A backup server used to duplicate all the processes and transactions of a primary server.
If the primary server was to fail, the backup server is put in the primary’s place so as to avoid any down time.
Server mirroring can be costly but it is effective for achieving fault tolerance, which is the ability of a system to quickly respond to an unexpected failure.
The expense comes in the fact that each server in the system must be mirrored by an identical server that’s sole purpose for running is in the event of a failure.
Server Farming is a much less expensive technique for achieving fault tolerance.