World Wide Web Consortium Issues Web Ontology Language Candidate Recommendations

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Emerging Ontology Standard, OWL, strengthens Semantic Web Foundations

Frequently Asked Questions

 

http://www.w3.org/ -- 19 August 2003 -- Today, the World Wide Web Consortium (W3C) issued Web Ontology Language (OWL) as a W3C Candidate Recommendation. Candidate Recommendation is an explicit call for implementations, indicating that the document has been reviewed by all other W3C Working Groups, that the specification is stable, and appropriate for implementation.

OWL is a language for defining structured, Web-based ontologies which enable richer integration and interoperability of data across application boundaries. Early adopters of these standards include bioinformatics and medical communities, corporate enterprise and governments. OWL enables a range of descriptive applications including managing web portals, collections management, content-based searches, enabling intelligent agents, web services and ubiquitous computing.

"OWL is an important step for making data on the Web more machine processable and reusable across applications, " said Tim Berners-Lee, W3C Director. "We're encouraged to see OWL already being used as an open standard for deploying large scale ontologies on the Web."

OWL is specified in 6 documents: The OWL Overview; OWL Semantics and Abstract Syntax; OWL Use Cases and Requirements; OWL Test Cases, OWL Guide, and the OWL Reference.

Read the FAQ for more details on OWL.

OWL Delivers Ontologies that Work on the Web

OWL is a Web Ontology language. Where earlier languages have been used to develop tools and ontologies for specific user communities (particularly in the sciences and in company-specific e-commerce applications), they were not defined to be compatible with the architecture of the World Wide Web in general, and the Semantic Web in particular.

OWL rectifies this by using both URIs for naming and the linking provided by RDF to add the following capabilities to ontologies:

  • Ability to be distributed across many systems
  • Scalable to Web needs
  • Compatible with Web standards for accessibility and internationalization.
  • Open and extensible

OWL provides a language for defining structured, Web-based ontologies which delivers richer integration and interoperability of data among descriptive communities.

OWL builds on RDF Model and Schema and adds more vocabulary for describing properties and classes: among others, relations between classes (e.g. disjointness), cardinality (e.g. "exactly one"), equality, richer typing of properties, characteristics of properties (e.g. symmetry), and enumerated classes.

Already there are multiple implementations and demonstrations of OWL, which are available to the public.

The OWL Documents Produced by W3C

The W3C Web Ontology Working Group has produced six OWL documents. Each is aimed at different segments of those wishing to learn, use, implement or understand the OWL language. Documents include - a presentation of the use cases and requirements that motivated OWL - an overview document which briefly explains the features of OWL and how they can be used - a comprehensive Guide that walks through the features of OWL with many examples of the use of OWL features - a reference document that provides the details of every OWL feature - a test case document, and test suite, providing over a hundred tests that can be used for making sure that OWL implementations are consistent with the language design - a document presenting the semantics of OWL and details of the mapping from OWL to RDF.The Candidate Recommendation phase for the OWL documents is estimated to last at least four weeks, at which time the Working Group will evaluate new implementations and comments on the drafts.

OWL's Place in the Architecture of the Semantic Web: XML, RDF, and Ontologies

Much has been written about the Semantic Web, as if it is a replacement technology for the Web we know today. In fact, the Semantic Web is made through incremental changes, by bringing machine-readable descriptions to the data and documents already on the Web. With both descriptions and ways to connect, compare, and contrast them, it's possible to build applications, tools, search engines, agents - all with no apparent change to Web pages.

W3C's Semantic Web Activity builds on work done in other W3C Activities, such as the XML Activity. Its focus is to develop standard technologies, on top of XML, that support the growth of the Semantic Web.

At the foundation, XML provides a set of rules for creating vocabularies that can bring structure to both documents and data on the Web. XML gives clear rules for syntax; XML Schemas then serve as a method for composing XML vocabularies. XML is a powerful, flexible surface syntax for structured documents, but imposes no semantic constraints on the meaning of these documents.

RDF - the Resource Description Framework - is a standard a way for simple descriptions to be made. What XML is for syntax, RDF is for semantics - a clear set of rules for providing simple descriptive information. RDF Schema then provides a way for those descriptions to be combined into a single vocabulary. What's needed next is a way to develop subject - or domain - specific vocabularies. That is the role of an ontology.

An ontology defines the terms used to describe and represent an area of knowledge. Ontologies are used by people, databases, and applications that need to share subject-specific (domain) information - like medicine, tool manufacturing, real estate, automobile repair, financial management, etc. Ontologies include computer-usable definitions of basic concepts in the domain and the relationships among them. They encode knowledge in a domain and also knowledge that spans domains. In this way, they make that knowledge reusable.

Industrial and Academic Leaders Move OWL Forward

The W3C Web Ontology Working Group carries a complement of industrial and academic expertise, lending the depth of research and product implementation experience necessary for building a robust ontology language system. Participants include representatives from Agfa-Gevaert N. V; Daimler Chrysler Research and Technology; DARPA; Defense Information Systems Agency (DISA); EDS; Fujitsu; Forschungszentrum Informatik (FZI); Hewlett Packard Company; Ibrow; IBM; INRIA; Ivis Group; Lucent; University of Maryland; Mondeca; Motorola; National Institute of of Standards and Technology (NIST); Network Inference, Nokia; Philips, University of Southampton; Stanford University; Sun Microsystems; Unicorn Solutions along with invited experts from German Research Center for Artificial Intelligence (DFKI) Gmbh; the Interoperability Technology Association for Information Processing, Japan (INTAP); and the University of West Florida.

OWL brings together research from a number of groups that have been developing languages in which to express ontological expressions on the web. OWL has its origins in two major research efforts: a draft language known as the DARPA Agent Markup Language Ontology notations (DAML-ONT) and Ontology Interface Layer (OIL) developed by European researchers with the support of the European Commission. Since then, an ad hoc group of researchers formed the Joint US/EU committee on Agent Markup Languages and released a new version of this language which merges DAML with the OIL. The documents released today reflect the collaborative work of international researchers with industrial participants working together the World Wide Web Consortium.

About the World Wide Web Consortium [W3C]

The W3C was created to lead the Web to its full potential by developing common protocols that promote its evolution and ensure its interoperability. It is an international industry consortium jointly run by the MIT Laboratory for Computer Science (MIT LCS) in the USA, the European Research Consortium for Informatics and Mathematics (ERCIM) headquartered in France and Keio University in Japan. Services provided by the Consortium include: a repository of information about the World Wide Web for developers and users, and various prototype and sample applications to demonstrate use of new technology. To date, nearly 400 organizations are Members of the Consortium. For more information see http://www.w3.org/

 

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Janet Daly, <janet@w3.org>, +1.617.253.5884 or +1.617.253.2613

Frequently Asked Questions on W3C's Web Ontology Language (OWL)

Status: this FAQ is no longer maintained. For a new, up-to-date FAQ, see the Semantic Web FAQ.

Q. What is an ontology?

A. Although the concept of ontology has been around for a very long time in philosophy, in recent years it has become identified with computers as a machine readable vocabulary that is specified with enough precision to allow differing terms to be precisely related.

More precisely, from the OWL Requirements Document:

An ontology defines the terms used to describe and represent an area of knowledge. Ontologies are used by people, databases, and applications that need to share domain information (a domain is just a specific subject area or area of knowledge, like medicine, tool manufacturing, real estate, automobile repair, financial management, etc.). Ontologies include computer-usable definitions of basic concepts in the domain and the relationships among them [...]. They encode knowledge in a domain and also knowledge that spans domains. In this way, they make that knowledge reusable.

 

Q. How is OWL different from earlier ontology languages?

A. OWL is a Web Ontology language. Where earlier languages have been used to develop tools and ontologies for specific user communities (particularly in the sciences and in company-specific e-commerce applications), they were not defined to be compatible with the architecture of the World Wide Web in general, and the Semantic Web in particular.

OWL rectifies this by providing a language which uses the linking provided by RDF to add the following capabilities to ontologies:

  • Ability to be distributed across many systems
  • Scalable to Web needs
  • Compatible with Web standards for accessibility and internationalization.
  • Open and extensible

 

Q. What can Web Ontologies be used for?

A. The Web Ontology Working Group identified major use cases of ontologies on the Web and described these in the Use Cases and Requirements document. A survey of implemented applications (using earlier web ontology languages) was made with about 25 actually deployed systems identified.

The WG categorized these into six main areas, as follows:

 

  • Web Portals
    • Categorization rules used to enhance search
  • Multimedia Collections
    • Content-based searches for non-text media
  • Corporate Web Site Management
    • Automated Taxonomical Organization of data and documents
    • Mapping Between Corporate Sectors (mergers!)
  • Design Documentation
    • Explication of "derived" assemblies (e.g. the wing span of an aircraft)
    • Explicit Management of Constraints
  • Intelligent Agents
    • Expressing User Preferences and/or Interests
    • Content Mapping between Web sites
  • Web Services and Ubiquitous Computing
    • Web Service Discovery and Composition
    • Rights Management and Access Control

 

Q. Who is implementing OWL tools and applications?

A. A large number of organizations have been exploring the use of OWL, with many tools currently available. The Working Group is maintaining a list of implementations and demonstrations. In addition, both the US government (via DARPA and NSF) and the European Union (via the 5th and 6th generation frameworks of the IST program) have invested in web ontology language development. Most of the systems currently using DAML, OIL and DAML+OIL (the predecessor languages that OWL was based on) are now migrating to OWL. In addition, a number of ontology language tools, such as the widely used Protege system, now provide OWL support.

 

Q. Are there OWL ontologies available already?

A. There are a large number of ontologies available on the Web in OWL. There is an ontology library at DAML ontology library, which contains about 250 examples written in OWL or DAML+OIL (a converter from DAML+OIL to OWL is available on the web). In addition, several large ontologies have been released in OWL. These include a cancer ontology in OWL developed by the US National Cancer Institute's Center for Bioinformatics, which contains about 17,000 cancer related terms and their definitions, and an OWL version of the well-known GALEN medical ontology, developed at the University of Manchester.

 

Q. What does OWL add that RDF-schema doesn't?

A. Owl extends RDFS to allow for the expression of complex relationships between different RDFS classes and of more precise constraints on specific classes and properties. Example of these include: - the means to limit the properties of classes with respect to number and type, - the means to infer that items with various properties are members of a particular class - the means to determine if all members of a class will have a particular property, or if only some of them might - the means to distinguish one-to-one from many-to-one or one-to-many relationships, allowing the "foreign keys" of a database to be represented in an ontology - the means to express relationships between classes defined in different documents across the web, - the means to construct new classes out of the unions, intersections and complements of other classes, and - the means to constrain range and domain to specific class/property combinations. The OWL Guide provides examples of all of these in the area of describing food and wine.

 

Q. What documents are in the OWL document set?

A. The Working Group has produced six documents each aimed at different segments of those wishing to learn, use, implement or understand the OWL language. Our documents include - a presentation of the use cases and requirements that motivated OWL - an overview document which briefly explains the features of OWL and how they can be used - a comprehensive Guide that provides a walk-through of the features of OWL with many examples of the use of OWL features - a reference document that provides the details of every OWL feature - a test case document, and test suite, providing over a hundred tests that can be used for making sure that OWL implementations are consistent with the language design - a document presenting the semantics of OWL and details of the mapping from OWL to RDF (This document presents the model theoretical details of every feature of OWL so that those implementing complete OWL reasoners can guarantee algorithmic compliance with all aspects of the language design).

Q. What is new about ontologies on the Semantic Web? How do they differ from expert systems and the other artificial intelligence (AI) technologies promoted in the 1980s?

A. The relation between the Semantic Web, and OWL in particular, to work in AI is somewhat parallel to the relation between the Web and the hypertext community -- based on some of the same motivations, but with a very different architecture that drastically changes the ways in which the technology can be deployed. In a widely cited article from Scientific American, Berners-Lee, Hendler and Lassila wrote:

For the semantic web to function, computers must have access to structured collections of information and sets of inference rules that they can use to conduct automated reasoning. Artificial-intelligence researchers have studied such systems since long before the Web was developed. Knowledge representation, as this technology is often called, is currently in a state comparable to that of hypertext before the advent of the Web: it is clearly a good idea, and some very nice demonstrations exist, but it has not yet changed the world. It contains the seeds of important applications, but to realize its full potential it must be linked into a single global system.

The OWL language is a major step towards developing that potential.

Q. What does the acronym "OWL" stand for?

A. Actually, OWL is not a real acronym. The language started out as the "Web Ontology Language" but the Working Group disliked the acronym "WOL." We decided to call it OWL. The Working Group became more comfortable with this decision when one of the members pointed out the following justification for this decision from the noted ontologist A.A. Milne who, in his influential book "Winnie the Pooh" stated of the wise character OWL:

"He could spell his own name WOL, and he could spell Tuesday so that you knew it wasn't Wednesday..."

Jim Hendler, co-chair of the W3C Web Ontology Working Group, and the W3C Communications Team

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