Galera Cluster

Galera Cluster is available for MySQL via the wsrep patch, for MariaDB through the MariaDB Galera Cluster package, and as Percona XtraDB Cluster from Percona. All three distributions use the same wsrep provider (Galera replication library) under the hood. PostgreSQL does not use Galera; it relies on native streaming replication or tools like Patroni for HA.

Galera uses a weighted quorum voting system where the partition containing the majority of nodes (by weight) remains operational as the primary component. The minority partition automatically switches to a read-only, non-operational state to prevent split-brain writes. A minimum of three nodes is recommended so that a single-node failure always leaves a majority quorum intact.

Galera uses optimistic certification-based conflict detection: each node applies writes locally first, then broadcasts the transaction for certification across the cluster. If two nodes commit conflicting writes to the same rows near-simultaneously, the certification process on each node determines a winner based on transaction ordering and rolls back the loser. This means application-level conflict rates are low but developers should be aware that multi-master writes to the same rows can occasionally result in deadlocks.

Galera Clusters typically use 3 or 5 nodes, with odd numbers preferred to simplify quorum calculations. While Galera technically supports more nodes, each additional node increases the certification and replication overhead for every committed transaction because all writes must be replicated synchronously. This means write throughput does not scale linearly with node count the way read throughput does.