"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..."