Clustering

J2EE application server vendors define a cluster as a group of machines working together to transparently provide enterprise services (support for JNDI, EJB, JSP, HttpSession and component failover, and so on). They leave the definition purposely vague because each vendor implements clustering differently. At one end of the spectrum rest vendors who put a dispatcher in front of a group of independent machines, none of which has knowledge of the other machines in the cluster. In this scheme, the dispatcher receives an initial request from a user and replies with an HTTP redirect header to pin the client to a particular member server of the cluster. At the other end of the spectrum reside vendors who implement a federation of tightly integrated machines, with each machine totally aware of the other machines around it along with the objects on those machines.

Purpose -
1. Reducing downtime - Another machine in cluster transperently takeover in case of failure of a particular machine.
2. Scalibility - More server can be added to handle the increased users.
3. Spreading load - Different machines in the cluster will be responsible for the indivudual module of the application hence decreasing the load on individual machine.

A cluster is a group of application servers that transparently run your J2EE application as if it were a single entity. To scale, you should include additional machines within the cluster. To minimize downtime, make sure every component of the cluster is redundant.

For More Information - Java World - Clustering