semantic web محمد احمدی نیا ahmadinia@gmail.com. 2 of 73 outline why semantic web? ...
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Semantic Web
محمد احمدی نیاahmadinia@gmail.com
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Outline
Why Semantic Web? What is Semantic Web? Semantic Web Main Technologies
XML RDF RDFs OWL
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Mom
Physician’s Agent
Lucy’s Agent
requiredtreatment
Schedule appointment
Insurance Co.
Provider sites
Rating
in-plan?close-by?
Specialist?
Pete’ Agent
Driving schedule
Semantic Web Scenario
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The Vision
The World Wide Web (the syntactic web) is a big and impressive success story, both in terms of the amount of available information and the growth rate of human users
It has entered most areas of our daily life and business.
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The Vision
This success is based on its simplicity It’s easy to publish information on the web It’s easy to access information (but if you know where
they are)
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Why Semantic Web?
Tasks often require to combine data on the Web: hotel and travel information may come from different
sites searches in different digital libraries
Humans combine these information easily even if different terminologies are used the information is incomplete, or buried in images,
videos, … But Machines?
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Impossible using the Syntactic Web…
Complex queries involving background knowledge Find information about “animals that use sonar but are
not either bats or dolphins” Locating information in data repositories
Travel enquiries Prices of goods and services Results of human genome experiments
Finding and using “web services” Delegating complex tasks to web “agents”
Book me a holiday next weekend somewhere warm, not too far away, and where they speak French or English
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What is the Problem?
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Challenge
Query What is the name of a person who is a member of an
organization in Iran, and likes a kind of sport?
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Why Semantic Web?
(Some) data should be available for machines for further processing Metadata: Data about data
Data should be possibly combined, merged on a Web scale
Machines may also need to reason about that data
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What is Semantic Web?
Tim Berners-Lee:
“The Semantic Web is an extension of the current web in which information is given well-defined meaning, better enabling computers and people to work in cooperation.” 2001
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Semantic Web “The Semantic Web will bring structure
to the meaningful content of Web pages, creating an environment where software agents roaming from page to page can readily carry out sophisticated tasks for users” 2001
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Current Web
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Current Web
Resources identified by URI's untyped
Links href, src, ... limited, non-descriptive
User Exciting world - semantics of the resource, however, gleaned from content
Machine: Very little information available - significance of the links only evident from
the context around the anchor.
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Semantic Web
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Semantic Web
Resources Globally Identified by URI's Extensible Relational
Links Identified by URI's Extensible Relational
User Even more exciting world, richer user experience
Machine More processable information is available (Data Web)
Computers and people Work, learn and exchange knowledge effectively
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Semantic Web Technologies
The Semantic Web extends the Web through the use of standards, markup languages and related processing tools
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Semantic Web Architecture
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A Simple Web Page
Markup consists of: rendering information (e.g.,
font size and colour) Hyper-links to related
content
Semantic content is accessible to humans but not (easily) to computers…
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Challenge
Query What is the name of a person who is a member of an
organization in Iran, and likes a kind of sport?
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What information can we see…
Hi!
My name is Ali!
This is my homepage. I am a student at Ferdowsi University of Mashhad.
I like football.
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The Page Source Code in HTML
<html> <head> <title>Ali's Homepage </title></head><body> <p><b>Hi!</b></p> <p><b>My name is Ali!</b></p> </b> <p>This is my homepage. I am a student at <a href=
“http://um.ac.ir">Ferdowsi University of Mashhad</a>.</p> <p>I like football.</p>
</body></html>
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What information can a machine see…
<html> <head> <title>…………………..</title></head><body> <p><b>………</b></p> <p><b>....................</b></p> </b> <p>…………………………………………..……<a
href=“………………”>…………………..</a>.</p> <p>............................</p>
</body></html>
XML
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XML
eXtensible Markup Language XML allows users to add arbitrary structure to their
documents but says nothing about what the structures mean
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Solution: XML “meaningful” tags
………………………….
<student>………………….</student><university>……………………..<university>
…………………………………………………………………………………………………….
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HTML vs. XML
HTML is for humans HTML describes web pages Browsers ignore and/or correct as many HTML errors as they can,
so HTML is often sloppy XML is for computers
XML describes data The rules are strict and errors are not allowed
In this way, XML is like a programming language Current versions of most browsers can display XML
However, browser support of XML is spotty at best
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XML Building blocks The XML declaration looks like this:
<?xml version="1.0" encoding="UTF-8“ ?>
Identify the nature of the content they surround General format:
<element attribute=“value” > … </element>
Empty element: <empty-Element attribute=“value” />
XML Elements have Relationships Elements are related as parents and children <name>
<first>David</first> <last>Matuszek</last>
</name>
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XML Applications
XML is used to Exchange Data Reduce the complexity of interpreting data
XML can be used to Store Data Accessible to more applications
XML can be used to Create new Languages, e.g. : WML (Wireless Markup Language), MusicXML
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XML Validation
"Well Formed" XML document correct XML syntax
"Valid" XML document “well formed” Conforms to the rules of a DTD
XML DTD defines the legal building blocks of an XML document Can be inline in XML or as an external reference
XML Schema an XML based alternative to DTD, more powerful Support namespace and data types
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An Example XML with DTD
<?xml version="1.0"?><!DOCTYPE note [
<!ELEMENT note (to,from,heading,body)> <!ELEMENT to (#PCDATA)> <!ELEMENT from (#PCDATA)> <!ELEMENT heading (#PCDATA)> <!ELEMENT body (#PCDATA)>
]> <note>
<to>Tove</to> <from>Jani</from> <heading>Reminder</heading> <body>Don't forget me this weekend</body>
</note>
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XML Schemas
“Schema” is a general term DTDs are a form of XML schemas
When we say “XML Schemas,” we usually mean the W3C XML Schema Language This is also known as “XML Schema Definition”
language, or XSD.
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But What if?
………………………….
<boy>………………….</boy><college>……………………..<college>…………
………………………………………………………………………………………….
RDF
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RDF
Resource Description Framework A data model
o Meaning encoded in sets of ‘triples’: entities have properties which have values
o Each triple: subject-predicate-object
o Entities, properties and values all have distinct URIs
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RDF Model
A model is a collection of statements Statement := (predicate,subject,object) Predicate is a resource Subject is a resource Object is either a resource or a literal
Subject Object
Predicate
Statement
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RDF
AliFerdowsi
University of Mashhad
studentOF
http://um.ac.ir
hasHomePage
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RDF
<rdf:Description about="some.uri/person/ali"><studentOf resource="some.uri/Ferdowsi_University_of_Mashhad"/>
</Description>
<Description about="Ferdowsi_University_of_Mashhad"> <hasHomePage>http://um.ac.ir</hasHomePage></rdf:Description>
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Example
We are looking for a woman who works for one of my client and her son studies at University of Rovaniemi
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XML vs. RDF RDF is not just an XML dialect.
XML: Has a tree structure data model. Only nodes are labeled.
RDF: Has a graph structure data model. Both edges (properties) and nodes (subjects/objects) are
labeled.
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Challenge
Query What is the name of a person who is a member of an
organization in Iran, and likes a kind of sport?
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But…
What is the relation between a Student and a Person? Football and Sport? University and College? University and Website?
Ontology is the solution
Ontology
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Ontology
To answer the query it is required to tell the machine that There is a class of objects named ‘Student’ There is a class of objects named ‘Person’ Each Student is a Person Each Person has a Name There is a class of objects named ‘Sport Type’ Football is a kind of Sport There is an instance of a Student that its Name is “Ali” ….
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Ontology
Philosophy The study of what is, what has to be true for
something to exist, the kinds of things that can exist
AI and computer science Co-opted the term. Something exists if it can be
represented, described, defined (in a formal, hence, machine-interpretable way).
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Ontology Ontologies are about vocabularies and their meanings, with
explicit, expressive, and well-defined semantics, possibly machine-interpretable.
“Ontology is a formal specification of a conceptualization.” Gruber, 1993
Main elements of an ontology: Concepts Relationships
Hierarchical Logical
Properties Instances (individuals)
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Ontology
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Ontologies in the Semantic Web Provide shared data structures to exchange
information between agents Can be explicitly used as annotations in web sites Can be used for knowledge-based services using
other web resources
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Ontology
Ontology Languages RDFS (RDF Schema) OWL SHOE DAML+OIL
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RDF Schema (RDFS)
RDF gives a formalism for meta data annotation, and a way to write it down in XML, but it does not give any special meaning to vocabulary such as subClassOf
RDF Schema allows you to define vocabulary terms and the relations between those terms in special domain it gives “extra meaning” to particular RDF predicates and
resources
this “extra meaning”, or semantics, specifies how a term should be interpreted
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Core Classes & PropertiesCore Classes
Core Properties
rdfs:Resource
rdfs:Literal
rdfs:Statement
rdfs:Class
rdfs:Property
rdfs:Type
rdfs:SubClassOf
rdfs:SubPropertyOf
rdfs:Domain
rdfs:Range
rdfs:Label
rdfs:Comment
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RDF &RDFS (a sample)
زبانهای برنامه سازی وب
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RDFS Sample Code<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#">
<rdfs:Class rdf:ID=“professor">
<rdfs:comment>
The class of professor All professors are academic staff members.
</rdfs:comment>
<rdfs:subClassOf rdf:resource="#academicStaffMember"/>
</rdfs:Class>
<rdfs:Class rdf:ID="academicStaffMember">
<rdfs:comment>
The class of academic staff members
</rdfs:comment>
<rdfs:subClassOf rdf:resource="#staffMember"/>
</rdfs:Class>
<rdfs:Class rdf:ID="staffMember">
<rdfs:comment>The class of staff members</rdfs:comment>
</rdfs:Class>
زبانهای برنامه سازی وب
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RDFS Sample Code …<rdfs:Class rdf:ID="course">
<rdfs:comment>The class of courses</rdfs:comment>
</rdfs:Class>
<rdf:Property rdf:ID="involves">
<rdfs:comment>
It relates only courses to lecturers.
</rdfs:comment>
<rdfs:domain rdf:resource="#course"/>
<rdfs:range rdf:resource="#academicStaffMember "/>
</rdf:Property>
<rdf:Property rdf:ID="isTaughtBy">
<rdfs:comment>
Inherits its domain ("course") and range ("lecturer")
from its superproperty "involves"
</rdfs:comment>
<rdfs:subPropertyOf rdf:resource="#involves"/>
</rdf:Property>
زبانهای برنامه سازی وب
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Problems with RDFS RDFS too weak to describe resources in sufficient detail
No localised range and domain constraints Can’t say that the range of hasChild is person when applied to
persons and elephant when applied to elephants No existence/cardinality constraints
Can’t say that all instances of person have a mother that is also a person, or that persons have exactly 2 parents
No transitive, inverse or symmetrical properties Can’t say that isPartOf is a transitive property, that hasPart is the
inverse of isPartOf or that touches is symmetrical …
Difficult to provide reasoning support No “native” reasoners for non-standard semantics May be possible to reason via FO axiomatisation
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OWL (Ontology Web Language) OWL is now a W3C Recommendation The purpose of OWL is identical to RDFS i.e. to provide an
XML vocabulary to define classes, properties and their relationships. RDFS enables us to express very rudimentary relationships
and has limited inferencing capability. OWL enables us to express much richer relationships, thus
yielding a much enhanced inferencing capability. The benefit of OWL is that it facilitates a much greater degree
of inference than you get with RDF Schema.
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Origins of OWL
RDFRDF
DAML+OILDAML+OIL
DARPA Agent Markup Language
A W3C Recommendation
OILOIL
OWLOWL
All influenced by RDF
Ontology Inference Layer
EU/NSF Joint Ad hoc Committee
DAMLDAML
OWL LiteOWL DL OWL Full
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OWL OWL and RDF Schema enable rich machine-processable semantics
XML/DTD/XML Schemas
RDF Schema
OWL
Semantics
Syntax
<rdfs:Class rdf:ID="River"> <rdfs:subClassOf rdf:resource="#Stream"/></rdfs:Class>
<owl:Class rdf:ID="River"> <rdfs:subClassOf rdf:resource="#Stream"/></owl:Class>
RDFS
OWL
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Full: Very expressive, no computation guarantees
DL (Description Logic): Maximum expressiveness, computationally complete
Lite: Simple classification hierarchy with simple constraints.
Versions of OWL
Depending on the intended usage, OWL provides three increasingly expressive sublanguages
OWL Full
OWL DL
OWL Lite
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OWL Ontologies What’s inside an OWL ontology
Classes + class-hierarchy Properties (Slots) / values Relations between classes
(inheritance, disjoints, equivalents) Restrictions on properties (type, cardinality) Characteristics of properties (transitive, equivalence,
symmetry …) Annotations Individuals
Reasoning tasks: classification,consistency checking
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owl:Class
Top-most class: owl:Thing
>owl:Class rdf:ID=“Person</">owl:Class rdf:ID=“Man<"
>rdfs:subClassOf rdf:resource="#Person </ "/>owl:Class<
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Properties
What is a Property? e.g., has_father, has_pet, service_number a collection of relationships between individuals (and
data) a way of describing a kind of relationship between
individuals an object in the world (OWL Full)
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OWL PropertiesObject
Properties
Ana has_father David
Is range a
literal / typed value ?
then ERROR
Data type
Properties
Ana age 25
XML Schema data types supported DB people happy
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Defining Properties
ObjectProperty DatatypeProperty rdfs:subPropertyOf rdfs:domain rdfs:range
<owl:ObjectProperty rdf:ID="madeFromGrape"> <rdfs:domain rdf:resource="#Wine"/> <rdfs:range rdf:resource="#WineGrape"/> </owl:ObjectProperty>
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Describing classes in OWLComplex Classes
Intersection of classes (owl:intersectionOf) OR (A B)
Union of classes (owl:unionOf) AND (A B)
Complement (owl:complementOf) NOT
Enumeration (owl:oneOf)
Disjoint Classes (owl:disjointWith)
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Describing classes in OWLRestrictions on Property Classes
Properties: allValuesFrom: rdfs:Class (lite/DL owl:Class) hasValue: specific Individual someValuesFrom: rdfs:Class (lite/DL owl:Class) minCardinality: xsd:nonNegativeInteger (in lite {0,1}) maxCardinality: xsd:nonNegativeInteger (in lite {0,1})
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Describing properties in OWLOWL vs. RDFS RDF Schema provides some of predefined properties:
rdfs:range used to indicate the range of values for a property. rdfs:domain used to associate a property with a class. rdfs:subPropertyOf used to specialize a property. …
OWL provides additional predefined properties: owl:cardinality (indicate cardinality) owl:hasValue (at least one of the specified property values) …
OWL provides additional property classes, which allow reasoning and inferencing: owl:FunctionalProperty owl:TransitiveProperty …
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OWL Cardinality
min Cardinality max Cardinality
“Cardinality” When min = max
has Value belongs to the class if it has the value
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An Example OWL ontology<owl:Class rdf:ID=“Person” /><owl:Class rdf:ID=“Man”>
<rdfs:subClassOf rdf:resource=“#Person” /><owl:disjointWith rdf:resource=“#Woman” />
</owl:Class><owl:Class rdf:ID=“Woman”>
<rdfs:subClassOf rdf:resource=“#Person” /><owl:disjointWith rdf:resource=“#Man” />
</owl:Class><owl:Class rdf:ID=“Father”>
<rdfs:subClassOf rdf:resource=“Man” /><owl:Restriction owl:minCardinality="1">
<owl:onProperty rdf:resource="#hasChild" /></owl:Restriction>
</owl:Class><owl:ObjectProperty rdf:ID=“hasChild">
<rdfs:domain rdf:resource="#Parent" /><rdfs:range rdf:resource="#Person" />
</owl:ObjectProperty>
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Semantic Web: Now
Numerous web technologies, languages, formats, and standards which provide the inferastructure URI, XML, XML Schema, RDF, RDFS, OWL,
SPARQL, SerQL, HTTP, Microformats, RDFa,
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Semantic Web: Now
A large body of research and activity is related to ontologies, since they play a main role Ontology engineering issues and tools
ontology development, ontology selection, ontology matching, ontology evaluation, ontology partitioning,….
Numerous Ontologies are developed in different domains, e.g. FOAF, DOAP, SIOC, SKOS, …
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Semantic Web: Now
Semantic Web technologies are used as an enabler in different areas to develop small semantic web applications Ontology-based ….. Ontology-enabled … Semantic-Web-Enabled … Semantic ….
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Semantic Web: Now
Semantic Web Search Engines Swoogle, SWSE, Watson, Falcons, Yahoo!
Microsearch, ….
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