The Semantic Web an introduction

1. The Semantic Web an introduction Luigi De Russis

2. The Web is a Web of document for people, not for machines

3. The Semantic Web is a Web of data Linking Open Data cloud diagram, by Richard Cyganiak and AnjaJentzsch. http://lod-cloud.net/

4. Let’s think!

5. Exercise: Build a Music Catalog Comprehensiveguide to music across the world Web-based With always-updated information about each artist

6. How? What are the problems? What about data replication? What about data synchronization? …?

7. Solution #1 How? Site editors roam the Web for new facts and update the site manually What are the problems? A lot of people need to continuously roam the Web; the site will get soon out-of-date What about data? Data is replicated and not up-to-date with new facts

8. Solution #2 How? Site editors roam the Web for new data and write a program to extract the information What are the problems? Code needs to be updated each time a new site is found; the site will get out-of-date, soon or later… What about data? Data is replicated and not up-to-date

9. Solution #3 How? Site editors browse the Web for new data via APIs, and write some code to incorporate the information What are the problems? Code needs to be updated each time a new site is found and/or an API is changed; the site will get out-of-date, soon or later… What about data? Data is replicated and not up-to-date

10. Solution #4 How? Site editors choose to use some external, public datasets (e.g., Wikipedia, MusicBrainz, …) What are the problems? No problem What about data? Data is immediately available, not as APIs or hidden on a Web site. Information can be extracted using standard queries or HTTP requests.

11. Solution #4 In short… Use the Web of Data as a Content Management System Use the community at large as content editor An example: BBC Music http://www.bbc.co.uk/music

12. Data on the Web is not enough! we need a proper infrastructure Data should be available on the Web accessible via standard Web technologies Data should be interlinked over the Web i.e., data can be integrated over the Web This is where Semantic Web come in

13. Fundamentals

14. To a computer, the Web is a flat, boring world, devoid of meaning. This is a pity, as in fact documents on the Web describe real objects and imaginary concepts. […] Adding semantics to the Web involves two things: allowing documents which have information in machine-readable forms, and allowing links to be created with relationship values. Only when we have this extra level of semantics we will be able to use computer power to help us exploit the information to a greater extent than our own reading. Tim Berners-Lee, 1994

15. What is the relationship with AI? Influence Some technologies in the Semantic Web benefited a lot from AI research and development (and viceversa) Different goals Artificial Intelligence approach: build smarter machines, teach computers to infer the meaning of data Semantic Web approach: have smarter data, make data easier for machines to find, access and process

16. Resource and description Resource every document “reachable” on the Web no matter the content, format, language, etc. Resource description independent from the format standard language (metadata)

17. Resource and description Resources

18. Resource and description Description

19. Resource and description Title Author Date Topic Quality Description Title Author Date Topic

20. URIs unambiguousnames for resources RDF a common data model to connectand describe resources SPARQL access to the data model RDFS, OWL common description languages OWL, RIF reasoning (mainly logic inference)

21. Modeling Data

22. Example: bookstore Represent the following data about the AI book as a set of relations Title: “Artificial Intelligence: A Modern Approach” Author:Russel, Stuart and Norvig, Peter Publisher: Prentice Hall ISBN: 978-0136042594

23. Example: bookstore http://...isbn/9780136042594 Resource

24. Example: bookstore http://...isbn/9780136042594 Artificial Intelligence: A Modern Approach Literal

25. Example: bookstore http://...isbn/9780136042594 title Artificial Intelligence: A Modern Approach

26. Example: bookstore http://...isbn/9780136042594 title Artificial Intelligence: A Modern Approach publisher author Prentice Hall author Norvig, Peter Russel, Stuart

27. RDF: Resource Description Framework Structured in statements Subject a resource (URI) Predicate a verb, property or relationship Object a resource or a literal string

28. Example: bookstore Object Subject Predicate http://...isbn/9780136042594 title Artificial Intelligence: A Modern Approach Predicate Predicate publisher Object author Prentice Hall Object author Norvig, Peter Object Russel, Stuart

29. Example: bookstore http://...isbn/9780136042594 title Artificial Intelligence: A Modern Approach RDF in XML syntax <rdf:RDFxmlns:rdf=http://www.w3.org/…/22-rdf-syntax-ns#> <rdf:Descriptionabout=“http://... isbn/9780136042594”> <title>Artificial Intelligence: A Modern Approach</title> </rdf:Description> </RDF>

30. Example: bookstore http://...isbn/9780136042594 title Artificial Intelligence: A Modern Approach RDF in Turtle <http://... isbn/9780136042594> title “Artificial Intelligence: A Modern Approach”

31. Linkin’ Data

32. Example: bookstore Represent the following data about the Italian translation of the AI book as a set of relations Title: “Intelligenzaartificiale. Un approcciomoderno” Author:Russel, Stuart and Norvig, Peter Publisher: Prentice Hall ISBN: 978-8871925936 Original ISBN: 978-0136042594

33. Example: bookstore IntelligenzaArtificiale. Un approcciomoderno title http://...isbn/9788871925936 publisher original Prentice Hall http://...isbn/9780136042594 creator creator Russel, Stuart Norvig, Peter

34. Example: bookstore IntelligenzaArtificiale. Un approcciomoderno title http://...isbn/9788871925936 publisher Prentice Hall original http://...isbn/9780136042594 http://...isbn/9780136042594 title Artificial Intelligence: A Modern Approach publisher Prentice Hall

35. Example: bookstore IntelligenzaArtificiale. Un approcciomoderno title http://...isbn/9788871925936 publisher Prentice Hall original http://...isbn/9780136042594 same URI, same resource http://...isbn/9780136042594 title Artificial Intelligence: A Modern Approach Prentice Hall creator

36. Example: bookstore IntelligenzaArtificiale. Un approcciomoderno title http://...isbn/9788871925936 publisher Prentice Hall original http://...isbn/9780136042594 title Artificial Intelligence: A Modern Approach publisher author creator Prentice Hall creator Russel, Stuart Norvig, Peter author Russel, Stuart Norvig, Peter

37. Example: bookstore IntelligenzaArtificiale. Un approcciomoderno title http://...isbn/9788871925936 What about merging creator and author? In RDF, it is not possible! publisher Prentice Hall original http://...isbn/9780136042594 title Artificial Intelligence: A Modern Approach publisher author creator Prentice Hall creator Russel, Stuart Norvig, Peter author Russel, Stuart Norvig, Peter

38. Problem: field names are arbitrary Synonyms : author or creator or maker or contributor or… Singular or plural: author or authors Solution: standards general or domain-specific

39. Dublin Core General vocabulary Dublin Core Metadata Initiative (DCMI) http://dublincore.org Building blocks to define metadata for the Semantic Web Define title, contributor, publisher, license, date, language, etc.

40. Problem: field values are arbitrary Value type: string, date, integer, … Value format: “Norvig, Peter” or “Norvig, P.” or “Peter Norvig” or… Value restrictions: one value or multiple values (how many?) Solutions Standards Controlled vocabulary (close list of terms) Semantically rich descriptions to support search (RDFS and/or OWL)

41. Friend Of A Friend (FOAF) General ontology Describe persons, their activities and their relations to other people and objects http://www.foaf-project.org Building blocks to define structured relations between people Define name, familyName, givenName, knows, age, nick, etc.

42. Example: bookstore http://...isbn/9780136042594 dc:title Artificial Intelligence: A Modern Approach dc:publisher dc:creator dc:creator foaf:name Prentice Hall foaf:name foaf:name Norvig, Peter Russel, Stuart foaf: http://xmlns.com/foaf/spec dc: http://purl.org/dc/terms

43. Why? http://...isbn/9780136042594 http://...isbn/9780136042594 dc:creator author foaf:name Norvig, Peter Norvig, Peter

44. RDF Schema

45. RDF Schema Schema Definition of the nodes and predicates used in a RDF document Domainand Range RDFS describes properties in terms of classes of resource to which they apply (from a “domain” to a “range”)

46. Example RDF data http://elite.polito.it/people/derussis http.//bit.ly/lingambmult teaches

47. Example Person subClassOf domain range Teacher Course teaches type type RDF schema RDF data http://elite.polito.it/people/derussis http.//bit.ly/lingambmult teaches

48. Back to the bookstore example… http://...isbn/9780136042594 http://...isbn/9780136042594 dc:creator Why? author foaf:name Norvig, Peter Norvig, Peter

49. Back to the bookstore example… http://...isbn/9780136042594 http://...isbn/9780136042594 dc:creator Why? anonymous class author foaf:name Norvig, Peter Norvig, Peter dc:creator has range Agent, i.e. a class (resource), not a literal: we use an anonymous class for this scope. Finally, foaf:Name has range rdfs:Literal.

50. RDFS Expressivity • Simple relationship between things • RDFS provides a vocabulary to express relationship between things (e.g., subClassOf or type) • Avoid complex relationship • RDFS cannot describe data in terms of set of operations (e.g., unionOf), equivalence (e.g., sameAs) or cardinality (e.g., allValueFrom)