...

Rebalance during scaling up is always painful. Every newly joined member will keep the group at rebalancing stage until all of new instances finished bootstrapping. There could be multiple shuffling of active tasks around existing and new instances, thus decreasing the entire system availability by a lot. This situation negative impact has been changed mitigated after we introduced KIP-345. Under static membership, user could provide a list of hard-coded `group.instance.id`s to pre-register their identities on broker, so that broker cooridnator coordinator could respond to scaling operations more intelligently. For example when we scale up the fleet by defining 4 new client instance ids, the server shall wait until all 4 new members to join the group before kicking out only one rebalance, instead of four in the worst case.

...

This change requires us to change JoinGroup protocol to batch mode in order to enable admin request to easily scale multiple members at once.

Public Interfaces

We will bump JoinGroup request/response version to support batch adding members.

...