Active Directory is the central repository in which all objects in an enterprise and their respective attributes are stored. It is a hierarchical, multi-master enabled database, capable of storing millions of objects. Because it is multi-master, changes to the database can be processed at any given domain controller (DC) in the enterprise regardless of whether the DC is connected or disconnected from the network.

Multi-Master Model

A multi-master enabled database, such as the Active Directory, provides the flexibility of allowing changes to occur at any DC in the enterprise, but it also introduces the possibility of conflicts that can potentially lead to problems once the data is replicated to the rest of the enterprise. One way Windows deals with conflicting updates is by having a conflict resolution algorithm handle discrepancies in values by resolving to the DC to which changes were written last (that is, "the last writer wins"), while discarding the changes in all other DCs. Although this resolution method may be acceptable in some cases, there are times when conflicts are just too difficult to resolve using the "last writer wins" approach. In such cases, it is best to prevent the conflict from occurring rather than to try to resolve it after the fact. 

For certain types of changes, Windows incorporates methods to prevent conflicting Active Directory updates from occurring.

Single-Master Model

To prevent conflicting updates in Windows, the Active Directory performs updates to certain objects in a single-master fashion. In a single-master model, only one DC in the entire directory is allowed to process updates. This is similar to the role given to a primary domain controller (PDC) in earlier versions of Windows (such as Microsoft Windows NT 3.51 and 4.0), in which the PDC is responsible for processing all updates in a given domain. 

Active Directory extends the single-master model found in earlier versions of Windows to include multiple roles, and the ability to transfer roles to any domain controller (DC) in the enterprise. Because an Active Directory role is not bound to a single DC, it is referred to as a Flexible Single Master Operation (FSMO) role. Currently in Windows there are five FSMO roles: 

• Schema master
• Domain naming master
• RID master
• PDC emulator
• Infrastructure master

Schema Master FSMO Role

The schema master FSMO role holder is the DC responsible for performing updates to the directory schema (that is, the schema naming context or LDAP://cn=schema,cn=configuration,dc=<domain>). This DC is the only one that can process updates to the directory schema. Once the Schema update is complete, it is replicated from the schema master to all other DCs in the directory. There is only one schema master per directory.

Domain Naming Master FSMO Role

The domain naming master FSMO role holder is the DC responsible for making changes to the forest-wide domain name space of the directory (that is, the Partitions\Configuration naming context or LDAP://CN=Partitions, CN=Configuration, DC=<domain>). This DC is the only one that can add or remove a domain from the directory. It can also add or remove cross references to domains in external directories.

RID Master FSMO Role

The RID master FSMO role holder is the single DC responsible for processing RID Pool requests from all DCs within a given domain. It is also responsible for removing an object from its domain and putting it in another domain during an object move. 

When a DC creates a security principal object such as a user or group, it attaches a unique Security ID (SID) to the object. This SID consists of a domain SID (the same for all SIDs created in a domain), and a relative ID (RID) that is unique for each security principal SID created in a domain. 

Each Windows DC in a domain is allocated a pool of RIDs that it is allowed to assign to the security principals it creates. When a DC's allocated RID pool falls below a threshold, that DC issues a request for additional RIDs to the domain's RID master. The domain RID master responds to the request by retrieving RIDs from the domain's unallocated RID pool and assigns them to the pool of the requesting DC. There is one RID master per domain in a directory.

PDC Emulator FSMO Role

The PDC emulator is necessary to synchronize time in an enterprise. Windows includes the W32Time (Windows Time) time service that is required by the Kerberos authentication protocol. All Windows-based computers within an enterprise use a common time. The purpose of the time service is to ensure that the Windows Time service uses a hierarchical relationship that controls authority and does not permit loops to ensure appropriate common time usage.

The PDC emulator of a domain is authoritative for the domain. The PDC emulator at the root of the forest becomes authoritative for the enterprise, and should be configured to gather the time from an external source. All PDC FSMO role holders follow the hierarchy of domains in the selection of their in-bound time partner. 

In a Windows domain, the PDC emulator role holder retains the following functions:

• Password changes performed by other DCs in the domain are replicated preferentially to the PDC emulator.
• Authentication failures that occur at a given DC in a domain because of an incorrect password are forwarded to the PDC emulator before a bad password failure message is reported to the user.
• Account lockout is processed on the PDC emulator.
• The PDC emulator performs all of the functionality that a Microsoft Windows NT 4.0 Server-based PDC or earlier PDC performs for Windows NT 4.0-based or earlier clients.

This part of the PDC emulator role becomes unnecessary when all workstations, member servers, and domain controllers that are running Windows NT 4.0 or earlier are all upgraded to Windows 2000. The PDC emulator still performs the other functions as described in a Windows 2000 environment. 

The following information describes the changes that occur during the upgrade process:

• Windows clients (workstations and member servers) and down-level clients that have installed the distributed services client package do not perform directory writes (such as password changes) preferentially at the DC that has advertised itself as the PDC; they use any DC for the domain.
• Once backup domain controllers (BDCs) in down-level domains are upgraded to Windows 2000, the PDC emulator receives no down-level replica requests.
• Windows clients (workstations and member servers) and down-level clients that have installed the distributed services client package use the Active Directory to locate network resources. They do not require the Windows NT Browser service.

Infrastructure FSMO Role

When an object in one domain is referenced by another object in another domain, it represents the reference by the GUID, the SID (for references to security principals), and the DN of the object being referenced. The infrastructure FSMO role holder is the DC responsible for updating an object's SID and distinguished name in a cross-domain object reference. 

NOTE: The Infrastructure Master (IM) role should be held by a domain controller that is not a Global Catalog server(GC). If the Infrastructure Master runs on a Global Catalog server it will stop updating object information because it does not contain any references to objects that it does not hold. This is because a Global Catalog server holds a partial replica of every object in the forest. As a result, cross-domain object references in that domain will not be updated and a warning to that effect will be logged on that DC's event log. 

If all the domain controllers in a domain also host the global catalog, all the domain controllers have the current data, and it is not important which domain controller holds the infrastructure master role.

When the Recycle Bin optional feature is enabled, every DC is responsible to update its cross-domain object references when the referenced object is moved, renamed, or deleted. In this case, there are no tasks associated with the Infrastructure FSMO role, and it is not important which domain controller owns the Infrastructure Master role. For more information, see 6.1.5.5 Infrastructure FSMO Role athttp://msdn.microsoft.com/en-us/library/cc223753.aspx