sanghyeok An created KAFKA-20601:
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Summary: ConsumerGroup static member rejoin can fail with
GroupMaxSizeReachedException at max size
Key: KAFKA-20601
URL: https://issues.apache.org/jira/browse/KAFKA-20601
Project: Kafka
Issue Type: Bug
Reporter: sanghyeok An
Assignee: sanghyeok An
GroupMetadataManager#throwIfConsumerGroupIsFull() only checks whether the
provided memberId already exists in the group:
{code:java}
group.numMembers() >= config.consumerGroupMaxSize()
&& (memberId.isEmpty() || !group.hasMember(memberId))
{code}
However, when a static member rejoins after leaving with the static leave epoch
(-2), the rejoining member may use a new member ID. In that case, the existing
static member is identified by instanceId, not by the new memberId.
Because the max-size check runs before resolving/replacing the static member, a
valid static member rejoin can be rejected with GroupMaxSizeReachedException
when the group is already at max size, even though the operation would replace
an existing static member and would not increase the group size.
*Expected behavior*
A static member rejoin for an existing instanceId should not fail the max-size
check solely because the rejoining memberId is new or empty. The check should
account for existing static members, similar to checking
group.hasStaticMember(instanceId).
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