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https://issues.apache.org/jira/browse/KAFKA-2984?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16319208#comment-16319208
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Matthias J. Sax commented on KAFKA-2984:
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The question is slightly different. But the idea is somewhat similar. But for
those use cases, you should use the Processor API and not rely on DSL
operators. The DSL has specific use-cases in mind and thus provides certain
semantics. If those semantics don't fix, use Processor API.
The question does not say anything about a changelog topic... If it does work
on a single key, and the single writer principle applies, I agree that there
should not be any out-of-order data.
> KTable should send old values along with new values to downstreams
> ------------------------------------------------------------------
>
> Key: KAFKA-2984
> URL: https://issues.apache.org/jira/browse/KAFKA-2984
> Project: Kafka
> Issue Type: Sub-task
> Components: streams
> Affects Versions: 0.10.0.0
> Reporter: Yasuhiro Matsuda
> Assignee: Yasuhiro Matsuda
> Fix For: 0.10.0.0
>
>
> Old values are necessary for implementing aggregate functions. KTable should
> augment an event with its old value. Basically KTable stream is a stream of
> (key, (new value, old value)) internally. The old value may be omitted when
> it is not used in the topology.
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