[jira] [Created] (FLINK-16039) Add API method to get last element in session window

["Manas Kale (Jira)" <ji...@apache.org>]

Manas Kale created FLINK-16039:
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             Summary: Add API method to get last element in session window
                 Key: FLINK-16039
                 URL: https://issues.apache.org/jira/browse/FLINK-16039
             Project: Flink
          Issue Type: Improvement
          Components: API / DataStream
    Affects Versions: 1.10.0
            Reporter: Manas Kale


Consider the events : 

[1, event], [2, event]

where first element is event timestamp in seconds and second element is event code/name.

Also consider that an Event time session window with inactivityGap = 2 seconds is acting on above stream.

When the first event arrives, a session window should be created that is [1,1].

When the second event arrives, a new session window should be created that is [2,2]. Since this falls within firstWindowTimestamp+inactivityGap, it should be merged into session window [1,2] and  [2,2] should be deleted.

This is my understanding of how session windows are created. *Please correct me if wrong.*

However, Flink does not follow such a definition of windows semantically. If I call the  getEnd() method of the TimeWindow() class, I get back _timestamp + inactivityGap_.

For the above example, after processing the first element, I would get 1 + 2 = 3 seconds as the window "end".

The actual window end should be the timestamp 1, which is the last event in the session window. 

A solution would be to change the "end" definition of all windows, but I suppose this would be breaking and would need some debate.

Therefore, I propose an intermediate solution : add a new API method that keeps track of the last element added in the session window. 

If there is agreement on this, I would like to start drafting a change document and implement this. 



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