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|Oracle GlassFish Server 3.1-3.1.1 High Availability Administration Guide|
A cluster is a named collection of GlassFish Server instances that share the same applications, resources, and configuration information. For information about GlassFish Server instances, see Chapter 5, Administering GlassFish Server Instances.
GlassFish Server enables you to administer all the instances in a cluster as a single unit from a single host, regardless of whether the instances reside on the same host or different hosts. You can perform the same operations on a cluster that you can perform on an unclustered instance, for example, deploying applications and creating resources.
A cluster provides high availability through failure protection, scalability, and load balancing.
Failure protection. If an instance or a host in a cluster fails, GlassFish Server detects the failure and recovers the user session state. If a load balancer is configured for the cluster, the load balancer redirects requests from the failed instance to other instances in the cluster. Because the same applications and resources are on all instances in the cluster, an instance can fail over to any other instance in the cluster.
To enable the user session state to be recovered, each instance in a cluster sends in-memory state data to another instance. As state data is updated in any instance, the data is replicated.
Scalability. If increased capacity is required, you can add instances to a cluster with no disruption in service. When an instance is added or removed, the changes are handled automatically.
Load balancing. If instances in a cluster are distributed among different hosts, the workload can be distributed among the hosts to increase overall system throughput.