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https://issues.apache.org/jira/browse/KAFKA-4217?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16149928#comment-16149928
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Richard Yu commented on KAFKA-4217:
-----------------------------------
[~guozhang] The flatTransform method that you mentioned earlier suggests that
there is no mapper in the the method input
parameters. Regardless, the method inherently needs a mapper (which implements
KeyValueMapper) for it to work.
Should there be a default mapper, or is one given to the method?
> 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
> Labels: api, needs-kip, newbie
>
> {{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.
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