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https://issues.apache.org/jira/browse/KAFKA-9987?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17108712#comment-17108712
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Travis Bischel commented on KAFKA-9987:
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Unless I'm mistaken, the algorithm in the first comment should be relatively
optimal in cases where the subscriptions of all members are equal, but _I
think_ has some edge cases with unequal subscriptions. This is (I think)
implied in the ticket description.
My code solves both cases by switching to a more complex algorithm (A*) to find
steal paths on unequal subscriptions, but sticks with a very simple algorithm
on equal subscription cases. The simple algorithm just moves from partitions
from the most loaded member to the least loaded until the partition delta
between the most and least is at most one.
> Improve sticky partition assignor algorithm
> -------------------------------------------
>
> Key: KAFKA-9987
> URL: https://issues.apache.org/jira/browse/KAFKA-9987
> Project: Kafka
> Issue Type: Improvement
> Components: clients
> Reporter: Sophie Blee-Goldman
> Assignee: Sophie Blee-Goldman
> Priority: Major
>
> In
> [KIP-429|https://cwiki.apache.org/confluence/display/KAFKA/KIP-429%3A+Kafka+Consumer+Incremental+Rebalance+Protocol]
> we added the new CooperativeStickyAssignor which leverages on the underlying
> sticky assignment algorithm of the existing StickyAssignor (moved to
> AbstractStickyAssignor). The algorithm is fairly complex as it tries to
> optimize stickiness while satisfying perfect balance _in the case individual
> consumers may be subscribed to different subsets of the topics._ While it
> does a pretty good job at what it promises to do, it doesn't scale well with
> large numbers of consumers and partitions.
> To give a concrete example, users have reported that it takes 2.5 minutes for
> the assignment to complete with just 2100 consumers reading from 2100
> partitions. Since partitions revoked during the first of two cooperative
> rebalances will remain unassigned until the end of the second rebalance, it's
> important for the rebalance to be as fast as possible. And since one of the
> primary improvements of the cooperative rebalancing protocol is better
> scaling experience, the only OOTB cooperative assignor should not itself
> scale poorly
> If we can constrain the problem a bit, we can simplify the algorithm greatly.
> In many cases the individual consumers won't be subscribed to some random
> subset of the total subscription, they will all be subscribed to the same set
> of topics and rely on the assignor to balance the partition workload.
> We can detect this case by checking the group's individual subscriptions and
> call on a more efficient assignment algorithm.
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