...

3. We add four APIs to PartitionWindowedStream,  including mapPartition, sortPartition, aggregate and reduce.

Proposed Changes

Why not support arbitrary window processing on non-keyed DataStream

If DataStream supports arbitrary window types for processing data on each subtask, it would not only support full window but also other window types such as count windows, sliding windows, and session windows. However, we have chosen to support only full window processing on DataStream. The main reason is that the DataStream is non-keyed and does not support keyed statebackend and keyed raw state. This issue results in two conflicts on the usage of windows:

...