The global catalog is a searchable repository of distributed data. It contains a partial representation of each object in a given domain. The said domain has to be in a multi-domain active directory forest.
Devices known as domain controllers store the data in the global catalog. These domain controllers have been specified as global catalog servers. The global catalog data within these controllers is distributed via multi-master replications.
The global catalog presents unique features to the system. This type of repository enables users to search for objects within a domain without having to know the domain name. Also, the global catalog is automatically created and updated by the system that manages the active directory replication. This ensures that accurate results are provided every time searches are performed. Aside from this, the global catalog does not refer to another server when used.
The global catalog is essential to the occurrence of a number of system events. In the user log-in process, the global catalog is required to provide the domain controllers with the IDs of the group in which the user is a member. The global catalog is also useful in group caching and updating. In sites that enable group caching, domain controllers contact the global catalog to cache group memberships and ensure that the cache is updated. Lastly, the global catalog is used in address book searches of e-mail client software. For instance, when a user creates a new e-mail and enters an address in the “To:” field, the global catalog instantly looks for matches in the global address book or other address lists.