TIL
Quorum is ceil(N/2)
Quorum is the smallest number of nodes that have to agree before a decision counts. The formula is ceil(N/2):
- 3 nodes need 2
- 5 nodes need 3
- 7 nodes need 4
This is why you almost always want an odd number of nodes. With an even count you can end up with the cluster split into two equal halves, neither side has a majority, and nobody can make progress (split-brain). An odd count guarantees one side wins.
Adding a node to go from 3 to 4 does not buy you more fault tolerance either: both tolerate one failure. Go 3 to 5 to actually gain something.