Software Architect's Handbook

Varying levels of redundancy

One way to recover from faults and achieve availability is to have a failover mechanism. There are varying levels of redundancy that can be provided.

In active/hot spare environments, each component has another one that performs the same processes with the same inputs, so that, if one fails, the other component can take over at any time. Failover is generally transparent because the recovery time is nearly instantaneous.

A passive/warm spare environment is one in which only active components perform processes from inputs, but the active components provide the backup components with periodic state updates. A passive redundancy approach is not as highly available as an active/hot spare, but it is less expensive ...