Redundancy is great, but it actually doesn’t buy you anything except the opportunity to recover from a failure. (Heck, you can get that with backups.) Redundancy doesn’t increase availability or reduce downtime one whit. High availability is built on top of redundancy, through the process of failover. When a component fails and there is redundancy, you can stop using the failed piece and start using its redundant standby instead. The combination of redundancy and failover can enable you to recover more quickly, and as you know, reducing MTTR reduces downtime and improves availability.

Before we continue, we should talk about a few terms. We use “failover” consistently; some people use “fallback” as a synonym. Sometimes people also say “switchover” to denote a switch that’s planned instead of a response to a failure. Po-tay-toe, po-tah-toe. We also use the term “failback” to indicate the reverse of failover. If you have failback capability, failover can be a two-way process: when server A fails and server B replaces it, you can repair server A and fail back to it.