The Linux-HA (High-Availability Linux) project provides a high-availability (clustering) solution for Linux, FreeBSD, OpenBSD, Solaris and Mac OS X which promotes reliability, availability, and serviceability (RAS).
The project’s main software product is Heartbeat, a GPL-licensed portable cluster management program for high-availability clustering. Its most important features are:

no fixed maximum number of nodes - Heartbeat can be used to build large clusters as well as very simple ones
resource monitoring: resources can be automatically restarted or moved to another node on failure
fencing mechanism to remove failed nodes from the cluster
sophisticated policy-based resource management, …

High availability is a system design protocol and associated implementation that ensures a certain absolute degree of operational continuity during a given measurement period.
Availability refers to the ability of the user community to access the system, whether to submit new work, update or alter existing work, or collect the results of previous work. If a user cannot access the system, it is said to be unavailable. Generally, the term downtime is used to refer to periods when a system is unavailable.
Planned and unplanned downtime
A distinction needs to be made between …