Consider an organization that wishes to make data about its employees available to a variety of people in the organization; example of the kinds of data would include name, designation, employee-id, address, email address, phone number, fax number, and so on. In the precomputerization days, organizations would create physical directories of employees and distribute them across the organization. Even today, telephone companies create physical directories of customers.
In general, a directory is a listing of information about some class of objects such as persons. Directories can be used to find information about a specific object, or in the reverse direction to find objects that meet a certain requirement. In the world of physical telephone directories, directories that satisfy lookups in the forward direction are called white pages, while directories that satisfy lookups in the reverse direction are called yellow pages.
Directory Access Protocols
Directory information can be made available through Web interfaces, as many organizations, and phone companies in particular do. Such interfaces are good for humans. However, programs too, need to access directory information. Directories can be used for storing other types of information, much like file system directories. For instance, Web browsers can store personal bookmarks and other browser settings in a directory system. A user can thus access the same settings from multiple locations, such as at home and at work, without having to share a file system.
Several directory access protocols have been developed to provide a standardized way of accessing data in a directory. The most widely used among them today is the Lightweight Directory Access Protocol (LDAP). Obviously all the types of data in our examples can be stored without much trouble in a database system, and accessed through protocols such as JDBC or ODBC. The question then is, why come up with a specialized protocol for accessing directory information? There are at least two answers to the question.
• First, directory access protocols are simplified protocols that cater to a limited type of access to data. They evolved in parallel with the database access protocols.
• Second, and more important, directory systems provide a simple mechanism to name objects in a hierarchical fashion, similar to file system directory names,which can be used in a distributed directory system to specify what information is stored in each of the directory servers.
Lightweight Directory Access Protocol
Clients use the application programmer interface defined by directory system to communicate with the directory servers. Directory access protocols also define a data model and access control.
The X.500 directory access protocol, defined by the International Organization for Standardization (ISO), is a standard for accessing directory information. However, the protocol is rather complex, and is not widely used. The Lightweight Directory Access Protocol (LDAP) provides many of the X.500 features, but with less complexity, and is widely used.
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