Github user aljoscha commented on the pull request: https://github.com/apache/flink/pull/1802#issuecomment-205394604 No, we introduced the `InternalWindowFunction` before 1.0.0 to decouple the user `WindowFunction` from the window operator implementation. There are now special `InternalWindowFunctions` that don't forward the key to the user function. The API for the user does not change, there an `AllWindowFunction` is used, as before. And that function does not get a key.
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