Directory service

In computing, a directory service or name service maps the names of network resources to their respective network addresses. It is a shared information infrastructure for locating, managing, administering and organizing everyday items and network resources, which can include volumes, folders, files, printers, users, groups, devices, telephone numbers and other objects. A directory service is a critical component of a network operating system. A directory server or name server is a server which provides such a service. Each resource on the network is considered an object by the directory server. Information about a particular resource is stored as a collection of attributes associated with that resource or object. A directory service defines a namespace for the network. The namespace is used to assign a name (unique identifier) to each of the objects. Directories typically have a set of rules determining how network resources are named and identified, which usually includes a requirement that the identifiers be unique and unambiguous. When using a directory service, a user does not have to remember the physical address of a network resource; providing a name locates the resource. Some directory services include access control provisions, limiting the availability of directory information to authorized users. Several things distinguish a directory service from a relational database. Data can be made redundant if it aids performance (e.g. by repeating values through rows in a table instead of relating them to the contents of a different table through a key, which technique is called denormalization; another technique could be the utilization of replicas for increasing actual throughput). Directory schemas are object classes, attributes, name bindings and knowledge (namespaces) where an object class has: Must - attributes that each instances must have May - attributes which can be defined for an instance but can be omitted, with the absence similar to NULL in a relational database Attributes are sometimes multi-valued, allowing multiple naming attributes at one level (such as machine type and serial number concatenation, or multiple phone numbers for "work phone").