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On Windows Server Systems, a domain controller (DC) is a server that responds to security authentication requests (logging in, checking permissions, etc.) within the Windows Server domain.[1]. A domain is a concept introduced in Windows NT whereby a user may be granted access to a number of computer resources with the use of a single username and password combination.

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Windows NT

In older versions of Windows such as Windows NT server, one domain controller per domain was configured as the Primary Domain Controller (PDC); all other domain controllers were Backup Domain Controllers (BDC).

A BDC could authenticate the users in a domain, but all updates to the domain (new users, changed passwords, group membership, etc) could only be made via the PDC, which would then propagate these changes to all BDCs in the domain. If the PDC was unavailable (or unable to communicate with the user requesting the change), the update would fail. If the PDC was permanently unavailable (e.g. if the machine failed), an existing BDC could be promoted to PDC.

Because of the critical nature of the PDC, best practices dictated that the PDC should be dedicated solely to domain services, and not used for file/print/application services that could slow down or crash the system. Some network administrators took the additional step of having a dedicated BDC online for the express purpose of being available for promotion if the PDC failed.

Windows 2000

Windows 2000 and later versions introduced Active Directory ("AD"), which largely eliminated the concept of primary and backup domain controllers in favor of multi-master replication.

However, there are still a number of roles that only one domain controller can perform, called the Flexible single master operation roles (some of these roles must be filled by one DC per domain, while others only require one DC per AD Forest). If the server performing one of these roles is lost the domain can still function, and if the server will not be available again, an administrator can designate an alternate DC to assume the role (a process known as "seizing" the role).

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