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https://issues.apache.org/jira/browse/KAFKA-3745?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15297250#comment-15297250
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Greg Fodor commented on KAFKA-3745:
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sure. we are left joining a KTable against a KStream. The entry in the KTable
may be null since it's an outer join, so it can't be relied upon to provide the
key, and the join key for the KStream is derived data from one of the fields
(in particular it's a parsed substring of one of the columns.) the workaround
right now is we do that parsing in a previous step and then emit a record with
the value.
> 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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