Since the focus of this book is more on the practical usage of RDF than the more theoretical Semantic Web, I wasn't sure about covering ontologies . After all, in a white paper at Stanford University, Tom Gruber described ontology thus:
It's a bit difficult to determine how to incorporate a discussion of a concept based on such an elusive definition into a book that begins with Practical . However, looking at examples of ontologies, in particular OIL, DAML+OIL, and the W3C's current OWL (Web Ontology Language) effort, it seemed to me that ontologies do fit into a book with Practical in the title, because an ontology is really the definition of the business rules associated with a vocabulary. In other words: ontologies are business models. According to the Web Ontology Language (OWL) Use Cases and Requirements document:
Following on the relational model analogy discussed in earlier chapters, if RDF is analogous to the relational data model and SQL is analogous to RDF/XML, then ontologies built on RDF/XML are equivalent to large architected business applications such as SAP, PeopleSoft, and Oracle's Financial and Warehouse applications. This equation definitely opened a home for ontologies in this book, and this chapter is it.
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