IS 208 - Metadata
Short Paper Resource Description Framework
May 15, 2000
The Resource Description Framework (RDF) is a metadata infrastructure that will improve discovery and access of web information. It is an application of XML (Extensible Markup Language) that provides structural constraints for encoding, exchange and reuse of metadata that describes web documents, web pages, and entire web sites. In a sense, it can be described as a cooperative attempt by participants in the World Wide Web Consortium (W3C) to formulate a structure and syntax to describe web resources that will transform the way in which information is found on the web. Much in the same way that standardized cataloging practices transformed the way bibliographic information is recorded and shared, RDF will provide the structure for creating metadata that will be used in the next generation of web search engines for web resource discovery.
There are several applications for RDF mentioned in current literature and these include:
RDF additionally provides a means for publishing both human-readable 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 metadata defined by different resource description communities.(3) In other words, RDF can provide interoperability between applications that exchange machine-understandable information on the Web.(4)
RDF provides a framework for metadata that is independent of the specific vocabulary or semantics of a particular institution. The Dublin core Initiative has adopted RDF. Educom's IMS Instructional Metadata System (IMS) that is designed for access to educational materials has adopted Dublin Core and RDF. The RDF infrastructure allows communities or institutions "to define vocabularies that can be reused, extended or refined to address domain specific descriptive requirements."(5) That means that vocabularies or anything that helps people to look up web information can be developed by any community and can be marketable commodities.(6)
RDF Background & Data Model
Web metadata development at W3C began with the Platform for Internet Content Selection (PICS) in 1995. This was developed so that different organizations could rate the content of web pages based on their own values and users. This began the collaborative effort to create a flexible architecture for web metadata. There are several W3C companies and institutions that are contributing to this effort to develop RDF, such as IBM, KnowledgeCite, Microsoft, Netscape, Nokia, OCLC, Reuters, SoftQuad, DVL, Grif, LANL and University of Michigan.(7) It draws on other metadata efforts such as the Dublin Core and Warwick Framework.
RDF uses XML (eXtensible Markup Language) as the common syntax. XML is a subset of Standardized Markup Language (SGML), the international standard for the printing industry specifically designed for web resources. XML cannot be used as it exists because although it allows the creation of tags for text data and to incorporate other tags, it cannot support scalability because the order in which the elements appear in an XML document is significant. In other words, although it works well to exchange resources on the web, it doesn't provide an adequate framework for metadata.(8)
RDF is the structure or a mathematical model for describing resources. "The RDF model groups together sets of simple metadata statements known as 'triples'. Each triple is made up of a 'resource' (or node), a 'propertyType' and a 'value'."(9) Resources in RDF are defined as unique objects that have an Uniform Resource Identifier (URI). An Uniform Resource Locator, or web address is only one form of an URI. Resources have 'propertyTypes' (attributes or characteristics) and these have corresponding 'values'. PropertyTypes express the relationship of values associated with resources. And values can be text strings or numbers.(10)
The RDF model is graphically represented using node and arc diagrams. An oval is used to represent a node or resource, a line with a directional arrow represents a propertyType and a rectangle is used to represent simple values. Each triple in RDF is known as a 'property'. "Nodes or resources can have more than one arc originating from them, that indicate multiple propertyTypes associated with the same resource."(11) A simple example of a property or a triple is this paper when it is published on the web. The node or resource is drawn in an oval that contains the 'URL' for the website for the paper; a line with a directional arrow is drawn with the propertyType, 'Author' written above it; and the value associated with the propertyType, or 'Karen Baxter' is written in quotes at the end of the arrow. In some cases, a second node (or resource) will have two propertyTypes associated with it (such as name and email). In these cases, the node will not have a corresponding URI, and is called an anonymous node.(12)
The semantics for propertyTypes (attributes) or the way in which different organizations define their attributes may vary. Since this is the case, XML namespaces are used to uniquely identify each propertyType. "Namespaces provide a prefix for each propertyType name, using a colon to separate the namespace prefix from the name."(13)
As an example, suppose we use Dublin Core's schema, then we would identify that the Dublin Core semantics for 'author' is used by defining the propertyType as DC:Creator, and the value is "Karen Baxter". "The corresponding syntactic way of expressing this statement using XML namespaces to identify the use of the Dublin Core Schema is:
<?xml:namespace ns=http://www.w3.org/RDF/RDF prefix="RDF"
<?xml:namespace ns=http://purl.oclc.org/DC/ prefix="DC" ?>
RDF is designed with these characteristics:
RDF will make the search for web resources more efficient and easier. It makes it possible to use different software to process the same metadata and to use a single piece of software to process in part many different metadata vocabularies. (15) RDF will improve access to information on the web by refining the way that search engines can look for information. It will provide the infrastructure to begin the next phase of organization for the multitude of web resources.
1. Lassila, Ora. "Introduction to RDF Metadata." W3C Note,
November 13, 1997. Online. Available:
2. Lassila. p.2
3. Miller, Eric. "An Introduction to the Resource Description Framework." D-Lib Magazine, May, 1998. Online. Available: http://www.dlib.org/dlib/may98/05miller.html
4. Lassila. p.2
5. Miller. p.5
6. Bray, Tim. "RDF and Metadata." Online. Available: http://www.com/xml/pub/98/06/rdf.html
7. Cover, Robin. "The XML Cover Pages: Resource Description Framework." Online. Available: http://www.oasis-open.org/cover/rdf.html
8. Bray. p.5
9. Powell, Andy. "Metadata for the Web: RDF and the Dublin Core." UK Office for Library and Information Networking, University of Bath. Online. Available: http://www.ukoln.ac.uk/metadata/presentations/ukolug98/paper/intro.html
10. Powell. p.1-2
11. Powell. p.3
12. Powell. p.4
13. Powell. p.4
14. Bray. p.4
15. Bray. p.6
Bibliography of Web Sites
Beckett, Dave. "Resource Description Framework (RDF)
Online. Available: 4/25/00
Excellent gateway to resources about RDF including, documents, publications, discussions, presentations, RDF editors and tools, RDF applications, tutorials and examples. Current, up to date information, through 1999.
Bray, Tim. "RDF and Metadata"
Online. Available: 5/2/00
Excellent article, simple and easy to understand introduction to RDF (Resource Description Framework). Includes reasons why it is needed, characteristics and its relationship with XML. Cited by many other sources.
Champin, Pierre-Antoine. "RDF Tutorial"
Online. Available: 4/25/00
Well organized site that describes RDF with sparse, concise definitions, but assumes familiarity with the vocabulary. Not easy to understand.
Cowan, John. "RDF Made (Fairly) Easy"
Online. Available: 5/4/00
Article reflects W3C's working draft dated February 16, 1998. Written to teach how to encode RDF compliant XML metadata. Explains RDF with simple cases. Assumes an understanding of XML 1.0.
Dublin Core Metadata Initiative
Guidance on expressing the Dublin Core within the
Resource Description Framework (RDF)
Eric Miller, Paul Miller and Dan Brickley
Online. Available: 4/25/00
A working draft of a technical report on how the Dublin Core can be expressed using XML, the formal syntax of RDF. Includes an introduction to XML and RDF, XML namespaces, encoding DC in RDF with both XML and HTML with examples.
Dublin Core Workshop Series
"Qualified Dublin Core Metadata for Simple Resource Discovery" July 10, 1998. Online. Available: 5/4/00
Working document or work in progress of the Internet Engineering Task Force (IETF) that defines how Dublin Core can be expressed using RDF. Very technical with examples.
Heery, Rachel. "What is... RDF?"
Ariadne, web version only. July 13, 1998. Online. Available: 5/5/00
An introduction to RDF that assumes familiarity with the vocabulary of data modeling. Points out that understanding RDF is contingent on seeing how it is used in tools and applications. Includes links to other articles and resources.
Lassila, Ora. "Introduction to RDF Metadata"
W3C NOTE 1997-11-13. Online. Available: 5/2/00
Excellent short article describing RDF. Compares the development of RDF with PICS (Platform for Internet Content Selection).
Miller, Eric. "An Introduction to the Resource Description
D-Lib Magazine. May 1998. Online. Available: 4/28/00
Excellent comprehensive introductory article to RDF. Clear, concise, and easy to understand. Discusses RDFs development background and its relation to other metadata initiatives (PICS, DC, WF), and the RDF data model with graphic examples.
Powell, Andy. "Metadata for the Web: RDF and the Dublin Core"
UK Office for Library and Information Networking, University of Bath.
Online. Available: 5/5/00
Excellent article, clear and concise introduction to RDF. Describes the RDF model in understandable terminology with illustrative examples. Brief description of Dublin Core and its relation to RDF as a likely key schema in RDF.
Rhyno, Art. "RDF and Metadata: Adding Value to the Web"
Information Technology September 1998. Online. Available: 5/2/00
General article on RDF. Describes the efforts of the library world to create standards (Z39.50) including standards for metadata (Dublin Core).
Sullivan, Eamonn. "RDF aims to tame the Web"
PC Week Labs. November 3, 1997. Online. Available: 5/4/00
An early article written just after the first draft release by W3C to describe RDF for laypersons. Describes RDF as a proposed XML-derived language for the Web and provides examples of how it might be used.
UKOLIN: The UK Office for Library and Information Networking
Metadata Resources: Resource Description Framework - RDF
Online. Available: 4/28/00
Gateway site maintained by UKOLN about RDF, includes papers, examples of RDF, seminar presentations and W3C RDF sites. Excellent source for online papers for all levels of expertise.
W3C (World Wide Web Consortium)
Resource Description Framework (RDF) Model and Syntax
W3C Working Draft February 22, 1999. Online. Available: 5/4/00
World Wide Web Consortium's most recent working draft specifications for RDF. Technical and detailed with examples of applying RDF. Necessary for learning how to use RDF.
"Frequently asked questions about RDF"
Online. Available: 5/4/00
Concise definition of RDF and how it can be used. Links to articles about Dublin Core is used within RDF.
email@example.com Mail Archives
Searchable archive of discussions about RDF.