Semantic Web and its Logical Foundations

1. Semantic Web and its Logical Foundations Serguei Krivov, Ecoinformatics CollaboratoryGund Institute for Ecological Economics, UVM

2. Overview • Semantic Web • Semantic Web Languages and Tools • Description Logics (DL) – Formalism of Web Ontology Languages • OWL –latest Web Ontology Language

3. Why Semantic Web? • Most information on the Web is designed for human consumption, and even if it was derived from a database with well defined meanings for its columns, that the structure of the data is not evident to a robot browsing the web. • Leaving aside the artificial intelligence problem of training machines to behave like people, the Semantic Web approach instead develops languages for expressing information in a machine processable form.

4. The Semantic Web Vision + Web made possible through established standards TCP/IP for transporting bits down a wire HTTP & HTML for transporting and rendering hyperlinked text + Applications able to exploit this common infrastructure Result is the WWW as we know it + 1st generation web mostly handwritten HTML pages + 2nd generation (current) web often machine generated/active + Both intended for direct human processing/interaction + In next generation web, resources should be more accessible to automated processes To be achieved via semantic markup Metadata annotations that describe content/function + Coincides with Tim Berners-Lee’s vision of a Semantic Web Ian Horroks, Logical Foundations for the Semantic Web

5. What is Semantic Web? • Semantic Web is an initiative of World Wide Web consortium (http://www.w3.org/2001/sw/) • Semantic Web is a set of Languages and Tools for machine processing of information stored in WWW • You can think of it as being an efficient way of representing data on the World Wide Web, or as a globally linked Knowledge Base . • Semantic Web is about efficient Knowledge Representation mechanism for WWW. (AI) • Semantic Web is about efficient Reasoning Systems requered for integration of distributed data .

6. Ontologies + Semantic markup must be meaningful to automated processes + Ontologies will play a key role Source of precisely defined terms (vocabulary) Can be shared across applications (and humans) + Ontology typically consists of: Hierarchical description of important concepts in domain Descriptions of properties of instances of each concept + Degree of formality can be quite variable + Increased formality and regularity facilitates machine understanding + Ontologies can be used, e.g.: To facilitate agent-agent communication in e-commerce In semantic based search To provide richer service descriptions that can be more flexibly interpreted by intelligent agents Ian Horroks, Logical Foundations for the Semantic Web

7. RDF –Ressource Definition Framework • [ressource] [property] [ressource] • [Hamlet] [Author] [W. Shakespeare ] <?xml version="1.0"?> <Classrdf:ID="Resource" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns="uri"> <property>value</property> <property>value</property> ... </Class>

8. RDF Usage • Add ”annotations” for web-pages ? • Add ”annotations” for web-services ? • Instead of building a menu for a web-site, declare the content in RDF, and let the user-agent build the menu dynamically ? • Describe all images on your website ? • Market the web.service your web.site provides ? RDF is used as a syntax for more expressive languages: DAML+OIL, OWL

9. ”Semantic Web In A Box” • A Semantic Web in A Box Architecture RDF enhanced Website Internet Search View Update Manage RDF Repository Agents *.jpg *.html *.gif *.asp *.jsp From Jens Jacob Anderson’s Semantic Web Tutorial

10. Companies ,Tools, and Resources Ontoprise : OntoEdit, OntoBrocker see: http://www.ontoprise.de/products/index_html_en Unicorn http://www.unicorn.com/ KAON : http://kaon.semanticweb.org/ Protégé -Free Ontology Editor http://protege.stanford.edu/ Semantic Web Tools Tutorial http://www.daml.org/2003/05/swmu-tools-tutorial/Overview.html First Semantic Web Website: http://owl.mindswap.org/

11. Web Languages + Web languages already extended to facilitate content description XML Schema (XMLS) RDF and RDF Schema (RDFS) + RDFS recognisable as an ontology language Classes and properties Range and domain of properties Sub/super-classes (and properties) + But RDFS not a suitable foundation for Semantic Web Too weak to describe resources in sufficient detail + Requirements for web ontology language: Compatible with existing Web standards (XML, RDF, RDFS) Easy to understand and use (based on familiar KR idioms) Formally specified and of “adequate” expressive power possible to provide automated reasoning support Ian Horroks, Logical Foundations for the Semantic Web

12. Formalism for Ontology Languages: Description Logicsextracts from Franconi’s course

13. DL-examples

14. DL definition of “Happy Father” (Example from Ian Horrocks, U Manchester, UK)

16. A Few Fundamental Definitions Example: KB={ A&B, (C or (not D)), D } The following is true: KB |= A KB |= C

17. Decidability: A formal system is decidable if it has inference procedure i that always terminates with definite answer (either KB|-A or not KB|-A)

18. Why DL are used as formalism of Semantic Web ? • FOL –sound, complete, but not decidable • Proposition Logic- sound, complete, decidable, but has low expressive power • DL- has sound, complete, decidable inference procedure; has reasonable expressive power • Alternatives to DL- decidable fragments of FOL- Horn logic (prolog), F-Logic

19. OWL-Web Ontology LanguageExtracts from:Ian Horroks, Logical Foundations for the Semantic Web + Three species of OWL OWL full is union of OWL syntax and RDF OWL DL restricted to FOL fragment ( DAML+OIL) OWL Lite is “easier to implement” subset of OWL DL + Semantic layering OWL DL OWL full within DL fragment DL semantics officially definitive + OWL DL based on SHIQ Description Logic + Benefits from many years of DL research Well defined semantics Formal properties well understood (complexity, decidability) Known reasoning algorithms Implemented systems (highly optimised)