Semantic Web - Lecture 09 - Web Information Systems (4011474FNR)

2 December 2005
Web Information Systems
Semantic Web
Prof. Beat Signer
Department of Computer Science
Vrije Universiteit Brussel
http://www.beatsigner.com

November 28, 2014 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 2
The Semantic Web
I have a dream for the Web [in which com-puters]
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.
Weaving the Web - The Original Design and Ultimate Destiny of the World Wide Web by Its Inventor,
Tim Berners-Lee, Harper San Francisco, September 1999
Tim Berners-Lee

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 auto-mation,
integration and reuse of data across various
applications. Metadata provides a means to make
statements and create machine-readable statements.
W3C, 2003

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 today's webpages
 machine-readable metadata for existing web content
 combination of data from different sources to derive new facts
 machines (agents) may use logical reasoning to infer facts that
are not explicitly recorded
 Crucial component of Web 3.0 or Giant Global Graph

Video: The Future Internet

Semantic Web Stack
 The Semantic Web Stack
(or Semantic Web Cake)
describes the architecture
of the Semantic Web
 URI/IRI
- unique identification of semantic
web resources
 Unicode
- representing/manipulating text
in different languages
 XML
- interchange of structured data
over the Web
Character set: UNICODE
Cryptography
Syntax: XML and XML Namespaces
Data interchange: RDF
Taxonomies: RDFS
Ontologies:
Querying: OWL
SPARQL
Unifying Logic
Trust
User interface and applications
Proof
Rules:
RIF/SWRL
Based on [http://en.wikipedia.org/wiki/File:Semantic-web-stack.png]
Identifiers:
URI/IRI

Semantic Web Stack ...
 XML Namespaces
- uniquely qualify markup from
multiple sources (integration)
 Resource Description
Framework (RDF)
- define RDF triples and repre-sent
resource information in
a graph structure
 RDF Schema (RDFS)
- create hierarchies of classes
and properties
Cryptography
Taxonomies: RDFS
Ontologies:
Querying: OWL
SPARQL
Unifying Logic
Trust
Proof
Rules:
RIF/SWRL
Identifiers:
URI/IRI

 Web Ontology Language
(OWL)
- language to define vocabularies
- extends RDFS with more ad-vanced
features (e.g. cardinality)
- enables reasoning based on
description logic
 SPARQL
- query language to query any
RDF-based data
 Rule Interchange Format
(RIF) and Semantic Web
Rule Language (SWRL)
- describe relations that cannot be
described in OWL
Cryptography
Taxonomies: RDFS
Ontologies:
Querying: OWL
SPARQL
Unifying Logic
Trust
Proof
Rules:
RIF/SWRL
Identifiers:
URI/IRI

 Unifying Logic
- logical reasoning (infer new
facts and check consistency)
 Proof
- explain logical reasoning steps
 Cryptography
- protect RDF data via encryption
- validate the source of facts by
digitally signing RDF data
 Trust
- authentification of sources and
trustworthiness of derived facts
 User Interface
- user interfaces for semantic web
applications
Cryptography
Taxonomies: RDFS
Ontologies:
Querying: OWL
SPARQL
Unifying Logic
Trust
Proof
Rules:
RIF/SWRL
Identifiers:
URI/IRI

Resource Description Framework
 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
 predicate (property)
 object (value)
{person-1, name, "Niklaus Wirth"}
subject predicate object

Resource Description Framework ...
 Subjects, predicates 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
 a literal cannot be the subject of an RDF statement
 Predicate
 relation between two resources or between a resource and a
literal
 RDF data is often stored in relational databases or so-called
triplestores (e.g. Apache Jena)
 up to billions of triples

RDF Graph
 A set of RDF statements can be represented as a
directed labelled graph
 note that in RDF we can only define statements about specific
instances but not about generic concepts
- ontologies have to be used to define statements about generic concepts
Beat
Signer
hasFamilyName
hasGivenName
http://wise.vub.ac.be/beat-signer

RDF Graph ...
 Anonymous resources have no explicit identifier
 in the example, the "office" is an anonymous resource
 anonymous resources are also called blank nodes or bnodes
 blank nodes can only be used as subjects or objects
Beat Signer
hasGivenName hasFamilyName
http://wise.vub.ac.be hasDirector
isMember
Lode
http://wise.vub.ac.be/lode-hoste
Hoste
hasFamily
Name
hasGivenName
isColleague
hasOffice
10F733 026293306
room phone

RDF Reification
 An RDF triple is not a resource and can therefore not
become subject of another statement
 we have to reify the original statement
- make a resource out of the statement
Beat Signer
http://wise.vub.ac.be hasDirector
http://wise.vub.ac.be/beat/
isMember
Lode
http://wise.vub.ac.be/lode/
Hoste
hasGivenName
rdf:subject rdf:object
rdf:statement isColleague
rdf:type
rdf:Property
1
forYears
hasFamily
Name

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
wearsShirt
http://shirt.org/shirt1
http://shirt.og/shirt2
RDF:_2
rdf:alternative

Advantages of RDF
 Simple
 Enables the combination (merging) of data from
different data models
 not easily possible in a relational 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
- triplestores, parsers, editors, frameworks, ...

RDF Schema (RDFS)
 Vocabulary description language for RDF
 domain vocabulary and structure
 Define common concepts and relationships
 classes (rdfs:Class) and subclasses (rdfs:subClassOf)
 properties and sub-properties (rdfs:subPropertyOf)
 domain (rdfs:domain) and range (rdfs:range) of a property
 rdfs:seeAlso, rdfs:isDefinedBy (utility properties)
 rdfs:label, rdfs:comment
 ...
 Provides the basic elements for the definition of
ontologies

RDF Schema Example
Beat Signer
Researcher
Lode
http://wise.vub.ac.be/lode-hoste
Hoste
isColleague
rdf:type rdf:type
Person isColleague
rdfs:Class rdf:Property
rdf:type rdf:type rdfs:domain
rdfs:range
rdfs:subClassOf
rdfs:Literal rdfs:Literal rdfs:Literal rdfs:Literal
rdf:type rdf:type rdf:type rdf:type

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 cannot 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 Lode then Lode is a colleague of Beat"
(transitivity)
 these issues are addressed by the Web Ontology
Language (OWL)

RDF(S) / XML Serialisation
 Syntax not so easy to learn
 many different ways to construct the same statement
 long URIs are hard to read
{http://wise.vub.ac.be/beat-signer, isColleague,
http://wise.vub.ac.be/lode-hoste}
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
<rdf:Description rdf:about="http://wise.vub.ac.be/beat-signer">
<isColleague rdf:resource="http://wise.vub.ac.be/lode-hoste"/>
<hasGivenName>Beat</hasGivenName>
...
</rdf:Description>
...
</rdf:RDF>

RDF Notation 3 (N3)
 Short non-XML serialisation
 separate predicates with a semicollon
 finish subject definition with a full stop
 Note that the N3 notation offers more features than are
necessary for RDF(S) serialisation
 e.g. support for RDF-based rules
<http://wise.vub.ac.be/beat-signer> isColleague <http://wise.vub.ac.be/lode-hoste>;
...
hasGivenName "Beat".

RDF Turtle Notation
 Terse RDF Triple Language
 Subset of N3 language
 only describes RDF features (RDF graph model)
 Syntax looks similar to Notation 3
 http://www.w3.org/TeamSubmission/turtle/
 Many RDF frameworks (e.g. Jena) offer Turtle parser
and serialisation features

RDF Applications
 Annotea project
 defines an RDF schema for the types of annotations that can be
used to annote webpages
 RSS
 some RSS versions use RDF(S) / XML serialisation
 Dublin Core
 widely used to describe digital media (also in standard HTML)
- bibliographic metadata such a 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>

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 defined
SELECT ?name ?mbox
WHERE { ?x foaf:name ?name .
?x foaf:mbox ?mbox }

Web Ontology Language (OWL)
 OWL evolved from DAML+OIL
 DAML is the DARPA Agent Markup Language
 OIL stands for Ontology Inference Layer
 There exist 3 different OWL sublanguages (flavours) with
different expressivness
 OWL Full
- maximum expressiveness (full language)
- no computational guarantee
 OWL DL
- maximal OWL Full subset that is still computationally decidable
 OWL Lite
- classification hierarchy and simple constraints (limited cardinality constraints)
- weakest of the three variants

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

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

Swoogle
 Search engine for seman-tic
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

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>

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
- identification by description
(pedicates and objects)
 "six degrees of separation" or
"small world phenomenon"
 FOAFNaut browser
[http://rdfweb.org/images/foaf/foafnaut-screenshot-path.jpg]

Semantic Wikis
 Use Semantic Web
technologies to provide
machine-processable
Wiki content
 page content
 link metadata
 Ontology reasoning
 much richer query interface
 Existing semantic Wikis
 DBPedia
 Semantic MediaWiki
 ...

Linked Data
 Link different data sources (URIs) on the Web
 provide metadata about the resources via RDF/XML, N3, etc.
 provide links to resources in other data sets on the Web
 Linked Open Data community project
 RDF tripes from DBPedia, GeneID, ACM, etc. (>30 billion triples)
 links between those triples (>500 million links)
http://lod-cloud.net/versions/2014-08-30/lod-cloud.svg

Semantic Desktops
 Apply Semantic Web tech-nologies
to personal infor-mation
management (PIM)
 inter-application data sharing
 enhancement of limited
filesystem functionality
- add document metadata
 Examples
 Haystack
 Nepomuk
Nepomuk Integration with Dolphin (KDE 4.0)

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

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
- Gleaning Resource Descriptions from Dialects of Language
- can convert from microformats to semantic web data (RDF)

hCard Microformat Example
 Some search engines (e.g. Google and Yahoo) start to
pay attention to different types of microformats
<head profile="http://www.w3.org/2006/03/hcard">
...
</head>
...
<div class="vcard">
<div class="fn">Lode Hoste</div>
<div class="org">Vrije Universiteit Brussel</div>
<div class="tel">32 2629 3306</div>
<a class="url" href="http://wise.vub.ac.be/members/lode-hoste">
http://wise.vub.ac.be/members/lode-hoste</a>
</div>

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>

Microdata
 Add machine readable metadata (semantics) to
HTML5 documents in the form of key/value pairs
 can be used by crawlers, search engines (SEO) and browsers to
provide a richer browsing experience
 alternative to Microformats and RDFa
W3C Working Draft
<section itemscope itemtype="http://data-vocabulary.org/Person">
Hello, my name is <span itemprop="name">Beat Signer</span> and I am a
<span itemprop="title">Professor</span> at the
<span itemprop="affiliation">Vrije Universiteit Brussel. </span>
<section itemprop="address" itemscope itemtype="http://data
-vocabulary.org/Address">My address is:
<span itemprop="street-address">Pleinlaan 2</span>,
<span itemprop="postal-code">1050 </span>
<span itemprop="locality">Brussels</span>,
<span itemprop="country-name">Belgium</span>.
</section>
</section>

Exercise 9
 Semantic Web
 working with linked data

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

References ...
 Linked Data
 http://linkeddata.org

2 December 2005
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