Microsoft's Active Directory

When Windows 2000 Advanced Server was released, it changed Windows networking dramatically. Before, computers and user accounts were organized into domains, and there were primary domain controllers (PDCs), which were used to hold the master copy of the security database, and there were backup domain controllers (BDCs), which held a copy of the security database and provided for redundancy. Changes could be made to the security database only on PDC and then propagated to the BDCs. In a large domain, this could take some time, and when the PDC was down, the administrator could either wait until it was online again to make changes like adding new users or promote a BDC to be the new PDC.

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