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
2 of XYZ

The initial Proposal (1989)
3 of XYZ
http://www.w3.org/History/1989/proposal.html

… but so far …
4 of XYZ
??
??
??

… 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
5 of XYZ

… 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
7 of XYZ

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.”
8 of XYZ

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.“
9 of XYZ

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/
 …
10 of XYZ

The Semantic Web stack
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http://www.w3.org/2007/03/layerCake.png

URIs
12
 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
13
 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
14
 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> .!
15

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
17
 RDF provide a way to write assertions about URIs
 But what about the semantic of these assertions
 E.g. how can one know that
http://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
19
 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
20
 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
21
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 .
21 of XYZ

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
22

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.
23

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
24

Which ontologies to use ?
 SearchMonkey Vocabularies
 http://developer.yahoo.com/searchmonkey/smguide/
profile_vocab.html
25

Which ontologies to use ?
 How to Publish Linked Data on the Web
 http://www4.wiwiss.fu-berlin.de/bizer/pub/
LinkedDataTutorial/
26

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
27

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
29
 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
29 of XYZ

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
30

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
31

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
32

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
33

34

35

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
36

RDFa and GRDDL
 The GRDDL Primer at
http://www.w3.org/TR/grddl-primer/#scheduling
shows the overall processing of XHTML+RDFa:
37

RDFa and GRDDL example
 http://sdow2009.semanticweb.org
38

 Browse source to check RDFa annotations
39

RDFa and GRDDL
 Header contains prefixes and links to the GRDDL
transformation
40

 Webpage can be translated to native RDF/XML using an
RDFa distiller - http://www.w3.org/2007/08/pyRdfa/
41

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/
42

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
43

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
44

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
45

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
46

Overview: SIOC for vBulletin
47

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
48

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
49

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
50

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
51

52
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
53

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 .!
}!
54

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”) .!
}!
55

Example query 3
 Irish cities at the east of Galway!
56

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) !
}!
57

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
58

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