High 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.

Users want their systems, for example computers, to be ready to serve them at all times. 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. The system is said to be unavailable if a user cannot access it. In general, the term downtime is used to refer to periods when a system is unavailable.

High-availability clusters (also known as HA Clusters or Failover Clusters) are computer clusters that are implemented primarily for the purpose of providing high availability of services which the cluster provides. They are often used for critical databases, business applications, file sharing on a network and customer services such as electronic commerce websites. They operate by having redundant computers or nodes which are then used to provide service when system components fail. Normally, if a server with a particular application crashes, the application will be unavailable until someone fixes the crashed server. HA clustering remedies this situation by detecting hardware/software faults, and immediately restarting the application on another system without requiring administrative intervention, a process known as Failover.

Alternative methods of high-availability include push-button failover solutions include backup appliances and advance backup software with the ability to provide a hot physical or virtual standby machine.