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https://issues.apache.org/jira/browse/KAFKA-12317?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17818138#comment-17818138
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Florin Akermann commented on KAFKA-12317:
-----------------------------------------
Thanks for the flag!
> Would you be interested to do a follow up PR to update the docs?
Yes.
> Relax non-null key requirement for left/outer KStream joins
> -----------------------------------------------------------
>
> Key: KAFKA-12317
> URL: https://issues.apache.org/jira/browse/KAFKA-12317
> Project: Kafka
> Issue Type: Improvement
> Components: streams
> Reporter: Matthias J. Sax
> Assignee: Florin Akermann
> Priority: Major
> Labels: kip
>
> Currently, for a stream-streams and stream-table/globalTable join
> KafkaStreams drops all stream records with a `null`{-}key (`null`-join-key
> for stream-globalTable), because for a `null`{-}(join)key the join is
> undefined: ie, we don't have an attribute the do the table lookup (we
> consider the stream-record as malformed). Note, that we define the semantics
> of _left/outer_ join as: keep the stream record if no matching join record
> was found.
> We could relax the definition of _left_ stream-table/globalTable and
> _left/outer_ stream-stream join though, and not drop `null`-(join)key stream
> records, and call the ValueJoiner with a `null` "other-side" value instead:
> if the stream record key (or join-key) is `null`, we could treat is as
> "failed lookup" instead of treating the stream record as corrupted.
> If we make this change, users that want to keep the current behavior, can add
> a `filter()` before the join to drop `null`-(join)key records from the stream
> explicitly.
> Note that this change also requires to change the behavior if we insert a
> repartition topic before the join: currently, we drop `null`-key record
> before writing into the repartition topic (as we know they would be dropped
> later anyway). We need to relax this behavior for a left stream-table and
> left/outer stream-stream join. User need to be aware (ie, we might need to
> put this into the docs and JavaDocs), that records with `null`-key would be
> partitioned randomly.
> KIP-962:
> [https://cwiki.apache.org/confluence/display/KAFKA/KIP-962%3A+Relax+non-null+key+requirement+in+Kafka+Streams]
>
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