What Is a Directory?

A directory is a way in which complex information is organized, making it easy to find. Directories list resources--for example, people, books in a library, or merchandise in a department store--and give details about each one. They can be either offline--for example, a telephone book or a department store catalog--or online.

Online directories are used by enterprises with distributed computer systems for fast searches, cost-effective management of users and security, and a central integration point for multiple applications and services. Online directories are also becoming critical to both e-businesses and hosted environments.

This section contains these topics:

• The Expanding Role of Online Directories

• The Problem: Too Many Special-Purpose Directories

The Expanding Role of Online Directories

An online directory is a specialized database that stores and retrieves collections of information about objects. Such information can represent any resources that require management: employee names, titles, and security credentials; information about partners; or information about shared network resources such as conference rooms and printers.

Online directories can be used by a variety of users and applications, and for a variety of purposes, including:

• An employee searching for corporate whitepage information, and, through a mail client, looking up e-mail addresses

• An application, such as a message transport agent, locating a user's mail server

• A database application identifying role information for a user

Although an online directory is a database--that is, a structured collection of data--it is not a relational database. The following table contrasts online directories with relational databases.

Table 1-1  Comparison of Online Directories and Relational Databases

Online Directories	Relational Databases
Primarily read-focused. Typical use involves a relatively small number of data updates, and a potentially large number of data retrievals.	Primarily write-focused. Typical use involves continuous recording of transactions, with retrievals done relatively infrequently.
Designed to handle relatively simple transactions on relatively small units of data. For example, an application might use a directory simply to store and retrieve an e-mail address, a telephone number, or a digital portrait.	Designed to handle large and diverse transactions using many operations on large units of data.
Designed to be location-independent. Directory-enabled applications expect, at all times, to see the same information throughout the deployment environment--regardless of which server they are querying. If a queried server does not store the information locally, then it must either retrieve the information or point the client application to it transparently.	Typically designed to be location-specific. While a relational database can be distributed, it usually resides on a particular database server.
Designed to store information in entries. These entries might represent any resource customers wish to manage: employees, e-commerce partners, conference rooms, or shared network resources such as printers. Associated with each entry is a number of attributes, each of which may have one or more values assigned. For example, typical attributes for a person entry might include first and last names, e-mail addresses, the address of a preferred mail server, passwords or other login credentials, or a digitized portrait.	Designed to store information as rows in relational tables.

The Problem: Too Many Special-Purpose Directories

According to some estimates, each of the world's largest companies has an average of 180 different directories, each designated for a special purpose. Add to this the various enterprise applications, each with its own additional directory of user names, and the actual number of special purpose directories becomes even greater.

Managing so many special purpose directories can cause problems:

• High cost of administration: Administrators must maintain essentially the same information in many different places. For example, when an enterprise hires a new employee, administrators must create a new user identity on the network, create a new e-mail account, add the user to the human-resources database, and set up all applications that the employee may need--for example, user accounts on development, testing, and production database systems. Later, if the employee leaves the company, administrators must reverse the process to disable all these user accounts.

• Inconsistent data: Because of the large administrative overhead, it can be difficult for multiple administrators, entering redundant information in multiple systems, to synchronize this employee information across all systems. The result can be inconsistent data across the enterprise.

• Security issues: Each separate directory may have its own password policy--which means that a user may struggle with a variety of user names and passwords, each for a different system.

Today's enterprises need a more general purpose directory infrastructure, one based on a common standard for supporting a wide variety of applications and services.