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Semantic Web Towards a Web of Knowledge - Introduction

Semantic Web Towards a Web of Knowledge - Introduction. Spring 2006 Computer Engineering Department Sharif University of Technology. Outline. Introduction From Original Web to Semantic Web Building Blocks URI (We talked before) XML (We talked before) RDF (We talked before)

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Semantic Web Towards a Web of Knowledge - Introduction

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  1. Semantic WebTowards a Web of Knowledge -Introduction Spring 2006 Computer Engineering Department Sharif University of Technology

  2. Outline • Introduction • From Original Web to Semantic Web • Building Blocks • URI (We talked before) • XML (We talked before) • RDF (We talked before) • Ontology (OWL) • Description Logic Semantic web - Computer Engineering Dept. - Fall 2005

  3. The Original Web • Web of relationships amongst named objects Information Management: A Proposal, Tim Berners-Lee, CERN, March 1989, May 1990, http://www.w3.org/History/1989/proposal.html Semantic web - Computer Engineering Dept. - Fall 2005

  4. Current Web: the Syntactic Web Semantic web - Computer Engineering Dept. - Fall 2005

  5. The Syntactic Web is… • A place where computers do the presentation (easy) and people do the linking and interpreting (hard). • Why not get computers to do more of the hard work? • “The bane of my existence is doing things that I know the computer could do for me.” —Dan Connolly, “The XML Revolution” Semantic web - Computer Engineering Dept. - Fall 2005

  6. Hard Work using the Syntactic Web… Find images Rev. Alan M. Gates, Associate Rector of the Church of the Holy Spirit, Lake Forest, Illinois Semantic web - Computer Engineering Dept. - Fall 2005

  7. Impossible using the Syntactic Web… • Complex queries involving background knowledge • Find information about “animals that use sonar but are not either bats or dolphins” • Locating information in data repositories • Travel enquiries • Prices of goods and services • Results of human genome experiments • Finding and using “web services” • Delegating complex tasks to web “agents” • Book me a holiday next weekend somewhere warm, not too far away, and where they speak French or English Semantic web - Computer Engineering Dept. - Fall 2005

  8. Consider a typical web page: What is the Problem? • Markup consists of: • rendering information (e.g., font size and colour) • Hyper-links to related content • Semantic content is accessible to humans but not (easily) to computers… Semantic web - Computer Engineering Dept. - Fall 2005

  9. WWW2002 The eleventh international world wide web conference Sheraton waikiki hotel Honolulu, hawaii, USA 7-11 may 2002 1 location 5 days learn interact Registered participants coming from australia, canada, chile denmark, france, germany, ghana, hong kong, india, ireland, italy, japan, malta, new zealand, the netherlands, norway, singapore, switzerland, the united kingdom, the united states, vietnam, zaire Register now On the 7th May Honolulu will provide the backdrop of the eleventh international world wide web conference. This prestigious event … Speakers confirmed Tim berners-lee Tim is the well known inventor of the Web, … Ian Foster Ian is the pioneer of the Grid, the next generation internet … What information can we see… Semantic web - Computer Engineering Dept. - Fall 2005

  10.          …   …  … What information can a machine see… Semantic web - Computer Engineering Dept. - Fall 2005

  11. <name> </name> <location> </location> <date></date> <slogan></slogan> <participants> </participants> <introduction> … </introduction> <speaker></speaker> <bio></bio>… Solution: XML markup with “meaningful” tags? Semantic web - Computer Engineering Dept. - Fall 2005

  12. <> </> <> </> <></> <></> <> </> <> … </> <></> <></> <></> <></> Still the Machine only sees… Semantic web - Computer Engineering Dept. - Fall 2005

  13. Need to Add “Semantics” • External agreement on meaning of annotations • E.g., Dublin Core for annotation of library/bibliographic information • Agree on the meaning of a set of annotation tags • Problems with this approach • Inflexible • Limited number of things can be expressed • Use Ontologies to specify meaning of annotations • Ontologies provide a vocabulary of terms • New terms can be formed by combining existing ones • “Conceptual Lego” • Meaning (semantics) of such terms is formally specified • Can also specify relationships between terms in multiple ontologies Semantic web - Computer Engineering Dept. - Fall 2005

  14. An example scenario (1)(Originally from Berners Lee, Scientific American paper) • The entertainment system was belting out the Beatles’ “We Can Work It Out” when the phone rang • When Pete answered, his phone turned the sound down by sending a message to all the other local devices that had a volume control • His sister, Lucy, was on the line from the doctor’s office: “Mom needs to see a specialist and then has to have a series of physical therapy sessions. Biweekly or something. I’m going to have my agent set up the appointments.” • Pete immediately agreed to share the work Semantic web - Computer Engineering Dept. - Fall 2005

  15. An example scenario (2) • At the doctor’s office, Lucy instructed her semantic web agent through her handheld web browser • The agent promptly retrieved information about Mom’s prescribed treatment from the doctor’s agent, looked up several lists of providers, and checked for the ones in plan for Mom’s insurance within a 20-mile radius of her home and with a rating of excellent or very good on trusted rating services • It then began trying to find a match between available appointment times (supplied by the agents of individual providers through their web sites) and Pete’s and Lucy’s busy schedules Semantic web - Computer Engineering Dept. - Fall 2005

  16. An example scenario (3) • In a few minutes the agent presented them with a plan • Pete didn’t like it – University hospital was all the way across the town from Mom’s place, and he’d be driving back in the middle of rush hour • He set his own agent to redo the search with stricter preferences about location and time • Lucy’s agent having complete trust in Pete’s agent in the context of the present task, automatically assisted by supplying access certificates and shortcuts to the data it had already been sorted through Semantic web - Computer Engineering Dept. - Fall 2005

  17. An example scenario (4) • Almost instantly the new plan was presented • A much closer clinic and earlier times – but there were two warning notes • First, Pete would have to reschedule a couple of his less important appointments. He checked what they were – not a problem • The other was something about insurance company’s list failing to include this provider under physical therapists: “Service type and insurance plan status securely verified by other means, “the agent reassured him.” Semantic web - Computer Engineering Dept. - Fall 2005

  18. An example scenario (5) • Lucy registered her assent at about same moment Pete was muttering, “Spare me the details,” and it was all set. • (Of course, Pete couldn’t resist the details and later that night had his agent explain how it had found that provider even though it wasn’t on the proper list.) Semantic web - Computer Engineering Dept. - Fall 2005

  19. Insurance Co. Rating Provider sites Physician’s Agent Mom required treatment in-plan? close-by? Specialist? Schedule appointment Driving schedule Lucy’s Agent Pete’ Agent Semantic web - Computer Engineering Dept. - Fall 2005

  20. The Building Blocks • URI • XML • RDF • Ontologies Semantic web - Computer Engineering Dept. - Fall 2005

  21. Youarehere Semantic Web Architecture Semantic web - Computer Engineering Dept. - Fall 2005

  22. The ultimate goal - Web of Trust Semantic web - Computer Engineering Dept. - Fall 2005

  23. URIs • URI=Uniform Resource Identifier • "The generic set of all names/addresses that are short strings that refer to resources" • URLs (Uniform Resource Locators) are a particular type of URI, used for resources that can be accessed on the WWW. Semantic web - Computer Engineering Dept. - Fall 2005

  24. XML “XML allows users to add arbitrary structure to their documents but says nothing about what the structures mean” Semantic web - Computer Engineering Dept. - Fall 2005

  25. CE Ganji http://ce.sharif.edu Sharif RDF • Meaning encoded in sets of ‘triples’: entities have properties which have values • Entities, properties and values all have distinct URIs studentOF departmentOF hasHomePage Semantic web - Computer Engineering Dept. - Fall 2005

  26. Example We are looking for a woman who works for one of my client and her son studies at University of Rovaniemi Semantic web - Computer Engineering Dept. - Fall 2005

  27. Ontologies • Database A and Database B may use different fields to contain ‘zip code’ (e.g. zip code and postal code) • Ontologies sort this out • Ontology = ‘a document or file that formally defines the relations among terms’ • Ontologies for the web normally have • A taxonomy • A set of inference rules • e.g. “If an animal eats a food, and this food is expensive, then that animal is not suitable for poor people.” Semantic web - Computer Engineering Dept. - Fall 2005

  28. 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. Semantic web - Computer Engineering Dept. - Fall 2005

  29. 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 Semantic web - Computer Engineering Dept. - Fall 2005

  30. 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> Semantic web - Computer Engineering Dept. - Fall 2005

  31. 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 Semantic web - Computer Engineering Dept. - Fall 2005

  32. OWL Design Goals • Shared ontologies • Ontology evolution • Ontology interoperability • Inconsistency detection • Expressivity vs. scalability • Ease of use • Compatibility with other standards • Internationalization Semantic web - Computer Engineering Dept. - Fall 2005

  33. 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 Semantic web - Computer Engineering Dept. - Fall 2005

  34. Comparison of versions • Full: • We get the full power of the OWL language. • It is very difficult to build a tool for this version. • The user of a fully-compliant tool may not get a quick and complete answer. • DL/Lite: • Tools can be built more quickly and easily • Users can expect responses from such tools to come quicker and be more complete. • We don't have access to the full power of the language. Semantic web - Computer Engineering Dept. - Fall 2005

  35. 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 Semantic web - Computer Engineering Dept. - Fall 2005

  36. 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 Semantic web - Computer Engineering Dept. - Fall 2005

  37. 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> Semantic web - Computer Engineering Dept. - Fall 2005

  38. Description Logics:Logic foundation of Semantic Web Semantic Web - Fall 2005 Computer Engineering Department Sharif University of Technology

  39. Outline • First order logic and Models • Introduction to Description Logics • Reasoning on Description Logics Semantic web - Computer Engineering Dept. - Fall 2005

  40. Prepositional Logic Semantic web - Computer Engineering Dept. - Fall 2005

  41. Truth Tables Semantic web - Computer Engineering Dept. - Fall 2005

  42. First Order Logic (FOL) Semantic web - Computer Engineering Dept. - Fall 2005

  43. Models Semantic web - Computer Engineering Dept. - Fall 2005

  44. Important Equivalences Semantic web - Computer Engineering Dept. - Fall 2005

  45. Example of a model Semantic web - Computer Engineering Dept. - Fall 2005

  46. Knowledge Representation with FOL Semantic web - Computer Engineering Dept. - Fall 2005

  47. Knowledge Representation with FOL Semantic web - Computer Engineering Dept. - Fall 2005

  48. What Are Description Logics? • A family of logic based Knowledge Representation formalisms • Based on concepts and roles • Concepts are interpreted as sets of objects. • Roles are interpreted as binary relations between objects. • Descendants of semantic networks and KL-ONE • Key features of DLs are: • Formal semantics • Decidable fragments of FOL • Closely related to Propositional Modal & Dynamic Logics • Provision of inference services • Sound and complete decision procedures for key problems • Implemented systems (highly optimised) • Trade-off between expressive power and computational complexity. Semantic web - Computer Engineering Dept. - Fall 2005

  49. Semantic web - Computer Engineering Dept. - Fall 2005

  50. Description Logic Family • Particular languages mainly characterised by: • Set of constructors for building complex concepts and roles from simpler ones. • Set of axioms for asserting facts about concepts, roles and individuals. Simplest logic in this family is named AL Others are specified by adding some suffixes like U ε NC: • ALC • ALCU • … Semantic web - Computer Engineering Dept. - Fall 2005

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