Normal HotStandby operation is highly reliable. The odds of both the primary and secondary servers failing at nearly the same time are low, if they use separate reliable power supplies. But you might want to reduce even the risk of power failure, or that the server that failed cannot be repaired rapidly. Ideally, when a primary server fails and the secondary server takes its place (or when a secondary server fails), you want to have a new secondary server that replaces the old secondary server so that you can continue to run with a complete pair of servers.

There are N HSB pairs, that is, every primary server has a secondary server. In addition, there are M spare servers, where M is at least one and usually less than N. When a primary or secondary server fails, a spare server is brought in as the new secondary server. Thus, a primary server never operates alone for long, even if its original partner has failed.

Spares are not dedicated to a particular server (or HSB pair of servers), and some configuration is required before the spare can replace the failed server.