High Availability

calm-adHigh 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.


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Chapter 1: Introduction: Why the Backup 1.0 Mentality is Killing You

Series: The Definitive Guide Series
Author: Don Jones
ISBN: pending

Synopsis:
The first backup—technically—was around 1951, when the first generation of digital computing appeared in the form of UNIVAC I. The “backups,” such as they were, were the punch cards used to feed instructions to the massive [...]