Quorum

Quorum is the minimum number of nodes that must agree for the cluster to be considered operational. In a 3-node cluster, the quorum is 2 (the majority). If one node disconnects, the other two recognise they are the majority and continue operating normally.

How it works #

Galera Cluster uses a group communication protocol that continuously checks how many nodes are reachable. The calculation is simple: quorum = (total nodes / 2) + 1. With 3 nodes the quorum is 2, with 5 nodes it’s 3. Nodes that lose quorum transition to Non-Primary state and refuse writes to avoid inconsistencies.

What it’s for #

Quorum prevents split-brain: the situation where two parts of the cluster operate independently, accepting different writes on the same data. Without quorum, a network partition could lead to divergent data impossible to reconcile automatically.

When to use it #

Quorum is automatically active in any Galera cluster. This is why three nodes is the minimum in production: with two nodes, losing one leaves the survivor without quorum and therefore blocked.