Ontology & OWL

1. Ontology & OWL Semantic Web - Spring 2008 Computer Engineering Department Sharif University of Technology

2. Outline • Introduction & Definitions • Ontology Languages • OWL

3. Where does it come from? • ontologyn. 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 • Co-opted the term. Something exists if it can be represented, described, defined (in a formal, hence, machine-interpretable way).

4. Ontologies

5. 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 conceptualization.” Gruber, 1993 • Main elements of an ontology: • Concepts • Relationships • Hierarchical • Logical • Properties • Instances (individuals)

6. A Definition • Informal • Terms • from a specific domain • uniquely defined, usually via natural language definitions • May contain additional semantics in the form of informal relations • machine-processing is difficult • Examples • Controlled vocabulary • Glossary • Thesaurus

7. A Definition • Formal • Domain-specific vocabulary • Well-defined semantic structure • Classes/concepts/types • E.g., a class { Publication } represents all publications • E.g., a class { Publication } can have subclasses { Newspaper }, { Journal } • Instances/individuals/objects • E.g., the newspaper Le Monde is an instance of the class { Newspaper } • Properties/roles/slots • Data • E.g., the class { Publication } and its subclasses { Newspaper }, { Journal } have a data property { numberOfPages } • Object • E.g., the class { Publication } and its subclasses { Newspaper }, { Journal } have an object property { publishes } • Is machine-processable

8. Ontologies in the Semantic Web • Provide shared data structures to exchange information between agents • Can be explicitly used as annotations in web sites • Can be used for knowledge-based services using other web resources • Can help to structure knowledge to build domain models (for other purposes)

9. Ontolgoies in the SW • They are means of representing the semantics of documents and enabling the semantics to be used by web applications and intelligent agents. • Ontologies are critical for applications that want to search across or merge information from diverse communities. • 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.

10. Meaning is in Connections is made from G a m Grape e a i e n r f W m i s p o r e d Wine

11. <Sentence> <Subject> Wine </Subject> <Verb> is made from </Verb> <Object> Grape </Object> </Sentence> <Sentence> <Subject> Wine </Subject> <Verb> is made from </Verb> <Object> Grape </Object> </Sentence> XML document For machines... The meaning of the document is not defined. Machines cannot understand it. Wine is made from Grape We are defining the structure of document by XML but now the meaning of the structure is not defined.

12. Natural Language <Sentence> <Subject> </Subject> <Verb> </Verb> <Object> </Object> </Sentence> Ontology gives the meaning... Document Ontology Wine is made from Grape

13. Why develop ontologies? • To 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 • Enable new users to learn about the domain • To distinguish domain knowledge from operational knowledge • e.g., biblio metadata

14. What they are good for • Informal • Controlled vocabulary • Beginnings of interoperability • Upper-level structures for extending further • E.g., AGRIS/CARIS categorization • Browsing support • E.g., IRS information search • Search • Limited query expansion • disambiguation • e.g., “Jordan” as a name of Basket-ball player and name of a country

15. What they are good for • Formal • Search • Concept-based query • User uses own words, language • Related terms • Intelligent query expansion: “fishing vessels in China” expands to “fishing vessels in Asia” • Consistency checking • e.g., “Goods” has a property called “price” that has a value restriction of number • Interoperability support • Terms defined in expressive ontologies allow for mapping precisely how one term relates to another

16. Ontology Languages • Graphical notations • Semantic networks • Topic maps • UML • RDF • Logic based • Description Logics (e.g., OIL, DAML+OIL, OWL) • Rules (e.g., RuleML, LP/Prolog) • First Order Logic

17. Ontology Languages • RDF(S) (Resource Description Framework (Schema)) • OIL (Ontology Interchange Language) • DAML+OIL (DARPA Agent Markup Language + OIL) • OWL (Ontology Web Language) • XOL (XML-based Ontology Exchange Language) • SHOE (Simple HTML Ontology Extension) • OML (Ontology Markup Language)

18. 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

19. 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 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.

20. Origins of OWL DARPA Agent Markup Language Ontology Inference Layer DAML OIL RDF EU/NSF Joint Ad hoc Committee DAML+OIL All influenced by RDF OWL Lite OWL DL OWL Full A W3C Recommendation OWL

21. OWL • OWL and RDF Schema enable richmachine-processable semantics RDFS OWL <rdfs:Class rdf:ID="River"> <rdfs:subClassOf rdf:resource="#Stream"/> </rdfs:Class> Semantics RDF Schema OWL Syntax XML/DTD/XML Schemas <owl:Class rdf:ID="River"> <rdfs:subClassOf rdf:resource="#Stream"/> </owl:Class>

22. Why Build on RDF • Provides basic ontological primitives • Classes and relations (properties) • Class (and property) hierarchy • Can exploit existing RDF infrastructure • Provides mechanism for using ontologies • RDF triples assert facts about resources • Use vocabulary from DAML+OIL ontologies

23. OWL Design Goals • Shared ontologies • Ontology evolution • Ontology interoperability • Inconsistency detection • Expressivity vs. scalability • Ease of use • Compatibility with other standards • Internationalization

24. Versions of OWL • Depending on the intended usage, OWL provides three increasingly expressive sublanguages Full: Very expressive, no computation guarantees DL (Description Logic): Maximum expressiveness, computationally complete Lite: Simple classification hierarchy with simple constraints. OWL Full OWL DL OWL Lite

25. Comparison of versions • OWL Lite supports those users primarily needing a classification hierarchy and simple constraints. For example, while it supports cardinality constraints, it only permits cardinality values of 0 or 1. It should be simpler to provide tool support for OWL Lite than its more expressive relatives, and OWL Lite provides a quick migration path for thesauri and other taxonomies. • OWL DL supports those users who want the maximum expressiveness while retaining computational completeness (all conclusions are guaranteed to be computable) and decidability (all computations will finish in finite time). OWL DL includes all OWL language constructs, but they can be used only under certain restrictions (for example, 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. It is unlikely that any reasoning software will be able to support complete reasoning for every feature of OWL Full.

26. Describing classes in OWL OWL vs. RDFS • OWL allows greater expressiveness • 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

27. 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, …) • Annotations • Individuals • Reasoning tasks: classification,consistency checking

28. 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)

29. owl:Class • Sub class of Class in RDF • Better to forget about classes of classes • Top-most class: owl:Thing <owl:Class rdf:ID=“Person"/> <owl:Class rdf:ID=“Man"> <rdfs:subClassOf rdf:resource="#Person" /> </owl:Class>

30. Individuals • Two equivalent declarations: • <Person rdf:ID=“Ganji" /> • <owl:Thing rdf:ID=“Ganji" /> <owl:Thing rdf:about="#Ganji"> <rdf:type rdf:resource="#Person"/> </owl:Thing>

31. 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 • an object in the world (OWL Full)

32. Object Properties Ana  owns  Cuba Is range a literal / typed value ? then ERROR Data type Properties Ana  age  25 XML Schema data types supported DB people happy OWL Properties

33. Defining Properties • ObjectProperty • DatatypeProperty • rdfs:subPropertyOf • rdfs:domain • rdfs:range • <owl:ObjectProperty rdf:ID="madeFromGrape"> <rdfs:domain rdf:resource="#Wine"/> <rdfs:range rdf:resource="#WineGrape"/> </owl:ObjectProperty>

34. 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 • Enumeration (owl:oneOf) • Disjoint Classes (owl:disjointWith)

35. Describing classes in OWL Property Restrictions • Defining a Class by restricting its possible instances via their property values • OWL distinguishes between the following two: • Value constraint • Cardinality constraint

36. Describing classes in OWL Restrictions on Property Classes • Properties: • allValuesFrom: rdfs:Class (lite/DL owl:Class) • hasValue: specific Individual • someValuesFrom: rdfs:Class (lite/DL owl:Class) • minCardinality: xsd:nonNegativeInteger (in lite {0,1}) • maxCardinality: xsd:nonNegativeInteger (in lite {0,1})

37. What’s in OWL, but not in RDF • Ability to be distributed across many systems • Scalable to Web needs • Compatible with Web standards for: • accessibility, and • Internationalization • Open and extensible

38. Describing properties in OWL 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 • …

39. Describing properties in OWL OWL Property Classes rdf:Property owl:InverseFunctionalProperty owl:DatatypeProperty owl:FunctionalProperty owl:ObjectProperty owl:TransitiveProperty owl:SymmetricProperty • An ObjectProperty relates one Resource to another Resource. • A DatatypeProperty relates one Resource to a Literal - an XML Schema data type.

40. Transitivity of properties X  p1  Y Y  p1  Z implies X  p1  Z • Transitivity existed already in RDF • “subClassOf”, and ??? • e.g. located_in, part_of

41. Symmetric properties X  p1  Y implies X  p1 Y • e.g., =

42. Functional Properties X  p1  Y X  p1  Z imply Z is the same as Y (they describe the same) What if Y, Z where explicitly defined as “different” ?

43. Inverse Functional Properties Y  p1  A Z  p1  A imply Z is the same as Y (they describe the same) What if Y, Z where explicitly defined as “different” ?

44. OWL distributed “equivalent class” “equivalent Property” Guitar Guitarra Internationalization standards ?

45. Male Students Married Female Professors Married Female Students Divorced Complex Classes Female Student Professor Married Human • Minority Students example

46. Disjoint Classes • Married disjoint with: • Divorced • Widowed • Single • Are “Divorced” and “Single” disjoint ? Married Widowed Single Divorced

47. OWL Cardinality • min Cardinality • max Cardinality • “Cardinality” • When min = max • has Value • belongs to the class if it has the value

48. An Example 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="#Parent" /> <rdfs:range rdf:resource="#Person" /> </owl:ObjectProperty>

49. Use cases – Web portals • A web portal is a web site that provides information content on a common topic, for example a specific city or domain of interest. • A web portal allows individuals that are interested in the topic to receive news, find and talk to one another, build a community, and find links to other web resources of common interest. • In order for a portal to be successful, it must be a starting place for locating interesting content. • Typically this content is submitted by members of the community, who often index it under some subtopic. • Another means of collecting content relies on the content providers tagging the content with information that can be used in syndicating it. Typically, this takes the form of simple metatags that identify the topic of the content, etc. • However, a simple index of subject areas may not provide the community with sufficient ability to search for the content that its members require. • In order to allow more intelligent syndication, web portals can define an ontology for the community. This ontology can provide a terminology for describing content and axioms that define terms using other terms from the ontology. • For example, an ontology might include terminology such as "journal paper," "publication," "person," and "author.“ • This ontology could include definitions that state things such as "all journal papers are publications" or "the authors of all publications are people." • When combined with facts, these definitions allow other facts that are necessarily true to be inferred. These inferences can, in turn, allow users to obtain search results from the portal that are impossible to obtain from conventional retrieval systems. • Examples of Semantic Web Portals are OntoWeb and The Open Directory Portal.

50. Use cases – Multimedia collections • Ontologies can be used to provide semantic annotations for collections of images, audio, or other non-textual objects. • these types of resources are typically indexed by captions or metatags. However, since different people can describe these non-textual objects in different ways, it is important that the search facilities go beyond simple keyword matching. • Multimedia ontologies can be of two types: media-specific and content-specific. • As an example of a multimedia collection, consider an archive of images of antique furniture. An ontology of antique furniture would be of great use in searching such an archive. • A taxonomy can be used to classify the different types of furniture. • It would also be useful if the ontology could express definitional knowledge. For example, if an indexer selects the value "Late Georgian" for the style/period of (say) an antique chest of drawers, it should be possible to infer that the data element "date.created" should have a value between 1760 and 1811 A.D. and that the "culture" is British. • Availability of this type of background knowledge significantly increases the support that can be given for indexing as well as for search.