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Data Replication
What is data replication?
Data replication is a process that happens when Active Directory domain controllers need to synchronize to ensure that the querying the database produces consistent results.
When a domain controller receives changes, they are automatically replicated to other domain controllers in a systematic fashion.
What happens during replication?
On receiving updates, each domain controller will update the database file called ntds.dit (found in C:\Windows\NTDS by default).
This single database file is where Active Directory physically stores all Active Directory objects for a single forest.
How do different domain controllers engage in data replication?
By default, Active Directory automatically designs a connection topology between domain controllers for replicating data.
This default connection topology maximizes directory consistency and minimizes the impact to network traffic by making optimal use of physical network connections.
Why is it important to consider data replication?
When designing Active Directory (i.e. defining forests, domains, and organizational units) for your organization, you should consider the amount of replication data and where that replication traffic takes place.
For example, since one database file (ntds.dit) stores all the information for a domain and that database file requires constant replication across all domain controllers for a domain, it may be more efficient to split users across different domains as that will reduce or split up the amount of replication traffic in your datacenter.
Resources
To read more about Active Directory internals, please read the Microsoft docs.