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From John Roesler <j...@confluent.io>
Subject Re: Behaviour of KStreamKTableJoinProcessor
Date Mon, 15 Jul 2019 16:35:58 GMT
Hi again, Ties,

I think I spoke too soon and also misread your email.

By any chance, are you doing a join of a KStream and a GlobalKTable?

In this case, it would make perfect sense to do what you're doing, but
unfortunately the current implementation doesn't support it.

Your workaround would be to use KStream.selectKey on the left side to
pick a key before the join. Unfortunately, this will create a
repartition topic that is unnecessary when you're joining with a

On the other hand, you could at that point switch to a regular
KStream/KTable join and reduce the memory/storage requirements, as
each node won't have to host the whole global data set anymore.

Please feel free to share your code in some form to clear up the
situation in case I got it wrong again.


On Mon, Jul 15, 2019 at 10:48 AM John Roesler <john@confluent.io> wrote:
> Hi Ties,
> You're on the right track. You need to use `KTable.map` ahead of the
> join to select the new key. This will allow Streams to make sure the
> data is correctly partitioned to perform the join.
> Thanks,
> -John
> On Mon, Jul 15, 2019 at 10:07 AM Ven, Ties Jens van de
> <ties.van.de.ven@alliander.com> wrote:
> >
> > I recently started working with kafka streams and I noticed some odd behavior.
> >
> > I was using a KTable left join with a null key, and ofcourse this will not work,
since it will join based on keys.
> > But I also supplied a KeyValueMapper, which takes a property from the value and
returns this as key, and uses this value to join.
> >
> > It turns out that in the code, it firsts checks if there is a null key, and if so,
it skips.
> > Would it be more logical to check the result of the keyMapper for null instead of
the actual key?
> >
> > https://github.com/apache/kafka/blob/trunk/streams/src/main/java/org/apache/kafka/streams/kstream/internals/KStreamKTableJoinProcessor.java
> >
> > Kind regards
> >
> > Ties

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