The Active Directory is a multi-master database. As such, is can be updated by any Domain controller that is writeable. There are some sensitive operations that need to be more strictly controlled than others (schema management and adding or removing one or more domains from an active directory forest) that are specified roles. This means that only one domain controller in the replica ring can provide a specific operation. To find the roles being held by the domain controllers, the best tool to use would be the ntdstil. Using this tool, do the following: 1 Go to the Start button 2 Type “roles”, hit enter 3 Type “connections”, hit enter 4 Type “connect to server (name of server) and (name of domain), press enter 5 Type “quit”, press enter 6 Type “select operation target”, press enter 7 Type “list roles for connected server”, press enter, and quit

A system can have only one schema master and domain naming master in the entire forest and only one PDC, RID and IM in your domain. If any of the domain controllers fail, then you can transfer the roles or seize them.
The Primary Domain Controller Emulator is the role which carries the highest importance in the domain (since it is responsible for the password changes and/or account lockout, as well as the Group Policy Object setting changes in the domain). However, if the domain controller holding PDC fails there will be no immediate impact. Once you find that the domain controller holding the PDC has gone down, then you can easily transfer or seize the role to another domain controller.
Additionally, Relative ID master is also one of the most important roles which need to be considered. If the DC holding the RID master goes down, then you might not be able to create new Active Domain objects in your domain (user accounts groups, etc.).