Failover is the capability to switch over automatically to a redundant or standby computer server, system, or network upon the failure or abnormal termination of the previously active server, system, or network. In Engineering, redundancy is the duplication of critical components of a system with the intention of increasing reliability of the System, usually A computer is a Machine that manipulates data according to a list of instructions. A server is a Computer dedicated to providing one or more services over a computer network typically through a request-response routine System (from Latin systēma, in turn from Greek systēma is a set of interacting or interdependent Entities, real or abstract A computer network is a group of interconnected Computers. Networks may be classified according to a wide variety of characteristics "Abend" redirects here For other uses see the wiktionary box Failover happens without human intervention and generally without warning, unlike switchover. Switchover is the capability to manually switch over from one system to a redundant or standby Computer server, System, or network

Systems designers usually provide failover capability in servers, systems or networks requiring continuous availability and a high degree of reliability. Systems design is the process or art of defining the architecture components modules interfaces and Data for a System to satisfy specified Requirements In Telecommunications and Reliability theory, the term availability has the following meanings 1

The automation is done using a "Heartbeat" cable that is connected to the two servers. As long as there is a "Pulse or heartbeat" from the main server to the second server, the second server will not initiate its systems. There may also be a third "Spare parts" server that has running spare components for "hot" switching to prevent down time.

The second server will immeditaly take over the work of the first as soon as it detects an alteration in the "heartbeat" of the first machine. Some systems have the ability to page or send a message to a pre-assigned technician or center.

In some cases, computer system failover is not desired to be automatic, but is required to allow human intervention to effect the failover. This is called "automated with manual approval", as the activity is automatic once approval is given.

Failback, conversely, is the process of restoring a system/component/service in a state of failover back to its original state (before failure).

See also

• Safety engineering
• Data reliability
• Fault-tolerance
• High-availability cluster

Safety engineering is an applied science strongly related to Systems engineering and the subset System Safety Engineering Data redundancy (sometimes incorrectly referred to as data reliability) in Computer data storage, is a property of some Disk arrays (most commonly in This article contains specific implementations of fault tolerant systems High-availability clusters (also known as HA Clusters or Failover Clusters) are Computer clusters that are implemented primarily for the purpose of providing

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