From SHIQ and RDF to OWL: The Making of a Web Ontology Language Ian Horrocks Peter F. Patel-Schneider and Frank van Harmelen Presented by Zonghui Lian

Ontology Languages So far, how many ontology languages we can know XOL (XML-based Ontology Exchange Language) SHOE (Simple HTML Ontology Extension) OML (Ontology Markup Language) RDF(S) (Resource Description Framework (Schema)) OIL (Ontology Interchange Language) DAML+OIL (DARPA Agent Markup Language + OIL) OWL (Ontology Web Language)

XML – XML Schema XML provides a standardized syntactical way to expose structural information XML schema allows to define a schema for XML documents and may already provide machine-understandable semantics of data Do not attach meaning to structural information

RDF Schema RDFS is too weak to describe resources in sufficient detail No localised range and domain constraints For example, can’t express such a subclass of person: the person who has children. No existence/cardinality constraints Can’t say that all instances of person have a mother that is also a person, or that persons have exactly 2 parents No transitive, inverse or symmetrical properties Can’t say that isPartOf is a transitive property, that hasPart is the inverse of isPartOf or that touches is symmetrical Difficult to provide reasoning support RDF has fact-stating ability. Symmetric: if P(x, y) then P(y, x) Transitive: if P(x,y) and P(y,z) then P(x, z) Functional: if P(x,y) and P(x,z) then y=z InverseOf: if P1(x,y) then P2(y,x) InverseFunctional: if P(y,x) and P(z,x) then y=z allValuesFrom: P(x,y) and y=allValuesFrom(C) someValuesFrom: P(x,y) and y=someValuesFrom(C) hasValue: P(x,y) and y=hasValue(v) cardinality: cardinality(P) = N minCardinality: minCardinality(P) = N maxCardinality: maxCardinality(P) = N equivalentProperty: P1 = P2 intersectionOf: C = intersectionOf(C1, C2, …) unionOf: C = unionOf(C1, C2, …) complementOf: C = complementOf(C1) oneOf: C = one of(v1, v2, …) equivalentClass: C1 = C2 disjointWith: C1 != C2 sameIndividualAs: I1 = I2 differentFrom: I1 != I2 AllDifferent: I1 != I2, I1 != I3, I2 != I3, … Thing: I1, I2, … Legend: Properties are indicated by: P, P1, P2, etc Specific classes are indicated by: x, y, z Generic classes are indicated by: C, C1, C2 Values are indicated by: v, v1, v2 Instance documents are indicated by: I1, I2, I3, etc. A number is indicated by: N P(x,y) is read as: “property P relates x to y”

Requirements Desirable features identified for a Web Ontology Language : Compatible with existing Web standards (XML, RDF, RDFS) Easy to understand and use Formally specified Has “adequate” expressive power Tools for reasoning support

Layers of Languages Attribution Explanation We are here! Identity Standard Syntax Metadata Ontologies Rules & Inference Explanation Attribution Complexity could be a problem

OWL (Ontology Web Language) OWL is now a W3C Recommendation The purpose of OWL is identical to RDFS i.e. to provide an XML vocabulary to define classes, properties and their relationships. RDFS enables us to express basic relationships and has limited inferencing capability. OWL enables us to express much richer relationships, thus yielding a much enhanced inferencing capability. The benefit of OWL is that it facilitates a much greater degree of inferencing than you get with RDF Schema.

Comparison RDF and OWL

Introduction of OWL (Ontology Web Language) Description logic and frames Many characteristics of RDF The frame includes the name of the class, identifies the more general class (or classes) that it specialises, and lists a set of “slots”. A slot may consist of a property-value pair, or a constraint on the values that can act as slot “fillers” (in this context, value means either an individual or a data value). This structure was used in the OIL language, with some enrichment of the syntax for specifying classes and slot constraints so as to enable the full power of a Description Logic style language to be captured.

D.L’s Influence on OWL Semantics are well defined. OWL uses D.L model theory to formalise the meaning of the language. Advantages Reasoning technique --- check the consistency of classes and ontologies, and to check entailment relationships. Assume a query …

D.L’s Influence on OWL Language constructors Expressive power Class (property) constructors Axioms Conflict with the computational complexity OWL entailment Blance Boolean connectives, restriction on properties, transtive properties, and property hierarchy

D.L’s Influence on OWL Datatypes Difference with Advantage Decrease the complexity & easy to answer question “-5 nonNegative ?” Totally seperated, 2nd indicates abstract concepts.

Origins of OWL DARPA Agent Markup Language DAML Ontology Inference Layer OIL RDF EU/NSF Joint Ad hoc Committee DAML+OIL All influenced by RDF Oil: the precise details of RDF semantics. OWL provides the complex inferences, undecidable. OWL Lite OWL DL OWL Full A W3C Recommendation OWL

Versions of OWL Depending on the intended usage, OWL provides three increasingly expressive sublanguages OWL Full Full: Consider the compatibility with RDF and RDFS as the primary importance. no computation guarantees DL (Description Logic): Friendly syntax, decidable inference. computationally complete Lite: Simpler syntax and more tractable inference. OWL DL OWL Lite

Advantages/Disadvantages of versions Full: The advantage of the Full version of OWL is that we get the full power of the OWL language. The disadvantage is that it is very difficult to build a computational tool for this version and may not get a quick and complete answer. DL/Lite: The advantage of the DL or Lite version of OWL is that tools can be built more quickly and easily, and users can expect responses from such tools to come quicker and be more complete. The disadvantage is that we don't have access to the full power of the language.

OWL as D.L

OWL as D.L

Conclusion Difference b/w OWL DL & D.L: Datatyping mechanisms (XML schema datatypes) RDF URI reference as name Entailments OWL DL is compatible with that of RDF and RDFS