Github user aljoscha commented on the issue:
https://github.com/apache/flink/pull/2982
@chenqin
I rebased and consolidated your PR a bit and played around with the APIs.
Some of the changes:
- Separation into internal changes/window operator changes and
user-function changes. I have a prototype commit that exposes side outputs
using `ProcessFunction`.
- I removed `CollectorWrapper`/`RichCollector` in favour of the
`ProcessFunction` approach
- The internal implementation now doesn't store the `OutputTag` on the
`StreamRecord` but instead adds an additional method on `Output` that should be
used for emitting data to a side output. Side outputs now also work with
chaining.
- `WindowedStream` is changed to add a `sideOutputLateData()` method that
is used to specify that late data side output is required. This is more general
than putting it into the method signature of `apply()` because it will simply
work for all different window types.
I quickly talked to @StephanEwen and we agreed that we need to further
think about how we want to expose side outputs for user-defined functions.
Especially, we have to think about what this addition means for the
`split()`/`select()` pattern. I will also do another change where
`sideOutputLateData()` is not required but instead will be added when the
late-date stream is requested.
What do you think?
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