Active Directory Replication

Multimaster Replication

Active Directory uses multimaster replication to accomplish the synchronization of directory information. True multimaster replication can be contrasted with other directory services that use a master-slave approach to updates wherein all updates must be made to the master copy of the directory and then be replicated to the slave copies. This system is adequate for a directory that has a small number of copies and for an environment where all of the changes can be applied centrally. But this approach does not scale beyond small-sized organizations nor does it address the needs of decentralized organizations. With Active Directory, no one domain controller is the master. Instead, all domain controllers within a domain are equivalent. Changes can be made to any domain controller, unlike a single-master system, where changes must be made to one server. In the single-master system, the primary server replicates the updated information to all other directory servers in the domain.

With multimaster replication, it is not necessary for every domain controller to replicate with every other domain controller. Instead, the system implements a robust set of connections that determines which domain controllers replicate to which other domain controllers to ensure that networks are not overloaded with replication traffic and that replication latency is not so long that it causes inconvenience to users. The set of connections through which changes are replicated to domain controllers in an enterprise is called the replication topology.

Multimaster update capability provides high availability of write access to directory objects because several servers can contain writable copies of an object. Each domain controller in the domain can accept updates independently, without communicating with other domain controllers. The system resolves any conflicts in updates to a specific directory object. If updates cease and replication continues, all copies of an object eventually reach the same value.

The manner in which a directory service stores information directly determines the performance and scalability of the directory service. Directory services must handle a large number of queries compared to the number of updates they must process. A typical ratio of queries to updates is 99:1. By creating multiple copies of the directory and keeping the copies consistent, the directory service can handle more queries per second.

Multimaster replication provides the following advantages over single-master replication:

If one domain controller becomes inoperable, other domain controllers can continue to update the directory. In single-master replication, if the primary domain controller becomes inoperable, directory updates cannot take place. For example, if the failed server holds your password and your password has expired, you cannot reset your password and therefore you cannot log on to the domain.
Servers that are capable of making changes to the directory, which in Windows 2000 are domain controllers, can be distributed across the network and can be located in multiple physical sites.