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https://issues.apache.org/jira/browse/BEAM-3568?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17139657#comment-17139657
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Beam JIRA Bot commented on BEAM-3568:
-------------------------------------
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> Overlapping sessions with zero allowed lateness due to window expiry rules
> --------------------------------------------------------------------------
>
> Key: BEAM-3568
> URL: https://issues.apache.org/jira/browse/BEAM-3568
> Project: Beam
> Issue Type: Bug
> Components: beam-model, runner-core
> Reporter: Kenneth Knowles
> Priority: P2
>
> Consider this sequence, with session gap durations of 5:
> - element arrives with timestamp 0, assigned to proto-window [0, 5)
> - watermark advances to 6, emitting the session and discarding it
> - element arrives with timestamp 3, assigned to proto-window [3, 8) so it is
> not dropped as the window is not expired
> - watermark advances to 8+, emitting that session
> While "technically correct" according to spec, this seems undesirable. It was
> introduced when late data dropping was tied to window expiry. I think either
> dropping the second element or including it and emitting a merged window
> would be OK.
> In the case of sessions, we could just retain the window until it cannot
> possibly merge with other non-expired data. Even with allowed lateness zero
> this is double the gap duration. The window would be in an interesting state
> where it would be expired and ineligible for further output but could still
> merge and the greater window could be output.
> The challenge is that sessions are just one kind of merging window - the
> merging logic has to be assumed opaque. So we cannot simply reason about how
> sessions work. The other, more drastic option, is to rethink how late data
> dropping is defined for merging windows, particularly in the "proto-window"
> phase.
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