[Muzzaf]

> What happens if (for instance) a 4 node cluster splits into 2 node clusters (I.E. a network fault between two data centers)- does each cluster choose a leader?

A Raft cluster must have an odd number of nodes.

> how are is "majority" calculated?

ceil(nodes/2).

> is the raft protocol unable to handle half of it's nodes being taken down? What happens if two clusters break off, both choose a leader (if it's possible), both gets new writes and then both clusters come back together?

They cannot each choose a leader, see above.

[deathanatos]

> A Raft cluster must have an odd number of nodes.

Why must a Raft cluster have an odd number of nodes?

> > how are is "majority" calculated?

> ceil(nodes/2).

A majority is defined as having greater than half the votes. I.e., you need nodes / 2 + ((nodes + 1) % 2) votes, or more simply votes > nodes / 2. Even in an even-sized cluster that can only hold true for one node, and not cause splits.

[Flenser]

> A Raft cluster must have an odd number of nodes.

what about a 7 to 3 / 3 / 1 split?

[Muzzaf]

Not sure I understand. A node in each split cluster would need at least 4 votes to be elected leader. Hence no node can be elected leader since all split clusters have strictly fewer than 4 nodes.

Theorem. With 2n + 1 nodes, there can not be two separate majorities after a net split.

Proof. By way of contradiction, assume there are two separate majorities. Each separate majority would contain at least ceil((2n + 1)/2) = n + 1 nodes. This implies that there are in total at least 2(n + 1) = 2n + 2 nodes in the system, contradiction.