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The Semantic Web

The Semantic Web. Riccardo Rosati Dottorato in Ingegneria Informatica Sapienza Università di Roma a.a. 2006/07. Overview. the aim of this course is to provide an introduction to the Semantic Web. ... with emphasis on two aspects:

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The Semantic Web

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  1. The Semantic Web Riccardo Rosati Dottorato in Ingegneria Informatica Sapienza Università di Roma a.a. 2006/07

  2. Overview the aim of this course is to provide an introduction to the Semantic Web... • ... with emphasis on two aspects: • emphasis on AI-related aspects, in particular knowledge representation and reasoning • emphasis on database-related aspects, in particulardata integration The Semantic Web - course overview

  3. Overview • Lecture 1: Introduction to the Semantic Web • Lecture 2: The XML layer • Lecture 3: The RDF layer • Lecture 4: The Ontology layer 1 • Description Logics, OWL • Lecture 5: The Ontology layer 2 • reasoning in OWL, OWL species The Semantic Web - course overview

  4. Overview • Lecture 6: The Ontology layer 3 • OWL technologies: OWL Tools, QuOnto • Lecture 7: The rule layer • Lecture 8: The RDF layer 2 • RDF semantics • Lecture 9: The Ontology layer 4 • query answering in OWL • Lecture 10: Handling inconsistency (Jan Chomicki)

  5. Lecture 1 Introduction to the Semantic Web

  6. What is the Semantic Web? • “The Semantic Web is a Web of actionable information—information derived from data through a semantic theory for interpreting the symbols.” • “The semantic theory provides an account of ‘meaning’ in which the logical connection of terms establishes interoperability between systems” (Shadbot, Hall, Berners-Lee, The Semantic Web revisited, IEEE Intelligent Systems, May 2006) Introduction to the Semantic Web

  7. The Semantic Web: why? • search on the Web: problems... • ...due to the way in which information is stored on the Web • Problem 1: web documents do not distinguish between information content and presentation (“solved” by XML) • Problem 2: different web documents may represent in different ways semantically related pieces of information • this leads to hard problems for “intelligent” information search on the Web Introduction to the Semantic Web

  8. Separating content and presentation Problem 1: web documents do not distinguish between information content and presentation • problem due to the HTML language • problem “solved” by current technology • stylesheets (HTML, XML) • XML • stylesheets allow for separating formatting attributes from the information presented Introduction to the Semantic Web

  9. Separating content and presentation • XML: eXtensible Mark-up Language • XML documents are written through a user-defined set of tags • tags are used to express the “semantics” of the various pieces of information Introduction to the Semantic Web

  10. XML: example HTML: <H1>Seminari di Ingegneria del Software</H1><UL> <LI>Teacher: Giuseppe De Giacomo<LI>Room: 7<LI>Prerequisites: none</UL> XML: <course><title>Seminari di Ingegneria del Software </title><teacher>Giuseppe De Giacomo</teacher><room>1AI, 1I</room><prereq>none</prereq> </course> Introduction to the Semantic Web

  11. Limitations of XML XML does not solve all the problems: • legacy HTML documents • different XML documents may express information with the same meaning using different tags Introduction to the Semantic Web

  12. The need for a “Semantic” Web Problem 2: different web documents may represent in different ways semantically related pieces of information • different XML documents do not share the “semantics” of information • idea: annotate (mark-up) pieces of information to express the “meaning” of such a piece of information • the meaning of such tags is shared!  shared semantics Introduction to the Semantic Web

  13. The Semantic Web initiative viewpoint: the Web = a web of data goal: to provide a common framework to share data on the Web across application boundaries main ideas: • ontology • standards • “layers” Introduction to the Semantic Web

  14. The Semantic Web Tower Introduction to the Semantic Web

  15. The Semantic Web Layers • XML layer • RDF + RDFS layer • Ontology layer • Proof-rule layer • Trust layer Introduction to the Semantic Web

  16. The XML layer • XML (eXtensible Markup Language) • user-definable and domain-specific markup • URI (Uniform Resource Identifier) • universal naming for Web resources • same URI = same resource • URIs are the “ground terms” of the SW • W3C standards Introduction to the Semantic Web

  17. The RDF + RDFS layer RDF = a simple conceptual data model W3C standard (1999) RDF model = set of RDF triples triple = expression (statement) (subject, predicate, object) • subject = resource • predicate = property (of the resource) • object = value (of the property) => an RDF model is a graph Introduction to the Semantic Web

  18. “W3C” dc:Publisher http://www.w3.org/TR/REC-rdf-syntax/ dc:Creator dc:Date “Ora Lassila” “1999-02-22” The RDF + RDFS layer Example of RDF graph: Introduction to the Semantic Web

  19. Person subClassOf subClassOf range domain Student Researcher hasSuperVisor type type Frank Jeen hasSuperVisor The RDF + RDFS layer • RDFS = RDF Schema • “vocabulary” for RDF • W3C standard (2004) example: RDFS RDF Introduction to the Semantic Web

  20. The Ontology layer ontology = shared conceptualization • conceptual model (more expressive than RDF + RDFS) • expressed in a true knowledge representation language OWL (Web Ontology Language) = standard language for ontologies Introduction to the Semantic Web

  21. The proof/rule layer beyond OWL: • proof/rule layer • rule: informal notion • rules are used to perform inference over ontologies • rules as a tool for capturing further knowledge (not expressible in OWL ontologies) Introduction to the Semantic Web

  22. The Trust layer • SW top layer: • support for provenance/trust • provenance: • where does the information come from? • how this information has been obtained? • can I trust this information? • largely unexplored issue • no standardization effort Introduction to the Semantic Web

  23. The Semantic Web: main ingredients • underlying web layer (URI, XML) • reusing and extending web technologies • basic conceptual modeling language (RDF) • ontology language (OWL) • rules/proof • reusing and extending AI technologies • knowledge representation • automated reasoning • ...and database technologies • data integration Introduction to the Semantic Web

  24. The Semantic Web from an AI perspective • the notion of ontology • the role of logic and Description Logics • the role of rule-based formalisms • ... (agent technology) Introduction to the Semantic Web

  25. The notion of ontology • ontology = shared conceptualization of a domain of interest • shared vocabulary => simple (shallow) ontology • (complex) relationships between “terms” => deep ontology • AI view: • ontology = logical theory (knowledge base) • DB view: • ontology = conceptual model Introduction to the Semantic Web

  26. Ontologies: example class-def animal % animals are a class class-def plant % plants are a class subclass-of NOT animal% that is disjoint from animals class-def tree subclass-of plant % trees are a type of plants class-def branch slot-constraint is-part-of % branches are parts of some tree has-valuetree max-cardinality 1 class-def defined carnivore % carnivores are animals subclass-of animal slot-constraint eats % that eat any other animals value-typeanimal class-def defined herbivore % herbivores are animals subclass-of animal, NOT carnivore % that are not carnivores, and slot-constraint eats % they eat plants or parts of plants value-type plant OR (slot-constraint is-part-of has-value plant) Introduction to the Semantic Web

  27. Ontologies: the role of logic • ontology = logical theory • why? • declarative • formal semantics • reasoning (sound and complete inference techniques) • well-established correspondence between conceptual modeling formalisms and logic Introduction to the Semantic Web

  28. Ontologies and Description Logics • OWL is based on a fragment of first-order predicate logic (FOL) • Description Logics (DLs) = subclasses of FOL • only unary and binary predicates • function-free • quantification allowed only in restricted form • (variable-free syntax) • decidable reasoning • DLs are one of the most prominent languages for Knowledge Representation Introduction to the Semantic Web

  29. Ontologies and Description Logics • expressive abilities of DLs have been widely explored • reasoning in DLs has been extensively studied • DL reasoners have been developed and optimized • DLs as a central technology for the SW Introduction to the Semantic Web

  30. Rule-based formalisms • Prolog • Logic programming • Constraint (logic) programming • Production rules • Datalog • ... Rule language for SW not standardized yet RIF (Rule Interchange Format) W3C working group Introduction to the Semantic Web

  31. The SW from a database perspective • ontology as a virtual database schema • ontology-based information access • the Semantic Web as a framework for data integration Introduction to the Semantic Web

  32. Virtual data integration abstract model of a virtual data integration system: triple (G,S,M) • G = global information schema • S = set of data sources • M = mapping between sources and global schema • the data sources are autonomous and heterogeneous information systems • the global schema is a virtual conceptualization of the domain of interest • the mapping is a declarative specification of the relationship between sources and global schema Introduction to the Semantic Web

  33. Ontology-based information access • ontology = virtual global information schema • ontology language = conceptual modeling language • the Web = distributed, heterogeneous, autonomous set of data sources but : • data integration considers different formalisms for expressing schema and data • the architecture of a data integration system is centralized Introduction to the Semantic Web

  34. Ontology-based information access • the data integration approach can be generalized to non-centralized architectures => peer data management systems • despite the differences in the formalisms adopted, there is a tight relationship between the SW and data integration => dealing with incomplete information Introduction to the Semantic Web

  35. The Semantic Web and data integration similarity between data integration and the SW: • global schema = ontology • sources = web sites (data) • mapping = ?? however: • different languages • different technologies => the SW is essentially a data integration technology Introduction to the Semantic Web

  36. Very quick overview of SW technologies main current technologies and standards for the SW: • RDF • RDFS • OWL Introduction to the Semantic Web

  37. RDF • RDF = Resource Description Framework • RDF data model is an abstract, conceptual layer (independent of XML) • RDF data model = set of RDF triples • triple = (subject, predicate, object) • subject = resource • predicate = property (of the resource) • object = value (of the property) • standardized in 1999 Introduction to the Semantic Web

  38. RDFS • RDFS = RDF Schema • set of predefined predicates: • subClassOf • subPropertyOf • domain • range • ... • ...with predefined semantics! • standardized in 2004 Introduction to the Semantic Web

  39. OWL • OWL = Web Ontology Language • the OWL family is constituted by 3 different languages (with different expressive power): • OWL Full • OWL-DL • OWL-Lite • technology at an early stage • standardized in 2004 • reasoning techniques and tools are very recent • “optimization” of reasoning is largely unexplored Introduction to the Semantic Web

  40. RDF(S) OWL • class-expressions • AND, OR, NOT • role-constraints • has-value, value-type • cardinality • role-properties • trans, symm... • class-def • subclass-of • property-def • subproperty-of • domain • range OWL vs. RDFS Introduction to the Semantic Web

  41. From RDFS to OWL Full with respect to the relative expressive power: OWL Full > OWL-DL > OWL-Lite > RDFS Introduction to the Semantic Web

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