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Transcript of The Semantic Web Riccardo Rosati Dottorato in Ingegneria Informatica Sapienza Università di Roma...
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The Semantic Web
Riccardo Rosati
Dottorato in Ingegneria InformaticaSapienza Università di Roma
a.a. 2006/07
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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
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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
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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)
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Lecture 1
Introduction to the Semantic Web
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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)
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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
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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
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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
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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>
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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
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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
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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”
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Introduction to the Semantic Web 14
The Semantic Web Tower
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Introduction to the Semantic Web 15
The Semantic Web Layers
• XML layer
• RDF + RDFS layer
• Ontology layer
• Proof-rule layer
• Trust layer
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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
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Introduction to the Semantic Web 17
The RDF + RDFS layer
RDF = a simple conceptual data modelW3C 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
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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
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Introduction to the Semantic Web 19
The RDF + RDFS layer
• RDFS = RDF Schema• “vocabulary” for RDF• W3C standard (2004)
example:
Person
Student Researcher
subClassOfsubClassOf
Jeentype
hasSuperVisordomain range
Frank
type
hasSuperVisor
RDFS
RDF
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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
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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)
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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
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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
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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)
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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
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Introduction to the Semantic Web 26
Ontologies: example
class-def animal % animals are a classclass-def plant % plants are a class subclass-of NOT animal % that is disjoint from animalsclass-def tree subclass-of plant % trees are a type of plantsclass-def branch slot-constraint is-part-of % branches are parts of some tree has-value tree max-cardinality 1class-def defined carnivore % carnivores are animals subclass-of animal slot-constraint eats % that eat any other animals value-type animalclass-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)
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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
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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
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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
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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
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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
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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
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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 sourcesbut :• data integration considers different formalisms for
expressing schema and data• the architecture of a data integration system is
centralized
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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
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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
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Introduction to the Semantic Web 36
Very quick overview of SW technologies
main current technologies and standards for the SW: • RDF • RDFS• OWL
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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
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Introduction to the Semantic Web 38
RDFS
• RDFS = RDF Schema• set of predefined predicates:
– subClassOf– subPropertyOf– domain– range– ...
• ...with predefined semantics!• standardized in 2004
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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
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Introduction to the Semantic Web 40
OWL vs. RDFS
• class-def• subclass-of• property-def• subproperty-of• domain• range
• 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...
• class-expressions• AND, OR, NOT
• role-constraints• has-value, value-type• cardinality
• role-properties• trans, symm...
RDF(S) OWL
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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