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https://issues.apache.org/jira/browse/KAFKA-12845?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17351308#comment-17351308
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Matthias J. Sax commented on KAFKA-12845:
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I think you confuse "input record key" with "join-key". As you correctly
mentioned, we apply a key-extractor to get the join-key for stream side
records. This join-key must still be non-null. K10277 only allows that the
input record key is null now. Of course, if you use the input record key as
join-key, the key-extractor would return the key as-is, and thus, if it's null
the record would still be dropped.
The issue K10277 addresses is, that if you have a null-key input record, but
you key-extractor returns a non-null join-key (from the value of the record),
the join should still work. However, before K10277 all records with null-key
were dropped, even if the key was not used in the join.
Does this make sense?
> Rollback change which requires join key to be non null on
> KStream->GlobalKTable
> -------------------------------------------------------------------------------
>
> Key: KAFKA-12845
> URL: https://issues.apache.org/jira/browse/KAFKA-12845
> Project: Kafka
> Issue Type: Improvement
> Components: streams
> Affects Versions: 2.7.0
> Reporter: Pedro Gontijo
> Priority: Major
>
> As part of [KAFKA-10277|https://issues.apache.org/jira/browse/KAFKA-10277]
> the behavior for KStream->GlobalKtable joins was changed to require non null
> join keys.
> But it seems reasonable that not every record will have an existing
> relationship (and hence a key) with the join globalktable. Think about a
> User>Car for instance, or PageView>Product. An empty/zero key could be
> returned by the KeyMapper but that will make a totally unnecessary search
> into the store.
> I do not think that makes sense for any GlobalKtable join (inner or left) but
> for left join it sounds even more strange.
>
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