Sun Java System Application Server Enterprise Edition 8.1 2005Q2 High Availability Administration Guide

Highly Available Clusters

A cluster is a collection of Application Server instances that work together as one logical entity. A cluster provides a runtime environment for one or more J2EE applications. A highly available cluster integrates a state replication service with clusters and load balancer.

Using clusters provides the following advantages:

All instances in a cluster:

Every cluster in the domain has a unique name; furthermore, this name must be unique across all node agent names, server instance names, cluster names, and configuration names. The name must not be domain. You perform the same operations on a cluster (for example, deploying applications and creating resources) that you perform on an unclustered server instance.

Clusters and Configurations

A cluster's settings are derived from a named configuration, which can potentially be shared with other clusters. A cluster whose configuration is not shared by other server instances or clusters is said to have a stand-alone configuration . By default, the name of this configuration is cluster_name -config, where cluster_name is the name of the cluster.

A cluster that shares its configuration with other clusters or instances is said to have a shared configuration.

Clusters, Instances, Sessions, and Load Balancing

Clusters, server instances, load balancers, and sessions are related as follows:

The cluster thus acts as a safe boundary for session failover for the server instances within the cluster. You can use the load balancer and upgrade components within the Application Server without loss of service.