[mjb]

Right. For the same reason, the Paxos protocol allows multiple prepare votes from each acceptor. It makes this safe by requiring that each acceptor tell the proposer of the value for the highest ballot number it has accepted, and requires the proposer to switch proposal values when receiving such a message. This is, at first glance, one of the strange edges of the Paxos (synod) protocol, but it makes a lot of sense once you see an example where it is needed.

Raft has somewhat similar constructs, but explained slightly differently.

[antirez]

Hey, actually Raft uses randomized retries, since I copied this part (and most of the rest) from Raft. Voting can result into split brain as this condition is recovered, and a retry time that is big enough compared to RTT guarantees liveness.

[gbhn]

It looked similar. Can you highlight the differences from Raft? Is it just that state is small so it can accompany the various voting/heartbeat messages?

[antirez]

Because the state is small and does not depend on the previous computation, the problem becomes trivial, and is accomplished just by broadcasting the full information with an attached unique version. No need for a log, rules to apply or not entries from the log to state machine, and so forth.