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Semantic Web

Morteza Amini

Ontology and OWL

Sharif University of Technology Fall 95-96

Outline

Introduction & Definitions

Ontology Languages

OWL (Ontology Web Language)

Sharif Univ. of Tech. Ontology and OWL - Morteza Amini 2

Outline


Ontology Languages



Where does it come from?

ontology n.

1692; lat. phil. onto- “being” + -logia “study of”

Philosophy The study of what is, what has to be true for something to

exist, the kinds of things that can exist.

AI and computer science Something exists if it can be represented, described, defined (in

a formal, hence, machine-interpretable way).


Ontologies


Ontologies (contd.)

Ontologies are about vocabularies and their meanings, with explicit, expressive, and well-defined semantics, possibly machine-interpretable.

“Ontology is a formal specification of a shared conceptualization.” Gruber, 1993

Main elements of an ontology: Concepts Relationships

Hierarchical Logical

Properties Instances (individuals)


Ontologies in Semantic Web

Although XML DTDs and XML Schemas are sufficient for exchanging data between parties who have agreed to definitions beforehand, their lack of semantics prevent machines from reliably performing this task given new XML vocabularies.

RDF and RDF Schema begin to approach this problem by allowing simple semantics to be associated with identifiers. With RDF Schema, one can define classes that may have multiple subclasses and super classes, and can define properties, which may have sub properties, domains, and ranges.

In this sense, RDF Schema is a simple ontology language. However, in order to achieve interoperation between numerous, autonomously developed and managed schemas, richer semantics are needed.

For example, RDF Schema cannot specify that the Person and Car classes are disjoint, or that a string quartet has exactly four musicians as members.


For Machines...

The meaning of the document is not defined.

Machines cannot understand it.

We are defining the structure of document by XML, but now the meaning of the structure is not defined.

<Sentence> <Subject> Paper </Subject> <Verb> is made from </Verb> <Object> Wood </Object> </Sentence>

XM

L do

cum

ent

<Σεντενχε> <Συβϕεχτ> Παπερ </Συβϕεχτ> <ςερβ> ισ µαδε φροµ </ςερβ> <Οβϕεχτ> Ωοοδ </Οβϕεχτ> </Σεντενχε>


<Σεντενχε> <Συβϕεχτ> </Συβϕεχτ> <ςερβ> </ςερβ> <Οβϕεχτ> </Οβϕεχτ> </Σεντενχε>

Παπερ

ισ µαδε φροµ

Ωοοδ

Ontology Gives the Meaning...

Document Ontology

Natural Language


Why Develop Ontologies?

To share common understanding of the structure of information among people or agents and share knowledge E.g., using an ontology for integrating terminologies

To reuse domain knowledge E.g., geography ontology

To make domain assumptions explicit Facilitate knowledge management, easier to validate, to change, … Enable new users to learn about the domain

To support interoperability Terms defined in expressive ontologies allow for mapping precisely how

one term relates to another

….


Outline


Ontology Languages



Ontology Languages

Graphical notations Semantic networks Topic maps UML RDF(S)

Logic based Description Logics (e.g., OIL, DAML+OIL, OWL) Rules (e.g., RuleML, LP/Prolog, SWRL) First Order Logic


Ontology Languages

RDF(S) (Resource Description Framework (Schema))

OIL (Ontology Interchange Language)

DAML+OIL (DARPA Agent Markup Language + OIL)


XOL (XML-based Ontology Exchange Language)

SHOE (Simple HTML Ontology Extension)

OML (Ontology Markup Language)


Object Oriented Model

Many languages use object oriented model:

Objects/Instances/Individuals Elements of the domain of discourse Equivalent to constants in FOL

Types/Classes/Concepts Sets of objects sharing certain characteristics Equivalent to unary predicates in FOL and Concepts in DL

Relations/Properties/Roles Sets of pairs (tuples) of objects Equivalent to binary predicates in FOL and Roles in DL


Outline


Ontology Languages




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 very rudimentary 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 inference than you get with RDF Schema.


Origins of OWL

RDF(S)

DAML+OIL

DARPA Agent Markup Language

A W3C Recommendation

OIL

OWL

All influenced by RDF

Ontology Interchange Language

EU/NSF Joint Ad hoc Committee

DAML

OWL Lite OWL DL OWL Full


OWL

OWL and RDF Schema enable rich machine-processable semantics.

XML/DTD/XML Schemas

RDF Schema

OWL Semantics

Syntax

<rdfs:Class rdf:ID="River"> <rdfs:subClassOf rdf:resource="#Stream"/> </rdfs:Class>

<owl:Class rdf:ID="River"> <rdfs:subClassOf rdf:resource="#Stream"/> </owl:Class>

RDFS

OWL


OWL Design Goals

Shared ontologies

Ontology interoperability

Inconsistency detection

Expressivity vs. scalability

Ease of use

Compatibility with other standards

Internationalization Sharif Univ. of Tech. Ontology and OWL - Morteza Amini 19

Versions of OWL

Depending on the intended usage, OWL provides three increasingly expressive sublanguages.

Full Very expressive, no computation (decidability)

guarantees.

DL (Description Logic) Maximum expressiveness, computationally

complete (decidable).

Lite Simple classification hierarchy with simple

constraints.

OWL Full

OWL DL

OWL Lite


Comparison of Versions

OWL Lite supports those users primarily needing a classification hierarchy and simple constraints. E.g., while it supports cardinality constraints, it only permits cardinality values of 0 or 1

OWL DL supports those users who want the maximum expressiveness while retaining decidability. OWL DL includes all OWL language constructs, but they can be used only under certain restrictions (e.g., while a class may be a subclass of many classes, a class cannot be an instance of another class). OWL DL is so named due to its correspondence with description logics.

OWL Full is meant for users who want maximum expressiveness and the syntactic freedom of RDF with no computational guarantees. For example, in OWL Full a class can be treated simultaneously as a collection of individuals and as an individual in its own right.


OWL vs. RDFS

OWL allows greater expressiveness (more meaning) Abstraction mechanism to group resources with similar

characteristics Much more powerful in describing constraints on relations

between classes Property transitivity, equivalence, symmetry, etc. …

Extensive support for reasoning


OWL vs. RDFS

RDF Schema provides some of predefined properties: rdfs:range used to indicate the range of values for a property. rdfs:domain used to associate a property with a class. rdfs:subPropertyOf used to specialize a property. …

OWL provides additional predefined properties: owl:cardinality (indicate cardinality) owl:hasValue (at least one of the specified property values) …

OWL provides additional property classes, which allow reasoning and inferencing: owl:FunctionalProperty owl:TransitiveProperty …


OWL/XML Declaration

Similar to RDF, containing owl name space. <rdf:RDF xmlns:owl=”http://www.w3c.org/2002/07/owl#” xmlns:rdf=“http://www.w3c.org/1999/02/22-rdf-syntax-ns#” xmlns:rdfs=“http://www.w3c.org/2000/01/rdf-schema#” xmlns:ex=“…”>

.

.

.

</rdf:RDF>


OWL Ontologies

What’s inside an OWL ontology Classes + class-hierarchy Properties (Slots) / values Relations between classes

(inheritance, disjoints, equivalents) Restrictions on properties (type, cardinality) Characteristics of properties (transitive, …) Individuals ...

Reasoning tasks: classification, consistency checking, …


Classes

What is a Class?

e.g., person, pet, old

a collection of individuals (object, things, . . . )

a way of describing part of the world

an object in the world (OWL Full)


OWL Classes

owl:Class Subclass of Class in RDF Better to forget about classes of classes in this course

Top-most class: owl:Thing

Bottom class: owl:Nothing

<owl:Class rdf:ID=“Person"/> <owl:Class rdf:ID=“Man"> <rdfs:subClassOf rdf:resource="#Person" /> </owl:Class>


Describing Classes in OWL

Complex Classes Union of classes (owl:unionOf)

OR (A B)

Intersection of classes (owl:intersectionOf) AND (A B)

Complement (owl:complementOf)

NOT (ØA)

Enumeration (owl:oneOf)

Disjoint Classes (owl:disjointWith)


Example

<owl:Class rdf:ID=“Parent” /> <owl:Calss rdf:ID=“Man”> <owl:subClassOf rdf:resource=#Person”/> <owl:disjointWith rdf:resource=#Woman”/> </owl:Class> <owl:Class rdf:ID=“Father”> <owl:intersectionOf rdf:parseType=“Collection”> <owl:Class rdf:about=“#Parent”/> <owl:Class rdf:about=“#Man”/> </owl:intersectionOf> </owl:Class> <owl:Class rdf:ID=“Nationality”> <owl:oneOf rdf:parseType=“Collection”> <owl:Thing rdf:about=“#Iranian”/> <owl:Thing rdf:about=“#Japanese”/> . . . </owl:oneOf> </owl:Class>


Example

<owl:Calss rdf:ID=“A”> <owl:intersectionOf rdf:parseType=“Collection”> <owl:Class> <owl:oneOf rdf:parseType=“Collection”> <owl:Thing rdf:about=“#x1”/> <owl:Thing rdf:about=“#x2”/> . . . </owl:oneOf> </owl:Class> <owl:Class> <owl:unionOf rdf:parseType=“Collection”> <owl:Class rdf:about=“#B”/> <owl:Class rdf:about=“#C”/> . . . </owl:unionOf> </owl:Class> <owl:Class> <owl:complementOf rdf:resource=“#D”/> </owl:Class> </owl:intersectionOf> </owl:Class>


A≡x1, x2 (BC) ¬D

Properties

What is a Property?

e.g., has_father, has_pet, service_number

a collection of relationships between individuals (and data)

a way of describing a kind of relationship between individuals


OWL Properties

Object Properties

Ana owns Cuba

Data type Properties

Ana age 25


Describing Properties in OWL

OWL Property Classes An ObjectProperty relates one Resource to another

Resource. A DatatypeProperty relates one Resource to a Literal - an

XML Schema data type.

rdf:Property

owl:ObjectProperty owl:DatatypeProperty owl:FunctionalProperty owl:InverseFunctionalProperty

owl:SymmetricProperty owl:TransitiveProperty


Defining Properties

We have RDFS facilities as well. rdfs:subPropertyOf rdfs:domain rdfs:range

<owl:ObjectProperty rdf:ID="madeFrom"> <rdfs:domain rdf:resource="#Book"/> <rdfs:range rdf:resource="#Paper"/> </owl:ObjectProperty>

<owl:ObjectProperty rdf:ID=“fatherOf"> <rdfs:subPropertyOf rdf:resource="#parentOf"/> </owl:ObjectProperty>

<owl:DatatypeProperty rdf:ID=“hasAge"> <rdfs:domain rdf:resource="#Person" /> <rdfs:range rdf:resource="&xsd;positiveInteger"/> </owl:DatatypeProperty>


Transitive Properties

X p Y

Y p Z

imply X p Z

Transitivity existed already in RDF “rdfs:subClassOf”, “rdfs:subPropertyOf”

Example: locatedIn <owl:TransitiveProperty rdf:ID=“locatedIn"> <rdfs:domain rdf:resource="#Region"/> <rdfs:range rdf:resource="#Region"/> </owl:TransitiveProperty>


Symmetric Properties

X p Y

implies Y p X

Example: colleagueOf <owl:SymmetricProperty rdf:ID=“colleagueOf"> <rdfs:domain rdf:resource="#Person"/> <rdfs:range rdf:resource="#Person"/> </owl:SymmetricProperty>


Functional Properties

X p Y

X p Z

imply Z is the same as Y (describe the same)

Example: hasHusband <owl:FunctionalProperty rdf:ID=“hasHusband"> <rdfs:domain rdf:resource="#Woman"/> <rdfs:range rdf:resource="#Man"/> </owl:FunctionalProperty>


Inverse Functional Properties

Y p X

Z p X

imply Z is the same as Y (describe the same)

Example: isMotherOf

<owl:InverseFunctionalProperty rdf:ID=“isMotherOf"> <rdfs:domain rdf:resource="#Woman"/> <rdfs:range rdf:resource="#Person"/> </owl:InverseFunctionalProperty>


Individuals and Their Properties

Two equivalent declarations: <Person rdf:ID=“ahmadi" />

OR <owl:Thing rdf:ID=“ahmadi" /> <owl:Thing rdf:about="#ahmadi"> <rdf:type rdf:resource="#Person"/> </owl:Thing>

Properties of an individual < Person rdf:ID=“reza" /> <owl:Thing rdf:about="#ahmadi"> <fatherOf rdf:resource="#reza"/> <hasAge rdf:datatype=“&xsd;positiveInteger “>42</hasAge> </owl:Thing>


Individuals

owl:sameAs Links two references to an individual.

owl:differentFrom Indicates two URI references refer to different individuals.

owl:AllDifferent All individuals in the list are all different from each other.

<owl:AllDifferent> <owl:distinctMembers rdf:parseType=“Collection”> <Person rdf:about=“#ali”/> <Person rdf:about=“#reza”/> <Person rdf:about=“#maryam”/> </owl:distinctMembers> </owl:AllDifferent> <Person rdf:ID=“ali"> <owl:differentFrom rdf:resource="#reza"/> <owl:sameAs rdf:resource="#morteza"/> </Person>


OWL Distributed

owl:equivalentClass for describing equivalent classes.

owl:equivalentProperty for describing equivalent properties.

Guitar

Guitarra

Internationalization standards ? Sharif Univ. of Tech. Ontology and OWL - Morteza Amini 41

Example

<owl:Class rdf:ID="DaPonteOperaOfMozart">

<owl:equivalentClass>

<owl:Class>

<owl:oneOf rdf:parseType="Collection">

<Opera rdf:about="#Nozze_di_Figaro"/>

<Opera rdf:about="#Don_Giovanni"/>

<Opera rdf:about="#Cosi_fan_tutte"/>

</owl:oneOf>

</owl:Class>

</owl:equivalentClass>

</owl:Class>


OWL Property Restriction

Property restriction is a kind of class description.

Property Restrictions

Defining a Class by restricting its possible instances via their

property values.

OWL distinguishes between the following two:

Value constraints

Cardinality constraints


OWL Value Constraints

Puts constraints on the range of the property when applied to this particular class description.

Properties: allValuesFrom: rdfs:Class (lite/DL owl:Class) hasValue: specific Individual someValuesFrom: rdfs:Class (lite/DL owl:Class)

<owl:Restriction> <owl:onProperty rdf:resource=“#hasParent”/> <owl:someValuesFrom rdf:resource=“#Physician”> </owl:Restriction> <owl:Class rdf:ID=“AliChildren”> <rdfs:equivalentClass> <owl:Restriction> <owl:onProperty rdf:resource=“#hasParent”/> <owl:hasValue rdf:resource=“#ali”> </owl:Restriction> </rdfs:equivalentClass> <owl:Class>


Class Description

OWL Cardinality Constraints

Any instance of a class may have an arbitrary number (zero or more) of values for a particular property.

To restrict the cardinality of properties locally with in a class context, the following constructs can be used. owl:minCardinality type: xsd:nonNegativeInteger (in lite 0,1) owl:maxCardinality type: xsd:nonNegativeInteger (in lite 0,1) owl:Cardinality type: xsd:nonNegativeInteger (in lite 0,1)

Example: Fathers have at least two children. <owl:Class rdf:ID=“Father”> <owl:Restriction owl:minCardinality=“2"> <owl:onProperty rdf:resource="#hasChild" /> </owl:Restriction> </owl:Class>


An Example of OWL Ontology

<owl:Class rdf:ID=“Person” /> <owl:Class rdf:ID=“Man”>

<rdfs:subClassOf rdf:resource=“#Person” /> <owl:disjointWith rdf:resource=“#Woman” />

</owl:Class> <owl:Class rdf:ID=“Woman”> <rdfs:subClassOf rdf:resource=“#Person” /> <owl:disjointWith rdf:resource=“#Man” /> </owl:Class> <owl:Class rdf:ID=“Father”> <rdfs:subClassOf rdf:resource=“Man” /> <owl:Restriction owl:minCardinality="1"> <owl:onProperty rdf:resource="#hasChild" /> </owl:Restriction> </owl:Class> <owl:ObjectProperty rdf:ID=“hasChild"> <rdfs:domain rdf:resource="#Person" /> <rdfs:range rdf:resource="#Person" /> </owl:ObjectProperty>


Introduction to OWL2: Version 2 of OWL

OWL2 has a very similar overall structure to OWL1.

Backwards compatibility with OWL1 is, to all intents and purposes, complete: all OWL1 Ontologies remain valid OWL2 Ontologies, with identical inferences in all practical cases.

OWL2 adds new functionality with respect to OWL1. Some of the new features are syntactic sugar (e.g., disjoint union of

classes) Others offer new expressivity, including:

Keys: a collection of properties can be assigned as a key to a class. Property chains: to define properties as a composition of other properties. Richer datatypes and data ranges: capability of custom datatype definition. Qualified cardinality restrictions: supporting ∃£nR.C and ∃£nR.C and ∃=nR.C Asymmetric, reflexive, and disjoint properties


OWL2 Profiles (1)

OWL2 also defines three new profiles.

Profiles considered Useful computational properties, e.g., reasoning complexity Implementation possibilities, e.g., using RDBs


OWL 2 three different tractable profiles:

OWL2 EL: polynomial time reasoning for schema (ontology consistency and subsumption) Useful for ontologies with large conceptual part

OWL2 QL: fast (logspace) query answering using RDBMs via SQL Useful for large datasets already stored in RDBs

OWL2 RL: fast (polynomial) query answering using rule extended DBs Useful for large datasets stored as RDF triples


OWL2 Profiles (2)

OWL2 EL is particularly useful in applications employing ontologies that contain very large numbers of properties and/or classes. Using this profile the basic reasoning problems (ontology consistency and class expression subsumption) can be performed in time that is polynomial with respect to the size of the ontology. The EL acronym reflects the profile's basis in the EL family of description logics [EL++],

logics that provide only Existential quantification.

OWL2 QL is aimed at applications that use very large volumes of instance data, and where query answering is the most important reasoning task. Using a suitable reasoning technique, sound and complete conjunctive query answering can be performed in LOGSPACE with respect to the size of the data (assertions). The QL acronym reflects the fact that query answering in this profile can be implemented

by rewriting queries into a standard relational Query Language.

OWL2 RL is aimed at applications that require scalable reasoning without sacrificing too much expressive power. OWL2 RL reasoning systems can be implemented using rule-based reasoning engines. The ontology consistency, class expression satisfiability, class expression subsumption, instance checking, and conjunctive query answering problems can be solved in time that is polynomial with respect to the size of the ontology. The RL acronym reflects the fact that reasoning in this profile can be implemented using a

standard Rule Language.


References

http://www.w3.org/TR/owl-ref/

http://www2.informatik.hu-berlin.de/~wandelt/ SW201213/9OWL2.pdf

Chapter 6 of the book


http://www.w3.org/TR/owl-ref/

http://www2.informatik.hu-berlin.de/%7Ewandelt/SW201213/9OWL2.pdf

http://www2.informatik.hu-berlin.de/%7Ewandelt/SW201213/9OWL2.pdf

Any Question... [email protected]


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