Semantic Web and

The Web Ontology Language

1

The World Wide Web today

• Information is represented • in natural language• in multiple formats• using graphics, multimedia

• Information overload• Though humans can process this information,

• machines (software agents) cannot understand it• vast majority of information remains untapped

2

Semantic Web Goals

• Make information on the Web machine processable• Provide a standard data model for data integration• Build machine reasoning infrastructure• Enhance the current Web• Enable software services

In short, “create a Web of Information”

3

Semantic Web Stack

4

Source: W3C http://www.w3.org/2000/Talks/1206-xml2k-tbl/slide10-0.html

Debunking Semantic Web Myths

Some Semantic Web myths …

• Large Scale application of AI on the Web• Need to understand logic, knowledge representation

techniques• Relies on centrally controlled ontologies• Meant to replace the current Web

5

Resource Description Framework• Data model of the Semantic Web

• Central to the data integration requirements of Semantic Web

• Allow data interchange without loss of meaning

6

RDF Basics

• Data is the form of facts called statements

• Each statement is a triple of resources of the form(Subject, Property, Object)

e.g. (“Ayn Rand”, authorOf, “The Fountainhead”)

7

Resource

There is a bit more to resources:• An entity in the data • Each resource identified by a URI (Universal Resource

Identifier)• These URI ground the RDF to the Web • Provide the basis for comparison of resources, hence

important• Resources can be literals too i.e. no URI associated with

them

8

RDF as a graph

9

Triples

•(index.html, dc:creator, staff-12345)

•(index.html, dc:language, ”en”)

•(index.html, creation-date, ”12 Aug 2006”)

How is RDF different from using XML?• Cleaner separation of syntax and semantics• Makes explicit the data model• However, XML is still used as a serialization mechanism• Other serialization mechanisms like Turtle and Notation

3 are available

10

Enter RDF-S (RDF-Schema)

• RDF-S is to RDF what XML-Schema is to XML• RDF-S provides mechanisms to define domain

vocabulary, just as a programming language allows you to define new types

• Applications can then define vocabularies• Lightweight ontology

11

RDF-S Basics and ConstructsRDF-S allows defining • resources as Classes, Properties or Datatypes• resources as instances of Classes • sub-class, sub-properties relations• domain/range of properties• resource containers like bag, list, etc.

12

A look at RDF-S

•Animal rdfs:type rdfs:Class•Horse rdfs:subClassOf Animal•Dog rdfs:subClassOf Animal•number_of_legs rdfs:type rdfs:Property •number_of_legs rdfs:domain Animal•number_of_legs rdfs:range Integer

13

RDF in action

• Dublin Core• vocabulary for describing networked, document like entities and

their properties• RSS 1.0

• lightweight way of content syndication

• SKOS (Simple Knowledge Organization System)• a model for expressing the basic structure and content of concept

schemes.

• XMP, FOAF, etc.

14

What is OWL?

• Web Ontology Language, a W3C Recommendation• Derived from the DAML+OIL• Builds on RDF, uses RDF/XML• Knowledge Representation layer of the Semantic Web• An ontology: A formal conceptualization of the world

• Concepts• Properties• Relationships• Additional Constraints

15

Why OWL?

• Supply richer semantics and knowledge base for agents

• Overcome limitations of RDF-S• Local Properties of classes• disjointness of classes• cardinality restrictions

• ... and others

16

OWL Features – Class Level (1)

• Class Equivalence• Faculty owl:equivalentClass AcademicStaffMember

• Class Disjointness• Professor owl:disjointWith AssociateProfessor

• Every class is a sub-class of owl:Thing and super-class of owl:Nothing

17

OWL Features: Class Level (2)

• Boolean combinations to create new classes

• Union• staff = owl:unionOf

( AcademicStaffMember, Non-AcademicStaffMember)

• Intersection• TA = owl:intersectionOf (student , stipendee)

• Complement• Non-AcademicStaffMember=

owl:complementOf (AcademicStaffMember)

18

OWL Features: Property Level (3)

• Inverse Relation • isTaughtBy owl:inverseOf teaches

• Special Properties• Transitive

• isBroaderThan owl:TransitiveProperty• Symmetric

• isPartner owl:SymmetricProperty• also, Function, One-One

19

OWL Features: Property Level (4)

• Property Restrictions: Constrain attributes to take only certain values for particular classes• Existential Quantification

• academicStaffMember teaches underGraduateCourse

• Universal Quantification• firstYearCourse isTaughtBy Professor

• Cardinality Constraints• academicStaffMember takes a maximum of 3

courses

20

OWL Expressiveness

• Powerful OWL constructs, but at a cost

• Trade-off between• Expressiveness of language• Decidability and completeness• Tractability

• 3 OWL flavours defined with different degrees of expressiveness

21

OWL Flavours

• OWL Full• Most expressive, undecidable• Any RDF valid OWL Full

• OWL DL• Based on Descriptive Logic• Guarantees computational completeness and decidability

• OWL Lite• Good enough for simple taxonomies

22

Semantic Web & OWL in action

• Knowledge Management

• Web Services

• Data Integration

23

Knowledge Management

• Use Semantic Web technologies for • Better organization of information• Improved search

• Portals: • Vodafone's Live Mobile Portal• Sun’s SwordFish

• Organizing data in biosciences: GoPubMed

24

Web Services

• Where does the Semantic Web come in?• Automatic Discovery of Web Services• Automatic Web Service Invocation• Automatic Web Service Composition and Interoperation

• OWL-S markup language for describing services profiles, process models and grounding for interoperability

25