During the design period of a new cluster, it is always advised to have at least 3 nodes (this is the case with PXC but it’s also the same with PRM). But why and what are the risks ?
The goal of having more than 2 nodes, in fact an odd number is recommended in that kind of clusters, is to avoid split-brain situation. This can occur when quorum (that can be simplified as “majority vote”) is not honoured. A split-brain is a state in which the nodes lose contact with one another and then both try to take control of shared resources or provide simultaneously the cluster service.
On PRM the problem is obvious, both nodes will try to run the master and slave IPS and will accept writes. But what could happen with Galera replication on PXC ?
Ok first let’s have a look with a standard PXC setup (no special galera options), 2 nodes:
two running nodes (percona1 and percona2), communication between nodes is ok