[jira] [Comment Edited] (KAFKA-4084) automated leader rebalance causes replication downtime for clusters with too many partitions

["Evan Williams (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/KAFKA-4084?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17029058#comment-17029058 ] 

Evan Williams edited comment on KAFKA-4084 at 2/3/20 4:40 PM:
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 We are seeing the same (or at least similar) issues. When terminating a broker in AWS, and assigning the same broker ID to it's replacement (that has a empty data dir). 

We have auto.leader.rebalance.enable=true and when the broker get's assigned as leader - we get 100% CPU usage (ReplicaFetcher threads), and many clients (streams) fail to connect, as the broker both tries to fetch and serve data. 

Am just thinking outside of the box a bit for a work around, but until KIP-491 is implemented (if at all), is it possible to implement some logic/automation to set auto.leader.rebalance.enable=false on the replacement node, so that as soon as the Kafka service is started, it will prevent it from becoming leader, until under replication partitions = 0, and then reset auto.leader.rebalance.enable back to true, and restart the kafka service ? Or must auto.leader.rebalance.enable be set on all brokers (or all other brokers), for it to be effective ?

Or perhaps just script kafka-preferred-replica-election.sh to run, when CPU is lower on the new node, or under replication partitions = 0 ?


was (Author: blodsbror):
 We are seeing the same (or at least similar) issues. When terminating a broker in AWS, and assigning the same broker ID to it's replacement (that has a empty data dir). We have auto.leader.rebalance.enable=true and when the broker get's assigned as leader - we get 100% CPU usage (ReplicaFetcher threads), and many clients (streams) fail to connect, as the broker both tries to fetch and serve data. Am just thinking outside of the box a bit for a work around, but until KIP-491 is implemented (if at all), is it possible to implement some logic/automation to set auto.leader.rebalance.enable=false on the replacement node, so that as soon as the Kafka service is started, it will prevent it from becoming leader, until under replication partitions = 0, and then reset auto.leader.rebalance.enable back to true, and restart the kafka service ? Or must auto.leader.rebalance.enable be set on all brokers (or all other brokers), for it to be effective ?

> automated leader rebalance causes replication downtime for clusters with too many partitions
> --------------------------------------------------------------------------------------------
>
>                 Key: KAFKA-4084
>                 URL: https://issues.apache.org/jira/browse/KAFKA-4084
>             Project: Kafka
>          Issue Type: Bug
>          Components: controller
>    Affects Versions: 0.8.2.2, 0.9.0.0, 0.9.0.1, 0.10.0.0, 0.10.0.1
>            Reporter: Tom Crayford
>            Priority: Major
>              Labels: reliability
>             Fix For: 1.1.0
>
>
> If you enable {{auto.leader.rebalance.enable}} (which is on by default), and you have a cluster with many partitions, there is a severe amount of replication downtime following a restart. This causes `UnderReplicatedPartitions` to fire, and replication is paused.
> This is because the current automated leader rebalance mechanism changes leaders for *all* imbalanced partitions at once, instead of doing it gradually. This effectively stops all replica fetchers in the cluster (assuming there are enough imbalanced partitions), and restarts them. This can take minutes on busy clusters, during which no replication is happening and user data is at risk. Clients with {{acks=-1}} also see issues at this time, because replication is effectively stalled.
> To quote Todd Palino from the mailing list:
> bq. There is an admin CLI command to trigger the preferred replica election manually. There is also a broker configuration “auto.leader.rebalance.enable” which you can set to have the broker automatically perform the PLE when needed. DO NOT USE THIS OPTION. There are serious performance issues when doing so, especially on larger clusters. It needs some development work that has not been fully identified yet.
> This setting is extremely useful for smaller clusters, but with high partition counts causes the huge issues stated above.
> One potential fix could be adding a new configuration for the number of partitions to do automated leader rebalancing for at once, and *stop* once that number of leader rebalances are in flight, until they're done. There may be better mechanisms, and I'd love to hear if anybody has any ideas.



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