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OWL: Web Ontology Language

OWL: Web Ontology Language. Slides are from Grigoris Antoniou, Frank van Harmelen, “A Semantic Web Primer”. Limitations of RDF Schema. Equivalence of classes

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OWL: Web Ontology Language

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  1. OWL: Web Ontology Language Slides are from Grigoris Antoniou, Frank van Harmelen, “A Semantic Web Primer”

  2. Limitations of RDF Schema • Equivalence of classes • Two classes, same concept - people use different words to represent the same thing. It would be very useful to be able to state "this class is equivalent to this second class". • E.g. One person may create an ontology with a class called "Airplane". Another person may create an ontology with a class called "Plane". It would be useful to be able to indicate that the two classes are equivalent. • Boolean combinations of classes • Sometimes we wish to build new classes by combining other classes using union, intersection, and complement. • E.g. Person is the union of the classes Male and Female persons. • Disjointness of classes • Sometimes we wish to say that classes are disjoint • e.g. male and female are disjoint classes

  3. Limitations of RDF Schema (continued) • Cardinality constraints - oftentimes it is useful to indicate the allowable number of occurrences of a property • E.g. We would like to be able to express that a River has only "one" officialLength property. • E.g. We would like to be able to express that an Ocean has one maxDepth. • E.g. a person has exactly two parents, a course is taught by at least one lecturer • Special characteristics of properties • Transitive property (like “greater than”) • Unique property (like “is mother of”) • A property is the inverse of another property (like “eats” and “is eaten by”)

  4. RDF Schemas: Building Block to More Expressive Ontology Languages RDF Schema was designed to be extended. The ontology languages all use RDF Schema's basic notions of Class, Property, domain, and range.

  5. What is OWL? • OWL stands for Web Ontology Language • OWL is built on top of RDF • OWL is written in XML • OWL is for processing information on the web • OWL was designed to be interpreted by computers • OWL was not designed for being read by people • OWL is a web standard • OWL has three sublanguages (version 2004) • OWL Lite, DL, Full • OWL 2 defines several profiles with different applications in mind The following slides are from Grigoris Antoniou, Frank van Harmelen, “A Semantic Web Primer”

  6. Tradeoff between Expressive Power and Efficient Reasoning Support • The richer the language is, the more inefficient the reasoning support becomes • Sometimes it crosses the border of non-computability • We need a compromise: • A language supported by reasonably efficient reasoners • A language that can express large classes of ontologies and knowledge.

  7. OWL Sublanguages • OWL Lite • Simple constraints and classification hierarchy • OWL DL (includes OWL Lite) • Maximal expressiveness while retaining computational completeness (all conclusions are computable) • OWL Full (includes OWL DL) • Maximal expressiveness • Unlikely any reasoning software to support OWL Full. OWL Full OWL DL OWL Lite

  8. OWL 2 and sublanguages (profiles) • OWL DL is decidable, but the performance is poor for non-trivial knowledge base • OWL 2 (2009) gives several profiles of OWL language • Each is intended for different users/applications • Include EL, QL, RL • OWL 2 EL • Has polynomial time to determine consistency of an ontology • A restriction on DL • Disallowed class descriptions • Cardinality • All-values-from • Union-of, disjoint union, complement-of • Disallowed property description • Inverse, disjoint • Functional, symmetric, …

  9. OWL 2 QL • Efficient query • Similar to Entity-relationship modeling • Good to model information contained in existing database • Enable queries in logspacewrt knowledge base size

  10. Reasoning About Knowledge in Ontology Languages • Class membership • E.g., If x is an instance of a class C, and C is a subclass of D, then we can infer that x is an instance of D. • Subclass and Equivalence of classes • E.g., If class A is equivalent to class B, and class B is equivalent to class C, then A is equivalent to C, too. • Consistency • E.g., X is an instance of classes A and B, but A and B are disjoint. • This is an indication of an error in the ontology.

  11. Uses for Reasoning • Reasoning support is important for • checking the consistency of the ontology and the knowledge • checking for unintended relationships between classes • automatically classifying instances in classes • Checks like the preceding ones are valuable for • designing large ontologies, where multiple authors are involved • integrating and sharing ontologies from various sources

  12. Reasoning support in OWL • Semantics is a prerequisite for reasoning support • Formal semantics and reasoning support are usually provided by • mapping an ontology language to a known logical formalism • using automated reasoners that already exist for those formalisms • OWL is (partially) mapped on a description logic, and makes use of reasoners such as FaCT and RACER • Description logics are a subset of predicate logic for which efficient reasoning support is possible

  13. OWL Compatibility with RDFSchema • All varieties of OWL use RDF for their syntax • Instances are declared as in RDF, using RDF descriptions and typing information • OWL constructors are specializations of their RDF counterparts Rdfs:Resource Rdfs:Class Rdfs:Property Owl:ObjectProperty owl:Class Owl:DatatypeProperty OWL RDF/RDFS XML Schema

  14. OWL Syntactic Varieties • OWL builds on RDF and uses RDF’s XML-based syntax • Other syntactic forms for OWL have also been defined: • An abstract syntax, that is much more compact and readable than the XML languages • A graphic syntax based on the conventions of UML

  15. OWL XML/RDF Syntax: Header <rdf:RDF xmlns:owl ="http://www.w3.org/2002/07/owl#" xmlns:rdf ="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#" xmlns:xsd ="http://www.w3.org/2001/XLMSchema#>

  16. owl:Ontology • An OWL ontology may start with a collection of assertions for housekeeping purposes using owl:Ontology element <owl:Ontologyrdf:about=”…"> <rdfs:comment>An example OWL ontology </rdfs:comment> <owl:priorVersionrdf:resource="http://www.mydomain.org/uni-ns-old"/> <owl:importsrdf:resource="http://www.mydomain.org/persons"/> <rdfs:label>University Ontology</rdfs:label> </owl:Ontology> • owl:imports is a transitive property

  17. OWL Class • Classes are defined using owl:Class • owl:Class is a subclass of rdfs:Class <owl:Classrdf:ID=”associateProfessor”> <rdfs:subClassOfrdf:resource=“#academicStaffMember”/> </owl:Class> • owl:equivalentClassdefines equivalence of classes <owl:Classrdf:ID="faculty"> <owl:equivalentClassrdf:resource="#academicStaffMember"/> </owl:Class> • Disjointness is defined usingowl:disjointWith <owl:Classrdf:about="#associateProfessor"> <owl:disjointWithrdf:resource="#professor"/> <owl:disjointWithrdf:resource="#assistantProfessor"/> </owl:Class> • owl:Thingis the most general class, which contains everything • owl:Nothing is the empty class

  18. Properties • In OWL there are two kinds of properties • Object properties, which relate objects to other objects • E.g. is-TaughtBy, supervises <owl:ObjectProperty rdf:ID="isTaughtBy"> <rdfs:domain rdf:resource="#course"/> <rdfs:range rdf:resource= "#academicStaffMember"/> <rdfs:subPropertyOf rdf:resource="#involves"/> </owl:ObjectProperty> • Data type properties, which relate objects to datatype values • E.g. phone, age, etc. <owl:DatatypeProperty rdf:ID="age"> <rdfs:range rdf:resource= "http://www.w3.org/2001/XLMSchema#nonNegativeInteger"/> </owl:DatatypeProperty> OWL RDF/RDFS XML Schema

  19. Relations to other properties • Equivalent property <owl:ObjectProperty rdf:ID="lecturesIn"> <owl:equivalentProperty rdf:resource="#teaches"/> </owl:ObjectProperty> • Inverse property <owl:ObjectProperty rdf:ID="teaches"> <rdfs:range rdf:resource="#course"/> <rdfs:domain rdf:resource= "#academicStaffMember"/> <owl:inverseOf rdf:resource="#isTaughtBy"/> </owl:ObjectProperty>

  20. Property restriction • Value constraints: put constrains on the range of the property when applied to this particular class description. • owl:allValuesFrom specifies universal quantification • owl:hasValue specifies a specific value • owl:someValuesFromspecifies existential quantification • Cardinality constraints: the number of value a property can take. • It is a way to define classes

  21. owl:allValuesFrom • Define the class of persons whose parents are both physicians. <owl:Restriction> <owl:onProperty rdf:resource="#hasParent" /> <owl:allValuesFrom rdf:resource="#Physician" /> </owl:Restriction> { x | (x, y) ∈ hasParent implies y ∈ Physician } • Example: • Suppose: David hasParent Bill who is a Physician, and David hasParent Rose who is a Phycisian • Conclusion: David belongs to the class defined above. • Semantics restriction(p allValuesFrom(r)) : {x | (x,y) ∈ p implies y ∈ r}

  22. allValuesFrom • FirstYearCourse is a course that must be taught by professors <owl:Class rdf:about="#firstYearCourse"> <rdfs:subClassOf> <owl:Restriction> <owl:onProperty rdf:resource="#isTaughtBy"/> <owl:allValuesFrom rdf:resource="#Professor"/> </owl:Restriction> </rdfs:subClassOf> </owl:Class> { x | (x,y) ∈isTaughtBy implies y ∈ Professor }

  23. owl:someValuesFrom • Define the class of persons who has at least one physician parent. <owl:Restriction> <owl:onProperty rdf:resource="#hasParent" /> <owl:someValuesFrom rdf:resource="#Physician" /> </owl:Restriction> { x | ∃ (x, y) ∈ hasParent ∧ y ∈ Physician} • Semantics restriction(p someValuesFrom(e)) : {x | ∃ (x,y) ∈ p ∧ y ∈ e}

  24. someValuesFrom • AcademicStaff must teach at least one under course. <owl:Classrdf:about="#academicStaffMember"> <rdfs:subClassOf> <owl:Restriction> <owl:onPropertyrdf:resource="#teaches"/> <owl:someValuesFromrdf:resource="#undergraduateCourse"/> </owl:Restriction> </rdfs:subClassOf> </owl:Class> { x | ∃ (x, y) ∈ Teaches ∧ y ∈ undergraduateCourse}

  25. owl:hasValue <owl:Restriction> <owl:onProperty rdf:resource="#hasParent" /> <owl:hasValue rdf:resource="#Bill" /> </owl:Restriction> restriction(p value(i)), for i an individual ID {x | (x, i) ∈ p}

  26. Cardinality restrictions • We can specify minimum and maximum number using owl:minCardinality and owl:maxCardinality • It is possible to specify a precise numberby using the same minimum and maximum number • For convenience, OWL offers also owl:cardinality • E.g. Every course is taught by at least someone: <owl:Class rdf:about="#course"> <rdfs:subClassOf> <owl:Restriction> <owl:onProperty rdf:resource="#isTaughtBy"/> <owl:minCardinality rdf:datatype="&xsd;nonNegativeInteger"> 1 </owl:minCardinality> </owl:Restriction> </rdfs:subClassOf> </owl:Class>

  27. Example: Book Ontology <owl:Class rdf:ID="Book"> <rdfs:subClassOf rdf:resource="#REFERENCE" /> <rdfs:subClassOf> <owl:Restriction> <owl:onProperty rdf:resource="#title" /> <owl:cardinality rdf:datatype="http://www.w3.org/2001/XMLSchema#nonNegativeInteger">1 </owl:cardinality> </owl:Restriction> </rdfs:subClassOf> <rdfs:subClassOf> <owl:Restriction> <owl:onProperty rdf:resource="#volume" /> <owl:maxCardinality rdf:datatype="http://www.w3.org/2001/XMLSchema#nonNegativeInteger">1 </owl:maxCardinality> </owl:Restriction> </rdfs:subClassOf> <rdfs:subClassOf> <owl:Restriction> <owl:onProperty rdf:resource="#publisher" /> <owl:maxCardinality rdf:datatype="http://www.w3.org/2001/XMLSchema#nonNegativeInteger">1 </owl:maxCardinality> </owl:Restriction> …

  28. Special properties • owl:TransitiveProperty (transitive property) • E.g. “has better grade than”, “is ancestor of” • owl:SymmetricProperty (symmetry) • E.g. “has same grade as”, “is sibling of” • owl:FunctionalProperty defines a property that has at most one value for each object • E.g. “age”, “height”, “directSupervisor” • owl:InverseFunctionalProperty defines a property for which two different objects cannot have the same value <owl:ObjectProperty rdf:ID="hasSameGradeAs"> <rdf:type rdf:resource="&owl;TransitiveProperty"/> <rdf:type rdf:resource="&owl;SymmetricProperty"/> <rdfs:domain rdf:resource="#student"/> <rdfs:range rdf:resource="#student"/> </owl:ObjectProperty>

  29. Boolean Combinations: complementOf • We can combine classes using Boolean operations (union, intersection, complement) <owl:Class rdf:about="#course"> <rdfs:subClassOf> <owl:Restriction> <owl:complementOf rdf:resource="#staffMember"/> </owl:Restriction> </rdfs:subClassOf> </owl:Class>

  30. Boolean Combinations: unionOf <owl:Class rdf:ID="peopleAtUni"> <owl:unionOf rdf:parseType="Collection"> <owl:Class rdf:about="#staffMember"/> <owl:Class rdf:about="#student"/> </owl:unionOf> </owl:Class> • The new class is not a subclass of the union, but rather equal to the union • We have stated an equivalence of classes

  31. Boolean Combinations: intersectionOf <owl:Class rdf:ID="facultyInCS"> <owl:intersectionOf rdf:parseType="Collection"> <owl:Class rdf:about="#faculty"/> <owl:Restriction> <owl:onProperty rdf:resource="#belongsTo"/> <owl:hasValue rdf:resource= "#CSDepartment"/> </owl:Restriction> </owl:intersectionOf> </owl:Class> Exercise: describe the class of faculty who have joint appointment in CS and ECE.

  32. Nesting of Boolean Operators Administrative staff: staff members who are neither faculty nor technical support staff. <owl:Class rdf:ID="adminStaff"> <owl:intersectionOf rdf:parseType="Collection"> <owl:Class rdf:about="#staffMember"/> <owl:complementOf> <owl:unionOf rdf:parseType="Collection"> <owl:Class rdf:about="#faculty"/> <owl:Class rdf:about= "#techSupportStaff"/> </owl:unionOf> </owl:complementOf> </owl:intersectionOf> </owl:Class> staff admin tech faculty

  33. Enumerations with owl:oneOf <owl:oneOf rdf:parseType="Collection"> <owl:Thing rdf:about="#Monday"/> <owl:Thing rdf:about="#Tuesday"/> <owl:Thing rdf:about="#Wednesday"/> <owl:Thing rdf:about="#Thursday"/> <owl:Thing rdf:about="#Friday"/> <owl:Thing rdf:about="#Saturday"/> <owl:Thing rdf:about="#Sunday"/> </owl:oneOf> • Now we have talked about three ways to describe a class: • Enumeration • Set operation • Property restriction

  34. Declaring instances • Instances of classes are declared as in RDF: <rdf:Description rdf:ID="949352"> <rdf:type rdf:resource= "#academicStaffMember"/> </rdf:Description> Or alternatively, we can have the following simplified notation: <academicStaffMember rdf:ID="949352"> </academicStaffMember>

  35. No Unique-Names Assumption • OWL does not adopt the unique-names assumption of database systems • If two instances have different names or IDs, it does not imply that they are different individuals • Suppose we state • each course is taught by at most one staff member, and • a given course is taught by two staff members • An OWL reasoner does not flag an error; • Instead it infers that the two resources are equal.

  36. Distinct Objects • To ensure that different individuals are indeed recognized as such, we must explicitly assert their inequality: <lecturer rdf:about="949318"> <owl:differentFrom rdf:resource="949352"/> </lecturer> • OWL provides a shorthand notation to assert the pairwise inequality of all individuals in a given list <owl:allDifferent> <owl:distinctMembers rdf:parseType="Collection"> <lecturer rdf:about="949318"/> <lecturer rdf:about="949352"/> <lecturer rdf:about="949111"/> </owl:distinctMembers> </owl:allDifferent>

  37. Data Types in OWL • XML Schema provides a mechanism to construct user-defined data types • E.g., the data type of adultAge includes all integers greater than 18 • Such derived data types cannot be used in OWL • The OWL reference document lists all the XML Schema data types that can be used • These include the most frequently used types such as string, integer, Boolean, time, and date.

  38. An African Wildlife Ontology – Class Hierarchy Animal plant herbivore carnivore tree giraffe lion

  39. An African Wildlife Ontology – Properties <owl:TransitiveProperty rdf:ID="is-part-of"/> <owl:ObjectProperty rdf:ID="eats"> <rdfs:domain rdf:resource="#animal"/> </owl:ObjectProperty> <owl:ObjectProperty rdf:ID="eaten-by"> <owl:inverseOf rdf:resource="#eats"/> </owl:ObjectProperty>

  40. An African Wildlife Ontology –Plants and Trees <owl:Class rdf:ID=“animal“/> <owl:Class rdf:ID="plant"> <rdfs:comment>Plants are disjoint from animals. </rdfs:comment> <owl:disjointWith="#animal"/> </owl:Class> <owl:Class rdf:ID="tree"> <rdfs:comment>Trees are a type of plant. </rdfs:comment> <rdfs:subClassOf rdf:resource="#plant"/> </owl:Class>

  41. An African Wildlife Ontology –Branches <owl:Class rdf:ID="branch"> <rdfs:comment>Branches are parts of trees. </rdfs:comment> <rdfs:subClassOf> <owl:Restriction> <owl:onProperty rdf:resource="#is-part-of"/> <owl:allValuesFrom rdf:resource="#tree"/> </owl:Restriction> </rdfs:subClassOf> </owl:Class>

  42. An African Wildlife Ontology – Leaves <owl:Class rdf:ID="leaf"> <rdfs:comment>Leaves are parts of branches. </rdfs:comment> <rdfs:subClassOf> <owl:Restriction> <owl:onProperty rdf:resource="#is-part-of"/> <owl:allValuesFrom rdf:resource="#branch"/> </owl:Restriction> </rdfs:subClassOf> </owl:Class>

  43. An African Wildlife Ontology – Carnivores <owl:Class rdf:ID="carnivore"> <rdfs:comment> Carnivores are exactly those animals that eat also animals. </rdfs:comment> <owl:intersectionOf rdf:parsetype="Collection"> <owl:Class rdf:about="#animal"/> <owl:Restriction> <owl:onProperty rdf:resource="#eats"/> <owl:someValuesFrom rdf:resource="#animal"/> </owl:Restriction> </owl:intersectionOf> </owl:Class>

  44. An African Wildlife Ontology –Herbivores <owl:Class rdf:ID="herbivore"> <rdfs:comment> Herbivores are exactly those animals that eat only plants or parts of plants. <owl:intersectionOf rdf:parseType=“Collection”> <owl:Class rdf=about=“#animal”/> <owl:Restriction> <owl:onProperty rdf:resource=“#eats”/> <owl:allValuesFrom> <owl:Class> <owl:unionOf rdf:parseType=“Collection”> <owl:Class rdf:resource=“plant”/> <owl:Restriction> <owl:onProperty rdf;resource=“#is_part_of”/> <owl:allValuesFrom rdf:resource=“#plant”/> </owl:Restriction> </owl:unionOf> </class> </owl:allValuesFrom> </owl:Restrcition> </owl:intersectionOf> </owl:Class>

  45. An African Wildlife Ontology –Giraffes <owl:Class rdf:ID="giraffe"> <rdfs:comment>Giraffes are herbivores, and they eat only leaves. </rdfs:comment> <rdfs:subClassOf rdf:type="#herbivore"/> <rdfs:subClassOf> <owl:Restriction> <owl:onProperty rdf:resource="#eats"/> <owl:allValuesFrom rdf:resource="#leaf"/> </owl:Restriction> </rdfs:subClassOf> </owl:Class>

  46. An African Wildlife Ontology –Lions <owl:Class rdf:ID="lion"> <rdfs:comment>Lions are animals (carnivores) that eat only herbivores. </rdfs:comment> <rdfs:subClassOf rdf:type="#carnivore"/> <rdfs:subClassOf> <owl:Restriction> <owl:onProperty rdf:resource="#eats"/> <owl:allValuesFrom rdf:resource="#herbivore"/> </owl:Restriction> </rdfs:subClassOf> </owl:Class>

  47. An African Wildlife Ontology –Tasty Plants <owl:Class rdf:ID="tasty-plant"> <rdfs:comment>Plants eaten both by herbivores and carnivores </rdfs:comment> <rdfs:subClassOf rdf:resource=“#plant”/> <rdfs:subClassOf> <owl:Restriction> <owl:onProperty rdf:resource=“#eaten_by”/> <owl:someValuesFrom> <owl:Class rdf:about=“#herbivore”/> </owl:someValuefrom> </owl:Restriction> </rdfs:subClassOf> <rdfs:subClassOf> <owl:Restriction> <owl:onProperty rdf:resource=“#eaten_by”/> <owl:someValuesFrom> <owl:Class rdf:about=“#carnivore”/> </owl:someValuefrom> </owl:Restriction> </rdfsSublassOf> </owl:Class>

  48. Closed world assumption • In a closed world (like DBs), the information we have is everything • On the Semantic Web, we want people to be able to extend our models. In this open world, we assume there can always more information added later • Closed world assumption: what is currently not known to be true is false • If a statement can not be proved to be true, it is false. • Example • Statement: "Mary" "is a citizen of" "France" • Question: Is Mary a citizen of Canada? • "Closed world" (for example database or Prolog) answer: No.

  49. Open world assumption: • OWL adopts the open-world assumption: • system's knowledge is incomplete • On the huge and only partially knowable WWW, this is a correct assumption • if a statement cannot be inferred from what is expressed in the system, then it still cannot be inferred to be false. • The absence of a particular statement within the web means, in principle, that the statement has not been made explicitly yet. • Example • Statement: "Mary" "is a citizen of" "France" • Question: Is Mary a citizen of Canada? • "Open world" answer: unknown (Mary could have dual citizenship). • The open world assumption is related to the monotonic nature of first-order logic: adding new information never falsifies a previous conclusion. Namely, if we subsequently learn that Mary is also a citizen of Canada, this does not change any earlier positive or negative conclusions.

  50. Summary • OWL is the proposed standard for Web ontologies • OWL builds upon RDF and RDF Schema: • (XML-based) RDF syntax is used • Instances are defined using RDF descriptions • Most RDFS modeling primitives are used • Formal semantics and reasoning support is provided through the mapping of OWL to logics • Predicate logic and especially description logics have been used for this purpose • While OWL is sufficiently rich to be used in practice, extensions are in the making • They will provide further logical features, including rules

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