[jira] [Resolved] (KAFKA-20778) Improve classic consumer rack-aware double rebalance on replica bounce

["Lianet Magrans (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/KAFKA-20778?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Lianet Magrans resolved KAFKA-20778.
------------------------------------
    Resolution: Fixed

> Improve classic consumer rack-aware double rebalance on replica bounce
> ----------------------------------------------------------------------
>
>                 Key: KAFKA-20778
>                 URL: https://issues.apache.org/jira/browse/KAFKA-20778
>             Project: Kafka
>          Issue Type: Improvement
>          Components: clients, consumer
>            Reporter: Lianet Magrans
>            Assignee: Lianet Magrans
>            Priority: Major
>             Fix For: 4.4.0
>
>
> When a replica is bounced , the classic consumer goes through a double rebalance because when it notices a rack ID changed from rackX to null, then from null to rackX, unneeded rebalance noise. 
> Consider if we could avoid this double rebalance by identifying that the case is that the replica is down (based on the existing flow that knows that the replica lost its "host", not only the rack).
> This issue only affects the classic consumer, where rebalances are triggered client-side based on the metadata (a replica temporarily shows as "emtpy node", no host or rack), but the client blindly takes that as rack=null and compares that null to the previous rack ID. With the new consumer group protocol there is no rebalance triggered by the GC in this case of temporary replica bounce, as the rack info remains unchanged (replica metadata available, unchanged, while temporarily down)



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