What Is a Directory Service?

When asked, "What is a directory service?" most people would answer, "The friendly telephone operator who looks up people's phone numbers for you." That is, of course, a correct answer. If the directory is the actual data—the list of people and telephone numbers—the operators and the method for calling them is the directory service. It's not all that different in the electronic world. Figure 1-2 shows a comparison of a traditional phone directory service and an electronic directory service.

Figure 1-2 Directories and directory services.

A directory service provides for the storage and retrieval of directory information for users and applications. Areas that a directory service must address are performance, security, reliability, availability, and ease of use. ("Ease of use" means that developers can write applications to access directory information without a lot of difficult programming.)

Consider an example. Most large shopping malls have kiosks that list the stores in the mall, categorizing them by the types of products they sell. Services that the mall provides, such as restrooms, telephones, security, and information booths, are also listed and mapped. These kiosks are really directories. Now imagine that there was only one mall directory kiosk, located at the center of the mall. Dozens of people would walk up to the kiosk, everyone trying to get the information at the same time. What if it was very tiny and the text was hard to read? People would spend a lot of time searching and looking for that certain specialty store that sells only socks. (Strangely enough, this is the kind of store my girlfriend likes.) Imagine that while these people are trying to read the mall directory, a janitor walks up and takes the kiosk away to be updated or cleaned. This is an admittedly silly example, but it illustrates the need for performance, reliability, availability, and ease of use in directory services. Malls accommodate hundreds of shoppers by placing many kiosks around the shopping center and by making them easy to understand.

Network directories provide similar solutions. In order to ensure high availability, directory information is replicated to many servers (electronic "kiosks"). The computers accessing the information in the directory can do so from the nearest server, resulting in improved performance. Since directory information can be gathered and presented in a variety of ways, it's easily accessible to an end-user.

Directory services make tasks easier for developers, administrators, and end-users. If you are a developer, a directory service makes it easy to store and look up information about network resources. Applications can publish information to be stored in the network directory to be used by other applications; network administrators gain benefits that include increased security and ease of administration. Most of all, users benefit from a common security model (avoiding multiple passwords) by taking advantage of data being shared between applications and by no longer having to remember specific items for resources in the directory—like the network path to the printer down the hall.