Semantic web

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Computer
communication B

The Semantic Web


Bibliography


The Semantic Web
, Scientific American, May
2001, Tim Berners
-
Lee, James Hendler and
Ora Lassila.



Breitman, K.K., Casanova, M.A., &
Truszkowski, W. (2007)
Semantic web:
Concepts, Technologies and Applications.
Springer Verlag, London


http://www.w3.org/


Antoniou, G., Van Harmelen, F. `(2004) “A
Semantic web Primer”(see library or Pdf
copy)


The semantic web:

some definitions


The Semantic 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


(Berners
-
Lee, Hendlers, J. & Lassila, O., 2001)


“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
(W3C, 2003)


“Soon it will be possible to access the Web resources by
content rather than just by keywords (Anutariya et al, 2001)


The semantic web (SW): definitions


“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
(W3C, 2003)


“Soon it will be possible to access the Web
resources by content rather than just by
keywords (Anutariya et al, 2001)


Semantic Web: Introduction


The content of the present Word Wide Web is
nowadays only accessible and can be
elaborated only by people


The Semantic Web is an enlargement of the
WWW with semantic information that can be
used by computers


With the help of semantic information the
content of pages could be processed
automatically and computers could make
inferences about a search

The semantic web: characteristics


The semantic web is not different from the www,
is actually a developing part of it.


The infrastructures and characteristics should be
common


Use URI (Uniform resource Identifiers) addressing


Use protocols that a have a small and universally
understood set of commands (like HTTP: Hypertext
Transfer Protocol)


Be decentralized (like the www)


Function on a large scale

The semantic web: The layer cake


The semantic web


Two characteristics for the construction of the semantic web


1.
Downward compatibility



Agents fully aware of a layer should also be able to interpret and
use information written at lower levels. For example, agents
aware of the semantics of OWL can take full advantage of
information written in RDF and RDF Schema.


2.
Upward partial understanding




On the other hand, agents fully aware of a layer should take at

least partial advantage of information at higher levels. For

example, an agent aware only of the RDF and RDF Schema

semantics can interpret knowledge written in OWL partly, by

disregarding those elements that go beyond RDF and RDF

Schema.

XML: Extensible Markup Language 1


It is a general purpose markup Language for creating specific
purpose mark
-
up languages


Follows the SGML
-
standards (Standard Generlised Markup
Language)


With XML the single users can create their own tags (which is
not possible with HTML)


Differences between HTML and XML


HTML (Hypertext Markup Language)


Has a fixed set of tags


It is most frequently used to define the lay
-
out


Does not focus on the logical content or on the structure


XML


It is possible to personally define the tags


Tags reflect a content


The layout is defined in a separate document (stylesheet)


WWW: HTML

Semantic web: XML

XML: Extensible Markup Language 2


A XML document consists of plain text and markup, in the form of tags.


A XML document is interpreted by application programs


A XML document can be represented in a form of a “tree”


XML: Extensible Markup Language 3


A XML document consists of


Elements

formed by


A
start
-
tag


A
content


A matching
end
-
tag



Elements can be nested in a
tree

form


Every element is named after
the start
-
tag






XML: Extensible Markup Language 4


A start
-
tag can have zero or more
attributes


Name
(followed by the equal sign)


Value
(between double quotes)





Every XML
-
document has to follow a specific
syntax
:


Every start
-
tag needs a matching end
-
tag (see previous slide)


Elements need to be nested in other elements


An XML
-
document can contain a XML
-
schema (defines additional
constraints on the document structure)




value

Name

RESOURCE, URIs and
NAMESPACES


A
resource

is anything that has an identity


Digital (i.e an electronic document)


Physical (i.e. a book)


A URI (Uniform Resource Identifier) is a
character string that identifies a
resource

on the Web


URIs can follow different schemes


FTP (File Transfer Protocol)


HTTP (Hypertext Transfer Protocol

http://www.mysite.com/food.html

Namespaces


Namespaces are contexts, the domain of
specific elements


Namespaces are identified by a URI


URIref: It is a URI with an optional
fragment identifier attached to it, preceded
by #


The semantic web: The layer cake


RDF: Resource Description Framework 1


RDF is a general
-
purpose language for
representing information in the web


Useful to represent metadata about Web
resources


RDF describes
resources

(Both abstract or
concrete subjects) identifiable via an URI


The syntax of RDF is based on XML


RDF
-
documents are written as XML
-
documents with the tag
rdf:RDF


RDF Statements


A RDF
-
statement is described by a triple (S, P, O)


S=
Subject of the statement (It’s a URIref)


P= Property (Predicate) of the statement (URIref)


The value of a property can be a simple value (ordinary
number), or can be a literal (string of characters)


O= Object

RDF
-
Schema


A RDF
-
schema:


Offers the bases to model hierarchies and
classes of properties.

The semantic web: The layer cake


Ontology: definitions

Ontology comes from:


Ontos (greek)= Being + Logos=Word


Gruber (1993):

“An ontology is a formal explicit specification of a
shared conceptualization”





WC3
-
consortium


“Ontology is a term borrowed from philosophy that refers
to the science of describing the kinds of entities in the
world and how they are related “


Should be machine
readable

A abstract model

Ontology


Ontology categorizes
concepts

(which are defined by a
set of common properties) into classes based on
common characteristics



Ontology is the representation of the knowledge of a
domain where a set of objects and their relationships is
described by a
vocabulary
.



Ontologies should provide descriptions for


Classes (things) in the various domains


Relationships among things


Properties of these things


Ontology



Ontologies should satisfy certain demands:


Expressivity: domains should be described


Consistency: it should not give contradictory information


It should support reasoning processes



Ontologies are useful in
sharing

and exchanging
information

between software agents


Ontologies do not necessarily reflect the human way of
thinking of how knowledge is classified


Ontologies should therefore not be seen as a reflection of
human intelligence


Ontology vs Taxonomy


Taxonomy


Is a classification of terms in form of a hierarchy
using typically a father
-
son relationship (i.e. Type of)


Example The taxonomy of the leaving beings

Kingdom: Animalia


Filo: Chordata


Subfilo: Vertebrata


……

Web Ontology Languages


They are designed to define ontologies


They are based on RDF and RDF
-
schema


SHOE


Oil (Ontology inference Layer)


OWL (Web Ontology Language)


http://www.w3.org/TR/owl
-
features/


It is an ontology description language


It is a standard language for the modeling of ontologies


Facilitates the interpretability of the Web content (more
than XML or RDF)


Less complex than RDF
-
schema


Has additional vocabulary based on description logic


OWL 1


Describes classes, properties and relations to
facilitate machine interpretability of the Web
content



Owl is defined as a vocabulary (like RDF) but is
semantically richer



In OWL classes of entities can be specified in
different ways, for example:


Which individuals belong to a specific class


Which qualities should have the elements that belong
to that class


If individuals belong to a particular underclass



OWL 2


It is possible to limit a membership in a
subclass


With RDF
-
schema it is possible to produce
subclasses


OWL: example 1


Definition of “name”










Properties, classes and things are distinguished


Things can be summarized in classes


OWL: example 2


Definition of “marine mammal”










Precise definition of classes: How many
subclasses are there?


OWL: example 2


What is dolphin?










A class is defined like a cross
-
section of two
classes


The semantic web: The layer cake


Logic 1: Logic rules


With logic it is possible to go further than the explicit
information


It is possible to control if till now the collected information is
consistent or if it has to be revised


With the rule of logic it is possible to derive novel information


Two inference rules


Modus Ponens

If A
→ B

A

Therefore B

If: “if x is a person than this person has a mother”

“Jan is a person” → “Jan has a mother”


Modus Tollens (from “tollere” take away from)

If A
→ B

⌐B

Therefore ⌐A

If: “if x is a person than this person has a mother”

“Jan does not have a mother” → “Jan is not a person”


Logic 2


Logic is the study of the principles of valid inferences
and demonstrations (traditionally considered a part of
philosophy)


Logic rules are used to create reasonings


Their formulation is close to the formulation used in natural
language


Logic is about the rules themselves, and not about the
way they have to be applied.


Classical logic is unfortunately too limited to model all
types of human reasoning


Default
-
reasoning:


“if x is a bird then x can fly”


“if x a penguin is then x can not fly”


More complicated forms of logic are more difficult to
process



The semantic web: The layer cake


Proof


There is a difference between the rule
formulation and their application to solve a
problem


There must be an intelligent choice of rules
and facts (a good order for the rules)


There should be good strategies for efficient
argumentations


Proof
-
systems are important topics in
informatics


But not much has been done in the
domain of the Semantic Web

Trust


Trust can be perceived at different levels


Is the processed information plausible?


How can be trust be verified?


On human judgments


Based on social trust (trust people that are
trusted by other people)