Calendar Server can be configured for failover. See Chapter 7, Configuring for High Availability (Failover Service), in Sun Java System Calendar Server 6 2005Q4 Administration Guide as a guide to installation and configuration.
To install the necessary packages:
On the primary node, run the Java ES installer, installing Calendar Server and the HA Sun Java System Calendar Server subcomponent of the Sun Cluster Agents component. Select the Configure Later option. When specifying installation directories, use a location on the cluster file system for Calendar Server.
On the secondary nodes, run the Java ES installer, installing the HA Sun Java System Calendar Server, selecting the Configure Later option.