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https://issues.apache.org/jira/browse/KAFKA-4402?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15662198#comment-15662198
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Ewen Cheslack-Postava commented on KAFKA-4402:
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[~Jun Yao] Sorry, missed that update with the example code while I was
commenting on the PR. I understand how you can encounter the imbalance when
using multiple topics, see PR for questions around the solution and whether we
want to change the default vs provide an alternative when this is a problem.
> Kafka Producer's DefaultPartitioner is actually not round robin as said in
> the code comments "If no partition or key is present choose a partition in a
> round-robin fashion"
> ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>
> Key: KAFKA-4402
> URL: https://issues.apache.org/jira/browse/KAFKA-4402
> Project: Kafka
> Issue Type: Improvement
> Reporter: Jun Yao
> Priority: Minor
>
> From this code comments, it is said that Kafka client Producer's
> DefaultPartitioner will do round robin if "no partition or key is present",
> https://github.com/apache/kafka/blob/41e676d29587042994a72baa5000a8861a075c8c/clients/src/main/java/org/apache/kafka/clients/producer/internals/DefaultPartitioner.java#L34
> from the code it looks trying to do round robin as well, as it maintained a
> counter and try to increase it every time and then will decide which
> partition to go to;
> However the issue here is the counter is a global counter that is shared by
> all the topics, so it is actually not round robin per topic and sometimes
> caused unbalanced routing among different partitions.
> Although we can pass a custom implementation of interface
> "org.apache.kafka.clients.producer.Partitioner", it might be still good to
> make the default implementation true round robin as comment.
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