Unlike the centralized NT domain database maintained on the primary domain controller, the Active Directory database is stored as a global catalog (GC) on all domain controllers that have been designated as global catalog hosts throughout the enterprise. Updates to the listing of security principles may be enacted through any copy of the global catalog and then propagated to all others as each cycle of updates occurs until all copies of the global catalog contain the new changes. Obviously, in a global deployment scenario, this would provide greater redundancy in the event of inaccessibility or loss of function of a domain controller, as well as minimizing the potential for a replication bottleneck, as it is no longer necessary for a single primary domain controller to update all other backup domain controllers.
Using Windows 2000 Active Directory, all domain controllers perform the same tasks, such as user login authentication and resource management, equally. Unlike Windows NT 4.0 Domain controllers, all Windows 2000 Domain Controllers are peers, providing the same services. In order to provide backward compatibility to legacy servers, one server in each domain is assigned the role of PDC Emulator, but the global catalog functions identically on all domain controllers designated as global catalog servers. The PDC Emulator provides a centralized point for replication of data to NT 4.0 Backup Domain controllers, which remain in mixed-mode domains. (Native-mode and mixed-mode domains are discussed in greater detail in Chapter 9.)
If conflicting changes occur (such as in the case where two administrators modify the attributes on a single entity), the change made later will be the one that is kept.