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https://issues.apache.org/jira/browse/KAFKA-19354?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17956166#comment-17956166
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José Armando García Sancio commented on KAFKA-19354:
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Okay. Thanks.
> KRaft observer unable to recover after re-bootstrapping to follower
> -------------------------------------------------------------------
>
> Key: KAFKA-19354
> URL: https://issues.apache.org/jira/browse/KAFKA-19354
> Project: Kafka
> Issue Type: Bug
> Components: kraft
> Affects Versions: 4.0.0
> Reporter: Justin Chen
> Assignee: Alyssa Huang
> Priority: Major
>
> [Original dev mail
> thread|https://lists.apache.org/thread/ws3390khsxhdg2b8cnv2mzv8slz5xq7q]
> If an observer's FETCH request to the quorum leader experiences a
> failure/timeout, it is possible that when it re-bootstraps, it will connect
> to a follower node (random selection). Subsequently, the observer node will
> continually send FETCH requests to that follower, and in receive a response
> with a "partitionError" errorCode=6 (NOT_LEADER_OR_FOLLOWER), which does not
> trigger a re-bootstrap.
> Thus, the observer will be stuck sending FETCH requests to the follower
> instead of the leader, halting metadata replication and causing it to fall
> out of sync.
> To recover from this state, re-bootstrapping would need to occur by
> restarting the affected observer or follower, until it connects to the
> correct leader.
> *Steps to reproduce:*
> 1. Spin up Kafka cluster with 3 or 5 controllers. (ideally 5 to increase
> likelihood of bootstrapping to a follower instead of the leader)
> 2. Enable a network delay on a particular observer broker (e.g. `tc qdisc add
> dev eth0 root netem delay 2500ms`). I picked 2500ms since default timeout is
> 2s for
> `controller.quorum.fetch.timeout.ms`/`controller.quorum.request.timeout.ms`.
> After a few seconds, disable the network delay (e.g. `tc qdisc del dev eth0
> root netem`).
> 3. The observer node will re-bootstrap, potentially to a follower instead of
> the leader. If so, the observer will continuously send fetch requests to the
> follower node, receive `NOT_LEADER_OR_FOLLOWER` in response, and will no
> longer replicate metadata.
> *Debug logs demonstrating this scenario:*
> - https://gist.github.com/justin-chen/1f3eee79d9a5066a467818a0b1bc006f
> - kraftcontroller-3 (leader), kraftcontroller-4 (follower), kafka-0 (observer)
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