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https://issues.apache.org/jira/browse/KAFKA-4217?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16753295#comment-16753295
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Matthias J. Sax commented on KAFKA-4217:
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Using KAFKA-7875 to track the second part of KIP-313
> KStream.transform equivalent of flatMap
> ---------------------------------------
>
> Key: KAFKA-4217
> URL: https://issues.apache.org/jira/browse/KAFKA-4217
> Project: Kafka
> Issue Type: Improvement
> Components: streams
> Affects Versions: 0.10.0.1
> Reporter: Elias Levy
> Assignee: Bruno Cadonna
> Priority: Major
> Labels: api, newbie
> Fix For: 2.2.0
>
>
> {{KStream.transform}} gives you access to state stores while allowing you to
> return zero or one transformed {{KeyValue}}. Alas, it is unclear what method
> you should use if you want to access state stores and return zero or multiple
> {{KeyValue}}. Presumably you can use {{transform}}, always return {{null}},
> and use {{ProcessorContext.forward}} to emit {{KeyValues}}.
> It may be good to introduce a {{transform}}-like {{flatMap}} equivalent, or
> allow store access from other {{KStream}} methods, such as {{flatMap}} itself.
> KIP:
> [https://cwiki.apache.org/confluence/display/KAFKA/KIP-313%3A+Add+KStream.flatTransform+and+KStream.flatTransformValues]
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