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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