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https://issues.apache.org/jira/browse/KAFKA-2172?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14529436#comment-14529436
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Jason Rosenberg commented on KAFKA-2172:
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as a side note, the documentation here:
http://kafka.apache.org/documentation.html#consumerconfigs currently has
duplicate entries for 'partition.assignment.strategy', one of which is far more
in depth than the other!
> Round-robin partition assignment strategy too restrictive
> ---------------------------------------------------------
>
> Key: KAFKA-2172
> URL: https://issues.apache.org/jira/browse/KAFKA-2172
> Project: Kafka
> Issue Type: Bug
> Reporter: Jason Rosenberg
>
> The round-ropin partition assignment strategy, was introduced for the
> high-level consumer, starting with 0.8.2.1. This appears to be a very
> attractive feature, but it has an unfortunate restriction, which prevents it
> from being easily utilized. That is that it requires all consumers in the
> consumer group have identical topic regex selectors, and that they have the
> same number of consumer threads.
> It turns out this is not always the case for our deployments. It's not
> unusual to run multiple consumers within a single process (with different
> topic selectors), or we might have multiple processes dedicated for different
> topic subsets. Agreed, we could change these to have separate group ids for
> each sub topic selector (but unfortunately, that's easier said than done).
> In several cases, we do at least have separate client.ids set for each
> sub-consumer, so it would be incrementally better if we could at least loosen
> the requirement such that each set of topics selected by a groupid/clientid
> pair are the same.
> But, if we want to do a rolling restart for a new version of a consumer
> config, the cluster will likely be in a state where it's not possible to have
> a single config until the full rolling restart completes across all nodes.
> This results in a consumer outage while the rolling restart is happening.
> Finally, it's especially problematic if we want to canary a new version for a
> period before rolling to the whole cluster.
> I'm not sure why this restriction should exist (as it obviously does not
> exist for the 'range' assignment strategy). It seems it could be made to
> work reasonably well with heterogenous topic selection and heterogenous
> thread counts. The documentation states that "The round-robin partition
> assignor lays out all the available partitions and all the available consumer
> threads. It then proceeds to do a round-robin assignment from partition to
> consumer thread."
> If the assignor can "lay out all the available partitions and all the
> available consumer threads", it should be able to uniformly assign partitions
> to the available threads. In each case, if a thread belongs to a consumer
> that doesn't have that partition selected, just move to the next available
> thread that does have the selection, etc.
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