Download - Semantic Web and The Web Ontology Language
Semantic Web and
The Web Ontology Language
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The World Wide Web today
• Information is represented • in natural language• in multiple formats• using graphics, multimedia
• Information overload• Though humans can process this information,
• machines (software agents) cannot understand it• vast majority of information remains untapped
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Semantic Web Goals
• Make information on the Web machine processable• Provide a standard data model for data integration• Build machine reasoning infrastructure• Enhance the current Web• Enable software services
In short, “create a Web of Information”
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Semantic Web Stack
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Source: W3C http://www.w3.org/2000/Talks/1206-xml2k-tbl/slide10-0.html
Debunking Semantic Web Myths
Some Semantic Web myths …
• Large Scale application of AI on the Web• Need to understand logic, knowledge representation
techniques• Relies on centrally controlled ontologies• Meant to replace the current Web
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Resource Description Framework• Data model of the Semantic Web
• Central to the data integration requirements of Semantic Web
• Allow data interchange without loss of meaning
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RDF Basics
• Data is the form of facts called statements
• Each statement is a triple of resources of the form(Subject, Property, Object)
e.g. (“Ayn Rand”, authorOf, “The Fountainhead”)
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Resource
There is a bit more to resources:• An entity in the data • Each resource identified by a URI (Universal Resource
Identifier)• These URI ground the RDF to the Web • Provide the basis for comparison of resources, hence
important• Resources can be literals too i.e. no URI associated with
them
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RDF as a graph
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Triples
•(index.html, dc:creator, staff-12345)
•(index.html, dc:language, ”en”)
•(index.html, creation-date, ”12 Aug 2006”)
How is RDF different from using XML?• Cleaner separation of syntax and semantics• Makes explicit the data model• However, XML is still used as a serialization mechanism• Other serialization mechanisms like Turtle and Notation
3 are available
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Enter RDF-S (RDF-Schema)
• RDF-S is to RDF what XML-Schema is to XML• RDF-S provides mechanisms to define domain
vocabulary, just as a programming language allows you to define new types
• Applications can then define vocabularies• Lightweight ontology
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RDF-S Basics and ConstructsRDF-S allows defining • resources as Classes, Properties or Datatypes• resources as instances of Classes • sub-class, sub-properties relations• domain/range of properties• resource containers like bag, list, etc.
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A look at RDF-S
•Animal rdfs:type rdfs:Class•Horse rdfs:subClassOf Animal•Dog rdfs:subClassOf Animal•number_of_legs rdfs:type rdfs:Property •number_of_legs rdfs:domain Animal•number_of_legs rdfs:range Integer
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RDF in action
• Dublin Core• vocabulary for describing networked, document like entities and
their properties• RSS 1.0
• lightweight way of content syndication
• SKOS (Simple Knowledge Organization System)• a model for expressing the basic structure and content of concept
schemes.
• XMP, FOAF, etc.
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What is OWL?
• Web Ontology Language, a W3C Recommendation• Derived from the DAML+OIL• Builds on RDF, uses RDF/XML• Knowledge Representation layer of the Semantic Web• An ontology: A formal conceptualization of the world
• Concepts• Properties• Relationships• Additional Constraints
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Why OWL?
• Supply richer semantics and knowledge base for agents
• Overcome limitations of RDF-S• Local Properties of classes• disjointness of classes• cardinality restrictions
• ... and others
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OWL Features – Class Level (1)
• Class Equivalence• Faculty owl:equivalentClass AcademicStaffMember
• Class Disjointness• Professor owl:disjointWith AssociateProfessor
• Every class is a sub-class of owl:Thing and super-class of owl:Nothing
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OWL Features: Class Level (2)
• Boolean combinations to create new classes
• Union• staff = owl:unionOf
( AcademicStaffMember, Non-AcademicStaffMember)
• Intersection• TA = owl:intersectionOf (student , stipendee)
• Complement• Non-AcademicStaffMember=
owl:complementOf (AcademicStaffMember)
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OWL Features: Property Level (3)
• Inverse Relation • isTaughtBy owl:inverseOf teaches
• Special Properties• Transitive
• isBroaderThan owl:TransitiveProperty• Symmetric
• isPartner owl:SymmetricProperty• also, Function, One-One
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OWL Features: Property Level (4)
• Property Restrictions: Constrain attributes to take only certain values for particular classes• Existential Quantification
• academicStaffMember teaches underGraduateCourse
• Universal Quantification• firstYearCourse isTaughtBy Professor
• Cardinality Constraints• academicStaffMember takes a maximum of 3
courses
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OWL Expressiveness
• Powerful OWL constructs, but at a cost
• Trade-off between• Expressiveness of language• Decidability and completeness• Tractability
• 3 OWL flavours defined with different degrees of expressiveness
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OWL Flavours
• OWL Full• Most expressive, undecidable• Any RDF valid OWL Full
• OWL DL• Based on Descriptive Logic• Guarantees computational completeness and decidability
• OWL Lite• Good enough for simple taxonomies
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Semantic Web & OWL in action
• Knowledge Management
• Web Services
• Data Integration
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Knowledge Management
• Use Semantic Web technologies for • Better organization of information• Improved search
• Portals: • Vodafone's Live Mobile Portal• Sun’s SwordFish
• Organizing data in biosciences: GoPubMed
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Web Services
• Where does the Semantic Web come in?• Automatic Discovery of Web Services• Automatic Web Service Invocation• Automatic Web Service Composition and Interoperation
• OWL-S markup language for describing services profiles, process models and grounding for interoperability
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