"Failover clusters are used to ensure high availability of system services and applications even through crashes, hardware failures, and environmental mishaps. In this article, I'll show you how to implement a rock-solid two-node high availability Apache cluster with the heartbeat application from The High-Availability Linux Project. I tested the cluster on Fedora Core 5, CentOS 4.3, and Ubuntu 6.06.1 LTS server distributions.
"In a cluster environment, a high availability (HA) system is responsible for starting and stopping services, mounting and dismounting resources, monitoring the system availability in the cluster environment, and handling the ownership of the virtual IP address that's shared between cluster nodes. The heartbeat service provides the basic functions required for the HA system..."
