Primary Domain Controller

In older versions of Microsoft Windows, specifically in the Windows NT 4.0 and earlier environments, the concept of a Primary Domain Controller (PDC) was a key element in the network architecture for managing user accounts and security.

The PDC was responsible for storing and managing the master copy of the user and computer accounts database for the domain. It handled all changes to the domain's user accounts, computer accounts, and security policies. The PDC served as the main authentication server for the domain. It processed login requests and verified user credentials.

The PDC automatically replicated all changes to the domain's database to Backup Domain Controllers (BDCs) within the domain. This replication ensured that BDCs had an up-to-date, read-only copy of the domain database.

The PDC was responsible for processing password changes and account lockout information. When a user changed their password, the change was first made on the PDC before being replicated to BDCs. Each Windows NT domain had exactly one PDC. This setup was a single point of failure, as the loss of the PDC could significantly impact network operations until a BDC was promoted to take its place.

In a multiple domain environment, the PDC also played a key role in establishing and maintaining trust relationships with other domains.

Microsoft introduced Active Directory, replacing the PDC/BDC model with a multi-master replication model. In this new model:

• Multiple Writable Domain Controllers: All domain controllers hold a writable copy of the Active Directory database, eliminating the single point of failure inherent in the PDC/BDC model.
• Flexible Single Master Operations (FSMO) Roles: Certain specialized roles, similar in concept to the PDC's unique responsibilities, are assigned to individual domain controllers in an Active Directory domain.
• Improved Scalability and Reliability: The new model provided better scalability, reliability, and easier management of network resources.