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https://issues.apache.org/jira/browse/FLINK-4994?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15834541#comment-15834541
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Aljoscha Krettek commented on FLINK-4994:
-----------------------------------------
Implemented on release-1.2 in:
a9189c2055fd3c10d741f63208aadca7fb4218f5
> Don't Clear Trigger State and Merging Window Set When Purging
> -------------------------------------------------------------
>
> Key: FLINK-4994
> URL: https://issues.apache.org/jira/browse/FLINK-4994
> Project: Flink
> Issue Type: Sub-task
> Components: Streaming
> Reporter: Aljoscha Krettek
> Assignee: Aljoscha Krettek
>
> Right now, when a {{Trigger}} returns {{TriggerResult.PURGE}} from any of the
> {{on*()}} methods the {{WindowOperator}} will clear all state of that window
> (window contents, merging window set) and call {{Trigger.clear()}} so that
> the {{Trigger}} can clean up its state/timers.
> This is problematic in some cases. For example, with merging windows (session
> windows) this means that a late-arriving element will not be put into the
> session that was previously built up but will be put into a completely new
> session that only contains this one element.
> The proposed new behaviour is this:
> * Only clean window contents on {{PURGE}}
> * Register cleanup timer for any window, don't delete this on {{PURGE}}
> * When the cleanup timer fires: clean window state, clean merging window
> set, call {{Trigger.clear()}} to allow it to clean state/timers
> Together with FLINK-4415 this would give the trigger the power to purge state
> and window contents separately.
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