Failover Mechanism in an Active/Passive Cluster

Universal Messaging runs as a service in a cluster. Within an active/passive cluster, there only be a single instance of Universal Messaging server running at any given time. The other Universal Messaging servers are inactive.

In a clustered environment, when a client makes a request to a server, the server handles the request much the same as in an unclustered environment, except that the server writes the client information to a shared disk instead of a private data store.

Steps	Description
1	Universal Messaging clients use the virtual IP address of the cluster to connect to the active/passive Universal Messaging cluster.
2	Cluster control software forwards the client request to the active server in the cluster.
3	The active server reads data from or writes data to the shared storage.
4	Universal Messaging returns the results to the client application.

The following diagram illustrates the failover in a clustered environment. If a server fails, subsequent requests for the session are redirected to a spare server in the cluster that is currently active and running.

Steps	Description
1	Universal Messaging clients use the virtual IP address of the cluster to connect to the active/passive Universal Messaging cluster.
2	The active server experiences failure and shuts down.
3	The cluster software returns the error code to the client.
4	Cluster control software marks the spare server as active.
5	Cluster control software forwards the client request to the active server in the cluster.
6	The active server reads data from or writes data to the shared storage.
7	Universal Messaging returns the results to the client application.