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https://issues.apache.org/jira/browse/KAFKA-3745?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15297477#comment-15297477
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Greg Fodor commented on KAFKA-3745:
-----------------------------------
Yep, I admit this is definitely not the most common case. But when it happens,
the key is basically lost, so the workaround results in passing additional
state through the system which seems undesirable.
> 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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