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What Is the Resource Description Framework? : Page 2

You might be familiar with RDF as a technology for describing resources like images, audio, and video. However, take a deeper look into its core model, syntax, and features to see how this framework is shaping the semantic web.

Key RDF Triple Elements
The key elements of an RDF triple are as follows:
  • Subject. In grammar, this is the noun or noun phrase that is the doer of the action. In the sentence "The company sells batteries," the subject is "the company." The subject of the sentence tells us what the sentence is about. In logic, this is the term about which something is asserted. In RDF, this is the resource that is being described by the ensuing predicate and object. Therefore, in RDF, we want a URI to stand for the unique concept "company" like "http://www.business.org/ontology/#company" to denote that we mean a form of business ownership and not friends coming for a visit.

Note: An RDF resource stands for either electronic resources, like files, or concepts, like "person." One way to think of an RDF resource is as "anything that has identity."
  • Predicate. In grammar, this is the part of a sentence that modifies the subject and includes the verb phrase. Returning to our sentence "The company sells batteries," the predicate is the phrase "sells batteries." In other words, the predicate tells us something about the subject. In logic, a predicate is a function from individuals (a particular type of subject) to truth-values with an arity based on the number of arguments it has. In RDF, a predicate is a relation between the subject and the object. Thus, in RDF, we would define a unique URI for the concept "sells" like "http://www.business.org/ontology/#sells."
  • Object. In grammar this is a noun that is acted upon by the verb. Returning to our sentence "The company sells batteries," the object is the noun "batteries." In logic, an object is acted upon by the predicate. In RDF, an object is either a resource referred to by the predicate or a literal value. In our example, we would define a unique URI for "batteries" like "http://www.business.org/ontology/#batteries."

    Figure 3. A graph of two RDF statements.
    IsaViz is copyrighted by the W3C. All Rights Reserved.
  • Statement. In RDF, the combination of the preceding three elements—subject, predicate, and object—as a single unit. Figure 3 displays a graph representation of two RDF statements. These two statements illustrate the concepts in Figure 2. Note that the object can be represented by a resource or by a literal value. The graphing is done via a W3C application called IsaViz available at www.w3.org/2001/11/IsaViz/.

We should stress that resources in RDF must be identified by resource IDs, which are URIs with optional anchor IDs. This is important so that a unique concept can be unambiguously identified via a globally unique ID. This is a key difference between relying on semantics over syntax. The syntactic meaning of words is often ambiguous. For example, the word "bark" in the sentences "The bark felt rough" and "The bark was loud" has two different meanings; however, by giving a unique URI to the concept of tree bark like "www.business.org/ontology/plant/#bark," we can always refer to a single definition of bark.

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