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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