The Web Ontology Language (OWL) is a family of knowledge representation languages for authoring ontologies. The languages are characterised by formal semantics and RDF/XML-based serializations for the Semantic Web. OWL is endorsed by the World Wide Web Consortium (W3C) and has attracted academic, medical and commercial interest.
In October 2007, a new W3C working group was started to extend OWL with several new features as proposed in the OWL 1.1 member submission. This new version, called OWL 2, soon found its way into semantic editors such as Protégé and semantic reasoners such as Pellet, RacerPro, FaCT++ and HermiT. W3C announced the new version on 27 October 2009.
The OWL family contains many species, serializations, syntaxes and specifications with similar names. This may be confusing unless a consistent approach is adopted. OWL and OWL2 will be used to refer to the 2004 and 2009 specifications, respectively. Full species names will be used, including specification version (for example, OWL2 EL). When referring more generally, OWL Family will be used.
An ontology defines the terms used to describe and represent an area of knowledge. Ontologies are critical for applications that need to search across or merge information from diverse communities. Although XML DTDs and XML Schemas are sufficient for exchanging data between parties who have agreed to the definitions beforehand, their lack of semantics prevents machines from reliably performing this task with new XML vocabularies.
The Semantic Web is a vision for the future of the Web in which information is given explicit meaning, making it easier for machines to automatically process and integrate information available on the Web. The Semantic Web will build on XML’s ability to define customized tagging schemes and RDF’s flexible approach to representing data.
The next element required for the Semantic Web is the OWL Web ontology language (OWL), which can formally describe the semantics of classes and properties used in Web documents. For machines to perform useful reasoning tasks on these documents, the language must go beyond the basic semantics of RDF Schema. In this article, I’ll briefly review several use cases that show the need for the OWL.
OWL languageOntologies are usually expressed in a logic-based language, so that detailed, accurate, consistent, sound, and meaningful distinctions can be made among the classes, properties, and relations. Some ontology tools can perform automated reasoning using the ontologies, and thus provide advanced services to intelligent applications such as conceptual/semantic search and retrieval, software agents, decision support, speech and natural language understanding, knowledge management, intelligent databases, and electronic commerce.
The OWL language provides three increasingly expressive sublanguages designed for use by specific communities of implementers and users:
OWL Lite supports those users primarily needing a classification hierarchy and simple constraint features. OWL DL supports those users who want the maximum expressiveness without losing computational completeness (all entailments are guaranteed to be computed) and decidability (all computations will finish in finite time) of reasoning systems. OWL DL was designed to support the existing Description Logic business segment and has desirable computational properties for reasoning systems. OWL Full is meant for users who want maximum expressiveness and the syntactic freedom of RDF with no computational guarantees. OWL Full allowsan ontology to augment the meaning of the predefined RDF or OWL vocabulary.
Before you can use a set of terms, you need a precise indication of what specific vocabularies are being used.
A standard initial component of an ontology includes a set of XML namespace declarations enclosed in an opening rdf:RDF tag:
xmlns:drinks =" http://www.site.org/2003/ontology/drinks#"
xmlns:xsd ="http://www.w3.org/2000/10/XMLSchema >
Once namespaces are established, we normally include a collection of assertions about the ontology grouped under an owl:Ontology tag. These tags, shown below, support such critical housekeeping tasks as comments, version control, and inclusion of other ontologies:
<owl:Ontology rdf:about="" > <rdfs:comment >OWL ontology of drinks at site.org</rdfs:comment >
<owl:priorVersion rdf:resource="http://www.site.org/2003/ontology/drinks-prev.owl"/ >
The owl:Ontology element is the place to collect much of the OWL metadata for the document. The rdf:about attribute provides a name or reference for the ontology. Where the value of the attribute is empty (i.e., the standard case), the name of the ontology is the base URI of the owl:Ontology element.