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https://issues.apache.org/jira/browse/FLINK-24501?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17435175#comment-17435175
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Wenlong Lyu commented on FLINK-24501:
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[~qingru zhang] in you case，if the watermark can be restore，the late event is
still late event when restoring from sp/cp，because its event time is earlier
than watermark restored.
On the other hand, introducing per-key watermark state could increase the state
size of window operator in long run and affects performance, because the
progress state should never be clean up in order to recognize later event in
restore in the future. such side effect could be worse when the window size is
small because the space namespace will increase quickly.
> Unexpected behavior of cumulate window aggregate for late event after recover
> from sp/cp
> ----------------------------------------------------------------------------------------
>
> Key: FLINK-24501
> URL: https://issues.apache.org/jira/browse/FLINK-24501
> Project: Flink
> Issue Type: Bug
> Components: Table SQL / Runtime
> Reporter: JING ZHANG
> Assignee: JING ZHANG
> Priority: Major
> Labels: pull-request-available
>
> *Problem description*
> After recover from savepoint or checkpoint, unexpected behavior of cumulate
> window aggregate for late event may happened.
> *Bug analyze*
> Currently, for cumulate window aggregate, late events belongs to the cleaned
> slice would be merged into the merged window state, and would be counted into
> the later slice.
> For example, for a CUMULATE window, step is 1 minute, size is 1 day.
> {code:java}
> SELECT window_start, window_end, COUNT(USER_ID)
> FROM TABLE(
> CUMULATE(TABLE Bid, DESCRIPTOR(bidtime), INTERVAL '1' MINUTES, INTERVAL
> '1' DAY))
> GROUP BY window_start, window_end;{code}
> When the watermark already comes to 11:01, result of window [00:00, 11:01)
> would be emitted. Let's assume the result is INSERT (00:00, 11:01, 4)
> Then if a late record which event time is 11:00 comes, it would be merged
> into merged state, and would be counted into the later slice, for example,
> for window [00:00, 11:02), [00:00, 11:03)... But the emitted window result
> INSERT (00:00, 11:01, 4) would not be retracted and updated.
> The behavior would be different if the job recover from savepoint/checkpoint.
> Let's do a savepoint after watermark comes to 11:01 and emit (00:00, 11:01,
> 4).
> Then recover the job from savepoint. Watermarks are not checkpointed and they
> need to be repopulated again. So after recovered, the watermark may rollback
> to 11:00, then if a record which event time is 11:00 comes, it would not be
> processed as late event, after watermark comes to 11:01 again, a window
> result INSERT (00:00, 11:01, 5) would be emitted to downstream.
> So the downstream operator would receive two INSERT record for WINDOW (00:00,
> 11:01) which may leads to wrong result.
>
> *Solution*
> There are two solutions for the problem:
> # save watermark to state in slice shared operator. (Prefered)
> # update the behavior for late event. For example, retract the emitted
> result and send the updated result. It needs to change the behavior of slice
> state clean mechanism because we clean the slice state after watermark
> exceeds the slice end currently.
>
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