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https://issues.apache.org/jira/browse/KAFKA-4217?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16140694#comment-16140694
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Matthias J. Sax commented on KAFKA-4217:
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[~minfing] I guess this is TBD and should be part of a KIP discussion.
However, I am not sure if we should add something like this in the first place.
The current JavaDocs of {{Transformer}} do explain how to use it already:
{noformat}
/**
* Transform the record with the given key and value.
* Additionally, any {@link StateStore state} that is {@link
KStream#transform(TransformerSupplier, String...)
* attached} to this operator can be accessed and modified
* arbitrarily (cf. {@link ProcessorContext#getStateStore(String)}).
* <p>
* If more than one output record should be forwarded downstream {@link
ProcessorContext#forward(Object, Object)},
* {@link ProcessorContext#forward(Object, Object, int)}, and
* {@link ProcessorContext#forward(Object, Object, String)} can be used.
* If not record should be forwarded downstream, {@code transform} can
return {@code null}.
*
* @param key the key for the record
* @param value the value for the record
* @return new {@link KeyValue} pair—if {@code null} no key-value
pair will
* be forwarded to down stream
*/
R transform(final K key, final V value);
{noformat}
IMHO, we can just close this issue as "not an issue". [~elevy] [~guozhang] WDYT?
> 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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