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In the current consumer protocol, the field `member.id` is assigned by broker to track group member status. A new consumer joins the group with `member.id` field set as UNKNOWN_MEMBER_ID (empty string), since it needs to receive the identity assignment from broker first. For request with unknown member id, broker will blindly accept the new join group request, store the member metadata and return a UUID to consumer. The edge case is that if initial join group request keeps failing due to connection timeout, or the consumer keeps restarting, or the max.poll.interval.ms configured on client is set to infinite (no rebalance timeout kicking in to clean up the member metadata map), there will be accumulated MemberMetadata info within group metadata cache which will eventually burst broker memory. The detection and fencing of invalid join group request is crucial for broker stability.

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