With clustering, a group of Web servers are managed and administered from a single server. All Web servers in the cluster share the same configuration files. A simple cluster is shown in Figure 9. The master server automatically starts failed processes, handles fail-over and manages all initial connections, routing them only to those available slaves in the Web server farm. The master server also maintains connection information about the application servers the cluster can access to execute logic.
Application servers play a very significant role in today's dot-com environment.
Typically these application servers are multiprocess and multithreaded. They are expected to scale vertically anywhere between 4 to 8 CPUs. Application servers are typically deployed on such systems as Sun Enterprise 420s, 4500s, 5500s, 6500s or an E10000 domain. Using this technique, the scaling is achieved by locating a certain set of application logic on one server and another set of application logic on another. Based on user profiles and the state of the connection, the Web server connects to and invokes logic from the appropriate app server as requested by the client.