Deployments of OpenStack that rely on MariaDB+Galera benefit from a HA database thanks to Galera's synchronous replication. In such deployments, the Galera cluster is typically managed via Pacemaker, by means of a galera resource agent.
While Galera itself has its own notion of cluster management (membership, health check, write-set replication...), a resource agent is still necessary for Pacemaker to perform the basic cluster management duties, for example:
Starting up the Galera servers on the available nodes in the cluster
Health monitoring and recovery actions on failure (e.g. fencing)
This document describes the concepts involved in booting a Galera cluster, how the galera resource agent implements the boot process of a galera cluster, and how it can be overriden for recovery scenarios.