Failover support
Load Balancer acts as a single point-of-presence for your enterprise's content hosts. This is beneficial because you advertise the cluster host name and address in DNS, rather than the host name and address of each content host, which provides a level of protection against casual attacks and provides a unified feel for your enterprise's Web site. To further enhance Web site availability, configure another Load Balancer to act as a backup for the primary Load Balancer, as depicted in Figure 10. If one Load Balancer fails or becomes inaccessible due to a network failure, end users can still reach the content hosts.
In the normal case, a browser running on one of the machines marked 1 directs its request for a file X to the cluster host name that is mapped to the primary Load Balancer (4). The Dispatcher routes the request to the content host (6) selected on the basis of the Dispatcher's load-balancing criteria. The content host sends file X directly to the browser, routing it through the enterprise's gateway (3) across the Internet (2) but bypassing Load Balancer.
The backup Dispatcher (5) does not perform load balancing as long as the primary one is operational. The primary and backup Dispatchers track each other's status by periodically exchanging messages called heartbeats. If the backup Dispatcher detects that the primary has failed, it automatically takes over the responsibility for load balancing by intercepting requests directed to the primary's cluster host name and IP address.
It is also possible to configure two Dispatchers for mutual high availability. In this case, each actively performs load balancing for a separate cluster of content hosts, simultaneously acting as the backup for its colleague. (On Load Balancer for IPv4 and IPv6 installations simple high availability is supported, but mutual high availability is not.)
The Dispatcher does not generally consume many processing or memory resources, and other applications can run on the Load Balancer machine. If it is vital to minimize equipment costs, it is even possible to run the backup Dispatcher on one of the machines in the cluster it is load balancing. Figure 11 depicts such a configuration, in which the backup Dispatcher runs on one of the content hosts (5) in the cluster.