Version 2 of the GlassFish Java EE Application Server contains many new features, among them enhanced clustering capabilities. The new clustering capabilities enhance high availability and scalability for deployment architectures through in-memory session state replication. With in-memory state replication, clustered server instances replicate session state in a ring topology, storing the replicated information in memory. This article describes the clustering capabilities of GlassFish version 2 and helps you get started deploying your application to a GlassFish cluster.
Read the entire entry: http://developers.sun.com/appserver/reference/techart/glassfishcluster/index.html