Editing Domain controller (Windows) (section)

== History ==
One domain controller per domain was configured as the primary domain controller (PDC), all other domain controllers were backup domain controllers (BDC).
                               
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 or 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.

A BDC could authenticate the users in a domain, but all updates to the domain could only be made via the PDC, which would then propagate these changes to all BDCs in the domain.  If the PDC was unavailable the update would fail. If the PDC was permanently unavailable an existing BDC could be promoted to be and later versions introduced [[Active Directory]] ("AD"), which largely eliminated the concept of PDC and BDC in favor of [[multi-master replication]]. However, there are still several 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 [[Active Directory#Forests, Trees and Domains|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 in a process known as "seizing" the role.