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https://issues.apache.org/jira/browse/KAFKA-13435?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17451661#comment-17451661
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David Jacot commented on KAFKA-13435:
-------------------------------------
[~rleslie] I understand your point. Manually restarting consumers when adding
partitions is not even enough because the returning leader is not able to
detect new partitions until a rebalance has taken place. This is required for
two reasons: 1) to let it know that it is leader and 2) to let it rebuilt its
internal `assignmentSnapshot` state. The leader needs both in order to be able
to trigger a rebalance when the metadata has changed. However, you could force
a rebalance with `Consumer#enforceRebalance` when you add partitions.
I would like to better understand your case. Could you elaborate a bit more on
why having a single rebalance is such an issue? I suppose that the members
would retain their assignments so the disruption should be minimal (I am not
sure if this is true for all assignors though).
Alternatively, we could extend the group coordinator (on the broker side) to
monitor the new partitions and trigger a rebalance when required. This is a bit
tricky in the current implementation because the group coordinator does not
parse the assignments. I am not sure if we want to go down that route at the
moment.
> Group won't consume partitions added after static member restart
> ----------------------------------------------------------------
>
> Key: KAFKA-13435
> URL: https://issues.apache.org/jira/browse/KAFKA-13435
> Project: Kafka
> Issue Type: Bug
> Components: consumer
> Affects Versions: 3.0.0
> Reporter: Ryan Leslie
> Assignee: David Jacot
> Priority: Critical
>
> When using consumer groups with static membership, if the consumer marked as
> leader has restarted, then metadata changes such as partition increase are
> not triggering expected rebalances.
> To reproduce this issue, simply:
> # Create a static consumer subscribed to a single topic
> # Close the consumer and create a new one with the same group instance id
> # Increase partitions for the topic
> # Observe that no rebalance occurs and the new partitions are not assigned
> I have only tested this in 2.7, but it may apply to newer versions as well.
> h3. Analysis
> In {_}ConsumerCoordinator{_}, one responsibility of the leader consumer is to
> track metadata and trigger a rebalance if there are changes such as new
> partitions added:
> [https://github.com/apache/kafka/blob/43bcc5682da82a602a4c0a000dc7433d0507b450/clients/src/main/java/org/apache/kafka/clients/consumer/internals/ConsumerCoordinator.java#L793]
> {code:java}
> if (assignmentSnapshot != null &&
> !assignmentSnapshot.matches(metadataSnapshot)) {
> ...
> requestRejoinIfNecessary(reason);
> return true;
> }
> {code}
> Note that _assignmentSnapshot_ is currently only set if the consumer is the
> leader:
> [https://github.com/apache/kafka/blob/43bcc5682da82a602a4c0a000dc7433d0507b450/clients/src/main/java/org/apache/kafka/clients/consumer/internals/ConsumerCoordinator.java#L353]
> {code:java}
> // Only the leader is responsible for monitoring for metadata changes (i.e.
> partition changes)
> if (!isLeader)
> assignmentSnapshot = null;
> {code}
> And _isLeader_ is only true after an assignment is performed during a
> rebalance:
> [https://github.com/apache/kafka/blob/43bcc5682da82a602a4c0a000dc7433d0507b450/clients/src/main/java/org/apache/kafka/clients/consumer/internals/ConsumerCoordinator.java#L634]
> That is, when a consumer group forms, exactly one consumer in the group
> should have _isLeader == True_ and be responsible for triggering rebalances
> on metadata changes.
> However, in the case of static membership, if the leader has been restarted
> and rejoined the group, the group essentially no longer has a current leader.
> Even though the metadata changes are fetched, no rebalance will be triggered.
> That is, _isLeader_ will be false for all members.
> This issue does not resolve until after an actual group change that causes a
> proper rebalance. In order to safely make a partition increase when using
> static membership, consumers must be stopped and have timed out, or forcibly
> removed with {_}AdminClient.removeMembersFromConsumerGroup(){_}.
> Correcting this in the client probably also requires help from the broker.
> Currently, when a static consumer that is leader is restarted, the
> coordinator does recognize the change:
> e.g. leader _bbfcb930-61a3-4d21-945c-85f4576490ff_ was restarted
> {noformat}
> [2021-11-04 13:53:13,487] INFO [GroupCoordinator 4]: Static member
> Some(1GK7DRJPHZ0LRV91Y4D3SYHS5928XHXJQ6263GT26V5P70QX0) of group ryan_test
> with unknown member id rejoins, assigning new member id
> 1GK7DRJPHZ0LRV91Y4D3SYHS5928XHXJQ6263GT26V5P70QX0-af88ecf2-
> 6ebf-47da-95ef-c54fef17ab74, while old member id
> 1GK7DRJPHZ0LRV91Y4D3SYHS5928XHXJQ6263GT26V5P70QX0-bbfcb930-61a3-4d21-945c-85f4576490ff
> will be removed. (
> kafka.coordinator.group.GroupCoordinator){noformat}
> However, it does not attempt to update the leader id since this isn't a new
> rebalance, and JOIN_GROUP will continue returning the now stale member id as
> leader:
> {noformat}
> 2021-11-04 13:53:13,490 DEBUG o.a.k.c.c.i.AbstractCoordinator [Consumer
> instanceId=1GK7DRJPHZ0LRV91Y4D3SYHS5928XHXJQ6263GT26V5P70QX0,
> clientId=1GK7DRJPHZ0LRV91Y4D3SYHS5928XHXJQ6263GT26V5P70QX0,
> groupId=ryan_test] Received successful JoinGroup response:
> JoinGroupResponseData(throttleTimeMs=0, errorCode=0, generationId=40,
> protocolType='consumer', protocolName='range',
> leader='1GK7DRJPHZ0LRV91Y4D3SYHS5928XHXJQ6263GT26V5P70QX0-bbfcb930-61a3-4d21-945c-85f4576490ff',
>
> memberId='1GK7DRJPHZ0LRV91Y4D3SYHS5928XHXJQ6263GT26V5P70QX0-af88ecf2-6ebf-47da-95ef-c54fef17ab74',
> members=[]){noformat}
> This means that it's not easy for any particular restarted member to identify
> that it should consider itself leader and handle metadata changes.
> There is reference to the difficulty of leader restarts in KAFKA-7728 but the
> focus seemed mainly on avoiding needless rebalances for static members. That
> goal was accomplished, but this issue seems to be a side effect of both not
> rebalancing AND not having the rejoined member reclaim its leadership status.
> Also, I have not verified if it's strictly related or valid, but noticed this
> ticket has been opened too: KAFKA-12759.
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