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https://issues.apache.org/jira/browse/BEAM-7520?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16907502#comment-16907502
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Kenneth Knowles commented on BEAM-7520:
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... and that new timer will be committed as part of the result first, but will 
be fired right away in the next bundle. This is incorrect because of the 
example you showed.

> DirectRunner timers are not strictly time ordered
> -------------------------------------------------
>
>                 Key: BEAM-7520
>                 URL: https://issues.apache.org/jira/browse/BEAM-7520
>             Project: Beam
>          Issue Type: Bug
>          Components: runner-direct
>    Affects Versions: 2.13.0
>            Reporter: Jan Lukavský
>            Assignee: Jan Lukavský
>            Priority: Major
>          Time Spent: 4h 10m
>  Remaining Estimate: 0h
>
> Let's suppose we have the following situation:
>  - statful ParDo with two timers - timerA and timerB
>  - timerA is set for window.maxTimestamp() + 1
>  - timerB is set anywhere between <windowStart, windowEnd), let's denote that 
> timerB.timestamp
>  - input watermark moves to BoundedWindow.TIMESTAMP_MAX_VALUE
> Then the order of timers is as follows (correct):
>  - timerB
>  - timerA
> But, if timerB sets another timer (say for timerB.timestamp + 1), then the 
> order of timers will be:
>  - timerB (timerB.timestamp)
>  - timerA (BoundedWindow.TIMESTAMP_MAX_VALUE)
>  - timerB (timerB.timestamp + 1)
> Which is not ordered by timestamp. The reason for this is that when the input 
> watermark update is evaluated, the WatermarkManager,extractFiredTimers() will 
> produce both timerA and timerB. That would be correct, but when timerB sets 
> another timer, that breaks this.



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