What is failover?

Failover is the automatic switch to a standby path when the primary one stops working:

• Internet failover - a router that moves to a second service, often mobile broadband, when the main link drops.
• DNS failover - traffic redirected to a standby host when health checks fail.
• The switch has to be tested - failover that has never been exercised usually turns out to be configuration rather than capability.

Automatic is better than manual, but a tested manual process beats an untested automatic one.