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In addition, as we are promoting incremental rebalances such as KIP-415, later we hope to support stateful consumers such as KStream Kafka Streams group to have new member only taking in standby task and give them time to replay the state when first joined. These new followers need to indicate a change of status when they have finished replaying the state and ready to take active tasks. If no JoinReason is specified, brokers will not be able to distinguish the joiner's purpose: whether you are requiring an incremental rebalance, or you are just joining for restart? 

Furthermore, the JoinReason could serve as a helpful hint when we debug consumer rebalances in the historical perspective. Broker just needs to memorize the cause of the state transition to PrepareRebalance by the group.instance.id and JoinReason to sort out a past timeline that could cross validate the delay from topic metadata change towards group leaders' notification. There is a big room to improve upon this feature that worths wider discussion.

In conclusion, having JoinReason to gracefully handle the problem of rebalance necessity could simplify the implementation logic by a lot, and hide enough rebalance protocol iteration, reduce unintended state shuffle and hide low-level details to brokers' perspective on whether to move the group towards PrepareRebalance.  

Public Interfaces

We will add a new enum field to the JoinGroupRequest interface, and bump the version to v6:

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