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Chapter 5The Semantic Web

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The Semantic Web Initiated by Tim Berners-Lee, the

inventor of the World Wide Web. A common framework that allows data to

be shared and reused across application, enterprise, and community boundaries.

Link smaller elements of data and information and further assign meaning to the links between data elements.

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Semantic Web Initiative

To understand sentences is to represent Web content in a form that is

more easily machine-processible and use intelligent techniques to take

advantage of these representations.

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Trend in Semantic WebProviding a common syntax for

machine understandable statements

Establishing common vocabulariesAgreeing on a logical languageUsing the language for exchanging

proofs

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A layered Structure for the Semantic WEBA layered structure for Semantic Web

Unicode/URI XML/Name Spaces/ XML SchemaRDF/RDF Schema Ontology vocabulary Logic ProofTrust

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The layered-language model (Courtesy of W3C, 2001)

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UnicodeUnicode is a character set that provides a unique number to every character irrespective of the platform, program, and the language.

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Generic URI syntax <URI> := <scheme> : <scheme-specific-part>

Example:

Uniform Resource Identifiers (URIs)

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Uniform Resource Names (URNs)Are special kinds of URIs that

map other namespaces into URN-space, are required to remain globally unique

and persistent, can be specified in a single line of text, have the scheme urn.

<URN> := urn: <namespace> : <namespace-specific-part>

Examples urn:isbn:0-395-36341-1 urn:newsml:reuters.com:20000206:IIMFFH05

643_2004-08-03_17-54-01_L06154

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Uniform Resource Locator (URL)

Another special type of URI used to identify an Internet resource Difference between URL, URN, and URI:

URL: http://www.tmrf.org/kpr/issue1.htm

URN: www.tmrf.org/kpr/issue1.htm#one

URI: http://www.tmrf.org/kpr/issue1.htm#one

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Markup

Example of a web page of a Student Service Center

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Explicit Metadata

Example of a web page of a Student Service Center

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OntologyCaptures data relationships and their

associated meaning Is a specification of a conceptualization

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OntologyAn ontology is a precise narrative of concepts in a field of discourse (classes or concepts), properties of each concept narrating various features and attributes of the concept (slots or roles or properties), and restrictions on slots (facets or role restrictions).

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Ontology developmentDeveloping an ontology includes:

defining classes in the ontology arranging the classes in a taxonomic

(subclass–superclass) hierarchy defining attributes/relationships and

describing allowed values for these slots filling in the values of slots for

instances

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Reasons for creating ontologies

to distribute common understanding of the information structure between individuals or software agents

to facilitate reuse of domain knowledge to formulate exact domain assumptions to split domain knowledge from

operational knowledge to analyze domain knowledge

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Ontology languageXML presents syntax for structured documents

but enforces no semantic constraints on the meaning of these documents.

is a Hierarchical data model is a syntax specification for data markup encloses data in tags tags can relate to the meaning of the

enclosed text tags are extensible

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Related Technologies Document Type Definition (DTD) XML Schema DOM XPath XLinks XPointers XSLT

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Advantages of XML

An XML document holds tags that indicate data types of every element.

It is straightforward to insert related data to a node in an XML document without formatting the document cumbersome.

There is nothing about XML that binds it to a specific operating system or technology.

There are numerous object-based parsers available for XML.

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XML Example of a College <location name=Innsbruck> <college name= College of Engineering>

<deptcode>CIS</deptcode></college>

</location>

An XML document college.xml

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Data Model

An ordered labeled tree

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XML SchemaA language for limiting the structure of XML documents. Constructs in an XML Schema:

simple type definitions complex type definitions, attribute element declaration

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Example of an XML schema

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Illustration of XML Communication

To maintain record of books by ISBN <document id="1">Hawking’s <book isbn="0553103741">A brief history of time</book> gives truly intimate glimpses into the intricacies of both the universe and Hawking himself.</document>

Database of books

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XML Query Languages path expression Xpath, Xquery, XQL, and XML-QL XPath

syntax for defining parts of an XML document

It uses path expressions to navigate in XML documents.

It includes almost 100 built-in functions and has become a W3C standard from 1999.

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XML Document

XML document college.xml

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Tree Representation of an XML Document

Tree representation for college.xml

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XPath Query Language Query 1: Select all lecturer elements. /college/lecturer

The expression selects all lecturer elements, which are children of the college element node that resides immediately below the root node.

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Query 2: Select all lecturer elements. //lecturer In this expression, // says that you must

consider every element in the document and check whether they are of type lecturer. This path expression selects all lecturer elements present anywhere in the document.

The tree representation of this query is identical to the tree representation of Query 1.

XPath Query Language

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Tree representation for Queries 1 and 2

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XPath Query LanguageQuery 3: Select all courses with the title “Nonlinear Analysis”.//course[@title="Nonlinear Analysis"]

Here, the test within square brackets restricts the set of selected nodes. The expression selects course elements with the title that satisfies a particular condition.

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XPath Query Language

Query 4: Select all title attribute nodes within course elements anywhere in the document, which have the value “Nonlinear Analysis”. //course[@title="Nonlinear Analysis"]/@title

This expression collects title attribute nodes of the course elements. The symbol @ denotes the attribute nodes.

The tree representation of queries 3 and 4 is given in next figure.

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Tree representation for Queries 3 and 4

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RDF

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What is RDF?RDF stands for Resource Description

FrameworkRDF is a framework for describing

resources on the webRDF is designed to be read and

understood by computers

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What is RDF?RDF is not designed for being

displayed to peopleRDF is written in XMLRDF is a part of the W3C's

Semantic Web ActivityRDF is a W3C Recommendation

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RDF document<?xml version="1.0"?>

<rdf:RDFxmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"xmlns:si="http://www.w3schools.com/rdf/">

<rdf:Description rdf:about="http://www.w3schools.com">  <si:title>W3Schools</si:title>  <si:author>Jan Egil Refsnes</si:author></rdf:Description>

</rdf:RDF>

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RDF- Examples of use

Describing properties for shopping items, such as price and availability

Describing time schedules for web events Describing information about web pages

(content, author, created and modified date)

Describing content and rating for web pictures

Describing content for search engines Describing electronic libraries

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RDF Resource, Property, and Property valueRDF identifies things using Web identifiers (URIs), and describes resources with properties and property values.

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Explanation of Resource, Property, and Property value

A Resource is anything that can have a URI, such as "http://www.w3schools.com/rdf"

A Property is a Resource that has a name, such as "author" or "homepage"

A Property value is the value of a Property, such as "Jan Egil Refsnes" or http://www.w3schools.com.

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Example<?xml version="1.0"?>

<RDF>  <Description about="http://www.w3schools.com/rdf">    <author>Jan Egil Refsnes</author>    <homepage>http://www.w3schools.com</homepage>  </Description></RDF>

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RDF Statement The combination of a Resource, a Property, and a Property value forms a Statement (known as the subject, predicate and object of a Statement).

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ExampleStatement: "The author of http://www.w3schools.com/rdf is Jan Egil Refsnes".

Subject: http://www.w3schools.com/rdf Predicate: author Object: Jan Egil Refsnes

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RDF Graph The underlying structure of any

expression in RDF is a collection of triples, each consisting of a subject, a predicate and an object. A set of such triples is called an RDF graph

The direction of the arc is significant: it always points toward the object.

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RDF Graph Example

http://www.w3schools.c

om/rdf

Jan Egil Refsnes

author

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RDF Graph

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Use of RDF to Describe This Book

Using a graph to represent metadata

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Metadata graph with URI references

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Fragment of an RDF