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https://issues.apache.org/jira/browse/KAFKA-12972?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17366745#comment-17366745
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Matthias J. Sax commented on KAFKA-12972:
-----------------------------------------
Is your application stateful? How long did you wait? Kafka Streams does a
"lazy" migration for stateful processing. Ie, it will first assign so-called
"warmup task" to rebuild the state – only after the state was rebuilt, a new
rebalance will be triggered re-assigning the partitions.
The logs should contain more details about it.
> KStreams - Rebalance not happening after new consumer member joins group
> ------------------------------------------------------------------------
>
> Key: KAFKA-12972
> URL: https://issues.apache.org/jira/browse/KAFKA-12972
> Project: Kafka
> Issue Type: Bug
> Components: streams
> Affects Versions: 2.7.0
> Reporter: NEERAJ VAIDYA
> Priority: Major
>
> I have a kafka streams application which consumes from a topic with 20
> partitions.
> I run the application with
> {code:java}
> num.streams.threads = 10
> {code}
> When I check the JMX MBean _kafka_consumer_coordinator_assigned_partitions_,
> I can see that each thread has been assigned 2 partitions. That sounds
> reasonable balancing of partitions across the threads.
> Now, I add another instance of this application on a different machine/VM,
> but I run it with 4 stream threads.
> I was expecting these 4 threads to be assigned (via rebalancing) atleast 1
> partition each, but after multiple rebalancing schedules (every 10 mins), I
> do not see even 1 partition being assigned to any of these 4 threads in the
> second instance of my application.
> I would have expected some rebalancing to happen across the 14 (10 + 4)
> stream threads of my application instances.
>
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