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https://issues.apache.org/jira/browse/KAFKA-16434?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17831702#comment-17831702
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Ayoub Omari commented on KAFKA-16434:
-------------------------------------
I submitted a bug fix for this in the same PR as KAFKA-16407:
https://github.com/apache/kafka/pull/15615
> ForeignKey INNER join does not unset join result when FK becomes null
> ---------------------------------------------------------------------
>
> Key: KAFKA-16434
> URL: https://issues.apache.org/jira/browse/KAFKA-16434
> Project: Kafka
> Issue Type: Bug
> Components: streams
> Affects Versions: 2.8.2, 3.7.0
> Reporter: Ayoub Omari
> Assignee: Ayoub Omari
> Priority: Major
>
> We have two topics : _left-topic[String, LeftRecord]_ and
> _right-topic[String, String]_
> where _LeftRecord_ :
> {code:scala}
> case class LeftRecord(foreignKey: String, name: String){code}
> we do a simple *INNER* foreign key join on left-topic's foreignKey field. The
> resulting join value is the value in right-topic. (same topology example as
> in KAFKA-16407)
>
> *Scenario: Unset foreign key of a primary key*
> {code:scala}
> rightTopic.pipeInput("fk1", "1")
> leftTopic.pipeInput("pk1", ProductValue("fk1", "pk1"))
> leftTopic.pipeInput("pk1", ProductValue(null, "pk1"))
> {code}
>
> *+Actual result+*
> {code:java}
> KeyValue(pk1, 3) {code}
>
> *+Expected result+*
> {code:java}
> KeyValue(pk1, 3)
> KeyValue(pk1, null) // This unsets the join between pk1 and fk1{code}
>
> However, in {+}other cases{+}, where the join result should be unset (e.g.
> the primary key is deleted, or the foreign key changes to a non existing FK),
> that record is {+}correctly emitted{+}.
>
> Also, the importance of unsetting the join result is mentioned in the code:
> [https://github.com/apache/kafka/blob/trunk/streams/src/main/java/org/apache/kafka/streams/kstream/internals/foreignkeyjoin/SubscriptionSendProcessorSupplier.java#L161C21-L163C36]
> {code:java}
> //[...] Additionally, propagate null if no FK is found there,
> // since we must "unset" any output set by the previous FK-join. This is true
> for both INNER and LEFT join. {code}
>
>
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