Flexible single master operation

Flexible Single Master Operations (FSMO, F is sometimes floating ; pronounced Fiz-mo), or just single master operation or operations master, is a feature of Microsoft's Active Directory (AD). As of 2005, the term FSMO has been deprecated in favour of operations masters.

FSMO is a specialized domain controller (DC) set of tasks, used where standard data transfer and update methods are inadequate. AD normally relies on multiple peer DCs, each with a copy of the AD database, being synchronized by multi-master replication. The tasks which are not suited to multi-master replication, and are viable only with a single-master database, are the FSMOs.

These roles are applicable at the domain level (i.e., there is one of each for every domain in a forest):

These roles are unique at the forest level (both are located in the forest root domain):

By default AD assigns all operations master roles to the first DC created in a forest. To provide fault tolerance, there should be at least 2 domain controllers available within each domain of the Forest. If new domains are created in the forest, the first DC in a new domain holds all of the domain-wide FSMO roles. This is not a satisfactory position if an active directory has a large number of domain controllers. Microsoft recommends the careful division of FSMO roles, with standby DCs ready to take over each role.

The PDC emulator and the RID master should be on the same DC, if possible. The Schema Master and Domain Naming Master should also be on the same DC.

When a FSMO role is transferred to a different DC, the original FSMO holder and the new FSMO holder communicate to ensure no data is lost during the transfer. If the original FSMO holder experienced an unrecoverable failure, you can force another DC to seize the lost roles; however, there is a risk of data loss because of the lack of communications. If you seize a FSMO role instead of transferring the role, that domain controller can never be allowed to host that FSMO role again, except for the PDC emulator Master operation and the Infrastructure Master Operation. Corruption can occur within Active Directory. FSMO roles can be easily moved between DCs using the AD snap-ins to the MMC or using ntdsutil, which is a command line-based tool.

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