Introduction to OWL Web Ontology Language for Medical and Biosciences Applications (Continued)

Resource Description Framework

As mentioned above, OWL is based on RDF, a language for describing resources. One goal of RDF to help applications discover information on the web. An example of an RDF language, other than OWL, is RDF Site Summary (RSS) 1.0, popularly used in news feeds. RDF is described informally in the RDF Primer¹ and formally in the RDF/XML Syntax Specification².

RDF enables establishment and discovery of statements about resources. As an example, the statement

http://www.medicalcomputing.net/index.html has a creation-date whose value is April 12, 2006

can be represented in RDF has three parts

the subject is the document with uniform resource locator (URL) 'http://www.medicalcomputing.net/index.html'
the predicate is the word 'creation-date'
the object is the phrase 'April 12, 2006'

This can be displayed graphically as

A simple RDF Relation

According to RDF conventions, the subject, 'http://www.medicalcomputing.net/index.html', is represented as an ellipse because it is a uniform resource identifier (URI) reference. URI's are a generalization of the familiar URL used for addresses of web pages. They are described by Request for Comments (RFC) 2396: Uniform Resource Identifiers⁷. The object, 'April 12, 2006', is represented as a rectangle because it is a literal value (i.e. constant), and the relation 'creation-date' is represented as a line between them. The statement can also be written as the triple


<http://www.medicalcomputing.net/index.html> <http://www.medicalcomputing.net/terms/creation-date> "April 12, 2006"

This can be written in XML as


<?xml version="1.0"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
         xmlns:medcomterms="http://www.medicalcomputing.net/terms/">
  <rdf:Description rdf:about="http://www.medicalcomputing.net/index.html">
     <medcomterms:creation-date>April 12, 2006</medcomterms:creation-date>
  </rdf:Description>
</rdf:RDF>

There is no need for the subject to be an Internet resource, such as a web page example above. It may be a person as well. In this case a URI is used. For example, we could model the statement

Fred Flinstone as examined on April 12, 2006


<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" 
         xmlns:medcomterms="http://www.medicalcomputing.net/terms/">
  <rdf:Description rdf:about="http://www.w3.org/People/EM/contact#fred_flinstone">
     <medcomterms:was-examined>April 12, 2006</medcomterms:was-examined>
  </rdf:Description>
</rdf:RDF>

Fred Flinstone is not an Internet resource but he can be described using this RDF fragment, which makes use of a URI for Fred


<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
             xmlns:contact="http://www.w3.org/2000/10/swap/pim/contact#">
  <contact:Person rdf:about="http://www.w3.org/People/EM/contact#fred_flinstone">
    <contact:fullName>Fred Flinstone</contact:fullName>
    <contact:mailbox rdf:resource="fflinstone@medicalcomputing.net"/>
  </contact:Person>
</rdf:RDF>

The person, Fred Flinstone is described as a by the URI http://www.w3.org/People/EM/contact#fred_flinstone. Fred's full name and mailbox are also listed as properties. Properties are a fundamental idea in RDF. Properties are binary relationships between the individuals, or objects, or describe data values associated with objects. Instances of properties can be written as ordered pairs. For example, this instance of the property mailbox can be written (http://www.w3.org/People/EM/contact#fred_flinstone, "fflinstone@medicalcomputing.net").

The domain of a property restricts the individuals to which the property can be applied. For example, we could define the domain of was-examined property to be people. The range of a property restricts the values it can take. For example, the range of the was-examined property may be defined as a valid date before the current point in time.

The concept of a class is described in the RDF Schema as a group of objects that share properties. For example, person could be defined as a class because individuals belonging to that class all have names and some have email addresses. The use of the term individual here is not casual. RDF uses the term individuals to describe instances of classes. For example, Fred Flinstone is an instance of the class person, so he is an individual. Despite the example use, the term individual in RDF and OWL does not just refer to people. Classes may be organized in a hierarchy using the subClassOf relation. For example, employee could be defined as a subclass of person.

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