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Relative Identifier Allocation

It is fairly easy for the system to generate a unique relative identifier for each account and group created on a stand-alone computer, where accounts and groups are stored in an account database managed by a local Security Accounts Manager (SAM) The SAM on a stand-alone computer can simply keep track of relative identifier values it has used before, making sure that it never uses them again.

Generating unique relative identifiers is a more complex process in a network domain Windows 2000 network domains can have several domain controllers, each of them a host for Active Directory, where account information is stored. This means that in a network domain there are as many copies of the account database as there are domain controllers. What is more, every copy of the account database is a master copy. New accounts and groups can be created on any domain controller. Changes made to Active Directory on one domain controller are replicated to all other domain controllers in the domain. The process of replicating changes in one master copy of the account database to all other master copies is called a multimaster operation .

The process of generating unique relative identifiers is a single-master operation . One domain controller is assigned the role of relative identifier (RID) master , and it allocates a sequence of relative identifiers to each domain controller in the domain. When a new domain account or group is created in one domain controller's replica of Active Directory, it is assigned a SID, and the relative identifier for the new SID is taken from the domain controller's allocation of relative identifiers. When its supply of relative identifiers begins to run low, the domain controller asks the RID master for another block.

Each domain controller makes sure that when it has used one value in a block of relative identifiers, it never uses that value again. The RID master makes sure that when it has allocated a block of relative identifiers, it never allocates those values again. The result of this teamwork is that every account and group created in the domain has a unique relative identifier.

Several other tasks performed by domain controllers are single-master operations. For example, one domain controller in an enterprise is assigned responsibility for ensuring that each domain has a unique name and a unique domain identifier. The domain controller assigned that role is called the domain naming master . For more information about single-master operations, see "Managing Flexible Single Master Operations" in this book.

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