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Web Ontology Language

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OWL Web Ontology Language
AbbreviationOWL
StatusPublished
Year started2002
EditorsJames Hendler, Guus Schreiber
Base standardsResource Description Framework, RDFS
DomainSemantic Web
WebsiteOWL Reference
OWL 2 Web Ontology Language
AbbreviationOWL 2
StatusPublished
Year started2008
EditorsW3C OWL Working Group
Base standardsResource Description Framework, RDFS
DomainSemantic Web
WebsiteOWL2 Overview

The Web Ontology Language (OWL) is a family of knowledge representation languages for authoring ontologies endorsed by the World Wide Web Consortium.[1] They are characterised by formal semantics and RDF/XML-based serializations for the Semantic Web. OWL has attracted both academic, medical and commercial interest.

In October 2007, a new W3C working group[2] was started to extend OWL with several new features as proposed in the OWL 1.1 member submission.[3] This new version, called OWL 2, soon found its way into semantic editors such as Protégé and semantic reasoners such as Pellet[4], RacerPro [5] and FaCT++[6]. W3C announced the new version on 27 October 2009.[7]

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.

History

Early Ontology Languages

Further information: Knowledge_representation § History_of_knowledge_representation_and_reasoning

There is a long history of ontological development in philosophy and computer science. Since the 1990s, a number of research efforts have explored how the idea of knowledge representation (KR) from AI could be made useful on the World Wide Web. These included languages based on HTML (called SHOE), based on XML (called XOL, later OIL), and various frame-based KR languages and knowledge acquisition approaches.

Ontology Languages For The Web

In 2000 in the USA, DARPA started development of DAML led by James Hendler. [8] In March 2001, the Joint EU/US Committee on Agent Markup Languages decided that DAML should be merged with OIL.[8] The EU/US ad hoc Joint Working Group on Agent Markup Languages was convened to develop DAML+OIL as a web ontology language. This group was jointly funded by the DARPA (under the DAML program) and the EU's IST funding project. DAML+OIL was intended to a thin layer above RDFS[8], with formal semantics based on a Description Logic (DL) [9] .

OWL started as a research-based[10] revision of DAML+OIL aimed at the semantic web.

Semantic Web Standards

The Semantic Web provides a common framework that allows data to be shared and reused across application, enterprise, and community boundaries.

— World Wide Web Consortium, W3C Semantic Web Activity[11]
Further information: Semantic Web

Web Ontology Working Group

As of Monday, the 31st of May, our working group will officially come to an end. We have achieved all that we were chartered to do, and I believe our work is being quite well appreciated.

— James Hendler and Guus Schreiber, So Long and thanks for all the fish[12]

The World Wide Web Consortium (W3C) created the Web Ontology Working Group as part of their Semantic Web Activity. It began work on November 1, 2001 with co-chairs James Hendler and Guus Schreiber.[12] The first working drafts of the abstract syntax, reference and synopsis were published in July 2002.[12] OWL became a formal W3C recommendation on February 10, 2004 and the working group was disbanded on May 31, 2004.[12]

OWL Working Group

OWL 2, an extension and revision of the OWL Web Ontology Language developed by the W3C OWL Working Group, became a W3C recommendation in October 2009. OWL 2 introduces profiles to improve scalability in typical applications.[7].

Acronym

Why not be inconsistent in at least one aspect of a language which is all about consistency?

— Guus Schreiber, Why OWL and not WOL?[13]

The natural acronym for Web Ontology Language would be WOL instead of OWL. Although the character Owl from Winnie the Pooh wrote his name WOL, the acronym OWL was proposed without reference to that character, as an easily pronounced acronym that would yield good logos, suggest wisdom, and honor William A. Martin's One World Language KR project from the 1970s.

Adoption

A survey (published in 2006) of ontologies available on the web collected 688 OWL ontologies. Of these, 199 were OWL Lite, 149 were OWL DL and 337 OWL Full (by syntax). They found that 19 ontologies had in excess of 2000 classes, and that 6 had more than 10000. The same survey collected 587 RDFS vocabularies.[14]

Ontologies

Further information: Ontology (information science)

Introduction

An ontology is an explicit specification of a conceptualization.

— Tom Gruber, A Translation Approach to Portable Ontology Specifications[15]

The data described by an ontology in the OWL family is interpreted as a set of "individuals" and a set of "property assertions" which relate these individuals to each other. An ontology consists of a set of axioms which place constraints on sets of individuals (called "classes") and the types of relationships permitted between them. These axioms provide semantics by allowing systems to infer additional information based on the data explicitly provided. A full introduction to the expressive power of the OWL is provided in the W3C's OWL Guide.

Example

An ontology describing families might include axioms stating that a "hasMother" property is only present between two individuals when "hasParent" is also present, and individuals of class "HasTypeOBlood" are never related via "hasParent" to members of the "HasTypeABBlood" class. If it is stated that the individual Harriet is related via "hasMother" to the individual Sue, and that Harriet is a member of the "HasTypeOBlood" class, then it can be inferred that Sue is not a member of "HasTypeABBlood".


Species

OWL Sublanguages

The W3C-endorsed OWL specification includes the definition of three variants of OWL, with different levels of expressiveness. These are OWL Lite, OWL DL and OWL FULL (ordered by increasing expressiveness). Each of these sublanguages is a syntactic extension of its simpler predecessor. The following set of relations hold. Their inverses do not.

  • Every legal OWL Lite ontology is a legal OWL DL ontology.
  • Every legal OWL DL ontology is a legal OWL Full ontology.
  • Every valid OWL Lite conclusion is a valid OWL DL conclusion.
  • Every valid OWL DL conclusion is a valid OWL Full conclusion.

OWL Lite

OWL Lite was originally intended to support those users primarily needing a classification hierarchy and simple constraints. For example, while it supports cardinality constraints, it only permits cardinality values of 0 or 1. It was hoped that it would be simpler to provide toolsupport for OWL Lite than its more expressive relatives, allowing quick migration path for systems utilizing thesauri and other taxonomies. In practice, however, most of the expressiveness constraints placed on OWL Lite amount to little more than syntactic inconveniences: most of the constructs available in OWL DL can be built using complex combinations of OWL Lite features. Development of OWL Lite tools has thus proven almost as difficult as development of tools for OWL DL, and OWL Lite is not widely used.

OWL DL

OWL DL was designed to provide the maximum expressiveness possible while retaining computational completeness (either φ or ¬φ belong), decidability (there is an effective procedure to determine whether φ is derivable or not), and the availability of practical reasoning algorithms. OWL DL includes all OWL language constructs, but they can be used only under certain restrictions (for example, number restrictions may not be placed upon properties which are declared to be transitive). OWL DL is so named due to its correspondence with description logic, a field of research that has studied the logics that form the formal foundation of OWL.

OWL Full

OWL Full is based on a different semantics from OWL Lite or OWL DL, and was designed to preserve some compatibility with RDF Schema. For example, in OWL Full a class can be treated simultaneously as a collection of individuals and as an individual in its own right; this is not permitted in OWL DL. OWL Full allows an ontology to augment the meaning of the pre-defined (RDF or OWL) vocabulary. It is unlikely that any reasoning software will be able to support complete reasoning for OWL Full.

OWL2 Profiles

In OWL 2, there are three sublanguages of the language. OWL 2 EL is a fragment that has polynomial time reasoning complexity; OWL 2 QL is designed to enable easier access and query to data stored in databases; OWL 2 RL is a rule subset of OWL 2.

Syntax

High Level

OWL Abstract Syntax

This high level syntax is used to specify the OWL ontology structure and semantics.[16]

The OWL abstract syntax presents an ontology as a sequence of annotations, axioms and facts. Annotations carry machine and human oriented meta-data. Information about the classes, properties and individuals that compose the ontology is contained in axioms and facts only. Each class, property and individual is either anonymous or identified by an URI reference. Facts state data either about an individual or about a pair of individual identifiers (that the objects identified are distinct or the same). Axioms specify the characteristics of classes and properties. This style is similar to frame languages, and quite dissimilar to well known syntaxes for Description Logics (DLs) and Resource Description Framework (RDF). [16]

Sean Bechhofer et al argued that though this syntax is hard to parse, it is quite concrete. They conclude that the name abstract syntax may be somewhat misleading.[17]

OWL2 Functional Syntax

Exchange Syntaxes

RDF Syntax

OWL2 XML Syntax

Manchester Syntax

Example

Semantics

Further information: formal semantics

Relation to description logics

In the beginning, IS-A was quite simple. Today, however, there are almost as many meanings for this inheritance link as there are knowledge-representation systems.

— Ronald J. Brachman, What ISA is and isn't[18]

Early attempts to build large ontologies were plagued by a lack of clear definitions. Members of the OWL family have model theoretic formal semantics, and so have strong logical foundations.

Description logics (DLs) are a family of logics that are decidable fragments of first-order logic with attractive and well-understood computational properties. OWL DL and OWL Lite semantics are based on DLs[19]. They combine a syntax for describing and exchanging ontologies, and formal semantics that gives them meaning. For example, OWL DL corresponds to the SHOIN (D) description logic, while OWL 2 corresponds to the SROIQ(D) logic. Sound, complete, terminating reasoners (i.e. systems which are guaranteed to derive every consequence of the knowledge in an ontology) exist for these DLs.

OWL Full is intended to be compatible with RDF Schema (RDFS), and to be capable of augmenting the meanings of existing Resource Description Framework (RDF) vocabulary. [20] A model theory describes the formal semantics for RDF. [21] This interpretation provides the meaning of RDF and RDFS vocabulary. So, the meaning of OWL Full ontologies are defined by extension of the RDFS meaning, and OWL Full is a semantic extension of RDF. [22]

Open world assumption

The languages in the OWL family use the open world assumption, in contrast to SQL and Prolog, which adopt the closed world assumption. Under this open world assumption, if a statement cannot be proved to be true using current knowledge, we cannot draw the conclusion that the statement is false.

Terminology

Languages in the OWL family are capable of creating classes, properties, defining instances and its operations.

Instances

An instance is an object. It corresponds to a Description Logic individual.

Classes

A class is a collection of objects. It corresponds to a Description Logic (DL) concept. A class may contain individuals, instances of the class. A class may have any number of instances. An instance may belong to none, one or more classes.

A class may be a subclass of another, inheriting characteristics from its parent superclass. This corresponds to logical subsumption and DL concept inclusion notated .

All classes are subclasses of owl:Thing (DL top notated ), the root class. All classes are subclassed by owl:Nothing (DL bottom notated ), the empty class. No instances are members of owl:Nothing. Modelers use owl:Thing and owl:Nothing to assert facts about all or no instances.[23]

Example

For example, Employee could be the subclass of class owl:Thing while Dealer, Manager, and Labourer all subclass of Employee.

Properties

A property is a directed binary relation that specifies class characteristics. It corresponds to a Description Logic role. They are attributes of instances and sometimes act as data values or link to other instances. Properties may possess logical capabilities such as being transitive, symmetric, inverse and functional. Properties may also have domains and ranges.

Datatype Properties

Datatype properties are relations between instances of classes and RDF literals or XML schema datatypes. For example, modelName (String datatype) is the property of Manufacturer class.

Object Properties

Object properties are relations between instances of two classes. For example, ownedBy may be an object type property of the Vehicle class and may have a range which is the class Person.

Operators

Languages in the OWL family support various operations on classes such as union, intersection and complement. They also allow class enumeration, cardinality, and disjointness.

Public Ontologies

Pedagogic Examples

Libraries

Biomedical

General

Standards

Browsers

The following tools include public ontology browsers:

Search

Limitations

  • Relationships are directed
  • No direct language support for n-ary relationships. For example modelers may wish to describe the qualities of a relation, to relate more than 2 individuals or to relate an individual to a list. This cannot be done this within OWL. They may need to adopt a pattern instead which encodes the meaning outside the formal semantics.[31]

See also

References

  1. ^ Smith, Michael K. (2004-02-10). "OWL Web Ontology Language Guide". W3C. Retrieved 2008-07-15. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  2. ^ W3C working group
  3. ^ "Submission Request to W3C: OWL 1.1 Web Ontology Language". W3C. 2006-12-19.
  4. ^ Pellet
  5. ^ RacerPro
  6. ^ FaCT++
  7. ^ a b [1]
  8. ^ a b c Lacy, Lee W. (2005). "Chapter 10". OWL: Representing Information Using the Web Ontology Language. Victoria, BC: Trafford Publishing. ISBN 1-4120-3448-5.
  9. ^ Baader, Franz; Horrocks, Ian; Sattler, Ulrike (2005). "Description Logics as Ontology Languages for the Semantic Web". In Hutter, Werner; Stephan (eds.). Mechanizing Mathematical Reasoning: Essays in Honor of Jörg H. Siekmann on the Occasion of His 60th Birthday. Heidelberg: Springer Berlin. ISBN 978-3-540-25051-7. {{cite book}}: External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help)
  10. ^ "Feature Synopsis for OWL Lite and OWL: W3C Working Draft 29 July 2002". W3C. 2002-07-29.
  11. ^ World Wide Web Consortium (2010-02-06). "W3C Semantic Web Activity". Retrieved 18 April 2010.
  12. ^ a b c d "Web-Ontology (WebOnt) Working Group (Closed)". W3C.
  13. ^ Ivan Herman. "Why OWL and not WOL?". Tutorial on Semantic Web Technologies. World Wide Web Consortium. Retrieved 18 April 2010.
  14. ^ Wang, Taowei David; Parsia, Bijan; Hendler, James (2006). "A Survey of the Web Ontology Landscape". The Semantic Web - ISWC 2006. 5th International Semantic Web Conference. Heidelberg: Springer Berlin. pp. 682–694. doi:10.1007/11926078_49. ISBN 978-3-540-49029-6. {{cite conference}}: External link in |conferenceurl= (help); Unknown parameter |booktitle= ignored (|book-title= suggested) (help); Unknown parameter |conferenceurl= ignored (|conference-url= suggested) (help)
  15. ^ Tom Gruber (1993). "A Translation Approach to Portable Ontology Specifications". In: Knowledge Acquisition. 5: 199-199.
  16. ^ a b Patel-Schneider, Peter F.; Horrocks, Ian; Patrick, Hayes (10 February 2004). "OWL Web Ontology Language Semantics and Abstract Syntax". World Wide Web Consortium. Retrieved 18 April 2010.
  17. ^ Bechhofer, Sean; Patel-Schneider, Peter F.; Turi, Daniele (20031210). "OWL Web Ontology Language Concrete Abstract Syntax". University of Manchester. Retrieved 18 April 2010. {{cite web}}: Check date values in: |date= (help)
  18. ^ Brachman, R.J. (1983). What ISA is and isn't: An analysis of taxonomic links in semantic networks. IEEE Computer 16(10), 30-36.
  19. ^ Ian Horrocks & Peter F. Patel-Schneider. "Reducing OWL Entailment to Description Logic Satisfiability" (PDF).
  20. ^ Deborah McGuinness and Frank van Harmelen (10 February 2004). "OWL Web Ontology Language Overview". W3C Recommendation for OWL, the Web Ontology Language. World Wide Web Consortium. Retrieved 18 April 2010.
  21. ^ Patrick Hayes (10 February 2004). "RDF Semantics". Resource Description Framework. World Wide Web Consortium. Retrieved 18 April 2010.
  22. ^ Peter F. Patel-Schneider,Patrick Hayes and Ian Horrocks (10 February 2004). "OWL Web Ontology Language Semantics and Abstract Syntax Section 5. RDF-Compatible Model-Theoretic Semantics". W3C Recommendation for OWL, the Web Ontology Language. World Wide Web Consortium. Retrieved 18 April 2010. {{cite web}}: line feed character in |title= at position 26 (help)
  23. ^ Lacy, Lee W. (2005). "Chapter 12". OWL: Representing Information Using the Web Ontology Language. Victoria, BC: Trafford Publishing. ISBN 1-4120-3448-5.
  24. ^ OBO Foundry
  25. ^ OBO Download Matrix
  26. ^ NCBO BioPortal
  27. ^ TONES (Thinking ONtologiES)
  28. ^ SUMO download
  29. ^ TDWG LSID Vocabularies
  30. ^ Protégé web site
  31. ^ Natasha Noy and Alan Rector (12 April 2006). "Defining N-ary Relations on the Semantic Web". World Wide Web Consortium. Retrieved 17 April 2010.

External links

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