Resource Description Framework (RDF)
In 1990, Tim Berners-Lee laid the foundation for the World Wide Web with three
basic components: HTTP (HyperText Transfer Protocol ), URLs (Universal Resource
Locators), and HTML (HyperText Markup Language). These elements were the
essential ingredients leading to the explosive growth of the World Wide Web. The
original concept for HTML was a modest one, where browsers could simply view
information on Web pages. The HTML program can be written to a simple text file
and can be easily mastered by a high school student.
However, HTML wasn't extensible, in that it has specifically designed tags
requiring vendor agreement before changes could be made. The eXtensible Markup
Language (XML) overcame this limitation. Proposed in late 1996 by the
Web Consortium (W3C)
, XML offered a way to manipulate a developer's structured
data. XML simplified the process of defining and using metadata and provided a
good representation of extensible, hierarchical, formatted information.
For the Web to provide meaningful content and capabilities today, it will have
to add new layers of markup languages starting with Resource Description
Framework (RDF). Figure 1 shows the projected pyramid of Markup Languages for the
Resource Description Framework (RDF) has been developed by the W3C in order to
extend XML and make work easier for autonomous agents and automated services by
introducing a rudimentary semantic capability.
RDF uses a simple relational model for structured data to be mixed, exported and
shared across different applications. Resource Description Framework (RDF)
defines a subject, a predicate, and an object to form an RDF triplet.
Consider the statement: The book is entitled, Gone with the Wind
A simple XML representation might be:
<title> Gone with the Wind. </title>
The grammatical sentence has three basic parts: a subject [The book], a
predicate [ is entitled], and an object [Gone with the Wind]. A machine,
however, could not make an inference based upon the simple XML representation of
For machines to make an inference automatically, it is necessary to add RDF to
the traditional HTML and XML markup.
The basic RDF model produces a triple, where a resource (the subject) is linked
through an arc labeled with a property (the predicate) to a value (the object).
Figure 2 shows the graphical representation of the RDF statement.
The RDF statement can be represented as a triple: (subject, predicate, object)
and also serialized in XML syntax as:
A collection of interrelated RDF statements is represented by a graph of
interconnected nodes. The nodes are connected via various relationships. For
example, let's say each node represents a person. Each person might be related
to another person because they are siblings, parents, spouses, or friends.
There are many RDF applications available, for example see Dave Beckett's
Resource Description Framework (RDF) Resource Guide.
Many communities have proliferated on the Internet, from companies to
professional organizations to social groupings. The Friend of a Friend (FOAF)
vocabulary, originated by Dan Brickley and Libby Miller, gives a basic
expression for community membership. FOAF expresses personal information and
relationships, and is a useful building block for creating information systems
that support online communities. Search engines can find people with similar
interests through FOAF.
FOAF is simply an RDF vocabulary. Its typical method of use is akin to that of
For more on RDF and additional markup languages see the references below.
Thinking on the Web: Berners-Lee, Turing and Godel
Developing Semantic Web Services
Connections: Patterns of Discovery