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Semantic Web Ontologies & Data Models

Semantic Web Ontologies & Data Models. CS 502 – 20030303 Carl Lagoze – Cornell University. Acknowledgements: Eric Miller Dieter Fensel. Components of the Semantic Web. Knowledge Representation and AI meeting the Web. Knowledge representation is not new Formalisms Semantic networks

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Semantic Web Ontologies & Data Models

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  1. Semantic WebOntologies & Data Models CS 502 – 20030303 Carl Lagoze – Cornell University Acknowledgements: Eric Miller Dieter Fensel 20020307

  2. Components of the Semantic Web 20020307

  3. Knowledge Representation and AI meeting the Web • Knowledge representation is not new • Formalisms • Semantic networks • Frame systems • Frames (objects) • Slots (attributes and properties) • Languages • LOOM • Classic • Cyc-L • Logics • Description Logics (DLs) 20020307

  4. So why re-invent it for the Web • Not re-invention • Same underlying formalisms (frames, slots, description logic) • But new factors • Lack of central control • Inconsistency, lies, re-interpretations, duplications • Continue change • New facts appear and modify constantly • Massive scale • Tractability • Knowledge expressiveness must be limited or reasoning must be incomplete • Open world context • Contrast to most reasoning systems that assume anything absent from knowledge base is not true • Need to build on existing standards • URI, XML, RDF 20020307

  5. Why aren’t XML schema good enough? • XML schema is designed to primarily describe structure and constraints on documents • Rich datatypes • Grammar for describing structure of elements (and attributes) • Incomplete modeling of is-a relationships • Top-down inheritance of attributes from superclasses to subclasses with extension and restriction is clumsy • E.g., student as person with a student # and age<28 • Bottom-up inheritance of instances to superclasses is not automatic • Multiple inheritance is missing 20020307

  6. From modeling to implementation Modeling primitives: Class; Slot ERmodel OWLspecification Modeling Implementation Relational Model XMLschema Data- base XML documents 20020307

  7. Brief review of RDF • Data model for defining machine-processible semantics of data • Three main object types: • Resources • Primitive metadata is URI • Properties • Sub-class of resource • Statements • (:s :p :o) • Graph model that is serializable in XML 20020307

  8. RDF Schemas A building block for assembling ontology languages • Declaration of vocabularies • Classes and class hierarchies • properties defined by a particular community • characteristics of properties and/or constraints on corresponding values • Provides substructure for inferences based on existing triples • Schema language is an expression of basic RDF model • Schema Type System - Basic Types • Property, Class, SubClassOf, Domain, Range • Minimal (but extensible) at this time • Expressible in the RDF model and syntax 20020307

  9. rdfs:label rdfs:label “Nom” “Author” rdfs:label “$100 $a” Schema Vocabularies • Enables communities to share machine readable tokens and locally define human readable labels. dc:Creator 20020307

  10. Relationships among vocabularies dc:Creator marc:100 ms:director bib:Author 20020307

  11. rdfs: subPropertyOf ms:director dc:Creator rdfs:label “Director” dc:Creator Relationships among vocabulary elements URI:R ms:director “John Smith” 20020307

  12. RDF Schema: Specializing Properties • rdfs:subPropertyOf – allows specialization of relations • E.g., the property “father” is a subPropertyOf the property parent • subProperty semantics 20020307

  13. Sub-Property Semantics 20020307

  14. Constraints on Properties • Force objects to be of a certain type • rdfs:domain • Restricts the type of resources that may have a specific property • rdfs:range • Restricts the type of resources that may be the value of a specific property range 20020307

  15. Inferences from Constraints doris betty eve alice charles 20020307

  16. Inferences from Constraints 20020307

  17. Class Hierarchy • rdfs:Class • Resources denoting a set of resources; range of rdf:type • rdfs:subClassOf • Create class hierarchy rdf:type rdf:type rdfs:class rdfs:subClassOf rdf:type rdf:type rdf:class rdf:class 20020307

  18. Sub-Class Inferencing 20020307

  19. Sub-class Inferencing Example 20020307

  20. What is an Ontology? • A formal specification of conceptualization shared in a community • Vocabulary for defining a set of things that exist in a world view • Formalization allows communication across application systems and extension • Parallel concepts in other areas: • Domains: database theory • Types: AI • Classes: OO systems • Types/Sorts: Logic • Global vs. Domain-specific 20020307

  21. XML and RDF are ontologically neutral • No standard vocabulary just primitives • Resource, Class, Property, Statement, etc. • Compare to classic first order logic • Conjunction, disjunction, implication, existential, universal quantifier 20020307

  22. Components of an Ontology • Vocabulary (concepts) • Structure (attributes of concepts and hierarchy) • Logical characteristics of concepts & attributes • Domain and range restrictions • Properties of relations (symmetry, transitivity) 20020307

  23. Wordnet • On-line lexical reference system, domain-independent • >100,000 word meanings organized in a taxonomy with semantic relationships • Synonymy, meronymy, hyponymy, hypernymy • Useful for text retrieval, etc. • http://www.cogsci.princeton.edu/~wn/online/ 20020307

  24. CYC • Effort in AI community to accommodate all of human knowledge!!! • Formalizes concepts with logical axioms specifying constraints on objects and classes • Associated reasoning tools • Contents are proprietary but there is OpenCyc • http://www.opencyc.org/ 20020307

  25. Ontologies for the Web • Lots of Participants and $$$ • Web Ontology Working Group • Distributed Agent Markup Language • Ontology Inference Layer • OntoWeb • Schemas Project • OWL (Web Ontology Language) – develop standard for encoding ontologies on top of RDF Schema 20020307

  26. Extending RDF(S) with OWL RDFS OWL • Class definition: Conjunction, disjunction, negation • Property constraints: universality, existence, cardinality • Properties of properties: transitivity, symmetry • Class, sub-class definition • Property (attribute), sub-property definition • Domain, range constraints 20020307

  27. DAML class building operations • disjointWith • No vegetarians are carnivores • sameClassAs (equivalence) • Enumerations (on instances) • The Ivy League is Cornell, Harvard, Yale, …. • Boolean set semantics (on classes) • Union (logical disjunction) • Intersection (logical conjunction) • complimentOf (logical negation) • All non-carnivores are vegetarians 20020307

  28. DAML property building operations & restrictions • Transitive Property • P(x,y) and P(y,z) -> P(x,z) • SymmetricProperty • P(x,y) iff P(y,x) • Functional Property • P(x,y) and P(x,z) -> y=z • inverseOf • P1(x,y) iff P2(y,x) • InverseFunctional Property • P(y,x) and P(z,x) -> y=z • Cardinality 20020307

  29. DAML+OIL DataTypes • Full use of XML schema data type definitions • Examples • Define a type age that must be a non-negative integer • Define a type clothing size that is an enumeration “small” “medium” “large” 20020307

  30. DAML+OIL Instance Creation • Create individual objects filling in slot/attribute/property definitions <Person ref:ID=“William Arms”> <rdfs:label>Bill</rdfs:label> <age><xsd:integer rdf:value=“57”/></age> <shoesize><xsd:decimal rdf:value=“10.5”/></shoesize></Person> 20020307

  31. Language Comparison 20020307

  32. Some useful RDF tools • JENA toolkit for manipulating RDF models • http://www.hpl.hp.com/semweb/jena-top.html • RDFSviz for visualizing ontologies expressed as RDF schema • http://www.dfki.uni-kl.de/frodo/RDFSViz/ • W3C RDF validation service for parsing and view RDF instances • http://www.w3.org/RDF/Validator/ 20020307

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