As explained in the Clustering Overview (see Clusters: An Overview), a cluster is a collection of Universal Messaging Realm Servers (realms).

Each cluster has one realm which is elected as master, and all other realms are deemed slaves. The master is the authoritative source of state for all resources within the cluster.

Should a realm or location become unavailable for any reason, the cluster's remaining realms should be able to carry on servicing clients:

Note: Dotted lines represent interrupted communication owing to server or network outages.

For publish/subscribe resources (see Messaging Paradigms), each published event will be allocated a unique event ID by the master, which is then propagated to each slave.