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Semantic Web

Semantic Web. Andrejs Lesovskis. Semantic Web layers. Ontology spectrum (1). Strong semantics. Modal Logic. First Order Logic. Logical theory. Is Disjoint Subclass of with transitivity property. Description Logic. DAML+OIL, OWL. From less to more expressive. UML. Conceptual model.

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Semantic Web

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  1. Semantic Web Andrejs Lesovskis

  2. Semantic Web layers

  3. Ontology spectrum (1) Strong semantics Modal Logic First Order Logic Logical theory Is Disjoint Subclass of with transitivity property Description Logic DAML+OIL, OWL From less to more expressive UML Conceptual model Is subclass of RDF/S Semantic Interoperability XTM Extended ER Thesaurus Has narrower meaning than ER Structural Interoperability DB Schemas, XML Schema Taxonomy Is sublassification of Relational Model, XML Syntactic Interoperability Weak semantics

  4. Ontology spectrum (2) Strong semantics Modal Logic First Order Logic Logical theory Is Disjoint Subclass of with transitivity property Problem: Very General Semantic Expressivity: Very High Description Logic DAML+OIL, OWL From less to more expressive UML Conceptual model Is subclass of Problem: General Semantic Expressivity: High RDF/S Semantic Interoperability XTM Extended ER Thesaurus Has narrower meaning than Problem: General Semantic Expressivity: Medium ER Structural Interoperability DB Schemas, XML Schema Taxonomy Is sublassification of Problem: Local Semantic Expressivity: Low Relational Model, XML Syntactic Interoperability Weak semantics

  5. Open and Closed World Assumptions The Closed World Assumption (CWA) is the assumption that what is not known to be true must be false. Imagine, we have the following statement "Peteris is a citizen of Latvia." and we are asked "Is Peteris a citizen of Lithuania? ", under the CWA, the answer is no.

  6. Open and Closed World Assumptions The Open World Assumption (OWA) is the opposite. In other words, it is the assumption that what is not known to be true is simply unknown. Under the OWA, the answer to the question about Peteris is "I don’t know". The Web is a system with incomplete information. Absence of information on the Web means that the information has not been made explicit. Therefore, Semantic Web uses the Open World Assumption.

  7. Protégé OWL Plugin • Extension of Protégé for handling OWL ontologies. • Project started in April 2003. • Features: • Loading and saving OWL files and databases; • Graphical editors for class expressions; • Access to description logics reasoners; • Powerful platform for hooking in custom-tailored components.

  8. OWL GUI Plugins (SWRL Editors, ezOWL, OWLViz, Wizards, etc.) OWL Extension APIs (SWRL, OWL-S, etc.) Protégé OWL GUI (Expression Editor, Conditions Widget, etc.) Protégé OWL API (Logical class definitions, restrictions, etc.) OWL Plugin Jena API (Parsing, Reasoning) OWL File Storage OWL Plugin Architecture Protégé GUI (Tabs, Widgets, Menus) Protégé API (Classes, properties, individuals, etc.) Protégé Core System DB Storage

  9. Tutorial Scenario • Semantic Web for Tourism/Traveling domain • Goal: Find matching holiday destinations for a customer I am looking for a comfortable destination with beach access Tourism Web

  10. Scenario description • A search problem: Match customer’s expectations with potential destinations • Required: Web Service that exploits formal information about the available destinations: • Accomodation (Hotels, B&B, Camping, ...), • Activities (Sightseeing, Sports, ...), • etc.

  11. Semantic Web for tourism domain • Open World: • New hotels are being added, • New activities are offered. • Providers publish their services dynamically. • Standard format / grounding is needed→ Tourism Ontology.

  12. Tourism Semantic Web OWL Metadata (Individuals) OWL Metadata (Individuals) Tourism Ontology Destination Activity Accomodation OWL Metadata (Individuals) OWL Metadata (Individuals) Web Services

  13. Sydney SydneysOlympicBeach BondiBeach Individuals • Represent objects in the domain. • Specific things. • Two names (synonyms) could represent the same “real-world” individual.

  14. hasPart hasAccomodation Sydney BondiBeach FourSeasons ObjectProperties • Link two individuals together using particular relationships (0..n, n..m).

  15. isPartOf hasPart Sydney BondiBeach Inverse Properties • Represent bidirectional relationships. • Adding a value to one property also adds a value to the inverse property.

  16. Sydney DatatypeProperties • Link individuals to primitive values(integers, floats, strings, booleans, etc). • Often: AnnotationProperties without formal “meaning”. hasSize = 4,500,000 isCapital = true rdfs:comment = “Don’t miss the opera house”

  17. City Beach Cairns Sydney BondiBeach CurrawongBeach Classes • Sets of individuals with common characteristics. • Individuals are instances of at least one class (OWL supports multiple inheritance).

  18. hasAccomodation hasAccomodation Destination Accomodation Sydney BestWestern FourSeasons Range and Domain • Property characteristics • Domain: “left side of relation” (Destination); • Range: “right side” (Accomodation).

  19. Domains • Individuals can only take values of properties that have matching domain: • “Only Destinations can have Accomodations”. • Domain can contain multiple classes. • Domain can be undefined:Property can be used everywhere.

  20. Cairns Canberra Coonabarabran Sydney Superclass Relationships • Classes can be organized in a hierarchy. • Direct instances of subclass are also (indirect) instances of superclasses.

  21. RetireeDestination City Cairns BondiBeach Sydney Class Relationships • Classes can overlap arbitrarily.

  22. UrbanArea RuralArea City Destination Woomera CapeYork Sydney Sydney Class Disjointness • All classes could potentially overlap. • In many cases we want to make sure they don’t share instances. disjointWith

  23. (Create a new OWL project)

  24. (Create simple classes)

  25. (Create class hierarchy and set disjoints)

  26. (Create Contact class with datatype properties)

  27. (Edit details of datatype properties)

  28. (Create an object property hasContact)

  29. (Create an object property with inverse)

  30. (Create the remaining classes and properties)

  31. Things with three star accomodation Things with sightseeing opportunities RetireeDestination Sydney SanJose BlueMountains Class Descriptions • Classes can be described by their logical characteristics. • Descriptions are “anonymous classes”.

  32. Class Descriptions • Define the “meaning” of classes. • Anonymous class expressions are used: • “All national parks have campgrounds.”, • “A backpackers destination is a destination that has budget accomodation and offers sports or adventure activities.” • Expressions mostly restrict property values (OWL Restrictions).

  33. Class Descriptions: Why? • Based on OWL’s Description Logic support. • Formalize intentions and modeling decisions (comparable to test cases). • Make sure that individuals fulfill conditions. • Tool-supported reasoning.

  34. Reasoning with Classes • Tool support for three types of reasoning exists: • Consistency checking:Can a class have any instances? • Classification:Is A a subclass of B? • Instance classification:Which classes does an individual belong to?

  35. Restrictions (Overview) • Define a condition for property values • allValuesFrom • someValuesFrom • hasValue • minCardinality • maxCardinality • cardinality • An anonymous class consisting of all individuals that fulfill the condition

  36. Cardinality Restrictions • Meaning: The property must have at least/at most/exactly x values • is the shortcut for and • Example: A FamilyDestination is a Destination that has at least one Accomodation and at least 2 Activities

  37. allValuesFrom Restrictions • Meaning: All values of the property must be of a certain type. • Warning: Also individuals with no values fulfill this condition (trivial satisfaction). • Example: Hiking is a Sport that is only possible in NationalParks.

  38. someValuesFrom Restrictions • Meaning: At least one value of the property must be of a certain type. • Others may exist as well. • Example: A NationalPark is a RuralArea that has at least one Campground and offers at least one Hiking opportunity.

  39. hasValue Restrictions • Meaning: At least one of the values of the property is a certain value. • Similar to someValuesFrom but with Individuals and primitive values. • Example: A PartOfSydney is a Destination where one of the values of the isPartOf property is Sydney.

  40. BudgetAccomodation OneStarRating TwoStarRating ThreeStarRating Enumerated Classes • Consist of exactly the listed individuals.

  41. Logical Class Definitions • Define classes out of other classes • unionOf (or) • intersectionOf (and) • complementOf (not) • Allow arbitrary nesting of class descriptions (A and (B or C) and not D)

  42. Adventure Sports unionOf • The class of individuals that belong to class A or class B (or both) • Example: Adventure or Sports activities

  43. BudgetAccomodation Hotel intersectionOf • The class of individuals that belong to both class A and class B. • Example: A BudgetHotelDestination is a destination with accomodation that is a budget accomodation and a hotel.

  44. AccommodationWith3Stars Hotel LuxuryHotel Implicit intersectionOf • When a class is defined by more than one class description, then it consists of the intersection of the descriptions. • Example: A luxury hotel is a hotel that is also an accommodation with 3 stars.

  45. QuietDestination (grayed) Destination FamilyDestination complementOf • The class of all individuals that do not belong to a certain class • Example: A quiet destination is a destination that is not a family destination

  46. Class Conditions • Necessary Conditions:(Primitive / partial classes)“If we know that something is a X,then it must fulfill the conditions...” • Necessary & Sufficient Conditions:(Defined / complete classes)“If something fulfills the conditions...,then it is an X.”

  47. QuietDestination NationalPark Class Conditions (2) (not everything that fulfills theseconditions is a NationalPark) (everything that fulfills theseconditions is a QuietDestination)

  48. BackpackersDestination NationalPark Classification • A RuralArea is a Destination • A Campground is BudgetAccomodation • Hiking is a Sport • Therefore:Every NationalPark is a Backpackers-Destiantion (Other BackpackerDestinations)

  49. Classification (2) • Input: Asserted class definitions. • Output: Inferred subclass relationships.

  50. (Create an enumerated class out of individuals)

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