Github user pnowojski commented on the issue:
https://github.com/apache/flink/pull/4626
I moved everything to `flink-streaming-contrib`.
@StephanEwen I went one step further and created following helper function:
```
public static <K, IN, ACC, OUT, W extends Window>
SingleOutputStreamOperator<OUT> aggregateWithPreAggregation(
DataStream<IN> input,
KeySelector<IN, K> keySelector,
AggregateFunction<IN, ACC, OUT> aggregateFunction,
WindowAssigner<? super IN, W> windowAssigner)
```
It adds a final aggregation step as well. With version that you proposed
user would have to implement two slightly different `windowAssigner` and
`aggregationFunction` for both the pre aggregation and final aggregation step.
This could lead to a confusion and mistakes. Those functions have to be
different because assigning windows, creating accumulators and accumulating
happens only in the pre aggregation step and it works on `INPUT` data type,
while in final aggregation we are working on a `Tuple3<KEY, WINDOW,
ACCUMULATOR>` and we perform only merging of already created windows and
accumulators.
---
