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An Introduction To The Semantic Web

An Introduction To The Semantic Web. Information Access on the Web. Find an mp3 of a song that was on the Billboard Top Ten that features a cowbell. The Cowbell Project - http://www.geekspeakweekly.com/cowbell/. The Semantic Web. A Vision Of Possibilities

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An Introduction To The Semantic Web

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  1. An Introduction To The Semantic Web

  2. Information Access on the Web Find an mp3 of a song that was on the Billboard Top Ten that features a cowbell. The Cowbell Project - http://www.geekspeakweekly.com/cowbell/

  3. The Semantic Web • A Vision Of Possibilities • “The Semantic Web is an extension of the current web in which information is given well-defined meaning, better enabling computers and people to work in cooperation.” • -- Tim Berners-Lee, James Hendler and OraLassila, The Semantic Web, Scientific American, May 2001

  4. Importance Of URIs • The Semantic Web will build on the distributed nature of the Web: • No central naming authority • Schema definitions: • Not implied in applications (cf. Web browsers and HTML DTD) • Accessible in a machine-understandable format using a URI

  5. What’s Needed • Semantics Shared schemas: conventions about declaring meaning Agreed ontologies (both terms and ‘rules’ as to how terms relate) Agreed data model (RDF) • Infrastructure Schema Registries to share schemas Common syntax (XML) • The Web for connectivity: URI, HTTP...

  6. So what is the Semantic Web? • The semantic web is like the regular web, but its main unit is data, not pages • Data is online • Can link to other data in the same file or in other places on the web

  7. What are the basic units? • URIs • Triples • subject – predicate – object • Ontology • Used for reasoning about data

  8. Semantic Web • In the Semantic Web we will need: • Machines talking to machines – semantics need to be unambiguously declared • Joined-up data – enabling complex tasks based on information from various sources • Wide scope – from, say, home to government to commerce • Trust – both in data and who is saying it • This is not going to be easily achieved

  9. How do we represent them? • History • XML, SHOE, DAML, OIL, DAML+OIL • RDF, RDFS, OWL

  10. Standardization • World Wide Web Consortium (W3C) • http://w3.org

  11. Data looks like a graph

  12. Classes • Concepts • Hierarchies • Multiple Inheritance allowed

  13. Properties • (aka predicates) • Properties of classes • Object properties (connect instances of classes) • Datatype properties (connects an instance of a class to a literal value)

  14. Instances • Instances of a class • The class is its type • Properties are used on instances • Can be of multiple types

  15. Ontologies • Generally considered a collection of classes and properties • Files can contain any combination of instances, classes, and properties

  16. Common Ontologies • Dublin Core • Word Net

  17. Exercise 1 • List 10 things about yourself • Demographic info, job, age, personal info – whatever you like

  18. Syntax: RDF/XML

  19. Using rdf:about • To describe a resource: • <rdf:Description rdf:about=“http://www.example.com/example.rdf#foo”> • Ending a description • <rdf:Description rdf:about=“ http://www.example.com/example.rdf#foo”></rdf:Description> • <rdf:Description rdf:about=“http://www.example.com/example.rdf#foo”/>

  20. Using rdf:ID • rdf:ID is a local definition instead of a global one • <rdf:Description rdf:about=“foo”>

  21. Properties • To create a property <rdf:Property rdf:ID=“hasTitle”/> <rdf:Property rdf:about=“http://www.example.com/employment.rdf#hasTitle”/>

  22. Exercise 2 • Create descriptions of the things and properties for the attributes in your list of info • <rdf:Description rdf:about=“http://www.example.com/example.rdf#foo”> <rdf:Property rdf:about=“http://www.example.com/employment.rdf#hasTitle”/>

  23. Property With a literal (String) <rdf:Description rdf:ID=“Jen”> <hasTitle>Professor</hasTitle> </rdf:Description>

  24. Property: With a resource as the object <rdf:Description rdf:ID=“Jen”> <hasTitle rdf:resource=“http://www.example.com/employment.rdf#Professor”/> </rdf:Description> OR • <rdf:Description rdf:ID=“Jen”> • <hasTitle rdf:resource=“#Professor”/> • </rdf:Description>

  25. Striping <rdf:Description> <ex:editor> <rdf:Description> <ex:homePage> <rdf:Description> </rdf:Description> </ex:homePage> </rdf:Description> </ex:editor> </rdf:Description>

  26. Benefits • Anyone can talk about any other resource (using rdf:about) • Allows for annotation and expansion of existing resources • New statements are joined into the graph

  27. RDF Schema

  28. Purpose of RDF Schema • The purpose of RDF Schema is to provide an XML vocabulary to: • express classes and their (subclass) relationships. • define properties and associate them with classes. • The benefit of an RDF Schema is that it facilitates inferencing on your data, and enhanced searching.

  29. Schema and Inference • With definitions of classes, domains, and ranges, information about a resource that is not explicit can be inferred from the data

  30. RDFS Classes • Classes are categories into which resources can be grouped • Members of classes are instances • subClasses create a hierarchy of classes • Multiple inheritance is allowed

  31. Properties • RDFS adds domains and ranges • Limit what types of objects can be the subject of a property and what types can be the object • SubProperty adds inheritance to properties • E.g. hasMother is a SubProperty of hasParent

  32. Multiple Domains and Ranges • Multiple domains or ranges on a property are treated as intersection • Example: the property hasMother has a range of Female and also a range of Parent. That means the object of the property must be both a Parent and Female

  33. Mapping and Aggregability • Instances of the same class can easily be aggregated from separate files • Using proper inferences, an instance does not need to be created as the proper type • Example: Find all the Females • Jen is a Female • Irene is a Parent • Jen hasMother Irene • We now know Irene is a Female because all objects of hasMother must be female

  34. Other Additions • Rdfs:Container, a super class of the RDF containers • Labels and comments • rdfs:label - commonly used to give a real-world name to the resource being described • rdfs:comment - can be any text that you want to relate to the resource

  35. Web Ontology Language (OWL)

  36. Foundations • XML • RDF • RDFS • DAML+OIL • OWL

  37. OWL • Three Species • OWL Lite • OWL DL • OWL Full

  38. Equality • equivalentClass • equivalentProperty • sameAs • differentFrom • allDifferent

  39. Example • ZipCode equivalentClass PostalCode • If zip code and postal code are supposed to be different - e.g. zip is for american addresses and postal is for foreign ones - then we can say they are different • ZipCode differentFrom PostalCode <owl:Class rdf:ID=“ZipCode”> <owl:differentFrom rdf:resource=“http://example.com/ont.owl#PostalCode/> </owl:Class>

  40. Property Characteristics • inverseOf • hasParent is the inverseOf hasChild • TransitiveProperty • E.g. - ancestorOf - if Bob is an ancestorOf Joe and Joe is an ancestorOf Fred, then Bob is an ancestorOf Fred • SymmetricProperty • E.g. if Tom is marriedTo Michelle, then Michelle is marriedTo Tom • FunctionalProperty (unique value) • Wine hasMaker - hasMaker is functional (there can be only one) • InverseFunctionalProperty • The inverse of a functional property - makesWine is the inverse of hasMaker and is an inverseFunctionalProperty

  41. Restrictions • Property Type Restrictions • allValuesFrom • The hasMother property has allValuesFrom the class Woman • someValuesFrom • The hasChild property has someValuesFrom the class Woman • Restricted Cardinality (can be 0 or 1 in Lite) • minCardinality • maxCardinality • Cardinality

  42. Local Restrictions on Property Ranges • Instead of setting a range for a property, each class can have its own range • E.g. The range of eats for vegetarians is different than for non-vegetarians • Done with subclasses and a restriction <owl:Class rdf:ID="Vegetarian"> <rdfs:subClassOf> <owl:Restriction> <owl:onProperty rdf:resource="#eats"/> <owl:allValuesFrom rdf:resource="#VegetarianFood"/> </owl:Restriction> </rdfs:subClassOf>…

  43. OWL DL and Full

  44. Combinations • unionOf (uses ParseType) • E.g. European Union Citizenship is the unionOf the citizenship of the member states • intersectionOf (uses ParseType) • E.g. Fire engines are found in the intersection of RedThings and Trucks • complementOf (used like subClassOf) • E.g. the complementOf livingThings are all things that are non-living • disjointWith (used like subClassOf) • E.g. Man and Woman are disjoint classes <owl:Class rdf:ID=“Man”/> <owl:Class rdf:ID=“Woman”> <owl:disjointWith rdf:resource=”#Man”/> </owl:Class>

  45. Intersection Example <owl:Class> <owl:intersectionOf rdf:parseType="Collection"> <owl:Class> <owl:equivalentClass rdf:resource="http://www.ksl.stanford.edu/projects/DAML/UNSPSC.daml#Food-Beverage-and-Tobacco-Products"/> </owl:Class> <owl:Class> <owl:equivalentClass rdf:resource="http://www.ksl.stanford.edu/projects/DAML/UNSPSC.daml#Meat"/> </owl:Class> </owl:intersectionOf> </owl:Class>

  46. ComplementOf Example <owl:Class> <owl:intersectionOf> <owl:Class rdf:resource=”#Food"/> <owl:Class> <owl:complementOf> <owl:Class rdf:resource="#Meat"/> </owl:complementOf> </owl:Class> </owl:intersectionOf > </owl:Class>

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