Active Directory Replication Technologies
Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2, Windows Server 2008, Windows Server 2008 R2, Windows Server 2012, Windows Server 2012 R2
Active Directory is a distributed directory service that stores objects that represent real-world entities such as users, computers, services, and network resources. Objects in the directory are distributed among all domain controllers in a forest, and all domain controllers can be updated directly. Active Directory replication is the process by which the changes that originate on one domain controller are automatically transferred to other domain controllers that store the same data.
In Windows 2000 Server and Windows Server 2003, the directory service is named Active Directory. In Windows Server 2008 and Windows Server 2008 R2, the directory service is named Active Directory Domain Services (AD DS). The rest of this topic refers to Active Directory, but the information is also applicable to AD DS.
Active Directory Replication Model
The replication model comprises the mechanisms that support the multimaster update capabilities of Active Directory domain controllers. To ensure that replication data is transferred efficiently in the multimaster system, domain controllers track the changes that they have received and request only the updates that have occurred since the last replication. The update tracking is based on the state of the data as it exists on a replicating pair of domain controllers at the time of replication. Update tracking ensures that:
Only changes that have not been received are replicated to a destination.
Conflicts are resolved according to the last change that occurred, even when individual domain controller clocks are not synchronized or when administrators at different domain controllers make changes to the same object.
The replication model also accommodates multimaster updates by enabling replicated changes to be stored on destination domain controllers and forwarded to other domain controllers. This store-and-forward capability removes the need for every domain controller on which updates originate to contact every other domain controller that requires the updates.
Active Directory Replication Topology
The replication topology is the current set of Active Directory connections by which domain controllers in a forest communicate over local area networks (LANs) and wide area networks (WANs) to synchronize the directory partition replicas that they have in common. The replication topology ensures the transfer of changes to all directory partition replicas in the forest without redundancy. Replication topology generation is dynamic and adapts to network conditions and availability of domain controllers.
To ensure a consistent replication topology, domain controllers use global configuration data to arrive at the same view of domain controller data. They apply the same algorithm to this data to arrive at an identical replication topology. Operating independently, each domain controller contributes to a uniform and efficient replication topology.
Replication topology generation is optimized for speed within sites and for cost between sites. Replication between domain controllers in the same site occurs automatically in response to changes and does not require administrative management. Replication within a site is sent uncompressed to reduce processing time. Replication between domain controllers in different sites can be managed to control the scheduling and routing of replication over WAN links. Replication between sites is compressed so that it uses less bandwidth when sent across WAN links, thereby reducing the cost.