An introduction to the resource description framework

Bulletin of the American Society for Information Science, Oct/Nov 1998 by Miller, Eric

The Resource Description Framework (RDF) is an infrastructure that enables the encoding, exchange and reuse of structured metadata. RDF is an application of XML that imposes needed structural constraints to provide unambiguous methods of expressing semantics. RDF additionally provides a means for publishing both humanreadable and machine-processable vocabularies designed to encourage the reuse and extension of metadata semantics among disparate information communities. The structural constraints RDF imposes to support the consistent encoding and exchange of standardized metadata provides for the interchangeability of separate packages of meta-data defined by different resource description communities.

Introduction

The World Wide Web affords unprecedented access to globally distributed information. Metadata, or structured data about data, improves discovery of and access to such information. The effective use of metadata among applications, however, requires common conventions about semantics, syntax and structure. Individual resource description communities define the semantics, or meaning, of metadata that address their particular needs. Syntax, the systematic arrangement of data elements for machine-processing, facilitates the exchange and use of metadata among multiple applications. Structure can be thought of as a formal constraint on the syntax for the consistent representation of semantics.

The Resource Description Framework (RDF), developed under the auspices of the World Wide Web Consortium [W3C], is an infrastructure that enables the encoding, exchange and reuse of structured metadata. This infrastructure enables metadata interoperability through the design of mechanisms that support common conventions of semantics, syntax and structure. RDF does not stipulate semantics for each resource description community, but rather provides the ability for these communities to define metadata elements as needed. RDF uses XML (eXtensible Markup Language) as a common syntax for the exchange and processing of metadata. The XML syntax is a subset of the international text processing standard SGML (Standard Generalized Markup Language) [SGML] specifically intended for use on the Web. The XML syntax provides vendor independence, user extensibility, validation, human readability and the ability to represent complex structures. By exploiting the features of XML, RDF imposes structure that provides for the unambiguous expression of semantics and, as such, enables consistent encoding, exchange and machine-processing of standardized metadata.

RDF supports the use of conventions that will facilitate modular interoperability among separate metadata element sets. These conventions include standard mechanisms for representing semantics that are grounded in a simple, yet powerful, data model discussed below. RDF additionally provides a means for publishing both human-readable and machine-processable vocabularies. Vocabularies are the set of properties, or metadata elements, defined by resource description communities. The ability to standardize the declaration of vocabularies is anticipated to encourage the reuse and extension of semantics among disparate information communities. For example, the Dublin Core Initiative [DC], an international resource description community focusing on simple resource description for discovery, has adopted RDF [DCRDF]. Educom's IMS Instructional Metadata System [IMS], designed to provide access to educational materials, has adopted the Dublin Core and corresponding architecture and extended it with domain-specific semantics. RDF is designed to support this type of semantic modularity by creating an infrastructure that supports the combination of distributed attribute registries. Thus, a central registry is not required. This permits communities to declare vocabularies which may be reused, extended and/or refined to address application or domain specific descriptive requirements.

The goals of RDF are broad, and the potential opportunities are enormous. This introduction to RDF begins by discussing the background context of the RDF initiative and relates it to other metadata activities. A discussion of the functionality of RDF and an overview of the model, schema and syntactic considerations of this framework follow.

Background

The history of metadata at the W3C began in 1995 with PICS, the Platform for Internet Content Selection [PICS]. PICS is a mechanism for communicating ratings of Web pages from a server to clients. These ratings, or rating labels, contain information about the content of Web pages; for example, whether a particular page contains a peer-reviewed research article or was authored by an accredited researcher or contains sex, nudity, violence, foul language, etc. Instead of being a fixed set of criteria, PICS introduced a general mechanism for creating rating systems. Different organizations could rate content based on their own objectives and values, and users - for example, parents worried about their children's Web usage - could set their browsers to filter out any Web pages not matching their own criteria. Development of PICS was motivated by the anticipation of restrictions on Internet content in the United States and elsewhere.