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https://issues.apache.org/jira/browse/KAFKA-20640?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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David Jacot reassigned KAFKA-20640:
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Assignee: Dongnuo Lyu (was: David Jacot)
> Consumer group accepts new classic members when migration policy is disabled
> ----------------------------------------------------------------------------
>
> Key: KAFKA-20640
> URL: https://issues.apache.org/jira/browse/KAFKA-20640
> Project: Kafka
> Issue Type: Bug
> Affects Versions: 4.0.0, 4.1.0, 4.2.0, 4.3.0
> Reporter: David Jacot
> Assignee: Dongnuo Lyu
> Priority: Major
>
> When group.consumer.migration.policy=disabled, a new classic-protocol member
> is still admitted into an existing consumer group. Because the policy permits
> neither an online upgrade nor an online downgrade, the group can never
> converge back to a single protocol — it stays permanently mixed.
> This is inconsistent with the other direction. With disabled, a new
> consumer-protocol member joining a classic group is refused, because the
> upgrade it would require is turned off. A new classic member joining a
> consumer group should be refused for the same reason: the downgrade it would
> imply is also turned off. Under disabled the intent is that no migration
> happens and groups stay single-protocol, so a new classic member joining a
> consumer group should be rejected, mirroring the consumer-into-classic case
> that is already rejected today.
> This applies to disabled only. Under upgrade, classic members already in the
> group during an in-progress upgrade must still be able to rejoin after a
> transient fence, and under downgrade, new classic members are the mechanism
> of the downgrade rollout, so neither policy should reject them.
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