Web Information Systems
Semantic Web
Prof. Beat Signer
Department of Computer Science
Vrije Universiteit Brussel
http://vub.academia.edu/BeatSigner
2 December 2005

The Semantic Web
I have a dream for the Web [in which computers]
become capable of analyzing all the data on the
Web - the content, links, and transactions between
people and computers. A 'Semantic Web', which
should make this possible, has yet to emerge, but
when it does, the day-to-day mechanisms of trade,
bureaucracy and our daily lives will be handled by
machines talking to machines. The 'intelligent
agents' people have touted for ages will finally
materialize.
[Tim Berners-Lee, Weaving the Web - The Original Design and Ultimate Destiny of the World Wide Web by Its Inventor, 2000]
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 2

The Semantic Web ...
The Semantic Web is a vision: 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. Metadata
provides a means to make statements and create
machine-readable statements.
[W3C, 2003]
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 3

The Semantic Web ...
 Meaning of data on the Web can not only be infered by
people but also discovered by machines without (or with
less) human intervention
 Web of Data instead of Web of Documents
 the Web as a huge decentralised database (knowledge base)
 machine-accessible data
 data may be interconnected similar to todays webpages
 combination of data from different sources to derive new facts
 machine-readable metadata for existing Web content
 machine (agents) may use logical reasoning to infer facts that are
not explicitly recorded
 Crucial component of Web 3.0
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 4

Video: The Future Internet
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 5

Semantic Web Stack
 Semantic Web Stack
(or Semantic Web Cake)
describes the architecture
of the Semantic Web
 URI/IRI
- unique identification of semantic
web resources
 Unicode
- representating/manipulating text [http://en.wikipedia.org/wiki/File:W3c-semantic-web-layers.svg]
in different languages
 XML
- interchange of structured data
over the Web
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 6

Semantic Web Stack ...
 XML Namespaces
- integrate (qualify) markup from
multiple sources
 XML Query (XQuery)
- query collections of XML data
 XML Schema
- define structure (grammar) of
specific XML languages
 RDF Model & Syntax
- define RDF triples and represent [http://en.wikipedia.org/wiki/File:W3c-semantic-web-layers.svg]
resource information in a graph
- describe taxonomies based on
RDF Schema (RDFS)
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 7

Semantic Web Stack ...
 Ontology
- language to define vocabularies
- extends RDFS with more ad-
vanced features (e.g. cardinality)
- enables reasoning based on
description logic (e.g. OWL)
 Rules / Query
- description of additional rules via
Rule Interchange Format (RIF)
- query RDF (OWL) data based on
[http://en.wikipedia.org/wiki/File:W3c-semantic-web-layers.svg]
the SPARQL query language
 Logic
- logical reasoning (infer new facts
and check consistency)
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 8

Semantic Web Stack ...
 Proof
- explain logical reasoning steps
 Trust
- authentification (sources) and
trustworthiness of derived facts
 Signature
- validate the source of facts by
digitally signing RDF data
 Encryption
- protect RDF data via encryption [http://en.wikipedia.org/wiki/File:W3c-semantic-web-layers.svg]
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 9

Resource Description Framework (RDF)
 The Resource Description Framework (RDF) has been
designed to describe
 data and metadata about specific subjects
 structure of data sets
 relationships between bits of data
 An RDF statement (triple) consists of three parts
 subject
 property (predicate)
 object (value)
{person-1, name, "Niklaus Wirth"}
subject property object
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 10

Resource Description Framework ...
 Subject, properties and objects are all resources
 Resource
 anything that can be referenced by a URI
 Literal
 non-structured data (e.g. String, Integer, ...); is also a resource
 could also be represented by a URI
 Property
 relation between two resources or between a resource and a
literal
 Statement
 triple of the form <subject><property><object>
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 11

RDF Graph
 Directed labelled graphs are well suited for representing
RDF statements
 note that in RDF we can only define statements about specific
instances but not about generic concepts (ontologies are used to
achieve this)
hasGivenName
Beat
http://wise.vub.ac.be/beat/
Signer
hasFamilyName
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 12

RDF Graph ...
http://wise.vub.ac.be
hasDirector isMember
isColleague
http://wise.vub.ac.be/beat/ http://wise.vub.ac.be/sven/
hasGivenName
hasGivenName hasFamilyName office
hasFamily Sven
Beat Signer Name
room phone
Casteleyn
10F705 026293754
 Office represented by anonymous node
 sometimes called blank nodes or b-nodes
 identity defined by properties (no explicit identifier)
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 13

RDF Reification
http://wise.vub.ac.be
hasDirector isMember
rdf:subject rdf:object
http://wise.vub.ac.be/beat/ http://wise.vub.ac.be/sven/
forYears
hasGivenName
hasGivenName hasFamilyName
rdf:type 1 hasFamily Sven
Beat Signer rdf:Property Name
rdf:statement isColleague
Casteleyn
 An RDF triple is no resource and can therefore not
become subject of another statement
 we have to reify the original statement
- make a resource out of the statement
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 14

RDF Container Nodes
 Special container resource types
 bag
- number of unordered resources with potential duplicates
 sequence
- ordered collection of resources
 alternative
- one of the members can be selected
 collection
- closed; once it has been defined, the members can no longer be changed
RDF:_1 http://shirt.org/shirt1
wearsShirt
http://wise.vub.ac.be/beat/ rdf:alternative
RDF:_2
http://shirt.og/shirt2
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 15

Advantages of RDF
 Simple
 Combine (merge) data from different data models
 not easily possible in a relatitional database (different schemas)
 The same resource can be annotated by different people
 resource referenced by URI
 separation of data and metadata
 Well-defined standard
 many tools available
- repositories, parsers, editors, frameworks, ...
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 16

RDF Schema (RDFS)
 Vocabulary description language for RDF
 domain vocabulary and structure
 Define common concepts/relationships
 classes and sub-classes
 properties and sub-properties
 domain and range (of a property)
 seeAlso, isDefinedBy (utility properties)
 label, comment
 ...
 Provides the basic elements for ontologies
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 17

RDF Schema Example
rdfs:Class rdf:Property
rdf:type rdfs:domain rdf:type
Person isColleague
rdfs:range
rdfs:subClassOf
Researcher
rdf:type rdf:type
isColleague
http://wise.vub.ac.be/beat/ http://wise.vub.ac.be/sven
hasGivenName hasFamilyName hasGivenName hasFamilyName
Beat Signer Sven Casteleyn
rdf:type rdf:type rdf:type rdf:type
rdfs:Literal rdfs:Literal rdfs:Literal rdfs:Literal
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 18

RDF(S) / XML Serialisation
{http://wise.vub.ac.be/beat/, isColleague, http://wise.vub.ac.be/sven/}
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
<rdf:Description rdf:about="http://wise.vub.ac.be/beat/">
<isColleague rdf:resource="http://wise.vub.ac.be/sven/"/>
<hasGivenName>Beat</hasGivenName>
...
</rdf:RDF>
 Syntax not so easy to learn
 many different ways to construct the same statement
 long URIs hard to read
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 19

RDF Notation 3 (N3)
 Short non-XML serialisation
 offers more features than necessary for RDF(S) serialisation
 separate properties with a semicollon
 finish subject definition with a full stop
<http://wise.vub.ac.be/beat/> isColleague < http://wise.vub.ac.be/sven/>;
...
hasGivenName Beat.
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 20

RDF Turtle Notation
 Terse RDF Triple Language
 Subset of N3 language
 only RDF features
 Syntax looks similar to Notation 3
 http://www.w3.org/TeamSubmission/turtle/
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 21

RDF Applications
 Annotea project
 defines an RDF schema defining the types of annotations to be
used for annottaing webpages
 RSS
 some RSS versions use RDF(S) / XML serialisation
 Dublin Core
 widely used to descibe digital media (also in standard HTML)
- Title, Creator, Description, ...
 uses RDF(S) / XML serialisation as one possible representation
<head>
...
<meta name="DC.Subject" content="Interactive Paper, Cross-media ..."/>
<meta name="DC.Description" content="Beat Signer does research on ..."/>
</head>
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 22

SPARQL Query Language
 RDF query language which can be used to
 extract information as URIs, literals, blank nodes or subgraphs
 SPARQL SELECT queries return variable bindings
 SPARQL querying relies on graph pattern matching
 Example
 get the name and mbox of all subjects that have both of these
properties defines
SELECT ?name ?mbox
WHERE { ?x foaf:name ?name .
?x foaf:mbox ?mbox }
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 23

Advantages of RDFS
 With RDFS we have a richer expressiveness
(e.g. subClassOf) than with RDF
 Simple reasoning (e.g. type hierarchy)
 Many existing tools to deal with RDFS
 However, some things can not be expressed; for example
 a person must have a family name
 a person can have at most one family name (cardinality)
 if Beat is a colleague of Sven then Sven is a colleague of Beat
(transitivity)
 These issues are addressed by the Web Ontology
Language (OWL)
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 24

Web Ontology Language (OWL)
 OWL evolved from DAML+OIL
 DAML is the DARPA Agent Markup Language
 OIL stand for Ontology Inference Layer
 There exist 3 different OWL variants with different
expressivness
 OWL Lite
- classification hierarchy and some simple constraints (e.g. Limited cardinality
constraints)
- weakest of the three variants
 OWL DL
- maximum expressiveness while still retaining computational decidable
 OWL Full
- maximum expressiveness
- no computational guarantee
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 25

Jena Semantic Web Framework
 Open source Semantic Web framework for Java
 create and access data from RDF graphs via an RDF API
 offers an OWL API
 data can be stored in files, databases or accessed via URLs
 http://jena.sourceforge.net
 RDF graphs can be serialised into different formats
 RDF/XML
 Notation 3
 Turtle
 relational database
 SPARQL query interface
 Multiple reasoners
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 26

Protégé
 Free open source platform
to create, manipulate and
visualise ontologies
 Two modelling tools
 Protégé-Frames editor
- build and populate frame-based
ontologies
- Java API for plug-ins
 Protégé-OWL editor
- build Semantic Web ontologies
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 27

Swoogle
 Search engine for
semantic web data (RDF)
 ontologies
 instance data
 single terms
 Ranking of semantic web
documents
 inspired by Google's
PageRank
 Developed at the
University of Maryland
 http://swoogle.umbc.edu
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 28

Friend of a Friend (FOAF)
 Personal information and connections to friends in RDF
 http://www.foaf-project.org
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#" xmlns:foaf="http://xmlns.com/foaf/0.1/">
<foaf:Person>
<foaf:name>Beat Signer</foaf:name>
<foaf:title>Prof.</foaf:title>
<foaf:givenname>Beat</foaf:givenname>
<foaf:family_name>Signer</foaf:family_name>
<foaf:nick>Beat</foaf:nick>
<foaf:mbox_sha1sum>ce6d419869307d57839feef6445a9d64f784eb36</foaf:mbox_sha1sum>
...
<foaf:knows>
<foaf:Person>
<foaf:name>Moira C. Norrie</foaf:name>
<foaf:mbox_sha1sum>4cb61b36a6feaa48c78acbb51fcce7cb356afdd6</foaf:mbox_sha1sum>
<rdfs:seeAlso rdf:resource="http://www.globis.ethz.ch/people/norrie.rdf">
</foaf:Person>
</foaf:knows>
...
</foaf:Person>
</rdf:RDF>
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 29

Friend of a Friend (FOAF) ...
 First social Semantic Web
application
 Miller and Brickley, 2000
 Describe a social network
without a central database
 links can be followed by
spiders (data mining)
 no unique identifier [http://rdfweb.org/images/foaf/foafnaut-screenshot-path.jpg]
- identification by description
(properties)
 " six degrees of separation"
 FOAFNaut browser
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 30

Semantic Wikis
 Use Semantic Web
technologies to provide
machine-processable
Wiki content
 page content
 link metadata
 Ontology reasoning
 Existing semantic Wikis
 Semantic MediaWiki
 KiWi
 OntoWiki
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 31

Twine
 Knowledge networking
 "combination of Wikipedia
and Facebook"
 Based on Semantic Web
technologies
 RDF, OWL, SPARQL, ..
 semantic graph with relation-
ships between people and
topics
 http://www.twine.com
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 32

Talis Platform
 Semantic Web application
platform
 reduces the complexity and
costs for storing, searching
and augmenting large
quantities of data
 delivered as Software as a
Service (SaaS)
 store arbitrary content as well
as RDF data
 SPARQL and free text
search interface
 http://www.talis.com
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 33

Semantic Desktops
 Apply Semantic Web
technologies to personal
information management
 inter-application data sharing
 enhancement of limited
filesystem functionality
- add document metadata
 Examples
 Haystack Nepomuk Integration with Dolphin (KDE 4.0)
 Nepomuk
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 34

GoodRelations
 Lightweight ontology for expressing
product information in e-commerce web applications
 Product features
 offers
 prices
 units
 ...
 Adopted by various companies
 Yahoo
 BestBuy
 ...
 Leads to enhanced product search functionality
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 35

Microformats
 Add semantics to (X)HTML pages
 Makes use of specific (X)HTML tag attributes
 class and rel attributes
- e.g. rel="nofollow" for search engines
 Specific microformats
 hCard: contact information
 hCalendar: event information
 hProduct: product information
 Alternative solutions
 semantic web (RDFa)
 GRDDL
- convert from microformats to semantic web data
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 36

hCard Microformat Example
<head profile="http://www.w3.org/2006/03/hcard">
...
</head>
...
<div class="vcard">
<div class="fn">Frederic Kleinermann</div>
<div class="org">Vrije Universiteit Brussel</div>
<div class="tel">32 2629 3713</div>
<a class="url" href="http://wise.vub.ac.be/members/frederic/">
http://wise.vub.ac.be/members/frederic/</a>
</div>
 Some search engines (e.g. Google and Yahoo) start to
pay attention to different types of microformats
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 37

RDF in Attributes (RDFa)
 Add a set of attribute extensions to XHTML for
embedding RDF metadata
 Different vocabularies
 FOAF, video, audio, commerce, …
 Search engines (e.g. Yahoo and Google) process certain
RDFa metadata (e.g. product information)
<p xmlns:dc=http://purl.org/dc/elements/1.1/
about="http://www.amazon.com/...">
and the will to live. <span property="dc:creator">Simpson</span>
dedicates the book <cite property="dc:title">Touching the Void</cite> to
the... The book was published in <span property="dc:date"
content="1989-12-01">December 1989</span>.
</p>
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 38

VUB STARLab
 Group of Prof. Dr. Robert
Meersman
 research on the Semantic
Web, databases and
ontologies
 Course Open Information
Systems
 ontologies as main theme
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 39

References
 Tim Berners-Lee, James Hendler and Ora Lassila, The
Semantic Web, Scientific American Magazine, May 2001
 http://www.scientificamerican.com/article.cfm?id=the-
semantic-web
 The Future Internet: Service Web 3.0
 http://www.youtube.com/watch?v=off08As3siM
 Resource Description Framework (RDF)
 http://www.w3.org/RDF/
 Thomas B. Passin, Explorer's Guide to the Semantic
Web, Manning Publications, March 2004
November 12, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 40

Next Week
Mobile Information Systems
2 December 2005