WSREP

WSREP (Write Set Replication) is the API and protocol that Galera Cluster uses for synchronous multi-master replication. Each transaction is captured as a “write set” (a set of row-level changes) and replicated to all cluster nodes before commit.

How it works #

When a node executes a transaction, WSREP intercepts it at commit time, packages it as a write set and sends it to all cluster nodes via the group communication protocol. Each node performs a certification process: it verifies that the transaction doesn’t conflict with other concurrent transactions. If certification succeeds, all nodes apply the transaction. If it fails, the transaction is rolled back on the originating node.

What it’s for #

WSREP ensures all cluster nodes have the same data at all times (synchronous replication). Unlike traditional MySQL asynchronous replication, there is no lag between master and slave: when a transaction is committed on one node, it’s already present on all others.

When to use it #

WSREP is activated with the wsrep_on=ON parameter in MariaDB/Percona XtraDB Cluster configuration. Status variables starting with wsrep_ (such as wsrep_cluster_size, wsrep_cluster_status, wsrep_flow_control_paused) are the main indicators for monitoring cluster health.