Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

Information has various properties and descriptions called metadata. For example, a mailing address has a name, address, city, state, and ZIP code. The metadata schema is this description of the mailing address. A schema is usually defined by some authority to describe data in a standard way so that it may be accessed by other users or applications.

A schema defines the vocabulary of a particular set of metadata (i.e., element names and formatting rules). It is a structural model that defines how objects in the real world, such as people and computers, are represented in the directory database. The schema may define the structure of the database, the names of the objects in it, and the attributes of those objects. An attribute holds values that may need to conform to a particular syntax or range of values.

An analogy is a form that you fill out. It has labeled fields that you write information in. Some fields must be filled out, while others are optional. Some fields have specific rules, such as the date must be formatted. The person creating the form might also create a schema that defines the form.

The W3C (World Wide Web Consortium) defined RDF (Resource Description Framework), which provides a framework for sharing schemas. The descriptions of these vocabulary sets are called RDF schemas. A schema defines the meaning, characteristics, and relationships of a set of properties, and this may include constraints on potential values and the inheritance of properties from other schemas. The RDF language allows each document containing metadata to clarify which vocabulary is being used by assigning each vocabulary a Web address. The schema specification language is a declarative representation language influenced by ideas from knowledge representation (e.g., semantic nets, frames, and predicate logic), as well as database schema specification languages and graph data models.

RFC 2251 (LDAPv3, December 1997) provides a good description of schema. Also see "Metadata" and "Repository."

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
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