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https://issues.apache.org/jira/browse/KAFKA-13197?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17400653#comment-17400653
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Guozhang Wang commented on KAFKA-13197:
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Thanks for filing this [~twbecker]. The current doc says "If {@code
keyValueMapper} returns {@code null} implying no match exists, no output record
will be added to the resulting {@code KStream}." But I read the tickets and I
think you are right: this property is not very reasonable since users may want
to know if a single stream record does not find any matching results as well.
I think the reasonable behavior (and the java doc should be updated
accordingly) in KAFKA-10277 should be
{code}
If the keyValueMapper returns null implying no matching key found, the
ValueJoiner would still be triggered with (null, v, null).
{code}
Does that sound right to you?
> KStream-GlobalKTable join semantics don't match documentation
> -------------------------------------------------------------
>
> Key: KAFKA-13197
> URL: https://issues.apache.org/jira/browse/KAFKA-13197
> Project: Kafka
> Issue Type: Bug
> Affects Versions: 2.7.0
> Reporter: Tommy Becker
> Priority: Major
>
> As part of KAFKA-10277, the behavior of KStream-GlobalKTable joins was
> changed. It appears the change was intended to merely relax a requirement but
> it actually broke backwards compatibility. Although it does allow {{null}}
> keys and values in the KStream to be joined, it now excludes {{null}} results
> of the {{KeyValueMapper}}. We have an application which can return {{null}}
> from the {{KeyValueMapper}} for non-null keys in the KStream, and relies on
> these nulls being passed to the {{ValueJoiner}}. Indeed the javadoc still
> explicitly says this is done:
> {quote}If a KStream input record key or value is null the record will not be
> included in the join operation and thus no output record will be added to the
> resulting KStream.
> If keyValueMapper returns null implying no match exists, a null value will
> be provided to ValueJoiner.
> {quote}
> Both these statements are incorrect.
> I think the new behavior is worse than the previous/documented behavior. It
> feels more reasonable to have a non-null stream record map to a null join key
> (our use-case is event-enhancement where the incoming record doesn't have the
> join field), than the reverse.
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