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https://issues.apache.org/jira/browse/FLINK-9857?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16545004#comment-16545004
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Aljoscha Krettek commented on FLINK-9857:
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We can fix this at one of two places:
- {{SystemProcessingTimeService}} service, by moving firing of timers back by
1 ms
- {{HeapInternalTimerService}}, which uses {{SystemProcessingTimeService}} but
is one level higher in the layer of abstractions
> Processing-time timers fire too early
> -------------------------------------
>
> Key: FLINK-9857
> URL: https://issues.apache.org/jira/browse/FLINK-9857
> Project: Flink
> Issue Type: Bug
> Components: DataStream API
> Affects Versions: 1.3.4, 1.4.2, 1.5.1, 1.6.0
> Reporter: Aljoscha Krettek
> Priority: Blocker
> Fix For: 1.5.2, 1.6.0
>
>
> The firing of processing-time timers is off by one. This leads to problems in
> edge cases, as discovered [here (mailing
> list)|https://lists.apache.org/thread.html/e49748fa5fa1c9217b9dfb65eea7a37af1f2895c769528e77a1a93fa@%3Cuser.flink.apache.org%3E]
> when elements arrive at the timestamp that is the end of the window.
> The problem is [here
> (github)|https://github.com/apache/flink/blob/79b38f8f9a79b917d525842cf46087c5b8c40f3d/flink-streaming-java/src/main/java/org/apache/flink/streaming/api/operators/HeapInternalTimerService.java#L231].
> For event-time, we fire timers when the watermark is >= the timestamp, this
> is correct because a watermark T says that we will not see elements with a
> timestamp smaller or equal to T. For processing time, a time of T does not
> say that we won't see an element with timestamp T, which makes
> processing-time timers fire one ms too early.
> I think we can fix it by turning that {{<=}} into a {{<}}.
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