Definition of availability

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, updated 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.

A distinction needs to be made between planned downtime and unplanned downtime. Typically, planned downtime is a result of maintenance that is disruptive to system operation and usually cannot be avoided. Planned downtime events include patches to system software that require a reboot or system configuration changes that only take effect upon a reboot. In general, planned downtime is usually the result of some logical event. Unplanned downtime events typically arise from some physical event, such as a hardware failure or environmental anomaly. Examples of unplanned downtime events include power outages, failed CPU or RAM components (or possibly other failed hardware components) and or an over-temperature related shutdown.

Many computing sites typically exclude planned downtime from availability calculations, since planned downtime should have no impact upon the computing community. By excluding planned downtime, many systems can claim to have phenomenally high availability, which might give the illusion of continuous availability. Systems that exhibit truly continuous availability are rare, expensive and carefully implemented specialty designs that eliminate any single point of failure.

Availability is usually expressed as a percentage of uptime in a given year. (Shorter time periods can be used, but sites that pick artificially short measurement periods may be hiding latent problems in their systems which produce instability, leading to unplanned downtime.) In a given year, the number of minutes of unplanned downtime are tallied for a system; the aggregate unplanned downtime is divided by the total number of minutes in a year (approximately 525,600), producing a percentage of downtime; the complement is the percentage of uptime, which is what is typically referred to as the availability of the system.