KIP-394 — Require member.id for initial join group request

Discussion Kafka 2.2 ConsumerProtocol

Changes the consumer group join protocol so that the initial `JoinGroup` request with `member.id=UNKNOWN_MEMBER_ID` returns a `MEMBER_ID_REQUIRED` error with a server-assigned member ID, and the client must retry with that member ID to complete the join. When clients repeatedly retry `JoinGroup` without completing the handshake (due to connection timeouts, infinite `max.poll.interval.ms`, or restart loops), the broker accumulates dangling member state entries that are never cleaned up, bloating coordinator memory.

Protocol Impact

JoinGroup

Details