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https://issues.apache.org/jira/browse/KAFKA-12317?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Matthias J. Sax updated KAFKA-12317:
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Summary: Relax non-null key requirement for left KStream joins (was: Relax
non-null key requirement for left KStream-KTable join)
> Relax non-null key requirement for left 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
> Priority: Major
>
> Currently, for a stream-table join KafkaStreams drops all stream records with
> a null-key, because for a null-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_ join as: keep the
> stream record if no KTable record was found.
> We could relax the definition of _left_ join though, and not drop non-key
> stream records, and call the ValueJoiner with a `null` table record instead:
> if the stream record key is `null`, we could treat is as "failed table
> 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`-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/outer stream-table
> (and maybe left/outer
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