The
Web Ontology Language (
OWL) is a family of
knowledge representationKnowledge representation is an area of artificial intelligence research aimed at representing knowledge in symbols to facilitate inferencing from those knowledge elements, creating new elements of knowledge...
languages for authoring
ontologiesIn computer science and information science, an ontology formally represents knowledge as a set of concepts within a domain, and the relationships between those concepts. It can be used to reason about the entities within that domain and may be used to describe the domain.In theory, an ontology is...
.
The languages are characterised by formal semantics and
RDFThe Resource Description Framework is a family of World Wide Web Consortium specifications originally designed as a metadata data model...
/
XMLExtensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....
-based serializations for the
Semantic WebThe Semantic Web is a collaborative movement led by the World Wide Web Consortium that promotes common formats for data on the World Wide Web. By encouraging the inclusion of semantic content in web pages, the Semantic Web aims at converting the current web of unstructured documents into a "web of...
. OWL is endorsed by the
World Wide Web ConsortiumThe World Wide Web Consortium is the main international standards organization for the World Wide Web .Founded and headed by Tim Berners-Lee, the consortium is made up of member organizations which maintain full-time staff for the purpose of working together in the development of standards for the...
(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. W3C announced the new version of OWL on 27 October 2009. This new version, called OWL 2, soon found its way into semantic editors such as
ProtégéProtégé is a free, open source ontology editor and a knowledge acquisition system. Like Eclipse, Protégé is a framework for which various other projects suggest plugins. This application is written in Java and heavily uses Swing to create the rather complex user interface...
and
semantic reasonerA semantic reasoner, reasoning engine, rules engine, or simply a reasoner, is a piece of software able to infer logical consequences from a set of asserted facts or axioms. The notion of a semantic reasoner generalizes that of an inference engine, by providing a richer set of mechanisms to work with...
s such as Pellet, RacerPro, FaCT++ and HermiT.
The OWL family contains many species, serializations, syntaxes and specifications with similar names. OWL and OWL2 are 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.
Early ontology languages
There is a long history of
ontologicalIn computer science and information science, an ontology formally represents knowledge as a set of concepts within a domain, and the relationships between those concepts. It can be used to reason about the entities within that domain and may be used to describe the domain.In theory, an ontology is...
development in philosophy and computer science. Since the 1990s, a number of research efforts have explored how the idea of
knowledge representationKnowledge representation is an area of artificial intelligence research aimed at representing knowledge in symbols to facilitate inferencing from those knowledge elements, creating new elements of knowledge...
(KR) from
artificial intelligenceArtificial intelligence is the intelligence of machines and the branch of computer science that aims to create it. AI textbooks define the field as "the study and design of intelligent agents" where an intelligent agent is a system that perceives its environment and takes actions that maximize its...
(AI) could be made useful on the World Wide Web. These included languages based on
HTMLHyperText Markup Language is the predominant markup language for web pages. HTML elements are the basic building-blocks of webpages....
(called
SHOEIn the semantic web, Simple HTML Ontology Extensions are a small set of HTML extensions designed to give web pages semantic meaning by allowing information such as class, subclass and property relationships....
), based on XML (called XOL, later
OILOIL can be regarded as an Ontology infrastructure for the Semantic Web. OIL is based on concepts developed in Description Logic and frame-based systems and is compatible with RDFS....
), and various frame-based KR languages and knowledge acquisition approaches.
Ontology languages for the web
In 2000 in the United States, DARPA started development of DAML led by
James HendlerJames Hendler is an artificial intelligence researcher at Rensselaer Polytechnic Institute, USA, and one of the originators of the Semantic Web.-Background and research:...
.
In March 2001, the
Joint EU/US Committee on Agent Markup Languages decided that DAML should be merged with OIL.
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 European Union's Information Society Technologies (IST) funding project. DAML+OIL was intended to be a thin layer above RDFS,
with formal semantics based on a
description logicDescription logic is a family of formal knowledge representation languages. It is more expressive than propositional logic but has more efficient decision problems than first-order predicate logic....
(DL).
OWL started as a research-based revision of DAML+OIL aimed at the semantic web.
Semantic web standards
RDF schema
In the late 1990s, the World Wide Web Consortium (W3C)
Metadata Activity started work on RDF Schema (RDFS), a language for
RDFThe Resource Description Framework is a family of World Wide Web Consortium specifications originally designed as a metadata data model...
vocabulary sharing. The RDF became a W3C
RecommendationWeb standards is a general term for the formal standards and other technical specifications that define and describe aspects of the World Wide Web. In recent years, the term has been more frequently associated with the trend of endorsing a set of standardized best practices for building web sites,...
in February 1999, and RDFS a Candidate Recommendation in March 2000. In February 2001, the
Semantic Web Activity replaced the Metadata Activity. In 2004 (as part of a wider revision of RDF) RDFS became a W3C Recommendation.
Though RDFS provides some support for ontology specification, the need for a more expressive ontology language had become clear.
Web-Ontology Working Group
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. The first working drafts of the
abstract syntaxThe abstract syntax of data is its structure described as a data type , independent of any particular representation or encoding....
, reference and synopsis were published in July 2002. OWL became a formal
W3C recommendationWeb standards is a general term for the formal standards and other technical specifications that define and describe aspects of the World Wide Web. In recent years, the term has been more frequently associated with the trend of endorsing a set of standardized best practices for building web sites,...
on February 10, 2004 and the working group was disbanded on May 31, 2004.
OWL Working Group
In 2005, at the
OWL Experiences And Directions Workshop a consensus formed that recent advances in description logic would allow a more expressive revision to satisfy user requirements more comprehensively whilst retaining good computational properties.
In December 2006, the OWL1.1 Member Submission
was made to the W3C. The W3C chartered the
OWL Working Group as part of the Semantic Web Activity in September 2007. In April 2008, this group decided to call this new language OWL2, indicating a substantial revision.
OWL 2 became a W3C recommendation in October 2009. OWL 2 introduces profiles to improve scalability in typical applications.
Acronym
The natural acronym for
Web Ontology Language would be
WOL instead of
OWL. Although the character
OwlOwl is a fictional character in A. A. Milne's Winnie-the-Pooh books and in Disney's Winnie the Pooh cartoons. Owl's character is based on the archetype of the "wise old owl", although in the books, the quality of Owl's "wisdom" is sometimes questionable....
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. MartinWilliam A. Martin was a computer scientist from Oklahoma City, Oklahoma.After graduating from Northwest Classen High School, where he was a state wrestling champion, he attended MIT where he received a bachelor's degree , master's and a Ph.D. in electrical engineering...
's
One World Language knowledge representation 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 2,000 classes, and that 6 had more than 10,000. The same survey collected 587 RDFS vocabularies.
Introduction
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".
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 tool support for OWL Lite than its more expressive relatives, allowing quick migration path for systems utilizing
thesauriA thesaurus is a reference work that lists words grouped together according to similarity of meaning , in contrast to a dictionary, which contains definitions and pronunciations...
and other
taxonomiesTaxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
. 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
completenessIn mathematical logic, a theory is complete if it is a maximal consistent set of sentences, i.e., if it is consistent, and none of its proper extensions is consistent...
(either φ or ¬φ belong),
decidabilityIn logic, the term decidable refers to the decision problem, the question of the existence of an effective method for determining membership in a set of formulas. Logical systems such as propositional logic are decidable if membership in their set of logically valid formulas can be effectively...
(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
The OWL family of languages supports a variety of syntaxes. It is useful to distinguish
high level syntaxes aimed at specification from
exchange syntaxes more suitable for general use.
OWL abstract syntax
This high level syntax is used to specify the OWL ontology structure and semantics.
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 referenceÚriis a village and commune in the comitatus of Pest in Hungary....
. 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 languageA frame language is a metalanguage. It applies the frame concept to the structuring of language properties. Frame languages are usually software languages.-Description:...
s, and quite dissimilar to well known syntaxes for
description logicDescription logic is a family of formal knowledge representation languages. It is more expressive than propositional logic but has more efficient decision problems than first-order predicate logic....
s (DLs) and
Resource Description FrameworkThe Resource Description Framework is a family of World Wide Web Consortium specifications originally designed as a metadata data model...
(RDF).
Sean Bechhofer,
et al. argue that though this syntax is hard to parse, it is quite concrete. They conclude that the name
abstract syntax may be somewhat misleading.
OWL2 functional syntax
This syntax closely follows the structure of an OWL2 ontology. It is used by OWL2 to specify semantics, mappings to exchange syntaxes and profiles.
RDF syntaxes
Syntactic mappings into
RDFThe Resource Description Framework is a family of World Wide Web Consortium specifications originally designed as a metadata data model...
are specified
for languages in the OWL family. Several RDF
serialization formatsIn computer science, in the context of data storage and transmission, serialization is the process of converting a data structure or object state into a format that can be stored and "resurrected" later in the same or another computer environment...
have been devised. Each leads to a syntax for languages in the OWL family through this mapping. RDF/XML is normative.
OWL2 XML syntax
OWL2 specifies an
XMLExtensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....
serialization that closely models the structure of an OWL2 ontology.
Manchester Syntax
The Manchester Syntax is a compact, human readable syntax with a style close to frame languages.
Variations are available for OWL and OWL2. Not all OWL and OWL2 ontologies can be expressed in this syntax.
Tea ontology
Consider an ontology for tea based on a Tea class. But first, an ontology is needed. Every OWL ontology must be identified by an
URIÚriis a village and commune in the comitatus of Pest in Hungary....
(
http://www.example.org/tea.owl, say). This is enough to get a flavour of the syntax. To save space below, preambles and prefix definitions have been skipped.
OWL2 Functional Syntax
Ontology(
Declaration( Class( :Tea ) )
)
OWL2 XML Syntax
Manchester Syntax
Ontology:
Class: Tea
RDF/XML syntax
RDF/
TurtleTurtle is a serialization format for Resource Description Framework graphs. A subset of Tim Berners-Lee and Dan Connolly's Notation3 language, it was defined by Dave Beckett, and is a superset of the minimal N-Triples format. Unlike full N3, Turtle doesn't go beyond RDF's graph model...
rdf:type owl:Ontology .
:Tea rdf:type owl:Class .
Relation to description logic
Early attempts to build large ontologies were plagued by a lack of clear definitions. Members of the OWL family have
model theoreticIn mathematics, model theory is the study of mathematical structures using tools from mathematical logic....
formal semantics, and so have strong
logicIn philosophy, Logic is the formal systematic study of the principles of valid inference and correct reasoning. Logic is used in most intellectual activities, but is studied primarily in the disciplines of philosophy, mathematics, semantics, and computer science...
al foundations.
Description logicDescription logic is a family of formal knowledge representation languages. It is more expressive than propositional logic but has more efficient decision problems than first-order predicate logic....
s (DLs) are a family of logics that are decidable fragments of
first-order logicFirst-order logic is a formal logical system used in mathematics, philosophy, linguistics, and computer science. It goes by many names, including: first-order predicate calculus, the lower predicate calculus, quantification theory, and predicate logic...
with attractive and well-understood computational properties. OWL DL and OWL Lite semantics are based on DLs.
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.
Relation To RDFS
OWL Full is intended to be compatible with
RDF SchemaRDF Schema is a set of classes with certain properties using the RDF extensible knowledge representation language, providing basic elements for the description of ontologies, otherwise called RDF vocabularies, intended to structure RDF resources...
(RDFS), and to be capable of augmenting the meanings of existing Resource Description Framework (RDF) vocabulary.
A
model theoryIn mathematics, model theory is the study of mathematical structures using tools from mathematical logic....
describes the formal semantics for
RDF. 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.
Open world assumption
The languages in the OWL family use the
open world assumptionIn formal logic, the open world assumption is the assumption that the truth-value of a statement is independent of whether or not it is known by any single observer or agent to be true. It is the opposite of the closed world assumption, which holds that any statement that is not known to be true is...
. Under the open world assumption, if a statement cannot be proven to be true with current knowledge, we cannot draw the conclusion that the statement is false.
Contrast to other languages
A
relational databaseA relational database is a database that conforms to relational model theory. The software used in a relational database is called a relational database management system . Colloquial use of the term "relational database" may refer to the RDBMS software, or the relational database itself...
consists of sets of tuples with the same
attributesIn computing, an attribute is a specification that defines a property of an object, element, or file. It may also refer to or set the specific value for a given instance of such....
.
SQLSQL is a programming language designed for managing data in relational database management systems ....
is a query and management language for relational databases.
PrologProlog is a general purpose logic programming language associated with artificial intelligence and computational linguistics.Prolog has its roots in first-order logic, a formal logic, and unlike many other programming languages, Prolog is declarative: the program logic is expressed in terms of...
is a
logical programmingLogic programming is, in its broadest sense, the use of mathematical logic for computer programming. In this view of logic programming, which can be traced at least as far back as John McCarthy's [1958] advice-taker proposal, logic is used as a purely declarative representation language, and a...
language. Both use the
closed world assumptionThe closed world assumption is the presumption that what is not currently known to be true, is false. The same name also refers to a logical formalization of this assumption by Raymond Reiter. The opposite of the closed world assumption is the open world assumption , stating that lack of knowledge...
.
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
subsumptionIn knowledge representation, object-oriented programming and design, is-a or is_a or is a is a relationship where one class D is a subclass of another class B ....
and DL
concept inclusion notated
.
All classes are subclasses of owl:Thing (DL
topThe top type in type theory, commonly abbreviated as top or by the down tack symbol , is the universal type—that type which contains every possible object in the type system of interest. The top type is sometimes called the universal supertype as all other types in any given type system are...
notated
), the
root class.
All classes are subclassed by owl:Nothing (DL
bottomIn type theory, a theory within mathematical logic, the bottom type is the type that has no values. It is also called the zero or empty type, and is sometimes denoted with falsum .A function whose return type is bottom cannot return any value...
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.
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. They are formulated using
owl:DatatypeProperty type.
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. They are formulated using
owl:ObjectProperty.
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.
Biomedical
- OBO Foundry
Open Biomedical Ontologies is an effort to create controlled vocabularies for shared use across different biological and medical domains. As of 2006, OBO forms part of the resources of the U.S...
- NCBO BioPortal
- NCI
The National Cancer Institute is part of the National Institutes of Health , which is one of 11 agencies that are part of the U.S. Department of Health and Human Services. The NCI coordinates the U.S...
Enterprise Vocabulary Services
Browsers
The following tools include public ontology browsers:
- Protégé OWL
Protégé is a free, open source ontology editor and a knowledge acquisition system. Like Eclipse, Protégé is a framework for which various other projects suggest plugins. This application is written in Java and heavily uses Swing to create the rather complex user interface...
Limitations
- 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 within OWL. They may need to adopt a pattern instead which encodes the meaning outside the formal semantics.
See also
- Agris: International Information System for the Agricultural Sciences and Technology
- Bossam: A rule-based OWL reasoner
- Common logic
Common logic is a framework for a family of logic languages, based on first-order logic, intended to facilitate the exchange and transmission of knowledge in computer-based systems....
- FOAF
FOAF is a machine-readable ontology describing persons, their activities and their relations to other people and objects. Anyone can use FOAF to describe him or herself...
+ DOACDescription Of A Career is a semantic vocabulary created by Ramon A. Parada to describe professional capabilities of a worker. It has been designed to be compatible with the European curriculum so those can be generated from a FOAF+DOAC file...
- Geopolitical ontology
A geopolitical ontology is a mechanism to describe, manage and exchange data related to geopolitical entities such as countries, territories, regions and other similar areas.-Definitions and examples:...
- IDEAS Group
The IDEAS Group is the International Defence Enterprise Architecture Specification for exchange Group. The deliverable of the project is a data exchange format for military Enterprise Architectures. The scope is four nation and covers MODAF , DoDAF , and the Australian Defence Architecture...
- Meta-Object Facility (MOF)
The Meta-Object Facility is an Object Management Group standard for model-driven engineering. The official reference page may be found at OMG's website.- Overview :...
, a different standard for the Unified Modeling Language (UML)Unified Modeling Language is a standardized general-purpose modeling language in the field of object-oriented software engineering. The standard is managed, and was created, by the Object Management Group...
of the Object Management Group (OMG)Object Management Group is a consortium, originally aimed at setting standards for distributed object-oriented systems, and is now focused on modeling and model-based standards.- Overview :...
- Multimedia Web Ontology Language
Multimedia Web Ontology Language has been designed to facilitate semantic interactions with multimedia contents. It supports perceptual modeling of concepts using expected media properties. While the reasoning in traditional ontology languages, e.g...
- SKOS
Simple Knowledge Organization System is a family of formal languages designed for representation of thesauri, classification schemes, taxonomies, subject-heading systems, or any other type of structured controlled vocabulary. SKOS is built upon RDF and RDFS, and its main objective is to enable...
- SSWAP
Simple Semantic Web Architecture and Protocol is a lightweight, document-centric protocol and architecture for the Semantic Web...
: Simple Semantic Web Architecture and Protocol
- Website Parse Template
Website Parse Template is an XML-based open format which provides HTML structure description of website pages. WPT format allows web crawlers to generate Semantic Web’s RDFs for web pages...
External links