Applying Flexible Paxos to Raft
Flexible Paxos provides us the insight that Paxos only needs that election and replication quorums intersect, not all quorums. Before this it was assumed that majority quorums we required because majority quorums always intersect. So what does that mean exactly and can it be applied to Raft?
In standard Raft, an election quorum is a subset of the set of servers that have voted for the same server in the same election term and that quorum is formed of a majority. For a 3 node cluster we need 2 votes and a 5 node cluster we need 3 votes and so on.
The next question is: what are all the possible quorums that exist and are there any two quorums that do not intersect? The possible majority quorums are {n1, n2}, {n2, n3} and {n1, n3} and we see that there are no two quorums that do not intersect. This is the property we get from majority quorums.
If we only had minority quorums then we could have two quorums that don’t intersect. For example, a 5 node cluster has many strict minority quorums that do not overlap, for example a couple are:
