Piotr Nowojski commented on FLINK-8560:

[~phoenixjiangnan] why would we have to keep all of the keys in the memory? Can 
not we have in KeyedProcessOperator:
public void onEventTime(InternalTimer<K, VoidNamespace> timer) throws Exception 
   onTime(timer, TimeDomain.EVENT_TIME);

public void onProcessingTime(InternalTimer<K, VoidNamespace> timer) throws 
Exception {
   onTime(timer, TimeDomain.PROCESSING_TIME);

private void onTime(InternalTimer<K, VoidNamespace> timer, TimeDomain 
timeDomain) throws Exception 
  Supplier<K> currentKeyAccessor = () -> (K) getCurrentKey();
  onTimerContext.reinitialize(currentKeyAccessor, timeDomain, timer);
  userFunction.onTimer(timer.getTimestamp(), onTimerContext, collector);

Key would be only accessed when a user specifically asks for it, thus it 
shouldn't cost us any thing.




> Access to the current key in ProcessFunction after keyBy()
> ----------------------------------------------------------
>                 Key: FLINK-8560
>                 URL: https://issues.apache.org/jira/browse/FLINK-8560
>             Project: Flink
>          Issue Type: Wish
>          Components: DataStream API
>            Reporter: Jürgen Thomann
>            Assignee: Bowen Li
>            Priority: Minor
> Currently it is required to store the key of a keyBy() in the processElement 
> method to have access to it in the OnTimerContext.
> This is not so good as you have to check in the processElement method for 
> every element if the key is already stored and set it if it's not already set.
> A possible solution would adding OnTimerContext#getCurrentKey() or a similar 
> method. Maybe having it in the open() method could maybe work as well.
> http://apache-flink-user-mailing-list-archive.2336050.n4.nabble.com/Getting-Key-from-keyBy-in-ProcessFunction-tt18126.html

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