User:BDBJack/Failover

In computing, failover is switching to a redundant or standby computer server, system, hardware component, network or service upon the failure or abnormal termination of the previously active application, server, system, hardware component, or network. Failover and switchover are essentially the same operation, except that failover is automatic and usually operates without warning, while switchover requires human intervention.

Systems designers usually provide failover capability in servers, systems or networks requiring continuous availability -- the used term is High Availability -- and a high degree of reliability.

At server level, failover automation usually uses a "heartbeat" cable that connects two servers. As long as a regular "pulse" or "heartbeat" continues between the main server and 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 downtime. The second server takes 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 send a notification of failover.

Some systems, intentionally, do not failover entirely automatically, but require human intervention. This "automated with manual approval" configuration runs automatically once a human has approved the failover.

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

The use of virtualization software has allowed failover practices to become less reliant on physical hardware; see also teleportation (virtualization)

Types of Failovers
There are many forms of failover mechanisms, which can be activated both manually and automatically.

Network Failovers

 * Load balancing
 * Clustering

Data Failovers

 * Mirroring
 * Cold Backups
 * Warm Backups
 * Log Shipping
 * Replication
 * RAID 1 (Mirroring)
 * RAID 5 (Mirroring with parity)