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https://issues.apache.org/jira/browse/KAFKA-3745?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15297660#comment-15297660
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Guozhang Wang commented on KAFKA-3745:
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Just wanted to make sure that we are on the same page: even if the join key is
not accessible in {{ValueJoiner}}, the returned key-value pair will still
inherit the join key, so it is not "lost". I guess your scenario requires you
to get the join-key value while doing the join computation inside
{{ValueJoiner}}, but need to get confirmed.
> Consider adding join key to ValueJoiner interface
> -------------------------------------------------
>
> Key: KAFKA-3745
> URL: https://issues.apache.org/jira/browse/KAFKA-3745
> Project: Kafka
> Issue Type: Improvement
> Components: streams
> Affects Versions: 0.10.0.0
> Reporter: Greg Fodor
> Priority: Minor
> Labels: api, newbie
>
> In working with Kafka Stream joining, it's sometimes the case that a join key
> is not actually present in the values of the joins themselves (if, for
> example, a previous transform generated an ephemeral join key.) In such
> cases, the actual key of the join is not available in the ValueJoiner
> implementation to be used to construct the final joined value. This can be
> worked around by explicitly threading the join key into the value if needed,
> but it seems like extending the interface to pass the join key along as well
> would be helpful.
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