THE SEMANTIC WEB
AN INTRODUCTION
LUIGI DE RUSSIS
THE WEB IS A WEB OF DOCUMENT
FOR PEOPLE, NOT FOR MACHINES
THE SEMANTIC WEB IS A WEB OF DATA
Linking Open Data cloud diagram,
by Richard Cyganiak and Anja Jentzsch.
http://lod-cloud.net/
LET’S THINK!
EXERCISE: BUILD A MUSIC CATALOG
Comprehensive guide to music across the world
Web-based
With always-updated information about each artist
HOW?
WHAT ARE THE PROBLEMS?
WHAT ABOUT DATA REPLICATION?
WHAT ABOUT DATA
SYNCHRONIZATION?
SOLUTION #1
HOW?
Site editors roam the Web for new facts and update the site manually
WHAT ARE THE PROBLEMS?
A lot of people need to continuously roam the Web; the site will get
soon out-of-date
WHAT ABOUT DATA?
Data is replicated and not up-to-date with new facts
SOLUTION #2
HOW?
Site editors roam the Web for new data and write a program to
extract the information
WHAT ARE THE PROBLEMS?
Code needs to be updated each time a new site is found; the site will
get out-of-date, soon or later…
WHAT ABOUT DATA?
Data is replicated and not up-to-date
SOLUTION #3
HOW?
Site editors browse the Web for new data via APIs, and write some
code to incorporate the information
WHAT ARE THE PROBLEMS?
Code needs to be updated each time a new site is found and/or an
API is changed; the site will get out-of-date, soon or later…
WHAT ABOUT DATA?
Data is replicated and not up-to-date
SOLUTION #4
HOW?
Site editors choose to use some external, public datasets (e.g.,
Wikipedia, MusicBrainz, …)
WHAT ARE THE PROBLEMS?
WHAT ABOUT DATA?
No problem
Data is immediately available, not as APIs or hidden on a Web site.
Information can be extracted using standard queries or HTTP
requests.
SOLUTION #4
IN SHORT…
Use the Web of Data as a Content Management System
Use the community at large as content editor
AN EXAMPLE: BBC MUSIC
http://www.bbc.co.uk/music
DATA ON THE WEB IS NOT ENOUGH!
we need a proper infrastructure
DATA SHOULD BE AVAILABLE ON THE WEB
accessible via standard Web technologies
DATA SHOULD BE INTERLINKED OVER THE
WEB
i.e., data can be integrated over the Web
FUNDAMENTALS
To a computer, the Web is a flat, boring world, devoid of
meaning. This is a pity, as in fact documents on the Web
describe real objects and imaginary concepts. […]
Adding semantics to the Web involves two things: allowing
documents which have information in machine-readable
forms, and allowing links to be created with relationship
values. Only when we have this extra level of semantics we
will be able to use computer power to help us exploit the
information to a greater extent than our own reading.
TIM BERNERS-LEE,
WHAT IS THE RELATIONSHIP WITH AI?
INFLUENCE
Some technologies in the Semantic Web benefited a lot from AI
research and development (and viceversa)
DIFFERENT GOALS
Artificial Intelligence approach: build smarter machines, teach
computers to infer the meaning of data
Semantic Web approach: have smarter data, make data easier for
machines to find, access and process
RESOURCE AND DESCRIPTION
RESOURCE
every document “reachable” on the Web
no matter the content, format, language, etc.
RESOURCE DESCRIPTION
independent from the format
standard language (metadata)
RESOURCE AND DESCRIPTION
Resources
RESOURCE AND DESCRIPTION
Descriptio
n
RESOURCE AND DESCRIPTION
Title
Author
Date
Topic
Quality
Descriptio
n
Title
Date
Author
Topic
URIS
unambiguous names for resources
RDF
a common data model to connect and describe resources
SPARQL
access to the data model
OWL, RIF
reasoning (mainly logic inference)
RDFS, OWL
common description languages
MODELING DATA
EXAMPLE: BOOKSTORE
Represent the following data about the AI book as a set of relations
Title: “Artificial Intelligence: A Modern
Approach”
Author: Russel, Stuart and Norvig, Peter
Publisher: Prentice Hall
ISBN: 978-0136042594
EXAMPLE: BOOKSTORE
http://...isbn/9780136042594
Resource
EXAMPLE: BOOKSTORE
Artificial Intelligence: A Modern
Approach
http://...isbn/9780136042594
Literal
EXAMPLE: BOOKSTORE
http://...isbn/9780136042594
title
Artificial Intelligence: A Modern
Approach
EXAMPLE: BOOKSTORE
title
http://...isbn/9780136042594
Artificial Intelligence: A Modern
Approach
publisher
author
Prentice Hall
author
Norvig, Peter
Russel, Stuart
RDF: RESOURCE DESCRIPTION
FRAMEWORK
STRUCTURED IN STATEMENTS
PREDICATE
a verb, property or relationship
SUBJECT
a resource (URI)
OBJECT
a resource or a literal string
EXAMPLE: BOOKSTORE
Subject
Predicate
title
http://...isbn/9780136042594
Predicate
Object
Artificial Intelligence: A Modern
Approach
Predicate
publisher
author
Object
Prentice Hall
Object
author
Norvig, Peter
Object
Russel, Stuart
EXAMPLE: BOOKSTORE
http://...isbn/9780136042594
title
Artificial Intelligence: A Modern
Approach
RDF IN XML SYNTAX
<rdf:RDF xmlns:rdf=http://www.w3.org/…/22-rdf-syntax-ns#>
<rdf:Description about=“http://... isbn/9780136042594”>
<title>Artificial Intelligence: A Modern Approach</title>
</rdf:Description>
</RDF>
EXAMPLE: BOOKSTORE
http://...isbn/9780136042594
title
Artificial Intelligence: A Modern
Approach
RDF IN TURTLE
<http://... isbn/9780136042594>
title “Artificial Intelligence: A Modern Approach”
LINKIN’ DATA
EXAMPLE: BOOKSTORE
Represent the following data about the Italian translation of the AI
book as a set of relations
Title: “Intelligenza artificiale. Un approccio
moderno”
Author: Russel, Stuart and Norvig, Peter
Publisher: Prentice Hall
ISBN: 978-8871925936
Original ISBN: 978-0136042594
EXAMPLE: BOOKSTORE
title
http://...isbn/9788871925936
Intelligenza Artificiale. Un
approccio moderno
publisher
original
Prentice Hall
http://...isbn/9780136042594
creator
creator
Russel, Stuart
Norvig, Peter
EXAMPLE: BOOKSTORE
http://...isbn/9788871925936
title
Intelligenza Artificiale. Un
approccio moderno
publisher
original
Prentice Hall
http://...isbn/9780136042594
http://...isbn/9780136042594
title
publisher
Artificial Intelligence: A Modern
Approach
Prentice Hall
EXAMPLE: BOOKSTORE
title
http://...isbn/9788871925936
Intelligenza Artificiale. Un
approccio moderno
publisher
original
Prentice Hall
http://...isbn/9780136042594
same URI, same resource
title
http://...isbn/9780136042594
creator
Artificial Intelligence: A Modern
Approach
Prentice Hall
EXAMPLE: BOOKSTORE
Intelligenza Artificiale. Un
approccio moderno
title
http://...isbn/9788871925936
publisher
Prentice Hall
original
http://...isbn/9780136042594
author
Russel, Stuart
author
creator
creator
title
publisher
Artificial Intelligence: A Modern
Approach
Prentice Hall
Norvig, Peter
Russel, Stuart
Norvig, Peter
EXAMPLE: BOOKSTORE
Intelligenza Artificiale. Un
approccio moderno
title
http://...isbn/9788871925936
publisher
Prentice Hall
original merging creator and author?
What about
In
RDF,
it
is
not
possible!
title
http://...isbn/9780136042594
author
Russel, Stuart
author
creator
creator
publisher
Artificial Intelligence: A Modern
Approach
Prentice Hall
Norvig, Peter
Russel, Stuart
Norvig, Peter
PROBLEM: FIELD NAMES ARE ARBITRARY
Synonyms : author or creator or maker or contributor or…
Singular or plural: author or authors
SOLUTION: STANDARDS
general or domain-specific
DUBLIN CORE
GENERAL VOCABULARY
Dublin Core Metadata Initiative (DCMI)
http://dublincore.org
BUILDING BLOCKS TO DEFINE METADATA
FOR THE SEMANTIC WEB
Define title, contributor, publisher, license, date, language, etc.
PROBLEM: FIELD VALUES ARE ARBITRARY
Value type: string, date, integer, …
Value format: “Norvig, Peter” or “Norvig, P.” or “Peter Norvig” or…
Value restrictions: one value or multiple values (how many?)
SOLUTIONS
Standards
Controlled vocabulary (close list of terms)
Semantically rich descriptions to support search (RDFS and/or OWL)
FRIEND OF A FRIEND (FOAF)
GENERAL ONTOLOGY
Describe persons, their activities and their relations to other people
and objects
http://www.foaf-project.org
BUILDING BLOCKS TO DEFINE STRUCTURED
RELATIONS BETWEEN PEOPLE
Define name, familyName, givenName, knows, age, nick, etc.
EXAMPLE: BOOKSTORE
dc:title
http://...isbn/9780136042594
dc:creator
dc:publisher
dc:creator
foaf:name
Prentice Hall
foaf:name
Norvig, Peter
Artificial Intelligence: A Modern
Approach
foaf:name
Russel, Stuart
foaf: http://xmlns.com/foaf/spec
dc: http://purl.org/dc/terms
WHY?
http://...isbn/9780136042594
http://...isbn/9780136042594
dc:creator
author
Norvig, Peter
foaf:name
Norvig, Peter
RDF SCHEMA
RDF SCHEMA
SCHEMA
Definition of the nodes and predicates used in a RDF document
DOMAIN AND RANGE
RDFS describes properties in terms of classes of resource
to which they apply (from a “domain” to a “range”)
EXAMPLE
RDF data
http://elite.polito.it/people/derussis
teaches
http.//bit.ly/lingambmult
EXAMPLE
Person
subClassOf
domain
Teacher
range
teaches
type
http://elite.polito.it/people/derussis
Course
type
teaches
RDF schema
RDF data
http.//bit.ly/lingambmult
BACK TO THE BOOKSTORE EXAMPLE…
http://...isbn/9780136042594
http://...isbn/9780136042594
dc:creator
author
Norvig, Peter
foaf:name
Norvig, Peter
BACK TO THE BOOKSTORE EXAMPLE…
http://...isbn/9780136042594
http://...isbn/9780136042594
dc:creator
author
Norvig, Peter
anonymous
class
foaf:name
Norvig, Peter
dc:creator has range Agent, i.e. a class (resource), not a literal:
we use an anonymous class for this scope.
Finally, foaf:Name has range rdfs:Literal.
RDFS EXPRESSIVITY
SIMPLE RELATIONSHIP BETWEEN THINGS
RDFS provides a vocabulary to express relationship between things
(e.g., subClassOf or type)
AVOID COMPLEX RELATIONSHIP
RDFS cannot describe data in terms of set of operations (e.g.,
unionOf), equivalence (e.g., sameAs) or cardinality (e.g.,
allValueFrom)
OWL
WEB ONTOLOGY LANGUAGE
WHAT?
OWL (version 2): a knowledge representation language
Designed to formulate, exchange and reason with knowledge about
a domain of interest
WEB ONTOLOGY LANGUAGE
INDIVIDUALS, CLASSES AND PROPERTIES
“Politecnico di Torino is a university”
“Politecnico di Torino has a professor named Elio Piccolo”
“Politecnico di Torino” is a object: an individual in OWL2
“university” is a category: a class in OWL2
“has a professor” is a relation: a property in OWL2
“Elio Piccolo” is an individual, too
WEB ONTOLOGY LANGUAGE
EXPRESSIVITY
Designed to represent rich and complex knowledge about things,
group of things, and their relations
LOGIC-BASED
Knowledge expressed in OWL can be reasoned with a computer
program to verify its consistency or to make implicit knowledge
explicit
WEB ONTOLOGY LANGUAGE
LINKED
Ontologies in OWL can be published on the Web and may refer or be
referred from other OWL ontologies
CHOOSE THE SYNTAX YOU LIKE
Various syntaxes available for OWL, for different purposes
(RDF/XML, Turtle, Manchester, etc.)
EXAMPLE: BOOKSTORE
dc:title
http://...isbn/9788871925936
rdfs:type
dc:publisher
Libro
Intelligenza Artificiale. Un
approccio moderno
Prentice Hall
owl:sameAs
Book
rdf:type
dc:title
Artificial Intelligence: A Modern
Approach
http://...isbn/9780136042594
dc:publisher
Prentice Hall
HANDS ON OWL
EXAMPLE: BOOKSTORE
It is time to sell the books we modeled.
Users must have the possibility to search in our book catalog.
We need to describe our store
and add some other information about the books.
GoodRelations helps in realizing such an example:
http://www.heppnetz.de/projects/goodrelations/
“EUR”
gr:hasCurrency
120.0
1.0
gr:hasCurrencyValue
gr:amountOfThisGood
gr:Sell
UnitPriceSpecification_1
gr:hasBusinessFunction
TypeAndQuantity
Node_1
gr:hasPriceSpecification
Bookstore_1
gr:typeOfGood
Offering_1
gr:offers
gr:ActualProductOrServiceInstance
AIBook_en
rdf:type
rdf:type
gr:Offering
rdf:type
gr:includeObject
item:hasTotalPages
QuantitativeValue
Integer_1
gr:hasValue
1132
item:Book
rdfs:subClassOf
gr:ProductOrService
DESCRIBE THE BUSINESS ENTITY
default:BookStore_1
a gr:BusinessEntity ;
gr:legalName “bookstore.com Ltd.”^^xsd:string .
Bookstore_1
DESCRIBE THE OFFERED ITEMS
AIBook_en
default:AIBook_en
a item:Book, gr:ActualProductOrServiceInstance ;
item:hasTotalPages default:QuantitativeValueInteger_1 .
default:QuantitativeValueInteger_1
a gr:QuantitativeValueInteger ;
gr:hasValue “1132”^^xsd:integer .
QuantitativeValue
Integer_1
DESCRIBE THE OFFER
Offering_1
default:Offering_1
a gr:Offering ;
gr:hasBusinessFunction gr:Sell ;
gr:hasPriceSpecification default:UnityPriceSpecification_1 ;
gr:includeObject default:TypeAndQuantityNode_1 .
LINK THE OFFER TO THE BUSINESS
ENTITY
default:BookStore_1 gr:offers default:Offering_1
DESCRIBE THE OFFER
default:TypeAndQuantityNode_1
a gr:TypeAndQuantityNode ;
gr:amountOfThisGood “1.0”^^xsd:float ;
gr:typeOfGood default:AIBook_en .
TypeAndQuantity
Node_1
default:UnitPriceSpecification_1
a gr:UnitPriceSpecification ;
gr:hasCurrency “EUR”^^xsd:string ;
gr:hasCurrencyValue “120.0”^^xsd:float .
UnitPriceSpecification_1
QUERY THE WHOLE!
How to get all the available offer for the book?
SPARQL
PREFIX gr: <http://purl.org/goodrelations/v1#>
PREFIX item: <http://www.elite.polito.it/ontologies/example/item#>
SELECT ?offering
WHERE { ?offering rdf:type gr:Offering .
?offering gr:includesObject ?object .
?object gr:typeOfGood ?item .
?item rdf:type item:Book .
}
QUERY THE WHOLE!
SPARQL
?item rdf:type item:Book .
?item
rdf:type
item:Book
QUERY THE WHOLE!
SPARQL
?object gr:typeOfGood ?item .
?object
gr:typeOfGood
AIBook_en
rdf:type
item:Book
QUERY THE WHOLE!
SPARQL
?offering gr:includesObject ?object .
TypeAndQuantity
Node_1
gr:typeOfGood
?offering
gr:includeObject
AIBook_en
rdf:type
item:Book
QUERY THE WHOLE!
SPARQL
?offering rdf:type gr:Offering .
TypeAndQuantity
Node_1
gr:typeOfGood
?offering
gr:includeObject
AIBook_en
rdf:type
gr:Offering
rdf:type
item:Book
QUERY THE WHOLE!
SPARQL
SELECT ?offering
TypeAndQuantity
Node_1
gr:typeOfGood
Offering_1
gr:includeObject
AIBook_en
rdf:type
gr:Offering
rdf:type
item:Book
REFERENCES
Semantic Web standards: http://w3c.org/standards/semanticweb
Semantic Web Wiki: http://semanticweb.org
Semantic Web FAQ: http://www.w3c.org/2001/sw/SW-FAQ
Book: A Semantic Web Primer (http://www.semanticwebprimer.org)
Book: Semantic Web Programming
(http://semwebprogramming.org)
Last access: 04 June 2013
THANKS!
Luigi De Russis
http://elite.polito.it
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