Chapter 9The Semantic Web

By Malak Bagais

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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.

The Semantic Web

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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.

Semantic Web Initiative

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Providing a common syntax for machine understandable statements

Establishing common vocabularies Agreeing on a logical language Using the language for exchanging

proofs

Trend in Semantic Web

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A layered structure for Semantic Web Unicode/URI XML/Name Spaces/ XML Schema RDF/RDF Schema Ontology vocabulary Logic Proof Trust

A layered Structure for the Semantic WEB

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

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

Unicode

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

Example:

Uniform Resource Identifiers (URIs)

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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:IIMFFH05643_2004-08-

03_17-54-01_L06154

Uniform Resource Names (URNs)

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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

Uniform Resource Locator (URL)

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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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Captures data relationships and their associated meaning

Is a specification of a conceptualization

Ontology

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An 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).

Ontology

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Developing 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

Ontology development

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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

Reasons for creating ontologies

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XML 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

Ontology language

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

Related Technologies

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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.

Advantages of XML

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<location name=Innsbruck> <college name= College of Engineering>

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

</location>

XML Example of a College

An XML document college.xml

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Data Model

An ordered labeled tree

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

simple type definitions complex type definitions, attribute element declaration

XML Schema

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Example of an XML schema

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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>

Illustration of XML Communication

Database of books

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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.

XML Query Languages

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XML Document

XML document college.xml

27Tree representation for college.xml

Tree Representation of an XML Document

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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.

XPath Query Language

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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

30 Tree representation for Queries 1 and 2

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Query 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.

XPath Query Language

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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.

XPath Query Language

33Tree representation for Queries 3 and 4

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RDF

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RDF stands for Resource Description Framework

RDF is a framework for describing resources on the web

RDF is designed to be read and understood by computers

What is RDF?

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RDF is not designed for being displayed to people

RDF is written in XML RDF is a part of the W3C's Semantic

Web Activity RDF is a W3C Recommendation

What is RDF?

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<?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>

RDF document

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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

RDF- Examples of use

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

RDF Resource, Property, and Property value

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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.

Explanation of Resource, Property, and Property value

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

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

Example

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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.

RDF Graph

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RDF Graph Example

http://www.w3schools.c

om/rdfJan 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

49Fragment of an RDF