Introduction to the Semantic Web

© Copyright 2009 Digital Enterprise Research Institute. All rights reserved.

Digital Enterprise Research Institute www.deri.ie

Introduction to the Semantic Web Alexandre Passant

Digital Enterprise Research Institute, National University of Ireland, Galway

DM110 – Emerging Web Media Week 7 – 02 Nov. 2009


Agenda

  What is the Semantic Web ? What for ?   From Documents to Data

  URIs and RDF   To identify resources and define statements about these

resources

  Ontologies with RDFS/OWL   Shared semantics to improve interoperability between

applications

  Querying data with SPARQL   To make use of it and create new applications

  NB: The upcoming lectures will cover related topics / subtopics

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The initial Proposal (1989)

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http://www.w3.org/History/1989/proposal.html


… but so far …

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??

??


… however …

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, and give particular relationships between them. For example, a document might describe a person. The title document to a house describes a house and also the ownership relation with a person. 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 will we be able to use computer power to help us exploit the information to a greater extent than our own reading.

Tim Berners-Lee, 1st World Wide Web Conference, Geneva, May 1994

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… so ?

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http://www.slideshare.net/danbri/when-presentation-849447


The Semantic Web is about

  Bridging the gap from a Web of Documents to a Web of Data   With typed objects and typed relationships: The Web as a

giant decentralized database

  Adding machine-readable meta-data to existing content   So that information can be parsed, queried, reused

  Defining shared semantics for this meta-data   For interoperability between applications and for advanced

purposes, such as reasoning

  Enabling machine-readable knowledge at Web scale, making information more easy to find and process

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A Bit of History

  Memex   1945 ! - Vannevar Bush

  A memex is “a device in which an individual stores all his books, records, and communications.”

  Augmenting Human Intellect   1960 - Douglas Engelbart

  “By ‘augmenting human intellect’ we mean increasing the capability of a man to approach a complex problem situation, to gain comprehension to suit his particular needs, and to derive solutions to problems.”

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More recently

  SHOE   http://www.cs.umd.edu/projects/plus/SHOE/

  “SHOE is a small extension to HTML which allows web page authors to annotate their web documents with machine-readable knowledge. SHOE makes real intelligent agent software on the web possible.“

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The Semantic Web, right now

  Most standardisation work is done in the W3C   http://w3.org

  The Semantic Web activity   http://www.w3.org/2001/sw/

  Various Incubator Groups, Working Group, Interest Group   SPARQL - http://www.w3.org/2009/sparql/wiki/Main_Page

  RDB2RDF – http://www.w3.org/2005/Incubator/rdb2rdf

  RIF - http://www.w3.org/2005/rules/

  HCLS - http://www.w3.org/2001/sw/hcls/

  …

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The Semantic Web stack

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http://www.w3.org/2007/03/layerCake.png


URIs

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  A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource as of RFC3986

  URIs are used to identify everything in a unique and non-ambiguous way   Not only pages (as on the current Web), but any resource

(people, documents, books, interests …)

  A URI for a person is different from a URI for a document about the person, because a person is not a document !

  Example   http://apassant.net/alex - myself

  http://apassant.net - my homepage


Content-negociation

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  URI for resource, URI for documents   But documents made for people cannot be read by

computers (the issue with the current Web)

  Content negotiation   Provides a way, for a resource URI to redirect to the

document describing that resource

  Depending on who is accessing it –  Human-readable of machine-readable

  Example   http://dbpedia.org/resource/Galway

  http://dbpedia.org/page/Galway

  http://dbpedia.org/data/Galway


RDF

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  URI represent resources   But how define things about these resources ?

  RDF – Resource Description Framework   RDF abstract syntax, a data model: a directed, labeled

graph based on URIs

  RDF is not XML ! RDF/XML is only one of the multiple way to serialize RDF data (N3, RDFa …)

  RDF is based on triples   <subject> <predicate> <object> .


RDF

@prefix dct: <http://purl.org/dc/terms/> . !

<http://example.org/dm110-semweb>! dct:title “Introduction to the Semantic Web” ; ! dct:author <http://apassant.net/alex> ;! dct:subject <http://dbpedia.org/resource/Semantic_Web> .!

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RDF serializations

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  RDF/XML   The most used, probably the most complex !

  E.g. http://geonames.org/2988507/about.rdf

  N3/Turtle   Easier to read for humans

  E.g. http://dbpedia.org/data/Galway.n3

  RDFa   Embeds RDF in XHTML, one page for humans and

machines

  E.g. http://apassant.net (browse source)

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Ontologies

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  RDF provide a way to write assertions about URIs   But what about the semantic of these assertions

  E.g. how can one know thathttp://xmlns.com/foaf/0.1/knows identifies an acquaintance relationship ?

  Ontologies provide common semantics for resources on the Semantic Web   “An ontology is a specification of a conceptualization.”

  Developing ontologies for the Semantic Web   Main languages are RDFS (RDF Schema) and OWL (Web

Ontology Language)

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Ontologies

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  Classes and properties   :Person a rdfs:Class .

  :father a rdfs:Property .

  :father rdfs:domain :Person .

  :father rdfs:range :Person .

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RDFS

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  RDFS defines classes, properties and subsumption relations between classes and properties   ex:Person rdfs:subClassOf ex:humanLiving .

  ex:worksWith rdfs:subPropertyOf ex:knows .

  Such relationships are used to infer new statements   :alex rdf:type ex:Person .

  :Alex ex:worksWith :Axel .

  Is enough to say that Alex is a humanLiving and knows Axel

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OWL

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  OWL goes further than RDFS by introducing new axioms   Disjunction (e.g. person / document)

  Transitivity (e.g. ancestor)

  Symmetry (e.g. sibling)

  Cardinality constraints (e.g. ancestor > 1)

  OWL2 has just been standardized W3C and introduces a lot of useful features, especially for reasoning   Property Chains

  parent + brother -> uncle

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OWL2 Property chain example

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ex:uncle rdf:type owl:ObjectProperty .

ex:parent rdf:type owl:ObjectProperty .

ex:brother rdf:type owl:ObjectProperty .

[] rdfs:subPropertyOf ex:uncle;

owl:propertyChain (

ex:parent

ex:brother

).

:alice ex:parent :bob .

:bob ex:brother :joe .

=>

:alice ex:uncle :joe .

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Notable ontologies

  Social networks and social data   FOAF – Friend Of A Friend

  SIOC – Semantically-Interlinked Online Communities

  Software development   DOAP – Description Of A Project

  BEATLE - Bug And Enhancement Tracking LanguagE

  Comprehensive / Top-level   Yago (From Wikipedia)

  OpenCYC

  Taxonomies   SKOS – Simple Knowledge Organisation System

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Zooming in: FOAF Ontology

  A model to describe people and social networks   http://foaf-project.org

  Concepts   Person, OnlineAccount, Document, etc.

  Properties   name, homepage, holdsAccount, knows, etc.

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FOAF in use

  Google Social Graph API   http://code.google.com/intl/fr/apis/socialgraph/

  Uses FOAF information already there on the Web to find your contacts   http://socialgraph-resources.googlecode.com/svn/trunk/

samples/findcontacts.html

  E.g.: http://apassant.net –  http://socialgraph-resources.googlecode.com/svn/trunk/

samples/findcontacts.html?q=http%3A%2F%2Fapassant.net –  Contacts found in various FOAF files that link to myself and to

my profile

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Which ontologies to use ?

  SearchMonkey Vocabularies   http://developer.yahoo.com/searchmonkey/smguide/

profile_vocab.html

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Which ontologies to use ?

  How to Publish Linked Data on the Web   http://www4.wiwiss.fu-berlin.de/bizer/pub/

LinkedDataTutorial/

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Extending ontologies ?

  What if existing ontologies are not enough for your needs ?   Create a new ontology

  … or extend an existing one !

  Ontologies can be extended in a decentralized way   E.g. you can create a subproperty of foaf:knows,

“hasLecturer”, in your own ontology and publish it online

  Open.vocab.org   A collaborative platform to manage ontologies

  http://open.vocab.org

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Warning: Domain and range

  Domain and range of properties in ontologies are descriptive, not prescriptive   :father rdfs:domain :Person

–  Not only pre-defined Persons can be fathers

–  But every father is a Person !

  Consequence 1: One triple is enough to describe several informations

  Consequence 2: Don’t use foaf:homepage for a shoe !

  For details   Based on RDF semantics (Rule rdfs2)

  http://www.w3.org/TR/rdf-mt/

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Warning: Open World

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  The Open World Assumption   Might be complex to understand when coming from a

RDBMS or OOP background

  If a fact is not there, it does not means it is false   Bob’s father is Paul. Is Jim Paul’s father ?

–  Cannot be answered unless usin cardinality constraints in the ontology (in OWL), e.g. a Person has only 1 father.

  Is Axel speaking today ? –  Cannot be answered

  Bob’s daughters are Alice and June. Has Bob 3 daughters ? –  Cannot be answered

  In practice, most applications use close-world reasoning / querying

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Creating RDF data using ontologies

  Overview of different methods:   Create RDF manually (using your favourite text-editor or

Web-based interfaces)

  Create XHTML+RDFa documents and use GRDDL transformation

–  For both human and machines !

  Use exporters / wrappers for existing service

  Use applications that natively expose RDF data

  Provide mappings from RDBMS to RDF data

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Getting a FOAF profile

  Or how to give yourself a URI   Give yourself an identity on the Semantic Web

  Create your FOAF file   http://www.ldodds.com/foaf/foaf-a-matic (requires

hosting, e.g. your FTP space or uploaded via Drupal)

  http://foafbuilder.qdos.com/builder/ (requires OpenID)

  I already have an homepage, what about duplication of information ?   Use RDFa to embed RDF annotations in your homepage !

  More on this topic in a few slides

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Extend your FOAF profile

  The foaf:knows property aims to represent social connections between people   :alex foaf:knows :axel .

  … but it’s voluntary a weak relationship (no strongsemantics on why / how we know each other)

  Going further with the relationship vocabulary   http://vocab.org/relationship/

  Various properties can be used: colleagueOf, hasMet …

  You can extend your FOAF file to add colleagues, co-workers, and use different properties for each of them   Useful for querying a particular type of relationship only

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Defining personal interests

  Instead of modeling interests as plain-text strings, use URIs to describe them !   Since URI are unique and non ambiguous identifiers

  Allows interlinking of various resources for advanced query purposes: “find all people that like movies directed by Tarantino”

  Using the foaf:topic_interest properties   :me foaf:topic_interest :movie .

  But … where to get these URIs ?   Sindice, the Semantic Web index, can be used to find URIs

for a given concept

  http://sindice.com

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RDFa and GRDDL

  GRDDL is a mechanism to transform any kind of XML to RDF

  XHTML+RDFa is XML, hence GRDDL can extract it   Simply embeds RDFa annotations in your HTML code

  Indexed by Yahoo! SearchMonkey and Google

  Done via XSLT, available at http://www.w3.org/2008/07/rdfa-xslt

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RDFa and GRDDL

  The GRDDL Primer athttp://www.w3.org/TR/grddl-primer/#scheduling shows the overall processing of XHTML+RDFa:

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RDFa and GRDDL example

  http://sdow2009.semanticweb.org

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  http://sdow2009.semanticweb.org   Browse source to check RDFa annotations

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RDFa and GRDDL

  http://sdow2009.semanticweb.org   Header contains prefixes and links to the GRDDL

transformation

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  http://sdow2009.semanticweb.org   Webpage can be translated to native RDF/XML using an

RDFa distiller - http://www.w3.org/2007/08/pyRdfa/

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Other example

  Adding RDFa in one’s profile   Need to define prefixes in the header, or include them in

the markup

  See http://www.w3.org/TR/xhtml-rdfa-primer/

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Wrappers for existing sources

  Creating and maintaining a FOAF file by hand can be a time-consuming task   How can we automatically get RDF data from existing

sources ?

  What about Web 2.0 services in which we already give lots of personal information ?   Most of them provide APIs to get structured information

(JSON, XML …) about the user profiles, content, etc.

  API to RDF wrappers can easily be implemented

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Wrappers for Web 2.0 services

  Facebook wrapper   Generates a FOAF file from your Facebook profile

  If you have a Facebook profile, then you can have a related FOAF file (and escape the Facebook walled-garden !)

  http://www.dcs.shef.ac.uk/~mrowe/foafgenerator.html

  Flickr wrapper   Generates FOAF + SIOC + links to geographical information

(using geonames.org)

  http://apassant.net/home/2007/12/flickrdf

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More RDF-ification services

  Translates many structured sources into RDF   URIBurner

–  http://linkeddata.uriburner.com/

–  Open Source, C++ , Based on Virtuoso

  Any23 –  Sindice sponsored

–  Open Source, Java based

  Swignition –  http://buzzword.org.uk/swignition/

–  Perl based

  Triplr –  Purely syntactic, fast

–  http://triplr.org

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Native export of RDF data

  CMS can expose RDF data natively using dedicated plug-ins   SIOC Export for Drupal: http://drupal.org/project/SIOC

  Provide RDF export of each blog post –  http://apassant.net/blog/2009/03/07/call-suggested-

features-sparql-working-group

–  http://apassant.net/sioc/node/235

  Using RDF autodiscovery feature in the HTML header –  So that RDF can be discovered when browsing HTML

–  Semantic Radar: http://sioc-project.org/firefox

  RDFa to be included in Drupal7 core ! –  http://groups.drupal.org/node/16597

–  100.000’s of RDFa-powered websites

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Overview: SIOC for vBulletin

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Relational to RDF Mapping

  Relational data (RDB) is structured data and can be mapped to RDF straight-forward   Especially useful as various websites are back-ended by a

relational database (e.g. MySQL in the previous Drupal lecture)

  Main issues:   Closed-world vs. open-world modeling

  Assigning URIs for entities (records)

  Mapping language expressivity

  For a state-of-the-art see http://www.w3.org/2005/Incubator/rdb2rdf/RDB2RDF_SurveyReport.pdf

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Relational to RDF Mapping

  Standardization   W3C RDB2RDF Incubator Group 2008/2009

  Upcoming W3C RDB2RDF Working Group

  Current solutions (see state-of-the-art)   D2RQ

–  http://www4.wiwiss.fu-berlin.de/bizer/d2rq/

–  DBLP in RDF: http://dblp.l3s.de/d2r/

  OpenLink’s Virtuoso –  http://www.openlinksw.com/virtuoso/

  Triplify –  http://triplify.org

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SPARQL

  RDF(S)/OWL useful to produce data   A need to query it

  SPARQL   SPARQL Protocol and RDF Query Language

  The “SQL” of the Semantic Web

  FAQ   http://www.thefigtrees.net/lee/sw/sparql-faq

  SPARQL Query Recommendation / tutorial   http://www.w3.org/TR/rdf-sparql-query/

  Currently under standardization for new features   http://www.w3.org/2009/sparql/wiki/Main_Page

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How it works ?

  Basic concept of Graph Pattern Matching   RDF data is graph data, SPARQL checks if the graph you

are looking for belongs to the graph you are querying

  Four different operators   SELECT, DESCRIBE, CONSTRUCT, ASK

  Combined with the pattern you want to match and optional features (union, filters …)

  A Protocol   To query RDF data using SPARQL endpoints via HTTP

  Most of endpoints are associated with an RDF store   A place that stores RDF data and provides open access to

it – e.g. http://dbpedia.org/sparql

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Example of SELECT queries

“select persons older than 30”

SELECT ?X WHERE { ?X a foaf:Person. ?X ex:age ?Y. FILTER (?Y > 30) }


Query DBPedia

  The Semantic Web aims at creating structured data where there is only HTML data at the moment   Wikipedia: A great resource for humans, poor for machines

  DBPedia – RDF version of Wikipedia

  New kind of advanced queries   http://wiki.dbpedia.org/OnlineAccess#h28-5

  People born in Berlin before 1900

  German musicians born in Berlin

  Etc …

  The following queries can be run online   http://dbpedia.org/snorql

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Example query 1

  People Born in Galway   Simple triple pattern

  http://dbpedia.org/ontology/birthplace

  Answer SELECT ?who!

WHERE {!

?who !

<http://dbpedia.org/ontology/birthplace> :Galway .!

}!

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Example query 2

  Japanese name of Galway

  Using the FILTER by LANG clause   FILTER(lang(?x) = “ja”)

  Answer SELECT ?name!

WHERE {!

:Galway rdfs:label ?name .!

FILTER (lang(?name) = “ja”) .!

}!

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Example query 3

  Irish cities at the east of Galway!

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Example query 3

  FILTER by type and comparison of coordinates

  Answer PREFIX yago: <http://dbpedia.org/class/yago/>!

SELECT DISTINCT ?place ?long WHERE {!

:Galway dbpedia2:westCoord ?glong .!

?place rdf:type!

yago:CitiesInTheRepublicOfIreland ;!

dbpedia2:westCoord ?long .!

FILTER (?long < ?glong) !

}!

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Assignement

  Create a FOAF file   Define your social network (>3) using the relationships

vocabulary and add some interests using DBPedia URI (>3)

  Validate at http://www.w3.org/RDF/Validator/

  Add the same information in your Drupal profile as RDFa   Check if it translates well using

http://www.w3.org/2007/08/pyRdfa/

  Some SPARQL queries over Dbpedia (based on the interests defined in your FOAF file)   Will send the list by e-mail

  Deadline 16 November

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Introduction to the Semantic Web

Technology

Transcript of Introduction to the Semantic Web