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https://issues.apache.org/jira/browse/KAFKA-4835?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15905515#comment-15905515
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Matthias J. Sax commented on KAFKA-4835:
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If you use a custom partitioner that is solely based on the message key, there
is not problem like this. As long as you don't modify the key, no
repartitioning will happen. And if you modify the key (which does require a
repartitioning), you can give your custom partitioner to Streams, too. You only
can't enjoy auto-repartitioning, but need to do all re-partitioning manually
via {{through(StreamPartitionier partitoner, String topic)}}.
> Allow users control over repartitioning
> ---------------------------------------
>
> Key: KAFKA-4835
> URL: https://issues.apache.org/jira/browse/KAFKA-4835
> Project: Kafka
> Issue Type: Improvement
> Components: streams
> Affects Versions: 0.10.2.0
> Reporter: Michal Borowiecki
> Labels: needs-kip
>
> From
> https://issues.apache.org/jira/browse/KAFKA-4601?focusedCommentId=15881030&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-15881030
> ...it would be good to provide users more control over the repartitioning.
> My use case is as follows (unrelated bits omitted for brevity):
> {code}
> KTable<String, Activity> loggedInCustomers = builder
> .stream("customerLogins")
> .groupBy((key, activity) ->
> activity.getCustomerRef())
> .reduce((first,second) -> second, loginStore());
>
> builder
> .stream("balanceUpdates")
> .map((key, activity) -> new KeyValue<>(
> activity.getCustomerRef(),
> activity))
> .join(loggedInCustomers, (activity, session) -> ...
> .to("sessions");
> {code}
> Both "groupBy" and "map" in the underlying implementation set the
> repartitionRequired flag (since the key changes), and the aggregation/join
> that follows will create the repartitioned topic.
> However, in our case I know that both input streams are already partitioned
> by the customerRef value, which I'm mapping into the key (because it's
> required by the join operation).
> So there are 2 unnecessary intermediate topics created with their associated
> overhead, while the ultimate goal is simply to do a join on a value that we
> already use to partition the original streams anyway.
> (Note, we don't have the option to re-implement the original input streams to
> make customerRef the message key.)
> I think it would be better to allow the user to decide (from their knowledge
> of the incoming streams) whether a repartition is mandatory on aggregation
> and join operations (overloaded version of the methods with the
> repartitionRequired flag exposed maybe?)
> An alternative would be to allow users to perform a join on a value other
> than the key (a keyValueMapper parameter to join, like the one used for joins
> with global tables), but I expect that to be more involved and error-prone to
> use for people who don't understand the partitioning requirements well
> (whereas it's safe for global tables).
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