- Document title:
- OWL 2 Web Ontology Language
- W3C OWL Working Group
The OWL 2 Web Ontology Language, informally OWL 2, is an ontology language for the Semantic Web with formally defined meaning. OWL 2 ontologies provide classes, properties, individuals, and data values and are stored as Semantic Web documents. OWL 2 ontologies can be used along with information written in RDF, and OWL 2 ontologies themselves are primarily exchanged as RDF documents.
the OWL 2 document
Overview describes the overall state ofOWL 2,and should be read beforeother OWL 2 documents. This documentdescribes serves as an introduction tothe various OWL 2 documents
- Status of this Document
formalised vocabularies of terms, often covering a specific domain and shared by a community of users. They specify the definitions of terms by describing their relationships with other terms in the ontology. OWL 2 is an extension and revision of the OWL Ontology Language developed by the W3C OWL Working group. OWL and OWL 2 are designed to facilitate ontology development and sharing via the Web, with the ultimate goal of making Web content more accessible to machines.
Figure 1 gives an overview of the OWL 2 language, showing its main building blocks and how they relate to each other. The ellipse in the
centre represents the abstract notion of an ontology, which can be thought of either as an abstract structure or as an RDF graph. At the top are various concrete syntaxes that can be used to serialise and exchange ontologies. At the bottom are the two semantic specifications that define the meaning of OWL 2 ontologies.
The conceptual structure of OWL 2 ontologies is defined in the OWL 2 Structural Specification document [OWL 2 Specification]. This document uses UML [UML] to define the structural elements available in OWL 2, explaining their roles and functionalities in abstract terms and without reference to any particular syntax. It also defines the functional-style syntax, which closely follows the structural specification and allows OWL 2 ontologies to be written in a compact form.
An OWL 2 ontology can also be represented as an RDF graph. The relationship between these two forms is specified by the Mapping to RDF Graphs document [OWL 2 RDF Mapping], which defines a mapping from the structural form to the RDF graph form, and vice versa. The OWL 2 Quick Reference Guide[ OWL 2 Quick Reference] provides a simple overview of the structural and RDF “views” of OWL 2, with the two views laid out side by side.
In practice, a concrete syntax is needed in order to store OWL 2 ontologies and to exchange them among tools and applications. The primary exchange syntax for OWL 2 is RDF/XML
1[RDF Syntax]; this is indeed the only syntax that must be supported by all OWL 2 tools (see Section 2.1 of the OWL 2 Conformance document[ OWL 2 Conformance]).
RDF/XML[ RDF Syntax ] provides for interoperability among OWL 2 tools, other concrete syntaxes may also be used. These include alternative RDF serializations, such as Turtle[Turtle ], an XML serialisation [OWL XML ], and a more "readable" syntax used in several ontology editing tools [OWL 2 Manchester Syntax]. Finally, the functional-style syntax can also be used for serialisation, although its main purpose is specifying the structure of the language [OWL 2 Specification]. It must be emphasized, however, that OWL 2 tools are not required to support any of these alternative syntaxes.
The OWL 2 Specification document defines the abstract structure of OWL 2 ontologies, but it does not define
the meaning of OWL 2 ontologies, i.e., it does not tell us when the axioms in one ontology follow from those in another (often referred to as entailment ). This is achieved by assigning a (model-theoretic) semantics to OWL 2 ontology structures. This semantics is used by reasoners and other tools to answer queries about, e.g., class consistency, subsumption and instance retrieval. The Direct Semantics [OWL 2 Direct Semantics] and the RDF-Based Semantics [OWL 2 RDF-Based Semantics] provide two alternative ways of assigning meaning to OWL 2 ontologies, with the so-called correspondence theorem providing a link between the two.
The Direct Semantics assigns meaning directly to ontology
structures via a translation into the SROIQ description logic—a fragment of first order logic with useful computational properties. The advantage of this is that the extensive description logic literature and implementation experience can be directly exploited by OWL 2 applications. However, some restrictions must be placed on ontology structures in order to ensure that they can be translated into a SROIQ knowledge base; for example, there must be a strict separation of object and datatype properties (see Section 3 of the OWL 2 Specification[ OWL 2 Specification] for a complete list of these restrictions). Ontologies that satisfy these restrictions and are interpreted using the Direct Semantics are called "OWL 2 DL" ontologies.
The RDF-Based Semantics assigns meaning directly to RDF graphs and so indirectly to ontology structures via the Mapping to RDF-graphs.
This Semantics is fully compatible with theRDF Semantics specification[RDF Semantics], and itextends the semantics conditions defined in that document. It can be applied to any OWL 2 Ontology, without restrictions. 2 OWL 2 Ontologies that are interpreted using the RDF-based semantics are called “OWL 2 Full” ontologies. Editor's Note: I am not quite sure how to characterize those RDF graphs that cannot be mapped back on the structure.is there an easy way to characterize them? Are they useful? It may be essential to give a good description here to avoid destroyingthe overall view... OWL 2 DL and OWL 2 Full are closely related. Any OWL 2 DL ontology can be treated as an OWL Full ontology by interpreting it using the RDF-based semantics. Many OWL 2 Full ontologies can be transformed into ontology structures (via the reverse mapping) and, if they satisfy the relevant restrictions, can be interpreted using the Direct semantics.
so-called “correspondence theorem” (see Section 7.3 in the RDF Based Semantics Document [OWL 2 RDF-Based Semantics]) alsodefines a precise relationship between the Direct and RDF-Based Semantics. In essence,This theorem states that, given an OWL 2 DL ontology, inferences drawn using the Direct Semantics will still be valid if the ontology is treated as OWL 2 Full and interpreted using the RDF-Based Semantics.
OWL 2 Profiles [OWL 2
RDF-Based Semantics] are sub-languages of OWL 2 that offer important advantages in particular application scenarios. Three different profiles are defined: OWL EL, OWL QL, and OWL RL. Each profile is defined as a syntactic restriction of the OWL 2 Structural Specification, i.e., as a subset of the structural elements that can be used in a conforming ontology. Each of the profiles trades off different aspects of OWL's expressive power in return for different computational and/or implementational benefits.
OWL 2 EL enables polynomial time algorithms for all the standard reasoning tasks; it is particularly suitable for applications where very large ontologies are needed, and where expressive power can be traded for performance guarantees. OWL 2 QL enables conjunctive queries to be answered using standard relational database technology; it is particularly suitable for applications were relatively lightweight ontologies are used with very large datasets, and where it is useful or necessary to access the data directly via relational queries (e.g., SQL). OWL 2 RL enables the implementation of polynomial time reasoning algorithms using rule-extended database technologies operating directly on RDF triples; it is particularly suitable for applications where relatively lightweight ontologies are used with very large datasets, and where it is useful or necessary to operate directly on data in the form of RDF triples.
Any OWL EL, QL or RL ontology is, of course, also an OWL 2 ontology and can be interpreted using either the Direct or RDF-Based Semantics. When using OWL RL, a rule-based implementation can
operated directly on RDF triples and so can be applied to an arbitrary RDF graph, i.e., to any OWL 2 Full ontology. In this case, reasoning will always be sound (that is, only correct answers to queries will be computed), but it may not be complete (that is, it is not guaranteed that all correct answers to queries will be computed). Theorem PR1 of the Profiles document states, however, that when the ontology is consistent with the structural definition of OWL RL, a suitable rule-based implementation will be both sound and complete.
4Differences between OWL 2 and the previous version of OWL
The 2004 version of
OWL[ OWL 1 Overview] (referred to hereafter as “OWL 1”) has a very similar overall structure to OWL 2. Looking at Figure 1, almost all the building blocks of OWL 2 were present in OWL 1, albeit under a possibly different name.
- The role of the structure, as well as the functional syntax of OWL 2, was played by the OWL Abstract
Syntax[OWL 1 Abstract Syntax]. The two syntaxes are different but their respective role within the overall structure of OWL is identical. The new,functional syntax is much closer to the RDF graph representation and can capture more RDF graphs; it also has a formal equivalence to UML[UML ]
- Like OWL 1, OWL 2 specifies a precise mapping from ontology structures (represented using the abstract/functional syntax) to RDF graphs. OWL 2, however, also benefits from an explicitly specified mapping from RDF graphs back to ontology structures.
- The two semantics (i.e., RDF-Based and Direct) of OWL 2 have their direct counterparts in OWL 1, under the name of RDF-Compatible Model-Theoretic
Semantics[OWL 1 RDF Semantics] and Direct Model-Theoretic Semantics[OWL 1 Direct Semantics].
- An XML Presentation Syntax was also available for OWL
1[OWL 1 Direct Semantics] (although not as a Recommendation). On the other hand, the Manchester syntax did not exist for OWL 1.
The central role of RDF/XML and the role of other syntaxes, and the relationships between the
OWL DL and OWL Full semantics (ie, the correspondence theorem) has not changed. More importantly, backward compatibility is ensured: OWL 1 Ontologies remain valid OWL 2 Ontologies with identical inferences. Editor's Note: Is this correct? Or are there corner cases to be mentioned? OWL 1 had only one profile (although the term was not used), namely OWL Lite[ OWL Lite ]. Based on the implementation experiences since 2004 this profile was not retained in OWL 2.
alsoadds new functionality with respect to OWL 1. Some of this is purely
syntactic facilities (e.g., disjoint union of classes or pairwise disjoint classes); some is genuinely new, namely: keys
- richer datatypes, data
- qualified cardinality
- assymetric, reflexive, and disjoint
- enhanced annotation capabilities
The OWL 2 New Features and Rationale
Document[ OWL 2 New Features and Rationale] gives a detailed description on these new features, while the OWL 2 Quick Reference Guide[ OWL 2 Quick Reference] gives an overview of the various OWL 2 features in general, clearly indicating those that are new.
The restrictions applicable to OWL DL have also been relaxed somewhat in OWL 2. Whereas, in OWL 1, there was a very strict separation between, for example, classes and individuals, OWL 2’s corresponding restrictions are a bit more relaxed. Under some circumstances the same identifier
(ie, URI) can be used to denote, for example, both a class and an individual while still maintaining the usability of Direct Semantics. In other words, the set of RDF Graphs, that can also be handled by Decription Logics reasoners, has become a bit larger in OWL 2 compared to OWL 1.
The OWL 2 ontology language is defined by five core specification documents describing its conceptual structure, primary exchange syntax (RDF/XML), two alternative semantics (Direct and RDF-Based) and conformance requirements. Three additional specification documents describe optional features that may be supported by some implementations: the language profiles, and two alternative concrete syntaxes (XML and Manchester).
These documents are, however, all rather technical and mainly aimed at OWL 2 implementers and tool developers. Those looking for a more approachable guide to the features and usage of OWL 2 may prefer to consult one of the user documents (Primer, New Features and Rationale, and Quick Reference Guide).
|1||For Users||Document Overview. A quick overview of the OWL 2 specification that includes a description of its relationship to OWL 1. This it the starting point and primary reference point for OWL 2.|
|2||Core Specification||Structural Specification and Functional-Style Syntax defines the constructs of OWL 2 in terms of both their structure and a functional-style syntax, and defines OWL 2 DL in terms of global restrictions on OWL 2.|
|3||Core Specification||Mapping to RDF Graphs defines a mapping of the OWL 2 constructs into RDF graphs, and thus defines the primary means of exchanging OWL 2 ontologies in the Semantic Web.|
|4||Core Specification||Direct Semantics defines the meaning of OWL 2 ontologies in terms of a model-theoretic semantics.|
|5||Core Specification||RDF-Based Semantics defines the meaning of OWL 2 ontologies via an extension of the RDF Semantics.|
|6||Core Specification||Conformance provides requirements for OWL 2 tools and a set of test cases to help determine conformance.|
|7||Specification||Profiles defines three sub-languages of OWL 2 that offer important advantages in particular applications scenarios.|
|8||For Users||OWL 2 Primer (currently in need of revision) provides an approachable introduction to OWL 2, including orientation for those coming from other disciplines.|
|9||For Users||OWL 2 New Features and Rationale (currently being completed) provides an overview of the main new features of OWL 2 and motivates their inclusion in the language.|
|10||For Users||OWL 2 Quick Reference Guide (currently being revised) provides a quick reference for the constructs of OWL 2 suitable for use by OWL 2 ontology developers. Other documents making up the OWL 2 specification include.|
|11||Specification||XML Serialization defines an XML syntax for exchanging OWL 2 ontologies.|
|12||Specification||Manchester Syntax defines an easy-to-read, but less formal syntax for OWL 2 that is used in some OWL 2 user interface tools and is also used in the Primer.|
|13||Specification||Data Range Extension: Linear Equations (not yet published) specifies a syntax and semantics for incorporating linear equations with rational coefficients solved in the reals in OWL 2.|
- [OWL 2 Specification]
- OWL 2 Web Ontology Language: Structural Specification and Functional-Style Syntax. Boris Motik, Peter F. Patel-Schneider, and Bijan Parsia, eds., 2008.
- OMG Unified Modeling Language (OMG UML), Infrastructure V2.1.2. Object Management Group, OMG Available Specification November 2007.
- [OWL 2 RDF Mapping]
- OWL 2 Web Ontology Language: Mapping to RDF Graphs. Peter F. Patel-Schneider and Boris Motik, eds., 2008.
- [OWL 2 Quick
- OWL 2 Web Ontology Language: Quick Reference Guide. Jie Bao, Elisa F. Kendall, Deborah L. McGuinness, and Evan K. Wallace, eds., 2008.
- [RDF Semantics]
- RDF Semantics. Patrick Hayes, ed., W3C Recommendation 10 February 2004, http://www.w3.org/TR/2004/REC-rdf-mt-20040210/. Latest version available at http://www.w3.org/TR/rdf-mt/.
- [RDF Syntax]
- RDF/XML Syntax Specification (Revised). Dave Beckett, ed., W3C Recommendation 10 February 2004, http://www.w3.org/TR/2004/REC-rdf-syntax-grammar-20040210/. Latest version available at http://www.w3.org/TR/rdf-syntax-grammar/.
- [OWL 2 Conformance]
- OWL 2 Web Ontology Language: Conformance and Test Cases. Peter F. Patel-Schneider and Boris Motik, eds., 2008
- Turtle - Terse RDF Triple Language. Dave Beckett and Tim Berners-Lee, W3C Team Submission 14 January 2008, http://www.w3.org/TeamSubmission/2008/SUBM-turtle-20080114/. Latest version available at http://www.w3.org/TeamSubmission/turtle/.
- [OWL 2 XML]
- OWL 2 Web Ontology Language: XML Serialization. Boris Motik and Peter F. Patel-Schneider, eds., 2008.
- [OWL 2 Manchester Syntax]
- OWL 2 Web Ontology Language: Manchester Syntax. Matthew Horridge and Peter F. Patel-Schneider, eds., 2008.
- [OWL 2 RDF-Based Semantics]
- OWL 2 Web Ontology Language: RDF-Based Semantics. Michael Schneider, ed., 2008.
- [OWL 2 Direct Semantics]
- OWL 2 Web Ontology Language: Direct Semantics. Boris Motik, Peter F. Patel-Schneider and Bernardo Cuenca Grau, eds., 2008.
- [Description Logics]
- The Description Logic Handbook. Franz Baader, Diego Calvanese, Deborah McGuinness, Daniele Nardi, Peter Patel-Schneider, Editors. Cambridge University Press, 2003; and Description Logics Home Page.
- [OWL 2 Profiles]
- OWL 2 Web Ontology Language: Profiles. Boris Motik, Bernardo Cuenca Grau, Ian Horrocks, Zhe Wu, Achille Fokoue and Carsten Lutz, eds., 2008.
- [OWL 1 Overview]
OOWLWeb Ontology Language Overview. Deborah L. McGuinness and Frank van Harmelen, eds, W3C Recommendation 10 February 2004, http://www.w3.org/TR/2004/REC-owl-features-20040210/. Latest version available at hhttp://www.w3.org/TR/owl-features/.
- [OWL 1 Abstract Syntax]
- OWL Web Ontology Language Semantics and Abstract Syntax, Section 2. Abstract Syntax. Peter F. Patel-Schneider and Ian Horrocks, eds, W3C Recommendation 10 February 2004, http://www.w3.org/TR/2004/REC-owl-semantics-20040210/syntax.html. Latest version available at http://www.w3.org/TR/owl-semantics/syntax.html.
- [OWL 1 RDF Mapping]
- OWL Web Ontology Language Semantics and Abstract Syntax, Section 4. Mapping to RDF Graphs. Peter F. Patel-Schneider and Ian Horrocks, eds, W3C Recommendation 10 February 2004, http://www.w3.org/TR/2004/REC-owl-semantics-20040210/mapping.html. Latest version available at http://www.w3.org/TR/owl-semantics/mapping.html.
- [OWL 1 RDF Semantics]
- OWL Web Ontology Language Semantics and Abstract Syntax, Section 5. RDF-Compatible Model-Theoretic Semantics. Peter F. Patel-Schneider, Patrick Hayes, and Ian Horrocks, eds, W3C Recommendation 10 February 2004, http://www.w3.org/TR/2004/REC-owl-semantics-20040210/rdfs.html. Latest version available at http://www.w3.org/TR/owl-semantics/rdfs.html.
- [OWL 1 Direct Semantics]
- OWL Web Ontology Language Semantics and Abstract Syntax, Section 3. Direct Model-Theoretic Semantics. Peter F. Patel-Schneider and Ian Horrocks, eds, W3C Recommendation 10 February 2004, http://www.w3.org/TR/2004/REC-owl-semantics-20040210/direct.html. Latest version available at http://www.w3.org/TR/owl-semantics/direct.html.
- [OWL 1 XML Syntax]
- OWL Web Ontology Language, XML Presentation Syntax. Masahiro Rohi, Jérôme Euzenat, and Peter F. Patel-Schneider, eds, W3C Note 11 June 2003, http://www.w3.org/TR/2003/NOTE-owl-xmlsyntax-20030611/. Latest version available at http://www.w3.org/TR/owl-xmlsyntax/.
- [OWL 1 Lite]
- OWL Web Ontology Language Semantics and Abstract Syntax, Section 2. Abstract Syntax, OWL Lite Axioms. Peter F. Patel-Schneider and Ian Horrocks, eds, W3C Recommendation 10 February 2004, http://www.w3.org/TR/2004/REC-owl-semantics-20040210/syntax.html#2.3.1. Latest version available at http://www.w3.org/TR/owl-semantics/syntax.html#2.3.1.
- [OWL 2 New Features and Rationale]
- OWL 2 New Features and Rationale. Christine Golbreich and Evan K. Wallace, eds. 2008.