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Each member encodes the lag of its standby tasks in its metadata. We can not update the lag in every heartbeat request because that would constantly trigger reassignment in the group. Instead, when a) the task lag has been reduced within the acceptable.recovery.lag threshold or b) the task lag is consistently increasing for some time, the member should consider triggering a rebalance by sending its next heartbeat with the appropriate encoded reason and the updated task lags. 

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• Deploy the new version of the software
• Roll the cluster
• Enable the new protocol
• Roll the cluster

Group Upgrade

• Deploy the new software
• Roll the members
• Enable the new protocol
• Roll the members

Eager and Cooperative

• Eager revokes all partitions before re-joining the group.
• Cooperative does two rebalance rounds. One to revoke partitions. One to assign partitions.

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