What Is the Global Catalog?

[kmohaan]

The global catalog is a distributed data repository that contains a
searchable, partial representation of every object in every domain in
a multidomain Active Directory forest. The global catalog is stored on
domain controllers that have been designated as global catalog servers
and is distributed through multimaster replication. Searches that are
directed to the global catalog are faster because they do not involve
referrals to different domain controllers.

In addition to configuration and schema directory partition replicas,
every domain controller in a Windows 2000 Server or Windows Server
2003 forest stores a full, writable replica of a single domain
directory partition. Therefore, a domain controller can locate only
the objects in its domain. Locating an object in a different domain
would require the user or application to provide the domain of the
requested object.

The global catalog provides the ability to locate objects from any
domain without having to know the domain name. A global catalog server
is a domain controller that, in addition to its full, writable domain
directory partition replica, also stores a partial, read-only replica
of all other domain directory partitions in the forest. The additional
domain directory partitions are partial because only a limited set of
attributes is included for each object. By including only the
attributes that are most used for searching, every object in every
domain in even the largest forest can be represented in the database
of a single global catalog server.

The global catalog is built and updated automatically by the Active
Directory replication system. The attributes that are replicated to
the global catalog are identified in the schema as the partial
attribute set (PAS) and are defined by Microsoft. However, to optimize
searching, you can edit the schema by adding or removing attributes
that are stored in the global catalog.
In Windows 2000 Server environments, any change to the PAS results in
full synchronization (update of all attributes) of the global catalog.
Windows Server 2003 reduces the impact of updating the global catalog
by replicating only the attributes that change.