Failover

The second server takes over the work of the first as soon as it detects an alteration in the "heartbeat" of the first machine.

Failback is the process of restoring a system, component, or service previously in a state of failure back to its original, working state, and having the standby system go from functioning back to standby.

The use of virtualization software has allowed failover practices to become less reliant on physical hardware through the process referred to as migration in which a running virtual machine is moved from one physical host to another, with little or no disruption in service.

In the same way, a failback operation can be performed to the first node once the issue is resolved or maintenance is done on it.

The term "failover", although probably in use by engineers much earlier, can be found in a 1962 declassified NASA report.