Information Technology Reference
In-Depth Information
In the early 1990s, KIF could be seen as a standard for ontology mod-
eling. This language was used in prominent tools such as Ontolingua and
in important ontology engineering projects, such as TOVE (Toronto Virtual
Enterprise) [55] and the Enterprise Ontology [129].
Later on in the 1990s, ontologies began to be applied to the World Wide
Web. SHOE [62], for example, used ontologies to annotate Web pages using
formal ontologies embedded in HTML documents. Ontobroker and its succes-
sor On2broker [40] use ontologies not only to annotate Web pages, but also to
formulate queries and derive answers. Ontobroker and On2broker provide an
annotation language which is used to annotate HTML documents with ref-
erences to ontologies. The ontology, the terminology used by the annotation
language, is specified using a representation language, based on F-Logic [72].
Finally, Ontobroker uses a query language for the retrieval of documents based
on their annotations. This query language is a subset of the representation
language. All these languages have an impact on the current languages of the
Semantic Web.
3.2 The Resource Description Framework
The Resource Description Framework (RDF) [75] is the first language de-
veloped especially for the Semantic Web. RDF was developed as a language
for adding machine-readable metadata to existing data on the Web. RDF
uses XML for its serialization in order to realize the layering depicted in
the Semantic Web language layer cake (Fig. 3.1). RDF Schema [20] extends
RDF with some basic (frame-based) ontological modeling primitives. There
are primitives such as classes, properties, and instances. Also, the instance-of,
subclass-of, and subproperty-of relationships have been introduced, allowing
structured class and property hierarchies.
RDF has the subject-predicate-object triple, commonly written as P(S,O),
as its basic data model. An object of a triple can, in turn, function as the sub-
ject of another triple, yielding a directed labeled graph, where resources (sub-
jects and objects) correspond to nodes, and predicates correspond to edges.
Furthermore, RDF allows a form of reification (a statement about a state-
ment), which means that any RDF statement can be used as a subject in
a triple. An example RDF graph is shown in Fig. 3.3. The corresponding
RDF/XML serialization is shown in Fig. 3.4.
Fig. 3.3.
Example RDF graph
Search WWH ::
Custom Search