34
master: 11ebf484280314231d146dcfb0b973934448f00b

> Add documentation to forbid key-modifying ReduceFunction
> --------------------------------------------------------
>
>                 Key: FLINK-2883
>                 URL: https://issues.apache.org/jira/browse/FLINK-2883
>             Project: Flink
>          Issue Type: Task
>          Components: DataStream API, Documentation
>    Affects Versions: 0.10.0
>            Reporter: Till Rohrmann
>            Assignee: Greg Hogan
>             Fix For: 1.3.0, 1.2.1
>
>
> If one uses a combinable reduce operation which also changes the key value of the underlying data element, then the results of the reduce operation can become wrong. The reason is that after the combine phase, another reduce operator is executed which will then reduce the elements based on the new key values. This might be not so surprising if one explicitly defined ones {{GroupReduceOperation}} as combinable. However, the {{ReduceFunction}} conceals the fact that a combiner is used implicitly. Furthermore, the API does not prevent the user from changing the key fields which could solve the problem.
> The following example program illustrates the problem
> {code}
> val env = ExecutionEnvironment.getExecutionEnvironment
> env.setParallelism(1)
> val input = env.fromElements((1,2), (1,3), (2,3), (3,3), (3,4))
> val result = input.groupBy(0).reduce{
>   (left, right) =>
>     (left._1 + right._1, left._2 + right._2)
> }
> result.output(new PrintingOutputFormat[Int]())
> env.execute()
> {code}
> The expected output is 
> {code}
> (2, 5)
> (2, 3)
> (6, 7)
> {code}
> However, the actual output is
> {code}
> (4, 8)
> (6, 7)
> {code}
> I think that the underlying problem is that associativity and commutativity is not sufficient for a combinable reduce operation. Additionally we also need to make sure that the key stays the same.



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