Page History

...

A common optimization for this step is to wait until 4 has caught up near the end of the log before adding it. Similarly, we may choose to add another replica first if it has already caught up.

The Once the check passes, the leader will send a BeginQuorumEpoch request to replica 4 so that it is aware of the voter change. Once this finally send out the control record to modify the existing voter list. Once that entry has been committed to the log, we can the new membership takes effect.

Then we could remove one of the existing replicas (say 3) and write by writing a new message:

currentVoters: [1, 2, 4]
targetVoters: [4, 5, 6]

...