A server instance containing a copy of a database that can be brought online if the source copy of the database becomes unavailable. Log shipping can be used to maintain a "warm" standby server, called a secondary server, whose copy of the database is automatically updated from log backups at regular intervals. Before failover to a warm standby server, its copy of the database must be brought fully up to date manually. Database mirroring can be used to maintain a "hot" standby server, called a mirror server, whose copy of the database is continuously brought up to date. Failover to the database on a mirror server is essentially instantaneous.