SEMANTIC WEB TECHNOLOGIES Instructor Lecture Date

: Amna Basharat Haider :6 : 3rd February, 2009

A LITTLE RECAP!

FAST-NU, Islamabad

Fall 2008 - Lecture 1

Instructor: Amna Basharat Haider

Semantic Web Technologies

Problems with current search engines • Current search engines = keywords: ▫ high recall, low precision ▫ sensitive to vocabulary ▫ insensitive to implicit content

Semantic Web Technologies

Search engines g on the Semantic Web • concept search instead of keyword search g g of queries q • semantic narrowing/widening • query-answering over >1 document • document d t transformation t f ti operators t

Semantic Web Technologies

Semantic Web Wedding Cake

5

LOOKING BACK ON XML!

Semantic Web Technologies

XML: User definable and domain specific p markup p HTML:

Introduction to AI

• Teacher: Frank van Harmelen
• Students: 1AI, 1I
• Requirements: LI R i t none

XML: Frank van Harmelen <students>1AI, 1I <req>none /

Semantic Web Technologies

XML: document = labelled tree • node = label + attr/values + contents ... ... ... <students>...

course =

titl teacher title t h students t d t name

http p

• XML Schema: g grammars for describing g legal trees and datatypes • So:

why not use XML to represent semantics?

Semantic Web Technologies

Syntax versus Semantics • Syntax: y the structure of your y data • Semantics: the meaning of your data • Two conditions necessary y for interoperability: ▫ Adopt a common syntax: this enables applications to parse the data. ▫ Adopt a means for understanding the semantics: this enables applications to use the data.

Semantic Web Technologies

Drawbacks of XML • XML is a universal metalanguage for defining markup • It provides a uniform framework for interchange of data and metadata between applications • However, XML does not provide any means of talking about the semantics (meaning) of data • E.g., there is no intended meaning associated with the nesting of tags ▫ It is up to each application to interpret the nesting.

Semantic Web Technologies

Nesting of Tags in XML David Billington is a lecturer of Discrete Maths David Billington > Discrete Maths Opposite nesting, same information!

Semantic Web Technologies

XML and Semantics? … • Predator: a medium medium-altitude, altitude, long long-endurance endurance unmanned aerial vehicle system. • Predator : one that victimizes, plunders, or destroys, especially for one's own gain. • Predator : an organism that lives by preying on other organisms. • Predator: a company which specializes in camouflage attire.

Semantic Web Technologies

XML: limitations for semantic markup • XML makes no commitment on: n Domain-specific ontological vocabulary o Ontological modeling primitives

• Requires pre-arranged agreement on n & o • Only O l ffeasible ibl for f closed l d collaboration ll b i ▫ agents in a small & stable community ▫ pages on a small & stable intranet

• Not suited for sharing Web-resources

GOING ON TO RDF NOW!

FAST-NU, Islamabad

Fall 2008 - Lecture 1

Instructor: Amna Basharat Haider

Semantic Web Technologies

RDF basics… • RDF: ▫ is a W3C standard standard, which provides tool to describe Web resources ▫ provides interoperability between applications that exchange machine-understandable information • RDF Schema: ▫ is a W3C standard which defines vocabulary for RDF ▫ organizes this vocabulary in a typed hierarchy ▫ capable to explicitly declare semantic relations between vocabulary terms

Semantic Web Technologies

What is RDF ? • RDF is a data model x the model is domain-neutral, domain neutral application application-neutral neutral and ready for internationalization (i18n) x the model can be viewed as directed, labeled graphs or as an object-oriented model (object/attribute/value)

• RDF data model is an abstract, conceptual layer y independent p of XML x consequently, XML is a transfer syntax for RDF, not a component of RDF

Semantic Web Technologies

RDF model „ RDF “statements” consist of resources ((= nodes)) which have properties which have values (= nodes,strings)

resource

property

= subject j = predicate = object

value

“http://www.w3.org/TR/REC-rdf-syntax/ has the author Ora Lassila”

http://www.w3.org/TR/REC-rdf-syntax/ author

“Ora Lassila”

Semantic Web Technologies

RDF Model Example “W3C” dc:Publisher

h http://www.w3.org/TR/REC-rdf-syntax/ // 3 /TR/REC df / dc:Creator dc:Date

“ “1999-02-22”

“Ora Lassila”

Semantic Web Technologies

RDF – Semantic Web over Web Resources John Director

Ontology

has homepage has_homepage

has_job

to_be_friends _with

has_job Secretary

has_homepage

Mary

BASIC IDEAS OF RDF Detailed!

Semantic Web Technologies

Basic Ideas of RDF • Basic building block: object-attribute-value triple ▫ It is i called ll d a statement t t t ▫ Sentence about Billington is such a statement

• RDF has been given a syntax in XML ▫ This syntax inherits the benefits of XML ▫ Other Oth syntactic t ti representations t ti off RDF possible

Semantic Web Technologies

Basic Ideas of RDF (2) • The fundamental concepts of RDF are: ▫ resources ▫ properties ▫ statements

Semantic Web Technologies

Resources • We can think of a resource as an object, a “thing” thing we want to talk about ▫ E.g. authors, books, publishers, places, people, hotels

• Every resource has a URI, a Universal Resource Identifier • A URI can be b ▫ a URL (Web address) or ▫ some other kind of unique identifier

Semantic Web Technologies

Resources • All things being described by RDF expressions are called resources: ▫ ▫ ▫ ▫

entire Web page; a specific XML element; whole collection of pages; an object that is not directly accessible via the Web.

Semantic Web Technologies

Resources and URIs • A resource can be anything that has identity • Uniform Resource Identifiers (URI)* (URI) provide a simple and extensible means for identifying a resource • Not all resources are network "retrievable"; e.g., human beings, corporations, and books in a library can also be considered resources * The term "Uniform Resource Locator" (URL) refers to the subset b t off URI that th t identify id tif resources via i a representation t ti off their primary access mechanism (e.g., their network "location"), rather than identifying the resource by name or by some other attribute(s) of that resource. resource

Semantic Web Technologies

URI

Venn diagram of Uniform Resource Identifier (URI) scheme categories. g both function as Schemes in the URL ((locator)) and URN ((name)) categories resource IDs, so URL and URN are subsets of URI. They are also, generally, disjoint sets. However, many schemes can't be categorized as strictly one or the other, because all URIs can be treated as names, and some schemes embody y aspects p of both categories g – or neither.

Semantic Web Technologies

Properties • Properties are a special kind of resources • They describe relations between resources ▫ E.g. E g “written written by by”, “age” age , “title” title , etc. etc

• Properties are also identified by URIs • Advantages of using URIs: ▫ Α global, global worldwide, worldwide unique naming scheme ▫ Reduces the homonym problem of distributed data representation

Semantic Web Technologies

Statements • Statements assert the properties of resources • A statement is an object-attribute-value triple ▫ It consists of a resource, a property, and a value

• Values can be resources or literals ▫ Literals are atomic values (strings) ( g )

Semantic Web Technologies

RDF Statement • Subject of an RDF statement is a resource • Predicate of an RDF statement is a property of a resource • Object of an RDF statement is the value of a property of a resource

Semantic Web Technologies

Example of RDF Statement Ora Lassila is the creator of the resource http://www.w3.org/Home/Lassila. Subject (resource)

http://www.w3.org/Home/Lassila

Predicate (property)

Creator

Object (literal)

“Ora Ora Lassila” Lassila

Semantic Web Technologies

RDF Example (serialization syntax) Ora Lassila is the creator of the resource http://www.w3.org/Home/Lassila. <s:Creator>Ora Lassila 's' is a specific namespace prefix, e.g. xmlns:s="http://description.org/schema/"

Semantic Web Technologies

RDF Example (abbreviated syntax) Ora Lassila is the creator of the resource http://www.w3.org/Home/Lassila. < df D