In multi-server architectures, one or more Standby servers act as backup servers to the Main server. If the Main server goes offline, a Standby server switches from Standby to Main so that it can carry out the duties of the Main server. The switch from Standby to Main (or vice-versa) is called a changeover.
A changeover occurs due to:
- Hardware failure—If there is a problem with the server hardware, the server acting as the Main server may go offline. The server will be unavailable for use as a Main server or Standby server until the faulty hardware is repaired or replaced.
- Communications failure—If the Main server loses its connection with the Standby server, a Main-Main situations may occur. This is where both servers act as Main servers. In such situations, when the communication link is restored, the servers will arbitrate to determine which server is Main and which is Standby. Main-Main situations can cause problems, especially when PSTN outstations are being used, but these problems can usually be avoided by using IMCP Monitor points (see Define the ICMP Monitor Settings for a Server).
- Software failure—Incorrect configuration could place an excessive demand on system resources and cause the Main server to go offline.
- A manual changeover is performed—Manual changeovers are user controlled Main-Standby switches; a user stops or restarts the Main server to force the Standby server to switch to Main. This is an excellent way to check that the Main server and Standby server can switch correctly without having a significant impact on the system (see Perform a Manual Changeover on a Multi-Server System).
Typically, manual changeovers are performed on a regular basis at times when an unsuccessful changeover will have a minimal effect on the system.