The Semantic Web

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The Semantic Web

Riccardo Rosati


Dottorato in Ingegneria Informatica

Sapienza Università di Roma


a.a. 2006/07

The Semantic Web
-

course overview

2

Overview


the aim of this course is to provide an introduction
to the Semantic Web...

... with emphasis on two aspects:


emphasis on
AI
-
related aspects
, in particular
knowledge representation and reasoning


emphasis on
database
-
related aspects
, in particular

data integration


The Semantic Web
-

course overview

3

Overview


Lecture 1: Introduction to the Semantic Web



Lecture 2: The XML layer



Lecture 3: The RDF layer



Lecture 4: The Ontology layer 1


Description Logics, OWL


Lecture 5: The Ontology layer 2


reasoning in OWL, OWL species

Overview


Lecture 6: The Ontology layer 3


OWL technologies: OWL Tools, QuOnto


Lecture 7: The rule layer



Lecture 8: The RDF layer 2



RDF semantics


Lecture 9: The Ontology layer 4


query answering in OWL


Lecture 10: Handling inconsistency (Jan Chomicki)

Lecture 1

Introduction to the Semantic Web

Introduction to the Semantic Web

6

What is the Semantic Web?


“The Semantic Web is a Web of actionable
information

information derived from data
through a
semantic theory

for interpreting the
symbols.”


“The semantic theory provides an account of
‘meaning’ in which the logical connection of
terms establishes
interoperability

between
systems”



(Shadbot, Hall, Berners
-
Lee, The Semantic Web revisited, IEEE
Intelligent Systems, May 2006)

Introduction to the Semantic Web

7

The Semantic Web: why?


search on the Web: problems...


...due to the way in which information is stored on
the Web


Problem 1
: web documents do not distinguish
between information content and presentation


(“solved” by XML)


Problem 2
: different web documents may
represent in different ways semantically related
pieces of information


this leads to hard problems for “intelligent”
information search on the Web

Introduction to the Semantic Web

8

Separating content and presentation


Problem 1
: web documents do not distinguish
between information content and presentation


problem due to the HTML language


problem “solved” by current technology


stylesheets (HTML, XML)


XML


stylesheets allow for separating formatting
attributes from the information presented

Introduction to the Semantic Web

9

Separating content and presentation


XML: eXtensible Mark
-
up Language


XML documents are written through a user
-
defined set of tags


tags are used to express the “semantics” of the
various pieces of information

Introduction to the Semantic Web

10

XML: example

HTML:


<H1>
Seminari di Ingegneria del Software
</H1>

<UL>




<LI>
Teacher: Giuseppe De Giacomo


<LI>
Room: 7


<LI>
Prerequisites: none

</UL>


XML:


<course>

<title>
Seminari di Ingegneria del Software


</title>

<teacher>
Giuseppe De Giacomo
</teacher>

<room>
1AI, 1I
</room>

<prereq>
none
</prereq>

</course>

Introduction to the Semantic Web

11

Limitations of XML

XML does not solve all the problems:


legacy HTML documents


different XML documents may express
information with the
same meaning

using
different
tags

Introduction to the Semantic Web

12

The need for a “Semantic” Web


Problem 2
: different web documents may
represent in different ways semantically related
pieces of information


different XML documents do not share the
“semantics” of information


idea: annotate (mark
-
up) pieces of information to
express the “meaning” of such a piece of
information


the meaning of such tags is shared!




shared semantics

Introduction to the Semantic Web

13

The Semantic Web initiative

viewpoint:


the Web = a web of data

goal:


to provide a common framework to share data on
the Web across application boundaries

main ideas:


ontology


standards


“layers”

Introduction to the Semantic Web

14

The Semantic Web Tower

Introduction to the Semantic Web

15

The Semantic Web Layers


XML layer


RDF + RDFS layer


Ontology layer


Proof
-
rule layer


Trust layer


Introduction to the Semantic Web

16

The XML layer


XML (eXtensible Markup Language)


user
-
definable and domain
-
specific markup


URI (Uniform Resource Identifier)


universal naming for Web resources


same URI = same resource


URIs are the “ground terms” of the SW



W3C standards


Introduction to the Semantic Web

17

The RDF + RDFS layer

RDF = a simple conceptual data model

W3C standard (1999)


RDF model = set of RDF
triples


triple = expression (statement)


(subject, predicate, object)



subject = resource


predicate = property (of the resource)


object = value (of the property)

=> an RDF model is a
graph

Introduction to the Semantic Web

18

The RDF + RDFS layer

Example of RDF graph:


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

“Ora Lassila”

dc:Creator

“1999
-
02
-
22”

dc:Date

“W3C”

dc:Publisher

Introduction to the Semantic Web

19

The RDF + RDFS layer


RDFS = RDF Schema


“vocabulary” for RDF


W3C standard (2004)

example:

Person

Student

Researcher

subClassOf

subClassOf

Jeen

type

hasSuperVisor

domain

range

Frank

type

hasSuperVisor

RDFS


RDF

Introduction to the Semantic Web

20

The Ontology layer

ontology = shared conceptualization



conceptual model


(more expressive than RDF + RDFS)



expressed in a true knowledge representation
language



OWL

(Web Ontology Language) = standard
language for ontologies

Introduction to the Semantic Web

21

The proof/rule layer

beyond OWL:


proof/rule layer


rule: informal notion


rules are used to perform inference over
ontologies


rules as a tool for capturing further knowledge
(not expressible in OWL ontologies)



Introduction to the Semantic Web

22

The Trust layer


SW top layer:


support for provenance/trust


provenance:


where does the information come from?


how this information has been obtained?


can I trust this information?


largely unexplored issue


no standardization effort

Introduction to the Semantic Web

23

The Semantic Web: main ingredients


underlying web layer (URI, XML)


reusing and extending web technologies


basic conceptual modeling language (RDF)


ontology language (OWL)


rules/proof


reusing and extending AI technologies


knowledge representation


automated reasoning


...and database technologies


data integration

Introduction to the Semantic Web

24

The Semantic Web from an AI perspective


the notion of ontology


the role of logic and Description Logics


the role of rule
-
based formalisms


... (agent technology)

Introduction to the Semantic Web

25

The notion of ontology


ontology =
shared conceptualization

of a domain
of interest


shared vocabulary => simple (shallow) ontology


(complex) relationships between “terms” => deep
ontology


AI view:


ontology = logical theory (knowledge base)


DB view:


ontology = conceptual model

Introduction to the Semantic Web

26

Ontologies: example

class
-
def
animal



% animals are a class

class
-
def
plant



% plants are a class


subclass
-
of
NOT
animal

% that is disjoint from animals

class
-
def
tree


subclass
-
of
plant


% trees are a type of plants

class
-
def
branch


slot
-
constraint
is
-
part
-
of

% branches are parts of some tree


has
-
value

tree


max
-
cardinality

1

class
-
def
defined carnivore

% carnivores are animals


subclass
-
of
animal


slot
-
constraint
eats


% that eat any other animals


value
-
type

animal

class
-
def
defined herbivore

% herbivores are animals


subclass
-
of
animal,
NOT
carnivore
% that are not carnivores, and


slot
-
constraint
eats



% they eat plants or parts of plants


value
-
type
plant
OR (slot
-
constraint
is
-
part
-
of
has
-
value
plant)


Introduction to the Semantic Web

27

Ontologies: the role of logic


ontology = logical theory


why?


declarative


formal semantics


reasoning (sound and complete inference techniques)


well
-
established correspondence between
conceptual modeling formalisms and logic

Introduction to the Semantic Web

28

Ontologies and Description Logics


OWL is based on a fragment of first
-
order
predicate logic (FOL)


Description Logics (DLs) = subclasses of FOL


only unary and binary predicates


function
-
free


quantification allowed only in restricted form


(variable
-
free syntax)


decidable reasoning



DLs are one of the most prominent languages for
Knowledge Representation

Introduction to the Semantic Web

29

Ontologies and Description Logics


expressive abilities of DLs have been widely
explored


reasoning in DLs has been extensively studied


DL reasoners have been developed and optimized




DLs as a central technology for the SW


Introduction to the Semantic Web

30

Rule
-
based formalisms


Prolog


Logic programming


Constraint (logic) programming


Production rules


Datalog


...


Rule language for SW not standardized yet

RIF (Rule Interchange Format) W3C working group

Introduction to the Semantic Web

31

The SW from a database perspective


ontology as a virtual database schema


ontology
-
based information access


the Semantic Web as a framework for data
integration


Introduction to the Semantic Web

32

Virtual data integration

abstract model of a virtual data integration system:

triple (
G
,
S
,
M
)


G

= global information schema


S

= set of data sources


M

= mapping between sources and global schema


1.
the data sources are autonomous and heterogeneous
information systems

2.
the global schema is a virtual conceptualization of the
domain of interest

3.
the mapping is a declarative specification of the
relationship between sources and global schema

Introduction to the Semantic Web

33

Ontology
-
based information access


ontology = virtual global information schema


ontology language = conceptual modeling
language


the Web = distributed, heterogeneous, autonomous
set of data sources

but :


data integration considers
different formalisms for
expressing schema and data


the architecture of a data integration system is
centralized


Introduction to the Semantic Web

34

Ontology
-
based information access



the data integration approach can be generalized to
non
-
centralized architectures


=> peer data management systems


despite the differences in the formalisms adopted,
there is a tight relationship between the SW and
data integration


=> dealing with incomplete information



Introduction to the Semantic Web

35

The Semantic Web and data integration

similarity between data integration and the SW:


global schema = ontology


sources = web sites (data)


mapping = ??


however:


different languages


different technologies


=>
the SW is essentially a data integration technology

Introduction to the Semantic Web

36

Very quick overview of SW technologies

main current technologies and standards for the SW:


RDF


RDFS


OWL


Introduction to the Semantic Web

37

RDF


RDF = Resource Description Framework


RDF data model is an abstract, conceptual layer
(
independent

of XML)


RDF data model = set of RDF
triples



triple =
(subject, predicate, object)


subject = resource


predicate = property (of the resource)


object = value (of the property)



standardized in 1999

Introduction to the Semantic Web

38

RDFS


RDFS = RDF Schema


set of predefined predicates:


subClassOf


subPropertyOf


domain


range


...


...with predefined semantics!


standardized in 2004

Introduction to the Semantic Web

39

OWL


OWL = Web Ontology Language


the OWL family is constituted by 3 different
languages (with different expressive power):


OWL Full


OWL
-
DL


OWL
-
Lite



technology at an early stage


standardized in 2004


reasoning techniques and tools are very recent


“optimization” of reasoning is largely unexplored

Introduction to the Semantic Web

40

OWL vs. RDFS



class
-
def



subclass
-
of



property
-
def



subproperty
-
of



domain



range



class
-
expressions



AND, OR, NOT



role
-
constraints



has
-
value, value
-
type



cardinality



role
-
properties



trans, symm...

RDF(S)

OWL

Introduction to the Semantic Web

41

From RDFS to OWL Full


with respect to the relative expressive power:


OWL Full > OWL
-
DL > OWL
-
Lite > RDFS