The master listener

A clustered listener configuration always includes a master listener. The master listener is a lightweight application whose job is to perform load balancing. It allocates requests to each of the running listeners in the cluster.

The master listener includes a load balancer component that coordinates load distribution across the cluster. The master listener and the load balancer function as a unit.

If the master listener goes down for any reason (hardware, software, or network failure), the Unica Campaign Web application detects the failure. The web application asks the next node to become the master listener. The requested listener performs the master listener election, and the highest priority node that is available becomes the master listener. Failover occurs automatically. Since the load balancer is a component of the master listener, the new master listener then handles load balancing.

There is always one master listener in a cluster. Any node in the cluster can perform as a master listener. The Unica Campaign configuration settings determine which node initially acts as the master listener (masterListenerPriority) and how load is balanced across clustered nodes (loadBalanceWeight).

If you have only a single listener, then load balancing and failover are not possible. A single listener performs all responsibilities without any additional listener nodes. However, if a failure occurs, and reconnection is possible, the listener is automatically restarted whenever possible. Upon restart, the listener recovers all of its back-end process connections.

For example, if a listener process is restarted, communication between the Web server and the listener are restored without user intervention. The Web server retries until the listener is available and then reconnects with the listener for each user session that was in progress.