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https://issues.apache.org/jira/browse/KAFKA-13359?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17425463#comment-17425463
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Luke Chen commented on KAFKA-13359:
-----------------------------------
The root cause of this issue should be KAFKA-9965.
> Round Robin Kafka Producer Routes to only half the partitions when even
> number of partitions
> --------------------------------------------------------------------------------------------
>
> Key: KAFKA-13359
> URL: https://issues.apache.org/jira/browse/KAFKA-13359
> Project: Kafka
> Issue Type: Bug
> Components: producer
> Affects Versions: 2.7.0, 3.0.0
> Reporter: David G
> Priority: Minor
>
> When you have 1 message per batch, in the round robin Partitioner. The
> messages go only to half the partitions beacuse it always skips 1 partition.
> This works out for odd number of partitions because skipping 1 will mean all
> partitions get a hit eventually, but with an even number half the partitions
> never get selected.
>
> So if you have partitions 1, 2, 3, 4
> Message 1: Partion 1
> Message 2: Partion 3
> Message 3: Partion 1 ... so on. [ Here 2 and 4 are never selected]
>
> If you have partitions 1, 2, 3
> Message 1: Partion 1
> Message 2: Partion 3
> Message 3: Partion 2 .. so on
>
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