Failover
A failover is the process of automatically or manually switching to a redundant or standby system, component, or network upon the failure or abnormal termination of the previously active system, component, or network. It is a core mechanism of fault tolerance and a standard tactic in disaster recovery planning.
Failover improves the availability of a system. It helps to remove single points of failure and keep a service operational even when some of its components fail. To fail over at all, a system needs redundancy. Standby instances are held in reserve so that traffic can be redirected without provisioning new capacity mid-incident.
Failover is the defining mechanic of an active-passive topology, where standby instances wait idle until the active one fails. In an active-active topology, by contrast, every instance serves live traffic and a failed node’s load is simply redistributed, so no failover step is required.
A failover has two phases: detecting that the active instance has failed, and promoting a standby to take its place. Detection relies on health signals such as heartbeats and health checks. Promotion may be automatic, driven by a load balancer or cluster manager, or manual, requiring an operator to intervene. The faster and more reliably both phases run, the shorter the resulting downtime.
Failover is not instant. Depending on the detection and promotion mechanism, recovery can take anywhere from milliseconds to minutes, during which the service is partly or wholly unavailable. A standby that has fallen behind on replication may also lose or reject recent writes when promoted, so the failover speed a system can honestly claim is bounded by how current its standby state is. Because a standby usually runs the same software as the primary, failover alone offers no protection against failures caused by a software bug. A stand-in running a different implementation can. This is why failing fast matters. A component that fails visibly gives the failover mechanism something to detect and route around.