Document Overview

From OWL

Revision as of 10:30, 25 February 2009 by Ivan ( Talk | contribs ) ( diff ) ←Older revision | Current revision ( diff ) | Newer revision→ ( diff )Jump to: navigation, search

[Hide Review Comments]

Document title:
OWL 2 Web Ontology Language
Document Overview

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.

This document, part 1 of 13 in the OWL 2 document Overview describes the overall state ofset, serves as an introduction to OWL 2,2 and should be read beforethe various other OWL 2 documents. This documentIt describes serves as an introduction tothe various syntaxes for OWL 2 documents2, the different kinds of semantics, the defined profiles (sub-languages), and the differences between OWL 1 and OWL 2.

Status of this Document

Copyright © 2008 W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark and document use rules apply.

[Show Short TOC]


1 Introduction

Ontologies are formalisedformalized 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 Web Ontology Language developed by the W3C Web Ontology Working Group and published in 2004. OWL 2 is being developed (and this document was written) by a follow-on group, the W3C OWL Working group.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.

2 Overview

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 centrecenter represents the abstract notion of an ontology, which can be thought of either as an abstract structure or as an RDF graph.graph (see 2.1 Ontologies). At the top are various concrete syntaxes (see 2.2 Syntaxes) that can be used to serialiseserialize and exchange ontologies. At the bottom are the two semantic specifications that define the meaning of OWL 2 ontologies.ontologies (see 2.3 Semantics).

Note that many users of OWL 2 will need only one syntax and one semantics; for them, this diagram could be much simpler, with only their one syntax at the top, their one semantics at the bottom, and rarely a need to see what's inside the ellipse in the center.

diagram showing that each syntax maps to/from ontologies and ontologies have two semantics
Figure 1. The Structure of OWL 2

2.1 Ontologies

The conceptual structure of OWL 2 ontologies is defined in the OWL 2 Structural Specification document [OWL 2 Structural 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.

AnAny OWL 2 ontology can also be representedwritten 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[Guide [OWL 2 Quick ReferenceGuide] provides a simple overview of the structural and RDF “views” of OWL 2, with the two views laid out side by side.

2.2 SyntaxSyntaxes

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[document [OWL 2 Conformance]).

While RDF/XML[ RDF Syntax ]RDF/XML provides for interoperability among OWL 2 tools, other concrete syntaxes may also be used. These include alternative RDF serializations, such as Turtle[Turtle ],[Turtle]; an XML serialisationserialization [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,serialization, although its main purpose is specifying the structure of the language [OWL 2 Structural Specification]. It must be emphasized, however, that OWL 2 tools are not required to support any of these alternative syntaxes.

Name of SyntaxSpecificationStatusPurpose
RDF/XML Mapping to RDF Graphs,
Mandatory Interchange (can be written and read by all conformant OWL 2 software)
Turtle Mapping to RDF Graphs,
Optional An alternative way to serialize RDF
OWL/XML XML Serialization Optional Easier processing using XML tools
Functional Syntax Structural Specification Optional For expressing the precise formal structure of OWL 2 ontologies
Manchester Syntax Manchester Syntax Optional Simpler reading and writing of ontologies

2.3 Semantics

The OWL 2 Structural 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.their meaning. 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-calleda correspondence theorem providing a link between the two. These two semantics are used by reasoners and other tools to answer queries about, e.g., class consistency, subsumption and instance retrieval.

The Direct Semantics assigns meaning directly to ontology structures viastructures, resulting in a translation intosemantics compatible with the model theoretic semantics of the SROIQ description logic—a fragment of first order logic with useful computational properties. The advantage of this close connection is that the extensive description logic literature and implementation experience can be directly exploited by OWL 2 However, some restrictionsconditions must be placed on ontology structures in order to ensure that they can be translated into a SROIQ knowledge base;base, for example, there must be a strict separation of object and datatypetransitive properties cannot be used in number restrictions (see Section 3 of the OWL 2 Specification[Structural Specification document [OWL 2 Structural Specification] for a complete list of these restrictions).conditions). Ontologies that satisfy these restrictions and are interpreted using the Direct Semanticssyntactic conditions are called "OWL“OWL 2 DL"DL” ontologies.

The RDF-Based Semantics assigns meaning directly to RDF graphs and so indirectly to ontology structures via the Mapping to RDF-graphs. ThisThe RDF-Based Semantics is fully compatible with theRDF Semantics specification[[RDF Semantics], and itextends the semanticssemantic conditions defined in that document. Itfor RDF. The RDF-Based Semantics can be applied to any OWL 2 Ontology, without restrictions.restrictions, as any OWL 2 Ontology can be mapped to RDF.2 This semantics for OWL 2 Ontologies that are interpreted usingis sometimes called the RDF-based“OWL 2 Full” semantics are calledand “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 there an easy way to characterize them? Are they useful? It may be essentialalso used to give a good description hererefer to avoid destroyingthe overall view... OWL 2 DL and OWL 2 Full are closely related. Anyentire OWL 2 DL ontology can be treatedlanguage, particularly when expressed 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.RDF graphs.

The so-called “correspondence theorem” (seecorrespondence theorem in Section 7.3 inof the RDF BasedRDF-Based Semantics Document [OWL 2 RDF-Based Semantics]) alsodefines a preciseprecise, close relationship between the Direct and RDF-Based Semantics. In essence,This theorem states that,states, in essence, 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 Fullmapped into an RDF graph and interpreted using the RDF-Based Semantics.

32.4 Profiles

OWL 2 Profiles [OWL 2 RDF-Based SemanticsProfiles] are sub-languages (syntactic subsets) of OWL 2 that offer important advantages in particular application scenarios. Three different profiles are defined: OWL 2 EL, OWL 2 QL, and OWL 2 RL. Each profile is defined as a syntactic restriction of the OWL 2 Structural Specification, i.e.,i.e, as a subset of the structural elements that can be used in a conforming ontology.ontology, and each is more restrictive than OWL DL. Each of the profiles trades off different aspects of OWL's expressive power in return for different computational and/or implementational benefits.

Venn Diagram showing DL as a subset of Full, and EL, QL, and RL as overlapping subsets of DL

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 2 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 2 RL, a rule-based implementation can operatedoperate 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 (in general) when the ontology is consistent with the structural definition of OWL 2 RL, a suitable rule-based implementation performing ground atomic queries will be both sound and complete.

3 4Differences between OWL 21 and the previous version of OWL2

The 2004 version of OWL[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[Syntax [OWL 1 Abstract Syntax]. The two syntaxes are different but their respective role within the overall structure of OWL is identical. The new,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 ][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[Semantics [OWL 1 RDF Semantics] and Direct Model-Theoretic Semantics[Semantics [OWL 1 Direct Semantics].
  • An XML Presentation Syntax was also available for OWL 1[1 [OWL 1 Direct SemanticsXML Syntax] (although not as a Recommendation). On the other hand, the Manchester syntax did not exist for OWL 1.
  • OWL 1 defined one sub-lanuage (OWL Lite), where OWL 2 defines three (EL, QL, and RL). OWL Lite has not been re-specified for OWL 2, but because of backward compatibility, OWL Lite ends up as a sub-language of OWL 2.

The central role of RDF/XML and the role of other syntaxes, and the relationships between the OWL DLDirect and OWL FullRDF-Based semantics (ie,(i.e., the correspondence theorem) has not changed. More importantly, the changes are backward compatibility is ensured:compatible: all 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 inremain valid OWL 2.2 Ontologies, with identical inferences (except in certain minor cases, detailed in 3.2 Modifications).

3.1 New Features

OWL 2 alsoadds new functionality with respect to OWL 1. Some of this is purelythe new features are syntactic facilitiessugar (e.g., disjoint union of classes or pairwise disjoint classes); some is genuinely new, namely: keysclasses) while others offer new reasoning capabilities, including:

  • keys;
  • property chainschains;
  • richer datatypes, data rangesranges;
  • qualified cardinality restrictionsrestrictions;
  • assymetric, reflexive, and disjoint propertiesproperties; and
  • enhanced annotation capabilities

The OWL 2 New Features and Rationale Document[document [OWL 2 New Features and Rationale] gives a detailed description on thesedescribes all the new features,features of OWL 2, while the OWL 2 Quick Reference Guide[Guide [OWL 2 Quick ReferenceGuide] givesprovides 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,(i.e., 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,Graphs that can also be handled by DecriptionDescription Logics reasoners,reasoners has become a bit larger in OWL 2 compared to OWL 1.

53.2 Modifications

Editor's Note: This section needs to be filled in carefully, enumerating all the cases where an OWL 1 ontology will have different semantics in OWL 2. The narrowness of the cases is expected to underscore the degree to which OWL 2 is substantially compatible with OWL 1.

4 Documentation Roadmap

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).

Part Type Document
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.

65 References

Editor's Note: Before publication, these need to be checked and re-written to point to the latest TR. They should also be sorted into alphabetic order
[OWL 2 Structural 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 Reference]Guide]
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, Latest version available at
[RDF Syntax]
RDF/XML Syntax Specification (Revised). Dave Beckett, ed., W3C Recommendation 10 February 2004, Latest version available at
[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, Latest version available at
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]
OOWLOWL Web Ontology Language Overview. Deborah L. McGuinness and Frank van Harmelen, eds, W3C Recommendation 10 February 2004, Latest version available at h
[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, Latest version available at
[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, Latest version available at
[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, Latest version available at
[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, Latest version available at
[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, Latest version available at
[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, Latest version available at
[OWL 2 New Features and Rationale]
OWL 2 New Features and Rationale. Christine Golbreich and Evan K. Wallace, eds. 2008.
New_Features_and_Rationale 7 Notes 1 There are some rare cases when RDF/XML may not be usable. Indeed, there may be (valid) RDF graphs that cannot be serialized into RDF/XML due to the restrictions of the XML syntax. An example is when a URI used for a predicate cannot be represented as an XML element. OWL 2 tools may use other RDF serializations syntaxes for those cases, but users are usually advised not to use such URI-s. 2 Formally, the RDF-based Semantics provides an interpretation for any RDF graphs using the OWL 2 vocabulary, even for those that cannot be mapped to the abstract structure. However, the RDF-based semantics only gives added' meaning (beyond their standard RDF meaning) to those (parts of) RDF graphs that correspond to the conceptual structures defined in the OWL 2 Structural Specification. 3 The theorem does impose a few additional restrictions on the structure of OWL RL ontologies, and only holds for ground atomic queries.Retrieved from ""
Personal tools