Directory Concepts

A directory object represents an object in a computing environment. A directory object can be used, for example, to represent a printer, a person, a computer, or a network. A distinguishing trait of a directory object is that it contains attributes for describing the object that it represents.
Attributes

A directory object can have attributes associated with it. For example, a printer might be represented by a directory object that has as attributes its speed, resolution, and color. A user might be represented by a directory object that has as attributes the user's email address, various telephone numbers, postal mail address, and computer account information. An attribute has an attribute identifier and a set of attribute values. The email address, for example, might have an attribute identifier of "mail" and value of "john.smith@somewhere.com".
Directories and Directory Services
A directory is a connected set of directory objects. A directory service provides operations for creating, adding, removing, and modifying the attributes associated with objects in a directory. The service is accessed through its interface.
There are many examples of directory services. The NDS is a directory service from Novell which provides information about many networking services such as the file and print services. Network Information Service (NIS) is a directory service available on the Solaris operating system for storing system-related information such as that relating to machines, networks, printers, and users. The Netscape Directory is a general-purpose directory service based on the emerging Internet standard Lightweight Directory Access Protocol (LDAP).
Searches and Search Filters
You can look up a directory object by supplying its name to the directory service. Alternatively, many directories, such as those based on the LDAP, support the notion of searches where instead of a name, you can supply a query consisting of a logical expression specifying the attributes that the object or objects must have. The query is called a search filter and this style of searching is sometimes called reverse lookup or content-based searching. The directory service searches for and returns the objects that satisfy the search filter.
For example, you can ask the directory service to find all users with the attribute "age" greater than 40 years. Similarly, you can ask the directory service to find all machines whose IP address starts with "192.113.50".
Combining Naming and Directory
Directories often arrange their objects in a hierachy. For example, the LDAP has the notion of a directory information tree (DIT) by which all objects in the directory are arranged. An organization object, for example, might contain group objects, which might in turn contain person objects. When directory objects are arranged in this way, they are playing the role of naming contexts in addition to being containers of attributes.

Naming and Directory Concepts