Stand-by Servers
Stand-by servers are a fault-tolerant measure in which a second server is configured identically to the first one. The second server can be stored remotely or locally and set up in a failover configuration. In a failover configuration, the secondary server is connected to the primary and ready to take over the server functions at a heartbeat's notice. If the secondary server detects that the primary has failed, it will automatically cut in. Network users will not notice the transition, as there will be little or no disruption in data availability.
The primary server communicates with the secondary server by issuing special notification notices referred to as heartbeats. If the secondary server stops receiving the heartbeat messages, it assume that the primary has died and so assumes the primary server configuration.