Chapter 5 The Semantic Web 1

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. 2

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. 3

Trend in Semantic Web  Providing a common syntax for machine understandable statements  Establishing common vocabularies  Agreeing on a logical language  Using the language for exchanging proofs 4

A layered Structure for the Semantic WEB A layered structure for Semantic Web  Unicode/URI  XML/Name Spaces/ XML Schema  RDF/RDF Schema  Ontology vocabulary  Logic  Proof  Trust 5

The layered-language model (Courtesy of W3C, 2001) 6

Unicode Unicode is a character set that provides a unique number to every character irrespective of the platform, program, and the language. 7

Generic URI syntax := : Example: Uniform Resource Identifiers (URIs) 8

9 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: : Examples  urn:isbn:  urn:newsml:reuters.com: :IIMFFH05643_ _ _L06154

10 Uniform Resource Locator (URL)  Another special type of URI  used to identify an Internet resource  Difference between URL, URN, and URI:  URL:  URN:  URI:

11 Markup Example of a web page of a Student Service Center

Explicit Metadata 12 Example of a web page of a Student Service Center

Ontology  Captures data relationships and their associated meaning  Is a specification of a conceptualization 13

Ontology 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 ). 14

Ontology development 15 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

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 16

Ontology language 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 17

Related Technologies  Document Type Definition (DTD)  XML Schema  DOM  XPath  XLinks  XPointers  XSLT 18

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. 19

XML Example of a College CIS 20 An XML document college.xml

Data Model An ordered labeled tree 21

XML Schema A language for limiting the structure of XML documents. Constructs in an XML Schema:  simple type definitions  complex type definitions,  attribute  element declaration 22

Example of an XML schema 23

Illustration of XML Communication To maintain record of books by ISBN Hawking’s A brief history of time gives truly intimate glimpses into the intricacies of both the universe and Hawking himself. Database of books 24

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

XML Document XML document college.xml 26

Tree Representation of an XML Document Tree representation for college.xml 27

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. 28

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 29

Tree representation for Queries 1 and 2 30

XPath Query Language Query 3 : Select all courses with the title “Nonlinear Analysis”. 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. 31

XPath Query Language Query 4 : Select all title attribute nodes within course elements anywhere in the document, which have the value “Nonlinear Analysis”.  This expression collects title attribute nodes of the course elements. The denotes the attribute nodes.  The tree representation of queries 3 and 4 is given in next figure. 32

Tree representation for Queries 3 and 4 33

RDF 34

What is RDF?  RDF stands for R esource D escription F ramework  RDF is a framework for describing resources on the web  RDF is designed to be read and understood by computers 35

What is RDF?  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 36

RDF document W3Schools Jan Egil Refsnes 37

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 38

RDF Resource, Property, and Property value RDF identifies things using Web identifiers (URIs), and describes resources with properties and property values. 39

Explanation of Resource, Property, and Property value  A Resource is anything that can have a URI, such as "  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

Example Jan Egil Refsnes 41

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). 42

Example Statement: "The author of is Jan Egil Refsnes".  Subject :  Predicate : author  Object : Jan Egil Refsnes 43

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. 44

RDF Graph Example ols.com/rdf Jan Egil Refsnes author 45

RDF Graph 46

Use of RDF to Describe This Book 47 Using a graph to represent metadata

48 Metadata graph with URI references

49 Fragment of an RDF