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 World Wide 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 Web.
Figure 1
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:
{book}
{title} Gone with the Wind. {/title}
{book}
(Note: we use "{" instead of "<" brackets around tags).
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 the sentence.
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.
Figure 2
The RDF statement can be represented as a triple: (subject, predicate, object) and also serialized in XML syntax as:
{?xml version="1.0"?
{rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:dc="http://purl.org/dc/elements/1.1/"}
{rdf:Description rdf:about=”SUBJECT”}
{dc:PREDICATE}”OBJECT”{/PREDICATE}
{/rdf:Description}
{/rdf:RDF}
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 RSS.
For more on RDF and additional markup languages see the references below.
REFERENCES
Connections: Patterns of Discovery
Developing Semantic Web Services
Web Site: Video Software Lab
