The Semantic Web – Part 1: Foundations of Semantic Web Technologies

The Semantic
Web – Part 1
Semantic CMS Community

Lecturer
Organization
The Vision and Basic
Date of presentation Technologies of the Semantic
Web.

Co-funded by the
1 Copyright IKS Consortium
European Union

Page:

Part I: Foundations

(1) Introduction of Content Foundations of Semantic
(2)
Management Web Technologies

Part II: Semantic Content Part III: Methodologies
Management

Knowledge Interaction Requirements Engineering
(3) (7)
and Presentation for Semantic CMS

(4) Knowledge Representation
and Reasoning
(8)
Designing
Semantic CMS

Semantifying
(5) Semantic Lifting (9) your CMS

Storing and Accessing Designing Interactive
(6) Semantic Data
(10) Ubiquitous IS

www.iks-project.eu Copyright IKS Consortium

Page: 3

What is this Lecture about?
 Semantic web technologies as a possible solution for
improving the „state of play“ in content management by
making semantics expressable in machine-readable way

 The Semantic Web
 The vision behind the semantic web
 Underlying technologies of the Semantic Web
Part I: Foundations

(1) Introduction of Content Foundations of Semantic
(2)
Management Web Technologies


Page:

Web evolution

Slide by Nova Spivack, Radar Networks
4

Page: 5

The Semantic Web
 Thevision of the Semantic Web has been originally
proposed by Tim Berners-Lee

 “TheSemantic Web is not a separate Web but an
extension of the current one, in which information is
given well-defined meaning, better enabling computers
and people to work in cooperation.” [The Semantic
Web, 2001]

 Standardized specification techniques for the semantic
annotation of content (RDF, OWL, ...)

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Semantic Web Stack
 W3C provides standardized
specifications for Semantic
Web technologies
 Semantic Web Layer Cake
as a conceptual architecture
describes an hierarchy of
languages
 Each layer exploits and uses
capabilities of the layers
below
Semantic Web Layer Cake, Image source: http://www.w3.org/2007/03/layerCake.svg


Page: 7

Unique Identification of
Resources
 “...more fundamental than either HTTP or HTML are
URIs, which are simple text strings that refer to Internet
resources -- documents, resources, people, and
indirectly to anything. URIs are the glue that binds the
Web together. IRIs extend and strengthen the glue, by
allowing people to identify Web resources
in their own language.”

 In a “Web of Data” the unique
identification of entities is required


Page: 8

How to identify resources?
 URI – Uniform Resource Identifier [RFC 3986]
 “A Uniform Resource Identifier (URI) is a compact
sequence of characters that identifies an abstract or
physical resource.”
 A URI consists of five parts: scheme, authority, path, query
and fragment
 URI = scheme ":" authority "/" path [ "?" query ] [ "#"
fragment ]
 Example:

http://user@example.com:8042/over/there?name=ferret#nose
scheme authority path query fragment

Page: 9

How to identify resources?
 IRI – Internationalized Resource Identifier [RFC 3987]
 “IRIs are defined similarly to URIs in [RFC3986], but the
class of unreserved characters is extended by adding the
characters of the UCS (Universal Character Set,
[ISO10646]) ..”
 Extends the character set used by URIs and thereby
allows the consideration of language specific syntaxes
(e.g. Japanese, Chinese, ...)

 Example
 Logical Hebrew http://ab. .ij/kl/mn/op.html
 ASCII notation http://ab.CDEFGH.ij/kl/mn/op.html

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Structuring Data
 XML – Extensible Markup Language
 XML can be used to specify the syntactical structure of
documents or complex data objects in a machine-readable
form
<car>
<brand>Jaguar</brand>
<model>XF</model>
<dimensions>
<length>4961</length>
<width>2070</width>
</dimensions>
<engine>5.0 L V8 Petrol</engine>
</car>


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Why isn´t XML enough?
 XML tags are only defining the structure of a document
 For a machine the tags have no semantics!

<car> <cat>
<brand>Jaguar</brand> <brand>Jaguar</brand>
... ...
</car> </cat>

The difference in meaning is only
recognized by a human.


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Why isn´t XML enough?
 The relation among the different tags is not expressed
explicitly

<car>
The relation between “car”, “brand”
and “Jaguar” is only recognizable
<brand>Jaguar</brand> by a human.

...

</car>


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What do we need?
 We want to express the statement:
 “The brand of the car is Jaguar.”

 We need ...
 ...a way to address the concrete resource car.
 ... to express the property brand of the resource car.
 ... to define the property value Jaguar for the property
brand.


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Resource Description
Framework (RDF)
 “TheResource Description Framework (RDF) identifies
things using Web identifiers (URIs), and describes
resources with properties and property values.”
 A Resource is an object that can be identified by an
URI, e.g. “http://example.org/Car”.
 A Property describes an aspect of a resource, e.g.
“http://example.org/Brand”. The property is also
identified by an URI.
 TheProperty value assigns a concrete value to a
property, e.g. “Jaguar” or ““http://example.org/Jaguar”.
www.iks-project.eu http://www.w3schools.com/rdf/ Copyright IKS Consortium

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RDF Statements
 RDF statements consist of subject (resource), predicate
(property) and object (property value)
Predicate Object
Subject
(URI)

Predicate

Object
(literal)

 Subjects (except Blank Nodes) and Predicates are
always defined by URIs
 Objects can be defined by URIs and literals

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RDF Statements - Example
 Exemplary statements:
 “The brand of the car is Jaguar.”
 “The model of the car is XF.”
Subject Predicate Object

http://example.org/rel/Brand
http://example.org/Car http://example.org/Jaguar

http://example.org/rel/Model

XF Predicate

Object

Page: 17

Resource Description
Framework (RDF)
 “The Resource Description Framework (RDF) is a
language for representing information about
resources...” [RDF Primer]

• W3C Standard (http://www.w3.org/RDF)

 RDF provides a graph-based data model
 for representing metadata
 for describing the semantics of
information in a machine-accessible way

Page: 18

RDF Serialization Formats
 RDF/XML
 N3
 N-Triples
 TRiG
 TRiX
 Turtle
 JSON
 JSON-LD
 RDFa


Page: 19

RDF/XML
 RDF/XML is the most common serialization format for
RDF statements

 Example:

<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:rel="http://example.org/rel/">
<rdf:Description rdf:about="http://example.org/Car">
<rel:Brand rdf:resource="http://example.org/Jaguar"/>
</rdf:Description>
<rel:Model>XF</rel:Model>
</rdf:Description>
</rdf:RDF>

http://www.w3.org/TR/2004/REC-rdf-syntax-grammar-20040210/


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Root Element and
Namespaces
Root element of the
RDF/XML document.
<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"

....

</rdf:RDF> Defined namespaces

<rdf:RDF> Root element of RDF documents
xmlns:rdf Specifies that elements with the “rdf” prefix are using
the namespace “..”
xmlns:abc Specifies that elements with the “abc” prefix are using
the namespace “..”

www.iks-project.eu
Copyright IKS Consortium

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Description Element
Node element

...
</rdf:Description>

...
Assigns a resource to
the node element.
</rdf:RDF>

<rdf:Description> Introduces node element (subject).
<rdf:about> Defines the URI reference for the node element.


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Objects defined by Resources
 Assigning resources to objects (property values) in
RDF/XML
Defines predicate “Brand”
in namespace “rel”.
<rel:Brand rdf:resource="http://example.org/Jaguar"/>
</rdf:Description>

</rdf:RDF> object
The is defined
by a resource. http://example.org/rel/Brand

http://example.org/Car http://example.org/Jaguar


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Objects defined by Literals
 Definition of objects (properties) with constant values
(literals)

<rel:Model>XF</rel:Model>
</rdf:Description>

</rdf:RDF>
The object is defined http://example.org/Car

by a literal.
http://example.org/rel/Model

XF

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RDF Container Elements
 “A container is a resource that contains things. The
contained things are called members. The members of
a container may be resources (including blank nodes)
or literals.”
 Container provide the ability to describe groups of
things, e.g. several authors of a book
 RDF defines three types of containers:
 Bag
 Sequence
 Alternatives
http://www.w3.org/TR/2004/REC-rdf-primer-20040210/

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Container Element - Bag
 “ABag (a resource having type rdf:Bag) represents a
group of resources or literals, possibly including
duplicate members, where there is no significance in
the order of the members.”

 Example:
 Describe equipment features of the car.


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http://www.w3.org/1999/0
2/22-rdf-syntax-ns#Bag
http://example.org/Car

http://www.w3.org/1999/02/22-rdf-syntax-ns#type

http://example.org/features#equipment

http://example.org/AC

http://www.w3.org/1999/02/22-rdf-syntax-ns#_1

http://example.org/CarRad
io

Blank Node

http://example.org/SeatHe
ating
www.iks-project.eu

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The subject “Car” is
<rdf:RDF described by the
predicate “equipment”.
xmlns:e="http://example.org/features#">

<e:equipment>
<rdf:Bag>
The different members
<rdf:li rdf:resource="http://example.org/AC"/>
<rdf:li rdf:resource="http://example.org/CarRadio"/>
of the Bag container
<rdf:li rdf:resource="http://example.org/SeatHeating"/>
are listed.
The object is </rdf:Bag>
a
container of type “Bag”
</e:equipment>
</rdf:Description>

</rdf:RDF>

<x:xyz> Defines predicate “xyz” in namespace “x”.
<rdf:Bag> Defines a container of type “Bag”.
<rdf:li> Defines members of the Bag container.

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Container Element - Sequence
 “ASequence or Seq (a resource having type rdf:Seq)
represents a group of resources or literals, possibly
including duplicate members, where the order of the
members is significant.”

 Example:
 Describe a list of previous owners of the car in
chronological order.


Page: 29

2/22-rdf-syntax-ns#Seq


http://example.org/owner#preOwners

http://example.org/JohnD
oe


http://example.org/RickMi
ller


http://example.org/Sarah
House

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<rdf:RDF described by the
predicate “preOwners”.
xmlns:p="http://example.org/owner#">

<p:preOwners>
<rdf:Seq>
<rdf:li rdf:resource="http://example.org/JohnDoe"/>
<rdf:li rdf:resource="http://example.org/RickMiller"/>
of the Sequence
<rdf:li rdf:resource="http://example.org/SarahHouse"/>
container are listed.
The object is a</rdf:Seq>
</p:preOwners>
container of type
</rdf:Description>
“Sequence”.
</rdf:RDF>

<rdf:Seq> Defines a container of type “Sequence”.
<rdf:li> Defines members of Sequence container.

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Container Element -
Alternative
 “An Alternative or Alt (a resource having type rdf:Alt)
represents a group of resources or literals that are
alternatives (typically for a single value of a property).”

 Example:
 Describe a list of possible colors of the car.


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Container Element - Alternative
2/22-rdf-syntax-ns#Alt

http://example.org/exterior#colors

http://example.org/Black



http://example.org/White


http://example.org/Green


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Container Element - Alternative
described by the
xmlns:c="http://example.org/exterior#"
predicate “colors”.
<c:colors>
<rdf:Alt>
<rdf:li rdf:resource="http://example.org/Black"/>
<rdf:li rdf:resource="http://example.org/White"/>
of the Alternative
<rdf:li rdf:resource="http://example.org/Green"/>
container are listed.
The object is a</rdf:Alt>
</c:colors>
container of type
</rdf:Description>
“Alternative”.
</rdf:RDF>

<rdf:Alt> Defines a container of type “Alternative”.
<rdf:li> Defines members of Alternative container.

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RDF Collections
 A RDF Container can be used to define groups, but
lacks the ability to “close” them. That means to define
that "these are all the members of the container".
 RDF Collections provide the ability to describe
groups, that contain only the specified resources.
 “An RDF collection is a group of things represented as
a list structure in the RDF graph.”

 Example:
 All equipment features of a car.

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RDF Collection

http://example.org/features#equipment

http://www.w3.org/1999/02/22-rdf-syntax-ns#_first
http://example.org/AC

http://www.w3.org/1999/02/22-rdf-syntax-ns#rest

http://example.org/CarRa
dio


http://example.org/SeatH
eating


2/22-rdf-syntax-ns#nil

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RDF Collection
The predicate
“equipment” is described
xmlns:e="http://example.org/features#">
by a Collection.
<e:equipment rdf:parseType="Collection">
<rdf:Description rdf:about="http://example.org/AC"/>
<rdf:Description rdf:about="http://example.org/CarRadio"/>
<rdf:Description rdf:about="http://example.org/SeatHeating"/>
</e:equipment>
</rdf:Description>

</rdf:RDF>

<x:xyz Defines that the predicate “xyz” is
rdf:parseType=“Collection”> described by a collection.


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Validation Tool

www.iks-project.eu http://www.w3.org/RDF/Validator/

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Semantic Layer Web Cake

A model for describing
resources with properties A format for specifying structured
and property values. data in a machine-readable form

Unique identification
of resources

Page: 39

RDF
 RDF provides a model for describing resources with
properties and property values.

@prefix ex: <http://www.example.org/>.

ex:Car1 ex:Brand ex:Jaguar
ex:Car1 ex:Colour “Black”
ex:Car2 ex:Brand ex:Jaguar
ex:Car2 ex:Colour “White”
ex:Car3 ex:Brand ex:VW
ex:Car3 ex:Colour “Black”


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SPARQL
 SPARQL Protocol and RDF Query Language
 W3C Recommendation since 2008

 SPARQL provides a standard for querying
information, that is specified in RDF

 SPARQL consists of three specifications
 Query language
 Query results XML format
 Data access protocol


Page: 41

Exemplary SPARQL Query
“Return the models and prices for all cars of brand
„Jaguar‟ ” Declares namespaces for
abbreviated resources
SPARQL Query: identifiers.
PREFIX ex: <http://example.org/> Identifies the variables to
SELECT ?model ?price
WHERE appear in the query
{ ?car ex:Brand ex:Jaguar . results.
?car ex:Model ?model .
?car ex:Price ?price . } Provides the basic graph
pattern to match against
Exemplary Result: the data graph.
Model Price
“XJ” “79.750,00”
“XF” “44.900,00”

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Basic Graph Patterns (BGP)
 A SPARQL query contains a set of triple patterns called
a basic graph pattern
 Triple patterns are like RDF triples except that each of
the subject, predicate and object may be a variable.
 A basic graph pattern matches a subgraph of the RDF
data when RDF terms from that subgraph may be
substituted for the variables and the result is a RDF
graph equivalent to the subgraph.
WHERE Variables are
{ ?car ex:Brand ex:Jaguar . defined by
?car ex:Model ?model . “?varname” or
?car ex:Price ?price . }
“$varname”

Page: 43

Group Graph Patterns
 “Fora Group Graph Pattern, a set of graph patterns
must all match”
 Ina SPARQL query string, a group graph pattern is
delimited with braces: {...}
PREFIX ex: <http://example.org/>
WHERE
{ { ?car ex:Brand ex:Jaguar .
?car ex:Model ?model . }

?car ex:Price ?price .
}


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Optional Pattern Matching
 Basic graph patterns allow applications to make queries
where the entire query pattern must match.
 It is useful to be able to have queries that allow
information to be added to the solution where the
information is available, but do not reject the solution
because some part of the query pattern does not
match.
 If the optional part does not match, it creates no
bindings but does not eliminate the solution.

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

Page: 45

Optional Pattern Matching
 Example:
WHERE
{ ?car ex:Brand ex:Jaguar .

OPTIONAL { ?car ex:Price ?price . }
}

 Result
Model Price
“XJ” “79.750,00”
“XF” “44.900,00”
“S-Type”

Page: 46

Scope of Filters
 “A constraint, expressed by the keyword FILTER, is a
restriction on solutions over the whole group in which
the filter appears.”
WHERE
{ ?car ex:Brand ex:Jaguar .
?car ex:Price ?price .
FILTER (?price < 50.000,00)
}

 Result:
Model Price
“XF” “44.900,00”

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Unary Operators (1/2)

Operator Type(A) Result Type

BOUND(A) variable xsd:boolean

IsURI(A) RDF term xsd:boolean

isBLANK(A) RDF term xsd:boolean

isLITERAL(A) RDF term xsd:boolean


Page: 48

Unary Operators (2/2)

Operator Type(A) Result Type

STR(A) literal simple literal

STR(A) URI / IRI simple literal

LANG(A) literal simple literal

DATATYPE(A) typed literal URI / IRI

DATATYPE(A) simple literal URI / IRI


Page: 49

Binary Operators (1/2)
Operator Type(A) Type(B) Result Type

A= B

A != B
numeric numeric
A<B simple literal simple literal
xsd:string xsd:string xsd:boolean
A> B xsd:boolean xsd:boolean
xsd:dateTime xsd:dateTime
A <= B

A >= B

Page: 50

Binary Operators (2/2)
Operator Type(A) Type(B) Result Type

sameTERM
RDF term RDF term xsd:boolean
(A,B)
langMATCHES
simple literal simple literal xsd:boolean
(A,B)
REGEX
simple literal simple literal xsd:boolean
(A,B)

 Filter can be combined by “&&” or “||”

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Alternative Graph Patterns
 “SPARQL provides a means of combining graph
patterns so that one of several alternative graph
patterns may match. If more than one of the alternatives
matches, all the possible pattern solutions are found.”
 Pattern alternatives are syntactically specified with the
UNION keyword.
SELECT ?office
WHERE
{ ?x rdf:type ex:CarManufacturer .
{ ?x ex:Headquarter ?office . } UNION
{ ?x ex:Office ?office . }
}


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Solution Sequences and
Modifiers
 “Query patterns generate an unordered collection of
solutions. These solutions are then treated as a
sequence (a solution sequence), initially in no specific
order; any sequence modifiers are then applied to
create another sequence.”

 There are several use cases in which the results shall
be presented in a defined order, e.g. alphabetical order

 Shall duplicates be presented or deleted?


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Order By
 “Followingthe ORDER BY clause is a sequence of
order comparators, composed of an expression and an
optional order modifier (either ASC() or DESC()). Each
ordering comparator is either ascending (indicated by
the ASC() modifier or by no modifier) or descending
(indicated by the DESC() modifier).”

SELECT ?constructionYear ?price
WHERE { ?car ex:Brand ex:Jaguar .
?car ex:ConstructionYear ?constructionYear .
?car ex:Price ?price . }

ORDER BY DESC(?constructionYear) ?price .


Page: 54

Some more Solution Sequence
Modifiers
 LIMIT
 “The LIMIT clause puts an upper bound on the number of
solutions returned. If the number of actual solutions is greater
than the limit, then at most the limit number of solutions will be
returned.”
 OFFSET
 “OFFSET causes the solutions generated to start after the
specified number of solutions. An OFFSET of zero has no
effect.”
 SELECT DISTINCT
 “The DISTINCT solution modifier eliminates duplicate solutions.
Duplicates are eliminated before either limit or offset is applied.”

Page: 55

Semantic Web Layer Cake

A language for querying
information specified in
RDF.

A model for describing
resources with properties A format for specifying structured
and property values. data in a machine-readable form

Unique identification
of resources

Page: 56

Lessons Learned
 Now you should know ...
 ... the general idea of the Semantic Web and the
differences to the World Wide Web.
 ... the underlying technologies of the Semantic Web.
 ... how resources can be identified and specified.
 .. the importance of XML for expressing data in a machine-
readable way.
 ... the concept of the RDF and the way it is expressed
(e.g.: RDF/XML).
 ... how SPARQL can used to query data specified in RDF.


Page: 57

References and Additional
Material
 Introduction to RDF
 http://www.w3schools.com/rdf/rdf_intro.asp
 RDF Syntax Specification
 http://www.w3.org/TR/2004/REC-rdf-syntax-grammar-
20040210
 RDF Validation Tool
 http://www.w3.org/RDF/Validator/


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