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https://issues.apache.org/jira/browse/FLINK-3674?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15399447#comment-15399447
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ASF GitHub Bot commented on FLINK-3674:
---------------------------------------

Github user StephanEwen commented on the issue:

    https://github.com/apache/flink/pull/2301
  
    Can we go back to the JIRA issue for a step, and first decide how we 
actually want his feature to look like?
    For API additions, it is crucial to not do "something fast", but discuss 
and understand deeply what the feature should actually be like.


> Add an interface for EventTime aware User Function
> --------------------------------------------------
>
>                 Key: FLINK-3674
>                 URL: https://issues.apache.org/jira/browse/FLINK-3674
>             Project: Flink
>          Issue Type: New Feature
>          Components: Streaming
>    Affects Versions: 1.0.0
>            Reporter: Stephan Ewen
>            Assignee: ramkrishna.s.vasudevan
>
> I suggest to add an interface that UDFs can implement, which will let them be 
> notified upon watermark updates.
> Example usage:
> {code}
> public interface EventTimeFunction {
>     void onWatermark(Watermark watermark);
> }
> public class MyMapper implements MapFunction<String, String>, 
> EventTimeFunction {
>     private long currentEventTime = Long.MIN_VALUE;
>     public String map(String value) {
>         return value + " @ " + currentEventTime;
>     }
>     public void onWatermark(Watermark watermark) {
>         currentEventTime = watermark.getTimestamp();
>     }
> }
> {code}



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