THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
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- After startup, a consumer learns it's consumer id in the very first JoinGroupResponse it receives from the co-ordinator. From that point onwards, the consumer is expected to include this consumer id in every request it sends to the co-ordinator (HeartbeatRequest, JoinGroupRequest, OffsetCommitRequest). If the co-ordinator receives a HeartbeatRequest or an OffsetCommitRequest with a consumer id that is different from the ones in the group, it sends an UnknownConsumer error code in the corresponding responses.
- The co-ordinator assigns a consumer id to a consumer on a successful rebalance and sends it in the JoinGroupResponse. The consumer should include this id in every subsequent JoinGroupRequest as well until it is shutdown or dies.
- The co-ordinator does consumer id assignment after it has received a JoinGroupRequest from all existing consumers in a group. At this point, it assigns a new id <group>-<consumer_host>-<sequence> to every consumer that did not have a consumer id in the JoinGroupRequest. The assumption is that such consumers are newly started up.
- If a consumer fails to send the same consumer id on subsequent JoinGroupRequests, it will cause a chain of rebalance attempts and can cause the group to never finish a rebalance operation successfully. This is because the way a co-ordinator knows that a rebalance operation should be triggered due to a new consumer, is by checking the consumer id in the JoinGroupRequest. If there is no consumer id, it assumes that a new consumer wants to join the group.
- If a consumer id is specified in the JoinGroupRequest but it does not match the ids in the current group membership, the co-ordinator sends an UnknownConsumer error code in the JoinGroupResponse and prevents the consumer from joining the group. This does not cause a rebalance operation for the rest of the consumers in the group, but also does not allow such a consumer to join an existing group.
Memory management
TBD
Request formats
For each consumer group, the coordinator stores the following information:
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