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https://issues.apache.org/jira/browse/KAFKA-3705?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16247892#comment-16247892
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Jan Filipiak commented on KAFKA-3705:
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[~thuey100] I am glad to see your interest in this. The pull request has
discussions regarding the client API. We are currenlty in the process setting
this fixed in the KIP
https://cwiki.apache.org/confluence/display/KAFKA/KIP-213+Support+non-key+joining+in+KTable.
The hardest part in the PR is still outstanding sadly. Merging Cache and
Persistent Stores in a prefix scan. We only run our code with 0.10.0.1 it was
easier back then. Or we just gonna flush the cache just every time. Feel
welcome to get involved!
> Support non-key joining in KTable
> ---------------------------------
>
> Key: KAFKA-3705
> URL: https://issues.apache.org/jira/browse/KAFKA-3705
> Project: Kafka
> Issue Type: Bug
> Components: streams
> Reporter: Guozhang Wang
> Labels: api
>
> Today in Kafka Streams DSL, KTable joins are only based on keys. If users
> want to join a KTable A by key {{a}} with another KTable B by key {{b}} but
> with a "foreign key" {{a}}, and assuming they are read from two topics which
> are partitioned on {{a}} and {{b}} respectively, they need to do the
> following pattern:
> {code}
> tableB' = tableB.groupBy(/* select on field "a" */).agg(...); // now tableB'
> is partitioned on "a"
> tableA.join(tableB', joiner);
> {code}
> Even if these two tables are read from two topics which are already
> partitioned on {{a}}, users still need to do the pre-aggregation in order to
> make the two joining streams to be on the same key. This is a draw-back from
> programability and we should fix it.
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