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Date: Thu, 24 Aug 2017 21:03:01 +0000 (UTC)
From: "Matthias J. Sax (JIRA)" <jira@apache.org>
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Subject: [jira] [Commented] (KAFKA-4217) KStream.transform equivalent of
 flatMap
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archived-at: Thu, 24 Aug 2017 21:03:07 -0000


    [ https://issues.apache.org/jira/browse/KAFKA-4217?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16140694#comment-16140694 ] 

Matthias J. Sax commented on KAFKA-4217:
----------------------------------------

[~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&mdash;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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