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https://issues.apache.org/jira/browse/FLINK-3715?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15305082#comment-15305082
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Elias Levy commented on FLINK-3715:
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As mentioned on the Google doc, this may not be flexible enough. If you have
windows where you are implementing early firing using multiple triggers, you
will want to keep the elements in the window during early firing, but purge
them at the end of the window firing.
> Move Accumulating/Discarding from Trigger to WindowOperator
> -----------------------------------------------------------
>
> Key: FLINK-3715
> URL: https://issues.apache.org/jira/browse/FLINK-3715
> Project: Flink
> Issue Type: Sub-task
> Components: Streaming
> Reporter: Aljoscha Krettek
>
> As mentioned in
> https://docs.google.com/document/d/1Xp-YBf87vLTduYSivgqWVEMjYUmkA-hyb4muX3KRl08/edit#heading=h.pyk1sn8f33q2
> we should move the decision of whether to {{PURGE}} a window upon firing
> from the {{Trigger}} to the {{WindowOperator}}. This also requires to add API
> so that the user can specify whether windows should be purged upon trigger
> firing (discarding) or kept (accumulating).
> As mentioned in the above doc, the {{Trigger}} can react with 4 results right
> now: {{CONTINUE}}, {{FIRE}}, {{PURGE}}, {{FIRE_AND_PURGE}}. The behavior of a
> trigger is not apparent if not looking at the code of the trigger, this has
> confused a number of users. With the new regime, a {{Trigger}} can just
> decide whether to {{CONTINUE}} or {{FIRE}}. The setting of
> accumulating/discarding decides whether to purge the window or keep it.
> This depends on FLINK-3714 where we introduce an "allowed lateness" setting.
> Having a choice between accumulating and discarding only makes sense with an
> allowed lateness greater zero. Otherwise no late elements could ever arrive
> that would go into the kept windows.
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