introduction to the semantic webeol/ssiim/1112/alberto... · introduction to the semantic web 2....

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Introduction to the Semantic Web

Alberto Fernández University Rey Juan Carlos [email protected]

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Profile: •  Public University •  Founded in 1997 •  >29000 students •  4 campus in greater Madrid

Fuenlabrada

Móstoles

Alcorcón

Vicálvaro Medicine & Health

Sciences

Polytechnic

Information and Communication

Sciences

Law & Social Sciences

University Rey Juan Carlos

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URJC: Computer Science Studies

•  Degrees –  Degree in Informatics Engineering –  Degree in Hardware Engineering –  Degree in Software Engineering –  Double Degree in Informatics Engineering + Software Engineering –  Double Degree in Informatics + Business Administration –  Double Degree in Informatics Engineering + Mathematics –  Double Degree in Software Engineering + Mathematics

•  Master/Doctorate –  Advanced Hardware and Software Systems –  Graphics, Games and Virtual Reality –  Interactive Informatics and Multimedia –  Information Systems Engineering –  Telematics and Informatics Systems –  Computer Vision

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1.  Introduction to the Semantic Web 2. Ontology languages 3. Querying the Semantic Web 4. Linked Data

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The Semantic Web

•  What is the Web? –  HTTP (how to transfer data)

GET /index.htm –  URI (how to address data)

http://www.ia.urjc.es/ –  HTML (how to present data to humans)

<html> <head> <title>Artificial Intelligence Group</title> …

•  The problem with the Web Millions of different documents online. Problems: –  How to find the right documents? –  How to extract relevant information (from those documents)? –  How to combine information from different sources?

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How to find the right documents?

What is the publications page of Tim Berners-Lee?

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How to extract relevant information?

What is a book about the Web?

What is the price?

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How to combine information?

I want the cheapest offer of ‘‘A Semantic Web Primer“

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How to combine information?

I want the cheapest offer of “A Semantic Web Primer“ including shipping costs Several clicks to get the shipping costs

Alberto Fernandez Universidad Rey Juan Carlos Tulipan s/n Mostoles 28933. Spain

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The Semantic Web

•  Solution –  Instead of natural language … –  … publish machine-understandable information –  Make queries in terms machine-understandable

•  How? –  Representing knowledge using standardised Ontologíes

•  Semantic Web: “the idea of having data on the Web defined and linked in a way that it can be used by machines not just for display purposes, but for automation, integration and reuse of data across various applications”

[W3C Semantic Web Activity (http://www.w3.org/2001/sw/)]

Ingeniería del Conocimiento 4º Ingeniería Informática

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Semantic Web Layered Architecture

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1.  Introduction to the Semantic Web 2. Ontology languages

1.  RDF 2.  RDF Schema 3.  OWL

3. Querying the Semantic Web 4. Linked Data

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RDF

•  Resource Description Framework •  W3C Recommentation

–  http://www.w3.org/RDF/ –  http://www.w3.org/TR/rdf-primer/

•  Data model: semantic net •  Sentence = triple (Subject, Predicate, Object)

–  Subject: resource (URI) or “blank node” –  Predicate/Property: binary relation (URI) –  Object: URI, literal or “blank node”

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RDF: Example

<http://www.example.org/index.html> <http://purl.org/dc/elements/1.1/creator> <http://www.example.org/staffid/85740> . <http://www.example.org/index.html> <http://www.example.org/terms/creation-date> "August 16, 1999" . <http://www.example.org/index.html> <http://purl.org/dc/elements/1.1/language> "en" . more compact (@prefix ex: <http://www.example.org/>.):

ex:index.html dc:creator exstaff:85740 . ex:index.html exterms:creation-date "August 16, 1999" . ex:index.html dc:language "en" .

Notation: Turtle

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RDF: XML syntax. Example

<?xml version="1.0"?> <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:exterms="http://www.example.org/terms/"> <rdf:Description rdf:about="http://www.example.org/index.html"> <exterms:creation-date>August 16, 1999</exterms:creation-date> </rdf:Description> <rdf:Description rdf:about="http://www.example.org/index.html"> <dc:language>en</dc:language> </rdf:Description> <rdf:Description rdf:about="http://www.example.org/index.html"> <dc:creator rdf:resource="http://www.example.org/staffid/85740"/> </rdf:Description> </rdf:RDF>

<rdf:Description rdf:about="http://www.example.org/index.html"> <exterms:creation-date>August 16, 1999</exterms:creation-date> <dc:language>en</dc:language> <dc:creator rdf:resource="http://www.example.org/staffid/85740"/> </rdf:Description>

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RDF: Blank Nodes

•  Nodes without URI (no needed) •  Anonymous resources •  Independent among each other

exstaff:85740 exterms:address _:johnaddress . _:johnaddress exterms:street "1501 Grant Avenue" . _:johnaddress exterms:city "Bedford" . _:johnaddress exterms:state "Massachusetts" . _:johnaddress exterms:postalCode "01730" .

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RDF: Literals

•  Data Types –  The only predefined data types is rdf:XMLLiteral –  Recommendation: XML Schema datatypes

(xsd=http://www.w3.org/2001/XMLSchema#): •  xsd:string, xsd:integer, xsd:date,…

•  Example ex:index.html exterms:creation-date "1999-08-16"^^xsd:date .

<creation-date rdf:datatype="http://www.w3.org/2001/XMLSchema#date"> 1999-08-16 </creation-date> <title rdf:datatype="http://www.w3.org/2001/XMLSchema#string">

RDF primer </title>

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RDF: Containers and Collections

•  Containers –  rdf:Bag: unordered. May contain duplicates. –  rdf:Seq: ordered. May contain duplicates. –  rdf:Alt: set of alternatives –  Note: containers are “open”

•  Collections –  “Closed” collections –  rdf:List, rdf:first ,rdf:rest, rdf:nil

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RDF: Reification

•  Mechanism to turn a statement into a resource •  So we can make statements about other statements •  Vocabulary

–  Sentence: rdf:Statement –  rdf:subject, rdf:predicate, rdf:object

•  Example: (ex:index.html dc:creator exstaff:85740 .) ex:triple1 rdf:type rdf:Statement . ex:triple1 rdf:subject ex:index.html . ex:triple1 rdf:predicate dc:creator . ex:triple1 rdf:object exstaff:85740 .

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RDF Schema (RDFS)

•  In RDF we talk about individual objects (resources) •  We would like to reason about classes that define

typos of objects –  For instance, to avoid sentences like (allowed in RDF):

•  SSIM is taught by SSIM (range restriction) •  Porto F.C. is taught by John (domain restriction)

•  Solution –  Classes, relations, domain and range restrictions, … –  Example:

•  Courses must be taught by academic staff members only

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RDF Schema (RDFS)

•  W3C Recommendation –  http://www.w3.org/TR/rdf-schema/

•  RDFS extends RDF con new primitives •  Defines a basic language for describing ontologies

–  Fixing the semantics of “subclass of” –  Classes and Properties hierarchies –  Inheritance –  Domain and Range restrictions

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RDF Schema (RDFS)

•  Clases (rdfs:Class) e Instancias (rdf:type) Class definition

<rdf:Description rdf:ID=“Course"> <rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/> </rdf:Description>

Or <rdfs:Class rdf:ID="Course"/>

Instances <Asignatura rdf:ID="SSIM "/>

•  Class hierarchies (rdfs:subClassOf) <rdfs:Class rdf:ID=”Professor"> <rdfs:subClassOf rdf:resource="#AcademisStaff"/> </rdfs:Class>

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RDF Schema (RDFS)

•  Properties (rdf:Property) <rdf:Property rdf:ID=”taught_by“/>

•  Property restrictions (rdfs:domain, rdfs:range) <rdf:Property rdf:ID="taught_by">

<rdfs:domain rdf:resource="#Course"/> <rdfs:range rdf:resource="#AcademicStaff"/>

</rdf:Property>

•  Property hierachies (rdfs:subPropertyOf) <rdf:Property rdf:ID=”taught_by">

<rdfs:domain rdf:resource="#Course"/> <rdfs:range rdf:resource="#AcademicStaff"/> <rdfs:subPropertyOf rdf:resource="#involves"/>

</rdf:Property>

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RDF Schema (RDFS)

taught_by Academic Staff

Assistant Professor

Professor

Course

property

Class Staff involves

domain

domain

range

range

subClassOf

subClassOf subClassOf

SSIM Eugenio Oliveira

type

type type

RDF Schema

RDF

subPropertyOf

taught_by

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RDF Schema (RDFS)

•  Primitives –  Classes

•  rdfs:Resource, rdfs:Literal, rdf:XMLLiteral, rdfs:Class, rdf:Property, rdfs:Datatype

–  Properties •  rdf:type, rdfs:subclassOf, rdf:subPropertyOf, rdfs:domain,

rdfs:range, rdfs:label, rdfs:comment –  Containers

•  rdfs:Container, rdf:Bag, rdf:Seq, rdf:Alt, rdfs:ContainerMembershipProperty, rdfs:member

–  Collections •  rdf:List, rdf:first, rdf:rest, rdf:nil

–  Reification •  rdf:Statement, rdf:predicate, rdf:subject, rdf:object

–  Others •  rdfs:seeAlso, rdfs:isDefinedBy, rdf:value

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RDF Schema (RDFS)

owns

Fish

Person

domain range

Wendy Wanda

type

type

owns

Marmaid?

Restriction violation: the range of owns is Fish.

OR There is not inconsistency: Wanda is a fish!

type

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RDFS

•  RDFS limitations –  It basically allows organise vocabularies in hierarchies –  Local scope of properties

•  Range restrictions cannot be applied to some classes only –  It cannot be expressed:

•  Disjoint classes –  Example: male y female

•  Boolean combinations of classes –  Example: Person = Man ∪ Woman

•  Cardinality restrictions •  Special characteristics of properties

–  Transitive, symmetric, inverse of, …

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OWL

•  Web Ontology Language •  W3C Recommendation

–  http://www.w3.org/2004/OWL/ –  http://www.w3.org/TR/owl-guide/

•  Language for describing ontologies •  Extends RDFS, adding primitives to augment

expressivity

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OWL

•  Heading –  owl:Ontology –  owl:imports: URI ontología –  owl:versionInfo –  owl:priorVersion: URI ontología –  owl:backwardCompatibleWith –  owl:incompatibleWith: it is not backward compatible –  owl:DeprecatedClass –  owl:DeprecatedProperty <owl:Ontology rdf:about=""> <owl:versionInfo>v 1.1</owl:versionInfo> <rdfs:comment>An example ontology</rdfs:comment> <owl:imports rdf:resource="http://www.example.org/foo"/> <owl:backwardCompatibleWith rdf:resource="http://www.example.org/vehicle-1.0"/> <owl:priorVersion rdf:resource="http://www.example.org/veh-1.0"/> </owl:Ontology>

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•  Classes –  owl:Class

•  owl:Class is subclass of rdfs:Class <owl:Class rdf:ID=”Course"/>

–  rdfs:subClassOf (v) –  owl:Thing: > (superclass of all classes) –  owl:Nothing: ? (subclass of all classes)

OWL

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•  Properties –  owl:ObjectProperty –  owl:DatatypeProperty –  rdfs:subPropertyOf (r1 v r2 in H)

–  rdfs:domain (9r.> v C or > v 8r –.C) –  rdfs:range (> v 8r –.C)

OWL

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•  Property characteristics –  owl:TransitiveProperty: (r+)

•  p(x,y) ∧ p(y,z) → p(x,z) (ej: “ancestor of”) –  owl:SymmetricProperty: (r ´ r –)

•  p(x,y) ↔ p(y,x) (ej: “relative of”) –  owl:FunctionalProperty: (> v ≤ 1 r)

•  p(x,y) ∧ p(x,z) → y = z (ej: “birth year”) –  owl:inverseOf: (r –)

•  p1(x,y) ↔ p2(y,x) (ej: p1=“teacher of”, p2=“student of”) –  owl:InverseFunctionalProperty: (> v ≤ 1 r -)

•  p(y,x) ∧ p(z,x) → y = z (ej: “mobile number”)

OWL

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•  Property restrictions –  owl:Restriction –  owl:onProperty –  owl:allValuesFrom: (8r.C)

<owl:Class rdf:ID=”Cow"> <rdfs:subClassOf rdf:resource="#Animal"/> <rdfs:subClassOf>

<owl:Restriction> <owl:onProperty rdf:resource="#eats" /> <owl:allValuesFrom rdf:resource="#Plant" /> </owl:Restriction> </rdfs:subClassOf> </owl:Class>

–  owl:someValuesFrom: (9r.C) –  owl:cardinality –  owl:maxCardinality (≤ n r) –  owl:minCardinality (≥ n r) –  owl:hasValue (9r.{a})

OWL

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•  Complex classes –  owl:intersectionOf: (C u D) –  owl:unionOf: (C t D) –  owl:complementOf: (¬C) –  owl:disjointWith: (C v ¬D) disjoint classes –  owl:oneOf: ({a,b,c} en O) enumerates individuals that belong to

a class. Example: {Red, Amber, Green}. <owl:Class rdf:ID=“TrafficLightColor">

<owl:oneOf rdf:parseType="Collection">

<Color rdf:about="#Red"/>

<Color rdf:about="#Amber"/>

<Color rdf:about="#Green"/>

</owl:oneOf>

</owl:Class>

OWL

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•  Ontology mapping –  owl:equivalentClass (´) –  owl:equivalentProperty (´ in H) –  owl:sameAs: ({x}´{x}) two URIs represent the same individual –  owl:differentFrom: ({x}´¬{x}) two URIs do not represent the

same individual –  owl:AllDifferent –  owl:distinctMembers

<owl:AllDifferent>

<owl:distinctMembers rdf:parseType="Collection">

<Color rdf:about="#Red"/>

<Color rdf:about="#Ambar"/>

<Color rdf:about="#Green"/>

</owl:distinctMembers>

</owl:AllDifferent>

OWL

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•  Annotations –  rdfs:label –  rdfs:comment –  rdfs:seeAlso –  rdfs:isDefinedBy –  owl:AnnotationProperty –  owl:OntologyProperty

OWL

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OWL: inference examples

•  disjointWith example Professor owl:subClassOf AcademicStaff. Book owl:subClassOf Publication . AcademicStaff owl:disjointWith Publication . Inferred: Professor and Book are disjoints

•  owl:equivalentClass example Man owl:subClassOf Person Person owl:equivalentClass Human Inferred: Man is a subclass of Human

•  Instances example “A Semantic Web Primer” rdf:type Book Book owl:subClassOf Publication Inferred: “A Semantic Web Primer” is a Publication

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SPARQL

•  Query Language for RDF •  W3C Recommendation

–  http://www.w3.org/TR/rdf-sparql-query/

•  Query language of RDF contents •  SQL-like sintax

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SPARQL

•  Basic patterns PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>

SELECT ?name ?sur WHERE { ?x :name ?nom. ?x :surname ?ape. ?x rdf:type :Employee. }

•  Group Patterns SELECT ?name ?sur WHERE { { ?x :name ?nom. ?x :surname ?ape. } {?x rdf:type :Employee.} }

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SPARQL

•  Optional patterns (OPTIONAL) SELECT ?name ?sur ?y WHERE { ?x :name ?name. ?x :surname ?sur. ?x rdf:type :Employee. OPTIONAL {?x :birth_year ?y} }

•  Alternative patterns(UNION) SELECT ?name ?y WHERE { {?x :name ?name} UNION {?x :surname ?sur}. ?x rdf:type :Employee. ?x :birth_year ?a. }

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SPARQL

•  Restrictions (FILTER) SELECT ?name ?sur WHERE { ?x :name ?name. ?x :surname ?sur. ?x rdf:type :Employee. ?x :birth_year ?y. FILTER (?y >= “1980").}

•  Result formats –  SELECT –  CONSTRUCT: generates an RDF graph –  ASK: indicates whether a query pattern matches or not –  DESCRIBE: returns RDF graph that describes the resources

found

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SPARQL

•  Result modifiers –  ORDER BY

SELECT ?name ?sur WHERE { ?x :name ?name. ?x :surname?sur. } ORDER BY ?sur

–  DISTINCT: avoid duplicates SELECT DISTINCT ?name ?sur

–  OFFSET / LIMIT SELECT ?name ?sur WHERE { ?x :name ?name. ?x :surname ?sur.} ORDER BY ?sur LIMIT 5 OFFSET 3

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Web of Data (Linked Data)

•  We want to query information like: All soccer players, who played as goalkeeper for a club that has a stadium with more than 40.000 seats and who are born in a country with more than 10 million inhabitants

•  From the Web of Documents to the Web of Data •  Linked Data

–  http://linkeddata.org/ –  http://www.w3.org/DesignIssues/LinkedData.html –  http://www.ted.com/talks/tim_berners_lee_on_the_next_web.html (vídeo 16 min)

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Web of Data (Linked Data)

•  Principles of Linked Data (Tim Berners-Lee) 1.  Use URIs as names for things 2.  Use HTTP URIs so that people can look up those names. 3.  When someone looks up a URI, provide useful information,

using the standards (RDF*, SPARQL) 4.  Include links to other URIs so that they can discover more

things.

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The Linking Open Data cloud diagram

“Linking Open Data cloud diagram, by Richard Cyganiak and Anja Jentzsch. http://lod-cloud.net/”

May 2007 12 datasets

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Sept 2008 45 datasets

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July 2009 95 datasets

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Sept. 2009 295 datasets

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Linked Data

•  Data Generation –  From existing repositories, e.g. Relational DB, XML, CSV

and spreadsheets, etc. to Linked Data –  Transforming information

•  R2O and ODEMapster •  OBDI •  NOR2O •  Jena •  geometry2rdf

–  Dynamic generation •  D2RQ Platform •  Triplify •  Ultrawrap

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Linked Data

•  Data Publication –  Virtuoso Open Source Edition –  D2R Server –  AllegroGraph RDFStore –  Joseki –  Sesame

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Linked Data

•  Data visualisation –  Pubby –  SNORQL –  Disco – Hyperdata Browser

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Linked Data: data integration

[http://www.w3.org/People/Ivan/CorePresentations/IntroThroughExample/]

[http://www.w3.org/People/Ivan/CorePresentations/IntroThroughExample/]

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Linked Data

•  Some existing Linked Data data sources –  Dbpedia: http://dbpedia.org/sparql –  UK Government: http://data.gov.uk/sparql –  USA Government: http://semantic.data.gov/sparql –  Musicbrainz: http://dbtune.org/musicbrainz/sparql –  ...

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Example: DBPedia

•  http://dbpedia.org •  Makes available information from Wikipedia •  Data generation

–  from structured information extracted from Wikipedia

•  Data publication –  OpenLink Virtuoso –  Public SPARQL endpoint: http://DBpedia.org/sparql

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Example: DBPedia

•  Data visualisation •  Leipzig query builder

–  http://querybuilder.dbpedia.org •  OpenLink Interactive SPARQL Query Builder (iSPARQL)

–  http://dbpedia.org/isparql; •  SNORQL query explorer

–  http://DBpedia.org/snorql •  any other SPARQL-aware client(s)

•  Other tools –  DBpedia Faceted Search

•  http://wiki.dbpedia.org/FacetedSearch •  Browser: http://dbpedia.neofonie.de/browse/

•  Resource example: –  http://wikipedia.org/wiki/Berlin à http://dbpedia.org/resource/Berlin