OWL Representing Information Using the Web Ontology Language

Section 4

Chapter 17: OWL DL ▫OWL DL dialect, restrictions, remaining OWL constructs Chapter 18: OWL Full ▫Elimination of all restrictions Chapter 19: OWL Dialect Selection ▫How to select the appropriate dialect Chapter 20: Applications ▫Provides use case for Semantic Web enabled application

Chapter 17

17 OWL DL OWL DL ▫DL: Description Logics  Information representation technique with well-defined semantics supporting inferencing ▫Contains all OWL vocabulary ▫Has fewer restrictions than OWL Lite  Restrictions ensure decidability Chapter covers ▫OWL DL ▫Complex classes ▫Required property values ▫Enumerated data values

17.1 OWL DL Restrictions ConstructRestrictions owl:cardinalityCannot be used on a owl:TransitiveProperty owl:TransitivePropertyCannot have an owl:cardinality constraint nor be functional owl:inverseOfCannot be specified for datatype properties owl:InverseFunctionalProperty owl:SymmetricProperty owl:TransitiveProperty owl:ObjectPropertyMust be disjoint from datatype properties owl:DatatypePropertyMust be disjoint from object properties Cannot be inverse functional owl:AnnotationPropertyAll annotations must subclass this class and cannot be used in property axioms ConstructRestrictions owl:importsCannot import an OWL Full ontology owl:sameAsMust reference a named individual owl:differentFrom owl:ClassCannot be individuals of other classes Class URIrefMust be stated to be a class (type separation) Property URIrefMust be stated to be a property (type separation) Individual URIrefMust belong to a class (type separation) rdf:PropertyMust be one of “owl:ObjectProperty”, “owl:DatatypeProperty”, “owl:AnnotationProperty”, or “owl:Ontology Property” Most important restriction: Type separation (classes, properties, and individuals are disjoint)

OWL DL Vocabulary CategoryConstructFeature Class expression owl:oneOfEnumerated classes owl:disjointWithRelating classes owl:unionOf Boolean combination owl:complementOf Enumerated valuesowl:dataRangeEnumerated values Property restrictionsowl:hasValueFiller information CategoryConstructFeature Class expressions owl:equivalentClassCan be applied to class expressions rdfs:subClassOf owl:intersectionOfBoolean combination Cardinality owl:minCardinality Arbitrary values owl:maxCardinality owl:cardinality Additional Constructs in OWL DLConstructs less restricted in OWL DL

Complex classes ▫Composed of class expressions  Analogous to math expression  Constructed of atomic symbols and anonymous classes in allowed patterns  Describe membership criteria Classes described by ▫Class names (URIrefs) ▫Enumerations ▫Property restrictions ▫Boolean combinations of class expressions 17.2 Complex Classes & Expressions

Enumerated Classes An enumeration is a predefined set of individuals owl:oneOf property ▫Exhaustively specifies member individuals known a priori in advance ▫Closed list; items cannot be added, so list should be very stable ▫Reasoners infer maxCardinality from count of members ▫Domain: instance of owl:Class; Range: instance of owl:List Syntax ▫ Example ▫

Disjoint Classes owl:disjointWith property ▫Asserts that 2 classes have no individuals in common  Instances of a class cannot be members of both classes ▫Symmetric ▫Classes can be disjoint with multiple classes ▫Every class is disjoint with owl:Nothing ▫Domain & Range: instance of owl:Class Syntax ▫ Example ▫ </owl:Class

Boolean Class Combinations Class extensions are sets OWL DL supplies Boolean properties ▫owl:intersectionOf ▫owl:unionOf ▫owl:complementOf

Intersection Property owl:intersectionOf property ▫OWL Lite restrictions removed ▫Can be used to define a class  Includes individuals common to all identified classes ▫Equivalent to AND operation in logic ▫Domain: instance of owl:Class ▫Range: instance of rdf:List Example ▫

Union Property owl:unionOf property ▫Defines a class  Includes individuals in any specified class (one or more) ▫Equivalent to OR operation in logic ▫Domain: instance of owl:Class ▫Range: instance of rdf:List Example ▫

Complement Property owl:complementOf property ▫Defines a disjoint class  All individuals who are not in the specified class ▫Equivalent to negation operation in logic ▫Usually used with intersectionOf ▫Domain & Range: instance of owl:Class Example: ▫

Set Operator Summary OperatorLogical Effect Resulting Class Contains ANDIndividuals in each specified class ORIndividuals in at least 1 of the specified classes NOTIndividuals not in the specified class

17.3 Requiring a Property Value owl:hasValue property ▫Local restriction that specifies that  Objects must have identified property  Property must have specified value  Can also have other values for the same property ▫Can use to define classes based on property values ▫Domain: instance of owl:Restriction Example ▫

17.4 Enumerated Data Values owl:DataRange class ▫Specified with owl:oneOf and rdf:Lists ▫Like enumerated classes, but lists of values instead Example ▫ No Yes

17.5 OWL DL Summary OWL DL ▫Restricted form of OWL Full  Contains all constructs of OWL Full ▫Extends OWL Lite by adding  Class expressions  Property value requirements  Enumerated data values

Chapter 18

18 OWL Full OWL Full ▫Complete specification of the OWL lanugage ▫Extension of RDF ▫Logical Layer ▫Allows developer to say anything about anything ▫Unrestricted use of RDFS ▫Superset of RDF and RDFS

18.1 OWL Full’s Differing Perspective OWL Full ▫Supports all OWL constructs ▫Relaxes all restrictions on OWL DL (17.1)

Type Separation No strict separation of classes, properties, individuals and data values ▫Classes can be treated as objects ▫Objects can be treated as classes ▫Etc.

Individuals In OWL Full, rdfs:Resource is equivalent to owl:Thing ▫Therefore all data values are individuals

Classes as Individuals A resource can be considered both ▫A class of individuals and ▫An individual of some other class Class can be a member of its own extension! In OWL Full, owl:Class is equivalent to rdfs:Class ▫In OWL DL & OWL Lite, owl:Class is a subclass of rdfs:Class

Properties In OWL Full, owl:ObjectProperty and rdf:Property are equivalent Object and datatype properties do not have to be disjoint owl:DatatypeProperty can be owl:InverseFunctionalProperty rdfs:domain & rdfs:range can have descriptions as values Annotation properties can be used anywhere; no need for owl:AnnotationProperty class

Property Restrictions OWL Full has owl:hasValue (same as OWL DL), but relaxes restrictions on XMLS datatypes

18.2 OWL Full Summary OWL Full ▫Complete, unrestricted species of OWL ▫Most expressive ▫No restrictions ▫Cost: decidability  Inference engines probably will never support all of OWL Full

Chapter 19

19.1 Choosing Your Weapon Select dialect that ▫Sufficient to meet requirements ▫Most restrictive ▫Most widely supported OWL Lite is usually best, if possible ▫OWL Lite is valid OWL DL and valid OWL Full Use OWL DL if you need its advantages ▫Fewer restrictions, better support cardinality, etc. OWL Full is the last resort ▫Use only if you need its expressive power

19.2 Migrating XML & RDF to OWL Existing RDF is only guaranteed to be valid in the OWL Full dialect ▫E.g., for OWL Lite & DL, URIrefs of classes, properties, & individuals must be explicitly typed Full list of restrictions in App. E of OWL Ref ▫ XML files should be migrated to RDF/XML first

19.3 Language Selection Summary Need decidability and computability? ▫OWL DL or OWL Lite Need all OWL constructs? ▫OWL DL or OWL Full Don’t need the kitchen sink? ▫OWL DL RequirementRecommendation Simple taxonomical relationships OWL Lite DecidabilityOWL DL or OWL Lite All OWL constructsOWL DL or OWL Full Maximum expressivityOWL Full

19.4 Satisfaction of Information Representation (IR) Requirements Building BlockOWL Support Description Class The OWL class construct represents a group or set of individual objects with similar characteristics. Subclass relationships support the definition of a subsumption hierarchy. Accomplished through rdfs:Class and owl:Class Property The RDF property construct associated attribute/value pairs with individuals. OWL datatype properties and object properties refine RDF’s property concept. Properties can be characterized as symmetric, transitive, functional, or inverse functional. They can be related through inverse and subproperty relationships. Accomplished through rdf:Property and subclasses owl:ObjectProperty, owl:DatatypeProperty, owl:AnnotationProperty, owl:OntologyProperty Individual Instances of user-defined classes are defined as RDF resources associated with classes. Accomplished through resources with rdf:type of user-defined classes

19.4 Satisfaction of IR Req’ts cont’d RelationshipOWL Support Concepts Relates classes to properties Restrictions (e.g. rdfs:domain, rdfs:range, owl:Restriction Relates classes to individuals Membership (instantiating individuals from classes using rdf:type and owl:Class) Relates properties to individuals Property values (implemented as XML elements and values) RelationshipOWL Support Constructs Synonymy owl:equivalentClass owl:equivalentProperty owl:sameAs Antonymy disjoint classes owl:differentFrom owl:AllDifferent/ owl:distinctMembers Hyponymy rdfs:subClassOf rdfs:subPropertyOf Meronymy Holonymy (user-defined properties)

19.4 Satisfaction of IR Req’ts cont’d Information Representation RequirementSatisfied By Producer descriptions of particular domainsontologies Distributed representations that can be extendedURIs, web-distributed ontologies imported with owl:imports, XML namespaces Extending resourcesrdf:about Work with current development toolsXML syntax, RDF structure Versioning and configuration managementowl:versionInfo property, deprecated classes and properties, compatibility properties Semantic join support and synonymyowl:sameAs, owl:equivalentProperty, owl:equivalentClass Modeling primitivesclasses, properties, individuals Leverage open, non-proprietary standardsXML, RDF(S), URIs

19.5 OWL Dialect Selection Summary DialectAdvantagesDisadvantages OWL LiteEasy software supportLimited expressiveness OWL DLDecidableRestricted OWL FullVery expressiveNot decidable

Chapter 20

20 OWL Applications Applications ▫Implementation layer ▫Composed of  Software  Ontologies  Instance files OWL designed for the web, but works fine in a stand-alone application Multiple uses for OWL with distributed ontologies and instance files

20.1 Application Example Mark wants to find an open restaurant that serves Key lime pie Possible solution ▫Mark enters search request on concierge web service provided by restaurant association ▫Web service extracts menu & hours of operation data published on restaurant websites in format compliant with restaurant ontology ▫Web service inferences & searches data ▫Results returned to Mark’s browser

OWL Lite Solution Need to develop ▫“Restaurant association” ontology ▫Instance data for restaurants (i.e. Knight Owl) Solution to be developed in OWL Lite

Ontologies Key classes ▫HoursOfOperation class  Related by hasDailyHours property to ▫DailyHours class  Related by openOn and closedOn properties to Hours of operation data to be stored in an RDF/XML instance file compliant with Hours Of Operation (HOO) ontology Restaurant class shown but not used in this demo

Ontologies cont’d <!DOCTYPE rdf:RDF[ ]> <rdf:RDF xmlns:xsd=" xmlns:rdf=" xmlns:rdfs=" xmlns:owl=" xmlns =" xml:base=" > Hours of Operation Ontology This ontology is used an example for various OWL constructs $Id: hoursOfOperation-ont.owl,v /11/02 12:34:56

Ontologies cont’d Daily Hours represents a set of opening hours for specified days of the week

Ontologies cont’d Sunday

Ontologies cont’d Monday Tuesday Wednesday Thursday Friday Saturday

Hours of Operation Individuals Knight Owl Restaurant hosts its Hours of Operation information on its webserver ▫Instance file imports & complies with restaurant association’s Hours of Operation Ontology Example Hours of Operation ▫“Effective 4/1/2004, Knight Owl’s hours are:”  Sunday Closed  Monday through Friday, 9am to 8pm  Saturday 9:30am to 9pm Assumptions ▫Restaurant association provides web service to search menus & hours ▫Association maintains registry of participating restaurant’s instance files

Hours of Operation cont’d <!DOCTYPE rdf:RDF[ <!ENTITY hoo " ]> <rdf:RDF xmlns:xsd=" xmlns:rdf=" xmlns:rdfs=" xmlns:owl=" xmlns:hoo=" xmlns =" xml:base=" > Hours of Operation Information for Knight Owl Restaurant This contains the hours of operation for the Knight Owl $Id: hours.rdf,v /11/02 12:34:56

Hours of Operation cont’d 09:00: :00: :30: :00:00+8

Hours of Operation cont’d

20.2 Supporting Applications The purpose of representing information in OWL is to support application functionality. By providing a consistent method for representing information and explicitly providing semantics, new and exciting Semantic Web applications will emerge OWL must be used correctly & consistently

Inappropriate Applications OWL ▫Suffers from many of the same challenges as XML ▫Relatively new  Not a large base of software tools  Not a large workforce conversant with OWL ▫Possible performance issues due to verbosity of RDF/XML ▫Reasoning hard to scale; rules somewhat limited ▫Only works with textual data, not binary

Appropriate Applications Appropriate OWL applications leverage OWL’s strengths OWL well-suited for applications like ▫Providing information on servers for software and human consumption ▫Well-defined object-oriented domains that can be easily described with text (taxonomical) Evaluate each potential application

20.3 Applications Summary Ultimately, OWL’s success will hinge on the benefits of OWL-based applications There are appropriate and inappropriate uses ▫Each situation must be analyzed separately Still waiting for the “killer app” for the Semantic Web