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https://issues.apache.org/jira/browse/KAFKA-10847?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17273421#comment-17273421
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Guozhang Wang commented on KAFKA-10847:
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Also, I'm wondering if we have considered if we should use the additional store
to temporarily hold records that do not have a join, or use that store to hold
records that DO have a join result that is already emitted. With latter
approach, we can still emit results if found a match immediately, but the
tradeoff may be that in practice, the likelihood a new record would find a
matching record from the other side right upon reception could be higher than
the likelihood that no match is found, and hence the size of that store would
be larger.
> Avoid spurious left/outer join results in stream-stream join
> -------------------------------------------------------------
>
> Key: KAFKA-10847
> URL: https://issues.apache.org/jira/browse/KAFKA-10847
> Project: Kafka
> Issue Type: Improvement
> Components: streams
> Reporter: Matthias J. Sax
> Assignee: Sergio Peña
> Priority: Major
>
> KafkaStreams follows an eager execution model, ie, it never buffers input
> records but processes them right away. For left/outer stream-stream join,
> this implies that left/outer join result might be emitted before the window
> end (or window close) time is reached. Thus, a record what will be an
> inner-join result, might produce a eager (and spurious) left/outer join
> result.
> We should change the implementation of the join, to not emit eager left/outer
> join result, but instead delay the emission of such result after the window
> grace period passed.
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