In email terms, a directory is an electronic repository of information about people, that also provides their email address. By browsing a directory, you can identify their email address, and thus send them email. In short, a directory is an email address book.
Early email systems normally connected a departmental group, perhaps up to 50 people. Here, the email system could easily maintain electronic address books and present them to users. However, as email systems grew, things got more complex, and the problems consumed a great deal of effort on the part of computing professionals. The main problems were:
- Local address books had to synchronized with other groups of people in the same organization. This was generally known as data propagation. Decisions had to be made on when and how such propagation would take place, and the processes were very prone to failure.
- The synchronization might have to be done with a different type of email program, which would use different address book structures, and different email address formats. Metadirectories were one attempt to deal with this.
- Address lookups might be need for external organizations or people, possibly in other countries. The X.500 standards tried to provide for this need.
This gallery illustrates the substantial efforts of computer vendors and IT professionals to develop and implement directories from about 1985 to 2000. Today, directories are an obscure and unimportant technology. Ultimately, the underlying problems were mitigated by:
- The very strong tendency of organizations to standardize on a single email package, which over time provided a good directory service within the organization
- The standardization of email address formats
- The standardization of a simple address book, known as LDAP
- The availability of web pages where people can look up email addresses
- The ability of people to exchange their email addresses over the phone
-–David Ferris & Nigel Dutt