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From Dmitry Minkovsky <dminkov...@gmail.com>
Subject Kafka Streams topology does not replay correctly
Date Tue, 16 Jan 2018 23:08:00 GMT
Earlier today I posted this question to SO
<https://stackoverflow.com/questions/48287840/kafka-streams-topology-does-not-replay-correctly>
:

> I have a topology that looks like this:

    KTable<ByteString, User> users = topology.table(USERS,
Consumed.with(byteStringSerde, userSerde), Materialized.as(USERS));

    KStream<ByteString, JoinRequest> joinRequests =
topology.stream(JOIN_REQUESTS, Consumed.with(byteStringSerde,
joinRequestSerde))
        .mapValues(entityTopologyProcessor::userNew)
        .to(USERS, Produced.with(byteStringSerde, userSerde));

    topology.stream(SETTINGS_CONFIRM_REQUESTS,
Consumed.with(byteStringSerde, settingsConfirmRequestSerde))
        .join(users, entityTopologyProcessor::userSettingsConfirm,
Joined.with(byteStringSerde, settingsConfirmRequestSerde, userSerde))
        .to(USERS, Produced.with(byteStringSerde, userSerde));

    topology.stream(SETTINGS_UPDATE_REQUESTS,
Consumed.with(byteStringSerde, settingsUpdateRequestSerde))
        .join(users, entityTopologyProcessor::userSettingsUpdate,
Joined.with(byteStringSerde, settingsUpdateRequestSerde, userSerde))
        .to(USERS, Produced.with(byteStringSerde, userSerde));

> At runtime this topology works fine. Users are created with join
requests. They confirm their settings with settings confirm requests. They
update their settings with settings update requests.
>
> However, reprocessing this topology does not produce the original
results. Specifically, the settings update joiner does not see the user
that resulted from the settings confirm joiner, even though in terms of
timestamps, many seconds elapse from the time the user is created, to the
time the user is confirmed to the time the user updates their settings.
>
> I'm at a loss. I've tried turning off caching/logging on the user table.
No idea what to do to make this reprocess properly.

----

The response by Matthias, also on SO:

> A KStream-KTable join is not 100% deterministic (and might never become
100% deterministic). We are aware of the problem and discuss solutions, to
at least mitigate the issue.
>
> One problem is, that if a Consumer fetches from the brokers, we cannot
control easily for which topics and/or partitions the broker returns data.
And depending on the order in which we receive data from the broker, the
result might slightly differ.
>
> One related issue: https://issues.apache.org/jira/browse/KAFKA-3514
>
> This blog post might help, too:
https://www.confluent.io/blog/crossing-streams-joins-apache-kafka/

----

I don't really know what to do with this response. I have been aware of
some "slight" discrepancy that might occur in edge cases with
KStream-KTable joins for some time now, but what I'm seeing is not a slight
discrepancy but very different results.

I looked at the JIRA Matthias linked
<https://issues.apache.org/jira/browse/KAFKA-3514>. However, my data has no
late arriving records. I don't know about the empty buffers. I have read
the blog post he linked several times already.

Can someone please suggest how I may obviate this problem? For example

   - Would it make sense for me to try launching the topology with fewer
   threads during the reprocess?
   - Would it make sense for launch the topology with fewer input tasks?
   - Would it make sense to increase size of the stream buffer?

I am at a total loss at this point. I cannot believe that there is nothing
I can do to replay this data and perform the migration I am trying to
perform, in order to release a next version of my application. Am I totally
screwed?


Thank you,
Dmitry

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