Semantic Web OWL. Dieter Fensel and Federico Facca

Semantic Web OWL Copyright @2009 Dieter Fensel and Federico Facca 1

Where are we? # Title 1 Introduction 2 Semantic Web architecture 3 Resource Description Framework 4 Semantic Web of hypertext and Web of data 5 Generating Semantic Annotations 6 Repositories 7 OWL 8 RIF 9 Web-scale reasoning 10 Social Semantic Web 11 Ontologies and the Semantic Web 12 Service Web 13 Semantic Web Tools 14 Semantic Web Applications 15 Exam 2

Agenda 1. Introduction and motivation 2. Technical solution 1. Design of OWL 2. OWL Layering 3. OWL and Description Logics 4. OWL Syntaxes 3. Illustration by a large example 4. Extensions 5. Summary 6. References 3

Semantic Web Stack Adapted from http://en.wikipedia.org/wiki/semantic_web_stack 4

MOTIVATION 5

Overview Limitations of RDFS Expressivity limitations Layering problems Web Ontology Language OWL Design of OWL OWL Layering OWL and Description Logics OWL Syntaxes 6

What we discussed so far RDF Schema RDFS Vocabulary Semantics of RDF and RDF Schema Semantic notions RDF(S) Entailment SPARQL SPARQL Queries Query answer RDF Vocabulary Classes: rdf:property, rdf:statement, rdf:xmlliteral RDFS Metadata rdf:seq, rdf:bag, rdf:alt, rdf:list Literals and datatypes Properties: in RDFS rdf:type, rdf:subject, rdf:predicate, RDFS Vocabulary rdf:object, rdf:first, rdf:rest, rdf:_n rdf:value Resources: RDFS Vocabulary rdf:nil RDFS Classes rdfs:resource rdfs:class rdfs:literal rdfs:datatype rdfs:container rdfs:containermembershipproperty RDFS Properties rdfs:domain rdfs:range rdfs:subpropertyof rdfs:subclassof rdfs:member rdfs:seealso rdfs:isdefinedby rdfs:comment rdfs:label 9

What we discussed so far RDF Schema RDFS Vocabulary RDFS Metadata Literals and datatypes in RDFS Semantics of RDF and RDF Schema Semantic notions RDF(S) Entailment SPARQL SPARQL Queries Query answer PREFIX vcard: <http://www.w3.org/2001/vcard-rdf/3.0#> SELECT?fullName WHERE {?x vcard:fn?fullname} 13

Requirements for Ontology Languages Well-defined syntax Convenience of expression Formal semantics Needed in reasoning, e.g.: Class membership Equivalence of classes Consistency Classification Efficient reasoning support Sufficient expressive power 14

Limitations of RDFS No semantics for: Containers Collections Reification Domain and range of property infer information rather than check data Conjunctive interpretation of multiple restrictions Use of properties as objects RDF/XML syntax very verbose 15

RDFS as an Ontology Language Classes Properties Class hierarchies Property hierarchies Domain and range restrictions 16

Expressiveness Limitations of RDF(S) Only binary relations Characteristics of Properties E.g. inverse, transitive, symmetric Local range restrictions E.g. for class Person, the property hasname has range xsd:string Complex concept descriptions E.g. Person is defined by Man and Woman Cardinality restrictions E.g. a Person may have at most 1 name Disjointness axioms E.g. nobody can be both a Man and a Woman 17

Layering Issues Syntax Only binary relations in RDF Verbose syntax No limitations on graph in RDF Every graph is valid Semantics Malformed graphs Use of vocabulary in language e.g. <rdfs:class, rdfs:subclassof, ex:a> Meta-classes e.g. <rdfs:class, rdfs:subclassof, ex:a>, <rdfs:class, rdfs:subclassof, ex:b>, <ex:a, rdf:type, ex:b> 18

Stack of Languages XML Surface syntax, no semantics XML Schema Describes structure of XML documents RDF Datamodel for relations between things RDF Schema RDF Vocabulary Definition Language OWL A more expressive Vocabulary Definition Language This and following slides in part due to Frank van Harmelen http://www.cs.vu.nl/~frankh/spool/semwebslides/owl.ppt 19

RDF Schema Recap RDFS provides Classes Class hierarchies Properties Property hierarchies Domain and range restrictions RDFS does not provide Property characteristics (inverse, transitive,...) Local range restrictions Complex concept definitions Cardinality restrictions Disjointness axioms 20

Extending RDF Schema OWL extends RDF Schema to a full-fledged knowledge representation language for the Web Logical expressions (and, or, not) (In)equality Local properties Required/optional properties Required values Enumerated classes Symmetry, inverse 21

The Web Ontology Language TECHNICAL SOLUTION 22

Design Goals for OWL Shareable Changing over time Interoperability Inconsistency detection Balancing expressivity and complexity Ease of use Compatible with existing standards Internationalization 23

Requirements for OWL Ontologies are object on the Web with their own meta-data, versioning, etc... Ontologies are extendable From Grigoris Antoniou and Frank van Harmelen: A Semantic Web Primer, MIT Press 2004 24

Requirements for OWL Ontologies are object on the Web with their own meta-data, versioning, etc... Ontologies are extendable They contain classes, properties, data-types, range/domain, individuals Equality (for classes, for individuals) Classes as instances Cardinality constraints XML syntax 25

Objectives for OWL Objectives: Layered language Complex datatypes Digital signatures Decidability (in part) Local unique names (in part) Disregarded: Default values Closed world option Property chaining Arithmetic String operations Partial imports View definitions Procedural attachments A logical system or theory is decidable if there exists an effective method such that for every formula in the system the method is capable of deciding whether the formula is valid in the system or not. [http://www.wikipedia.org] [http://www.w3.org/tr/webont-req/] 26

Language Layers of OWL OWL Lite Classification hierarchy Simple constraints OWL DL Maximal expressiveness while maintaining tractability Standard formalization in a DL OWL Full Very high expressiveness Losing tractability All syntactic freedom of RDF (self-modifying) 27

Language Layers of OWL OWL Lite Classification hierarchy Simple constraints Problems that are solvable in theory but cannot be solved in practice are called intractable. [http://www.wikipedia.org] OWL DL Maximal expressiveness while maintaining tractability Standard formalization in a DL OWL Full Very high expressiveness Losing tractability All syntactic freedom of RDF (self-modifying) 28

Features of OWL Language Layers OWL Lite (sub)classes, individuals (sub)properties, domain, range conjunction (in)equality cardinality 0/1 datatypes inverse, transitive, symmetric properties somevaluesfrom allvaluesfrom OWL DL Negation Disjunction Full cardinality Enumerated types hasvalue OWL Full Meta-classes Modify language 29

OWL Full No restriction on use of vocabulary (as long as legal RDF) Classes as instances (and much more) RDF style model theory Reasoning using FOL engine Semantics should correspond to OWL DL for restricted KBs 30

OWL DL Use of vocabulary restricted Can t be used to do nasty things (e.g. modify OWL) No classes as instances Defined by abstract syntax Standard DL-based model theory Direct correspondence with a DL Partial reasoning via DL engines Reasoning for full language via FOL engines No need for axiomatization Built-in datatypes required for performance 31

OWL Lite No explicit negation or union Restricted cardinality (0/1) No nominals (oneof) DL-based semantics Reasoning via DL engines (+ datatypes) Semantically, only small restriction on OWL DL No nominals No arbitrary cardinality 32

OWL and Description Logics OWL Lite corresponds to the DL SHIN(D) Named classes (A) Named properties (P) Individuals (C(o)) Property values (P(o, a)) Intersection (C D) Union(!) (C D) Negation(!) ( C) Existential value restrictions ( P.C) Universal value restrictions ( P.C) Unqualified number restrictions ( np, np, = np) OWL DL corresponds to the DL SHOIN(D) Property value ( P.{o}) Enumeration ({o 1,..., o n }) 33

OWL Constructs + XML Schema datatypes: int, string, real, etc... 34

OWL Axioms 35

More on OWL Species OWL Full is not a Description Logic OWL Lite has strong syntactic restrictions, but only limited semantics restrictions, compared with OWL DL Negation can be encoded using disjointness With negation and conjunction, you can encode disjunction Class(C complete unionof(b C)) is equivalent to: DisjointClasses(not B B) DisjointClasses(not C C) Class(not B and not C complete not B not C) DisjointClasses(notB and not C B or C) Class(C complete not B and not C) 36

More on Layering For an OWL DL-restricted KB, OWL Full semantics is not equivalent to OWL DL semantics John friend Susan OWL Full entails: John rdf:type owl:thing Susan rdf:type owl:thing friend rdf:type owl:objectproperty John rdf:type _:x _:x owl:onproperty friend _:x owl: mincardinality 1 ˆˆxsd:nonNegativeInteger 37

Syntaxes of OWL RDF Official exchange syntax Hard for humans RDF parsers are hard to write! UML Large user base XML Not the RDF syntax Better for humans More XML than RDF tools available Abstract syntax Not defined for OWL Full Human readable 38

OWL in RDF/XML Example from [OwlGuide]: <!ENTITY vin http://www.w3.org/tr/2004/rec owl guide 20040210/wine# > <!ENTITY food http://www.w3.org/tr/2004/rec owl guide 20040210/food# >... <rdf:rdf xmlns:vin= http://www.w3.org/tr/2004/rec owl guide 20040210/wine# xmlns:food= http://www.w3.org/tr/2004/rec owl guide 20040210/food#... > <owl:class rdf:id= Wine > <rdfs:subclassof rdf:resource= &food;potableliquid /> <rdfs:label xml:lang= en >wine</rdfs:label> <rdfs:label xml:lang= fr >vin</rdfs:label>... </owl:class> <owl:class rdf:id= Pasta > <rdfs:subclassof rdf:resource= #EdibleThing />... </owl:class> </rdf:rdf> 39

OWL Abstract Syntax Class(professor partial ) Class(associateProfessor partial academicstaffmember) DisjointClasses(associateProfessor assistantprofessor) DisjointClasses(professor associateprofessor) Class(faculty complete academicstaffmember) 41

OWL Abstract Syntax In DL syntax: associateprofessor academicstaffmember associateprofessor assistantprofessor professor associateprofessor faculty academicstaffmember Class(professor partial ) Class(associateProfessor partial academicstaffmember) DisjointClasses(associateProfessor assistantprofessor) DisjointClasses(professor associateprofessor) Class(faculty complete academicstaffmember) 42

More Examples DatatypeProperty(age range(xsd:nonnegativeinteger)) ObjectProperty(lecturesIn) ObjectProperty(isTaughtBy domain(course) range(academicstaffmember)) SubPropertyOf(isTaughtBy involves) ObjectProperty(teaches inverseof(istaughtby) domain(academicstaffmember) range(course)) EquivalentProperties(lecturesIn teaches) ObjectProperty(hasSameGradeAs Transitive Symmetric domain(student) range(student)) 43

More Examples In DL syntax: age.xsd:nonnegativeinteger istaughtby.course istaughtby.academicstaffmember istaughtby involves teaches istaughtby teaches.academicstaffmember teaches.course lecturesin teaches hassamegradeas + hassamegradeas hassamegradeas hassamegradeas hassamegradeas.student hassamegradeas.student DatatypeProperty(age range(xsd:nonnegativeinteger)) ObjectProperty(lecturesIn) ObjectProperty(isTaughtBy domain(course) range(academicstaffmember)) SubPropertyOf(isTaughtBy involves) ObjectProperty(teaches inverseof(istaughtby) domain(academicstaffmember) range(course)) EquivalentProperties(lecturesIn teaches) ObjectProperty(hasSameGradeAs Transitive Symmetric domain(student) range(student)) 44

More Examples Individual(949318 type(lecturer)) Individual(949352 type(academicstaffmember) value(age 39 ˆˆ&xsd;integer)) ObjectProperty(isTaughtBy Functional) Individual(CIT1111 type(course) value(istaughtby 949352) value(istaughtby 949318)) DifferentIndividuals(949318 949352) DifferentIndividuals(949352 949111 949318) 45

More Examples In DL syntax: 949318 : lecturer 949352 : academicstaffmember 949352, 39 ˆˆ&xsd; integer :age 1isTaughtBy CIT1111 : course CIT1111, 949352 :istaughtby CIT1111, 949318 :istaughtby 949318 949352 949352 949111 949111 949318 949352 949318 Individual(949318 type(lecturer)) Individual(949352 type(academicstaffmember) value(age 39 ˆˆ&xsd;integer)) ObjectProperty(isTaughtBy Functional) Individual(CIT1111 type(course) value(istaughtby 949352) value(istaughtby 949318)) DifferentIndividuals(949318 949352) DifferentIndividuals(949352 949111 949318) 46

More Examples Class(firstYearCourse partial restriction(istaughtby allvaluesfrom (Professor))) Class(mathCourse partial restriction(istaughtby hasvalue (949352))) Class(academicStaffMember partial restriction (teaches somevaluesfrom (undergraduatecourse))) Class(course partial restriction (istaughtby mincardinality (1))) Class(department partial restriction (hasmember mincardinality(10)) restriction (hasmember maxcardinality(30))) 47

More Examples In DL syntax: firstyearcourse istaughtby.professor mathcourse istaughtby.{949352} academicstaffmember teaches.undergraduatecourse course 1isTaughtBy department 10hasMember 30hasMember Class(firstYearCourse partial restriction(istaughtby allvaluesfrom (Professor))) Class(mathCourse partial restriction(istaughtby hasvalue (949352))) Class(academicStaffMember partial restriction (teaches somevaluesfrom (undergraduatecourse))) Class(course partial restriction (istaughtby mincardinality (1))) Class(department partial restriction (hasmember mincardinality(10)) restriction (hasmember maxcardinality(30))) 48

More Examples Class(course partial complementof(staffmember)) Class(peopleAtUni complete unionof(staffmember student)) Class(facultyInCS complete intersectionof(faculty restriction (belongsto hasvalue (CSDepartment)))) Class(adminStaff complete intersectionof(staffmember complementof(unionof(faculty techsupportstaff)))) 49

More Examples In DL syntax: course staffmember peopleatuni staffmember student facultyincs faculty belongsto.{csdepartment} adminstaff staffmember (faculty techsupportstaff ) Class(course partial complementof(staffmember)) Class(peopleAtUni complete unionof(staffmember student)) Class(facultyInCS complete intersectionof(faculty restriction (belongsto hasvalue (CSDepartment)))) Class(adminStaff complete intersectionof(staffmember complementof(unionof(faculty techsupportstaff)))) 50

Tool Support for OWL Ontology editors Protégé (http://protege.stanford.edu/) OilED (http://oiled.man.ac.uk/) Reasoners Hoolet (http://owl.man.ac.uk/hoolet/) Fact++ (http://owl.man.ac.uk/factplusplus/) KAON2 (http://kaon2.semanticweb.org/) Pellet (http://clarkparsia.com/pellet/) APIs OWL-API (http://owlapi.sourceforge.net/) Jena (http://jena.sourceforge.net/) 51

An example of usage of OWL ILLUSTRATION BY A LARGER EXAMPLE 52

The Wine Ontology An Ontology describing wine domain One of the most widely used examples for OWL and referenced by W3C. There is also a wine agent associated to this ontology that performs OWL queries using a web-based ontological mark-up language. That is, by combining a logical reasoner with an OWL ontology. The agent's operation can be described in three parts: consulting the ontology, performing queries and outputting results. Available here: http://www.w3.org/tr/owl-guide/ 53

Wine Ontology Schema [http://mysite.verizon.net/jflynn12/visioowl/visioowl.htm] 54

Wine Ontology OWL Example <owl:class rdf:id="cheninblanc"> <rdfs:subclassof> <owl:restriction> <owl:onproperty rdf:resource="#hascolor"/> <owl:hasvalue rdf:resource="#white"/> </owl:restriction> </rdfs:subclassof> <rdfs:subclassof> <owl:restriction> <owl:onproperty rdf:resource="#hasbody"/> <owl:allvaluesfrom> <owl:class> <owl:oneof rdf:parsetype="collection"> <owl:thing rdf:about="#full"/> <owl:thing rdf:about="#medium"/> </owl:oneof> </owl:class> </owl:allvaluesfrom> </owl:restriction> </rdfs:subclassof> <owl:intersectionof rdf:parsetype="collection"> <owl:class rdf:about="#wine"/> <owl:restriction> <owl:onproperty rdf:resource="#madefromgrape"/> <owl:hasvalue rdf:resource="#cheninblancgrape"/> </owl:restriction> <owl:restriction> <owl:onproperty rdf:resource="#madefromgrape"/> <owl:maxcardinality rdf:datatype="http://www.w3.org/2001/xmlschema#nonnegativeinteger">1</owl:maxcardinality> </owl:restriction> </owl:intersectionof> </owl:class> <CheninBlanc rdf:id="ventanacheninblanc"> <locatedin rdf:resource="#centralcoastregion"/> <hasmaker rdf:resource="#ventana"/> <hassugar rdf:resource="#offdry"/> <hasflavor rdf:resource="#moderate"/> <hasbody rdf:resource="#medium"/> </CheninBlanc> 55

EXTENSIONS 56

From Description Logic to Horn Logic The practical experience in building applications has revealed several shortcomings of OWL, and hence Description Logics for modeling knowledge Feature that ontology engineers are missing: Higher Relational Expressivity Polyadic Predicates Close-World Reasoning Integrity Constraints Modeling Exceptions Proper combination of Description Logic and Horn Logic can define extensions of the language to support such features OWL-Flight 57

SUMMARY 58

Summary Limitations of RDFS Expressivity limitations Problems with layering Web Ontology Language OWL Design of OWL OWL Layering OWL and Description Logics OWL Syntaxes 59

References Mandatory reading Semantic Web Primer Chapters 4 [OWL Guide] http://www.w3.org/tr/owl-guide/ Ian Horrocks, Peter F. Patel-Schneider, and Frank van Harmelen. From SHIQ and RDF to OWL: The making of a web ontology language. Journal of Web Semantics, 1(1):7, 2003. http://www.cs.vu.nl/%7efrankh/abstracts/jws03.html Further reading Jos de Bruijn: Using Ontologies. Enabling Knowledge Sharing and Reuse on the Semantic Web. DERI Technical Report DERI-2003-10-29, 2003. http://www.deri.org/publications/techpapers/documents/deri-tr-2003-10-29.pdf [OWL Reference] http://www.w3.org/tr/owl-ref/ [OWL Abstract syntax and Semantics] http://www.w3.org/tr/owl-semantics J. de Bruijn, A. Polleres, R. Lara, and D. Fensel: OWL DL vs. OWL Flight: Conceptual modeling and reasoning on the semantic web. In Proceedings of the 14th International World Wide Web Conference (WWW2005), Chiba, Japan, 2005. ACM. 60

Next Lecture # Title 1 Introduction 2 Semantic Web architecture 3 Resource Description Framework 4 Semantic Web of hypertext and Web of data 5 Generating Semantic Annotations 6 Repositories 7 OWL 8 RIF 9 Web-scale reasoning 10 Social Semantic Web 11 Ontologies and the Semantic Web 12 Service Web 13 Semantic Web Tools 14 Semantic Web Applications 15 Exam 61

Questions? 62 62