Github user StephanEwen commented on the pull request: https://github.com/apache/flink/pull/1562#issuecomment-191683975 Okay, that sounds like a design flaw in hiding the aggregating functions completely from the window operator in the state. I think the operator should be aware of the additional function, if only to open()/close() it. The fact that a RichReduceFunction cannot be used at all seems like a huge limitation.
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