Understanding Failover
Prior to Windows 2000, Windows NT supported multiple domain controllers: Primary (PDC) and Backup (BDC). You could query any controller for information, but changes could only be made to the PDC. Active Directory, introduced as part of the Windows 2000 Server, goes a step further. It allows all controllers to be primaries, and a change to any one controller is automatically propagated to the other controllers.
This allows CA IdentityMinder, which is used to manage an installation, to have failover support. For example, ADS is communicating with a single domain controller and it goes down. ADS then automatically connects to an alternate domain controller and retries the failed operation. Thereafter, all communications happen with the alternate controller.
For technical reasons, it is advantageous to establish an order in which the controllers are to be used. This can be done from the Failover page on the Endpoint property sheet. This page automatically displays the alternate controllers (as retrieved from DNS) and allows the user to prioritize them.
In the background, ADS periodically attempts to reconnect to any failed controllers. When ADS detects that a failed controller of a higher priority than the current controller is back online, it automatically reroutes the next request to the restored controller.
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