[jira] [Created] (KAFKA-20640) Consumer group accepts new classic members when migration policy is disabled

["David Jacot (Jira)" <ji...@apache.org>]

David Jacot created KAFKA-20640:
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             Summary: 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: 4.3.0, 4.2.0, 4.1.0, 4.0.0
            Reporter: David Jacot
            Assignee: David Jacot


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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