High Availability (HA) clusters are implemented in Alfresco to improve the availability of services and to improve performance of these services. Availability is enhanced through redundant nodes that provide services when other nodes fail. When integrated with a load balancer, performance is enhanced by distributing, or balancing, server workload across a collection of nodes.
A cluster represents a collection of nodes. For example, a set up of two tomcat nodes on two separate machines, talking to shared content store, shared database, but each with their own indexes. This is the simplest cluster to set up, gives redundancy due to the two machines, and can load-balance for performance or use the second node as a "hot spare" for fail over.