Active Directory Replication

State-based Replication

Active Directory uses a state-based approach to replication. In state-based replication, each master in the multimaster system applies updates to its replica as they arrive, without maintaining a change log file. (The database system used by Active Directory does use a transaction log file, but this log is part of the database system, not the replication system.) In a typical log-based replication system, each master keeps a log of the updates that it originated. In the log-based system, the goal of each master is to communicate its log to every other replica. After a log has arrived at a replica, the replica applies the log, bringing its state more up-to-date.

Active Directory replication is driven not by logs stored with the source replica, but by the current "state" (the current values of all objects) of the source replica. This state includes information that is used to resolve conflicts and avoid sending the full replica on each replication cycle. Each originating write operation is assigned a sequence number at its originating domain controller. All replicas maintain information about how up-to-date they are with respect to all other replicas, and values in the directory are tagged with the sequence numbers of their originating write updates. By using this information, the replication source can filter the state changes that it replicates.

A state-based approach uses a single mechanism for incremental and full synchronization, and performs fewer database updates because repeated or conflicting updates to an attribute are collapsed into a single state.

Generally speaking, an Active Directory partition replica maintains all of its objects in a list ordered by last modification time. This is a log of sorts, but a coarse one whose size is at most a tiny fraction of the size of the replica itself. A typical replication request can be satisfied by examining only the last few objects on this list because the replication destination server is aware of how much of its replication source's list has already been processed.