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https://issues.apache.org/jira/browse/KAFKA-8318?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17212583#comment-17212583
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Matthias J. Sax commented on KAFKA-8318:
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Sessions are merged here:
[https://github.com/apache/kafka/blob/trunk/streams/src/main/java/org/apache/kafka/streams/kstream/internals/KStreamSessionWindowAggregate.java#L141]
> Session Window Aggregations generate an extra tombstone
> -------------------------------------------------------
>
> Key: KAFKA-8318
> URL: https://issues.apache.org/jira/browse/KAFKA-8318
> Project: Kafka
> Issue Type: Improvement
> Components: streams
> Reporter: John Roesler
> Assignee: Ilia Pasynkov
> Priority: Minor
> Labels: newbie++
>
> See the discussion
> https://github.com/apache/kafka/pull/6654#discussion_r280231439
> The session merging logic generates a tombstone in addition to an update when
> the session window already exists. It's not a correctness issue, just a small
> performance hit, because that tombstone is immediately invalidated by the
> update.
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