Failover
Automatically switching to a backup system when the primary one fails — ideally without users noticing.
Think of it like
An elevator with a backup power system that kicks in if the main power cuts.
Example
A request gets routed to server A; server A crashes; the load balancer automatically sends it to server B.
How it actually works
Failover can be automatic (health checks detect failure and redirect) or manual. Automatic failover is complex — you need to detect failure quickly (false positives trigger unnecessary switches), coordinate state, and maintain data consistency. DNS failover is slow; load-balancer-based failover is fast.
For product teams
Good failover is invisible to users and crucial for uptime guarantees.
For engineers
Implement health checks carefully; test failover scenarios; use local retry logic.
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