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https://issues.apache.org/jira/browse/KAFKA-4217?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16140832#comment-16140832
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Guozhang Wang commented on KAFKA-4217:
--------------------------------------
The issue with the current recommended {{forward}} call is that it looses
strong typing. I.e. if users forward a <key, value> where value type is not R
it would only error-out at runtime. Thus this JIRA was maintained as a reminder
to improve on this, such that if we have a new API added we can then updated
the java doc such that {{forward}} can be used but not recommended because of
strong typing.
As for [~Yohan123]'s question, the original purpose was to add a
`flatTransform` and `flatTransformValues` functions that gives the following
API:
{code}
public <K1, V1> KStream<K1, V1> flatTransform(final TransformerSupplier<? super
K, ? super V, ? extends Iterable<? extends KeyValue<? extends K1, ? extends
V1>>> transformerSupplier, ...)
public <V1> KStream<K, V1> flatTransform(final ValueTransformerSupplier<? super
V, ? extends Iterable<? extends V1>> valueTransformerSupplier, ...)
{code}
> 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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