Mission-critical Web-based applications ­ customer self-service, distribution channel and supply chain management, online trading and banking ­ must be deployed on a cluster of servers in order to provide scalability and high availability. Scalability means that servers can be dynamically added or removed as needed to meet user demand, and that the overall load of requests is distributed among the servers so that resources remain fully utilized. High availability means that there is no "single point of failure" in either the system or the application, and that requests automatically failover from nonworking components to working components. Ideally, clustering should be transparent to applications: externally, the cluster should present a "single-system image." In addition to simplifying the task of application development, this allows off-the-shelf components to be deployed without modification.