Based on the "clusterjsp.war", I assume that yours is a web application?
High-availability of a web application is typically achieved by a cluster of application servers behind a load balancing tier, using distributed application state. In such a configuration, sessions created on any server are available to all servers, such that a failure of a single server does not impact overall availability of the application, and any sessions active on the failed server are able to be serviced by any remaining cluster members.
Primary/backup style setups are typically relegated to use cases where some service must be limited to a single instance/process.