Semantic Web Technology

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Semantic Web Technology

• COMP3150/4700 E-technology Architectures, Tools
  and Applications
• (Fall 2007)

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Lecture Outline

• Basic Ideas of RDF
• The Language of RDF
• Basic Ideas of RDF Schema
• ?he Language of RDF Schema
• Basic Ideas of OWL
• The Language of OWL

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Lecture Outline

• Basic Ideas of RDF
• The Language of RDF
• Basic Ideas of RDF Schema
• ?he Language of RDF Schema
• Basic Ideas of OWL
• The Language of OWL

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Why XML is not sufficient?

• XML is a universal metalanguage for defining
  markup
• However, XML does not provide any means of
  talking about the semantics (meaning) of data
• E.g., there is no intended meaning associated
  with the nesting of tags
• It is up to each application to interpret the
  nesting.

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Nesting of Tags in XML

• ipod
• Apple
• Opposite nesting, same information!
• How to solve this ambiguity?

ipod is a product of Apple.
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Basic Ideas of RDF

• RDF Resource Description Framework
• Basic building block (also called a statement)
  resource-property-value triple
• RDF has been given a syntax in XML
• This syntax inherits the benefits of XML

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Resources

• In RDF, everything is referred to as a resource
  (with a URI) which can be
• a thing (e.g., book, person, etc.) or
• a concept (e.g., ownership, role, etc.)
• Recall that URI stands for Universal Resource
  Identifier that can be
• a URL (Web address) or
• some other kind of unique identifier

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Properties

• A special kind of resources, e.g.,
• Peter (resource) Develop (property) Software
  Package ABC (value)
• Develop (resource) type-of (property) IT Job
  (value)
• Describe relations between resources
• E.g. writtenBy, hasAge, hasTitle, etc.
• Properties are also identified by URIs.

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Statements

• Statements assert the properties of resources
• A statement is an resource-property-value triple
• Values can be resources or literals
• Literals are atomic values (strings)
• E.g.,
• Peter (resource) Develop (property) Software
  Package ABC (value)
• Software Package ABC (resource) hasProgLang
  (property) Java (value)

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Statements as Graphs

• A directed graph with labeled nodes and arcs
• from the resource (the subject of the statement)
• to the value (the object of the statement)
• Known in AI as a semantic net
• The value of a statement may be a resource.

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A Set of Triples as a Semantic Net
producer
iphone
www.apple.com
hasCEO
site-owner
Steve Grove
www.steve-grove-website.com
interest
Calligraphy
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Statements as Triples

• (http//www.apple.com,
• http//www.williamshop.com/producer,
• iphone)
• The triple (x,P,y) can be considered as a logical
  formula P(x,y)
• Binary predicate P relates x to y
• E.g., producer(Apple,iphone)
• RDF offers only binary predicates (properties)

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Statements as XML Documents

• Graphs are a powerful tool for human
  understanding but
• The Semantic Web vision requires
  machine-accessible and machine-processable
  representations

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Statements as XML Documents

• xmlnsrdf"http//www.w3.org/1999/02/22-rdf-synta
  x-ns"
• xmlnsws"http//www.williamshop.com/my-rdf-ns"
• rdfabout"http//www.apple.com"
• iphone

resource
property
value
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Statements as XML Documents

• An RDF document is represented by an XML element
  with the tag rdfRDF
• The content of this element is a number of
  descriptions, which use rdfDescription tags.
• Every description makes a statement about a
  resource, identified in 3 ways
• an about attribute, referencing an existing
  resource
• an ID attribute, creating a new resource
• without a name, creating an anonymous resource

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An RDF document is

• Thus an RDF document can be viewed as
• A set of triples
• A graph (semantic net)
• An XML document

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Lecture Outline

• Basic Ideas of RDF
• The Language of RDF
• Basic Ideas of RDF Schema
• ?he Language of RDF Schema
• Basic Ideas of OWL
• The Language of OWL

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XML-Based Syntax of RDF

• An RDF document consists of an rdfRDF element
• The content of that element is a number of
  descriptions
• A namespace mechanism is used
• Disambiguation
• Namespaces are expected to be RDF documents
  defining resources that can be reused
• Large, distributed collections of knowledge

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Data Types

• The attribute rdfdatatype"xsdinteger" is used
  to indicate the data type of the value of the age
  property (via a URI)
• William Cheung
• Associate Professor
• 25

What if this is not a literal?
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The rdfresource Attribute

• E-Technology
• William Cheung
• Associate Professor

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The rdfresource Attribute

• The relationships between courses and lecturers
  (in the example) were not formally defined but
  existed implicitly through the use of the same
  name
• The use of the same name may just be a
  coincidence for a machine
• We can denote that two entities are the same
  using the rdfresource attribute

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Nested Descriptions are allowed

• E-Technology
• William Cheung
• Associate Professor

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Adding rdftype Element/Property

• - nscourse"/
• E-Technology
• bu- nsfaculty"/
• William Cheung
• Associate Professor

Instantiation!!
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Abbreviated Syntax

• Simplification rules
• Childless property elements within description
  elements may be replaced by XML attributes
• For description elements with a typing element we
  can use the name specified in the rdftype
  element instead of rdfDescription
• These rules create syntactic variations of the
  same RDF statement
• They are equivalent according to the RDF data
  model, although they have different XML syntax

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Abbreviated Syntax Example

• u- nscourse"/
• E-Technology

E-Technology" w.hkbu.edu.hk/bu-nscourse"/ rdfresource"9231999"/
hnology"

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Container Elements

• Collect a number of resources or attributes about
  which we want to make statements as a whole
• E.g., for courses taught by William Cheung

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3 Types of Container Elements

• rdfBag an unordered container, allowing multiple
  occurrences
• E.g. students in a class
• rdfSeq an ordered container, which may contain
  multiple occurrences
• E.g. telephone directory (order is imposed)
• rdfAlt a set of alternatives
• E.g. allowable document formats (ps or pdf)

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Example for a Bag

• Cheung" butitle" Dr "

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Lecture Outline

• Basic Ideas of RDF
• The Language of RDF
• Basic Ideas of RDF Schema
• ?he Language of RDF Schema
• Basic Ideas of OWL
• The Language of OWL

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From RDF To RDF Schema

• RDF
• define vocabularies for describing resources
• does not assume, nor does it define semantics of
  any particular application domain
• RDF Schema (RDFS) - with semantics definition
  related to
• Classes and Properties
• Class Hierarchies and Inheritance
• Property Hierarchies

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Classes and their Instances

• We must distinguish between
• Concrete things (individual objects) in the
  domain E-Technology, William Cheung, etc.
• Sets of individuals sharing properties called
  classes lecturers, courses etc.
• Individual objects that belong to a class are
  referred to as instances of that class
• The relationship between instances and classes in
  RDF is through rdftype

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Nonsensical Statements Disallowed Through the Use
of Classes

• E-Technology is taught by Web Intelligence
• We want courses to be taught by lecturers only
• Restriction on values of the property is taught
  by (range restriction)
• RRS Building is taught by William Cheung
• Only courses can be taught
• This imposes a restriction on the objects to
  which the property can be applied (domain
  restriction)

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Class Hierarchies

• Classes can be organised in hierarchies using
  RDFS
• A is a subclass of B if every instance of A is
  also an instance of B
• Then B is a superclass of A

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Inheritance Concept in Class Hierarchies

• Range restriction Courses must be taught by
  academic staff members only
• William Cheung is an Associate Professor.
• He inherits the ability to teach from the class
  of academic staff members
• This is done in RDF Schema by fixing the
  semantics of is a subclass of
• It is not up to an application (RDF processing
  software) to interpret is a subclass of.

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Property Hierarchies

• Hierarchical relationships for properties
• E.g., is taught by is a subproperty of
  involves
• If a course C is taught by an academic staff
  member A, then C also involves ?
• P is a subproperty of Q, if Q(x,y) is true
  whenever P(x,y) is true

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RDF vs RDF Schema
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Lecture Outline

• Basic Ideas of RDF
• The Language of RDF
• Basic Ideas of RDF Schema
• ?he Language of RDF Schema
• Basic Ideas of OWL
• The Language of OWL

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RDF Schema in RDF

• The modeling primitives of RDF Schema are defined
  using resources and properties (RDF itself is
  used!)
• E.g., to declare that lecturer is a subclass of
  academic staff member
• (subClassOf,lecturer,academicStaffMember)
• XML-based syntax of RDF is used.

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Core Classes and Properties

• rdfsResource, the class of all resources
• rdfsClass, the class of all classes
• rdfsLiteral, the class of all literals (strings)
  
• rdfProperty, the class of all properties.
• rdfStatement, the class of all reified
  statements
• rdftype, declares a resource an instance of a
  class
• rdfssubClassOf, relates a class to its
  superclasses
• rdfssubPropertyOf, relates a property to its
  superproperties
• rdfsdomain, specifies the domain of a property
• rdfsrange, specifies the range of a property

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Examples

• 2000/01/rdf-schemaLiteral"/

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Relationships Between Core Classes and Properties

• rdfssubClassOf and rdfssubPropertyOf are
  transitive, by definition (how about
  isMotherOf??)
• rdfsClass is a subclass of rdfsResource
• Because every class is a resource
• rdfsResource is an instance of rdfsClass
• rdfsResource is the class of all resources, so
  it is a class
• Every class is an instance of rdfsClass
• For the same reason

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Subclass Hierarchy of Some Modeling Primitives of
RDF Schema

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Instance Relationships of Some Core Properties of
RDF and RDF Schema

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Reification and Containers

• rdfsubject, relates a reified statement to its
  subject
• rdfpredicate, relates a reified statement to its
  predicate
• rdfobject, relates a reified statement to its
  object
• rdfBag, the class of bags
• rdfSeq, the class of sequences
• rdfAlt, the class of alternatives
• rdfsContainer, which is a superclass of all
  container classes, including the three above

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Lecture Outline

• Basic Ideas of RDF
• The Language of RDF
• Basic Ideas of RDF Schema
• ?he Language of RDF Schema
• Basic Ideas of OWL
• The Language of OWL

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Limitations of RDF Schema

• Local scope of properties
• rdfsrange defines the range of a property (e.g.
  eats) for resources of all classes
• In RDF Schema we cannot declare range
  restrictions to properties that apply to
  resources of some classes only
• E.g. we cannot say that cows eat only plants,
  while other animals may eat meat, too

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Limitations of RDF Schema (2)

• Disjointness of classes
• Sometimes we wish to say that classes are
  disjoint (e.g. male and female)
• Boolean combinations of classes
• Sometimes we wish to build new classes by
  combining other classes using union,
  intersection, and complement
• E.g. person is the disjoint union of the classes
  male and female

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Limitations of RDF Schema (3)

• Cardinality restrictions
• E.g. a person has exactly two parents, a course
  is taught by at least one lecturer
• Special characteristics of properties
• Transitive property (like greater than)
• Unique property (like is mother of)
• A property is the inverse of another property
  (like eats and is eaten by)

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Combining OWL with RDF Schema

• Ideally, OWL extends RDF Schema
• Consistent with the layered architecture of the
  Semantic Web
• Simply extending RDF Schema
• expressive power ? reasoning efficiency ?
• Combining RDF Schema with logic leads to
  uncontrollable computational properties

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Three Species of OWL

• OWL Full
• is fully upward-compatible with RDF, both
  syntactically and semantically
• is so powerful that it is undecidable, thus no
  complete (or efficient) reasoning support
• OWL DL
• permits efficient reasoning support
• lose full compatibility with RDF, ie., not every
  RDF doc. is a legal OWL DL doc.
• OWL Lite
• An even further restriction limits OWL DL to a
  subset of the language constructors

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OWL Compatibility with RDF Schema
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Lecture Outline

• Basic Ideas of RDF
• The Language of RDF
• Basic Ideas of RDF Schema
• ?he Language of RDF Schema
• Basic Ideas of OWL
• The Language of OWL

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Classes

• Classes are defined using owlClass
• owlClass is a subclass of rdfsClass
• Disjointness is defined using owldisjointWith
• sor"/

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Classes (2)

• owlequivalentClass defines equivalence of
  classes
• "academicStaffMember"/

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Properties

• In OWL there are two kinds of properties
• Object properties, which relate objects to other
  objects
• E.g. is-TaughtBy, supervises
• Data type properties, which relate objects to
  datatype values
• E.g. phone, title, age, etc.

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Object Properties

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Datatype Properties

• 1/XLMSchema
• nonNegativeInteger"/

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Inverse Properties

• "/

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Equivalent Properties

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Property Restrictions

• In OWL we can declare that the class C satisfies
  certain conditions
• All instances of C have certain properties taking
  certain values.
• E.g., First year courses (a class) are taught by
  (property) processors (a class)
• E.g., Tutorial sessions (a class) are taught by
  (property) teaching assistants (a class)
• This is equivalent to saying that C is subclass
  of an anonymous class C', where C' collects all
  objects that satisfy the conditions.

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Property Restrictions (2)

• A (restriction) class is achieved through an
  owlRestriction element
• This element contains an owlonProperty element
  and one or more restriction declarations
• One type defines cardinality restrictions (at
  least one, at most 3,)

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Property Restrictions (3)

• The other type defines restrictions on the kinds
  of values the property may take
• owlallValuesFrom specifies universal
  quantification
• owlhasValue specifies a specific value
• owlsomeValuesFrom specifies existential
  quantification

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owlallValuesFrom

• rdfresource"Professor"/

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owlhasValue

• "isTaughtBy"/
• "9231999"/

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owlsomeValuesFrom

• "undergraduateCourse"/

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Cardinality Restrictions

• "isTaughtBy"/
• "xsdnonNegativeInteger" 1

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Special Properties

• owlTransitiveProperty (transitive property)
• E.g. has better grade than, is ancestor of
• owlSymmetricProperty (symmetry)
• E.g. has same grade as, is sibling of
• owlFunctionalProperty defines a property that
  has at most one value for each object
• E.g. age, height, directSupervisor
• owlInverseFunctionalProperty defines a property
  for which two different objects cannot have the
  same value

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Boolean Combinations

• We can combine classes using Boolean operations
  (union, intersection, complement)
• "staffMember"/

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Boolean Combinations (2)

• The new class is not a subclass of the union, but
  rather equal to the union
• We have stated an equivalence of classes

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Boolean Combinations (3)

• t"/

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Nesting of Boolean Operators

• "/

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No Unique-Names Assumption

• OWL does not adopt the unique-names assumption of
  database systems
• If two instances having a different name or ID
  does not imply that they are different
  individuals
• Suppose we state that each course is taught by at
  most one staff member, and that a given course
  is taught by two staff members
• An OWL reasoner does not flag an error
• Instead it infers that the two staff members are
  identical

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Distinct Objects

• To ensure that different individuals are indeed
  recognized as such, we must explicitly assert
  their inequality

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Distinct Objects (2)

• OWL provides a shorthand notation to assert the
  pairwise inequality of all individuals in a given
  list

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A Example - A Printer Ontology
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A Printer Ontology - Beginning

• xmlnsrdf"http//www.w3.org/1999/02/22-rdf-synta
  x-ns"
• xmlnsrdfs"http//www.w3.org/2000/01/rdf-schema
  "
• xmlnsxsd"http//www.w3.org/2001/XMLSchema"
• xmlnsowl"http//www.w3.org/2002/07/owl"
• xmlns"http//www.cs.vu.nl/frankh/spool/printer.
  owl"
• My example version 1.2, 17 October 2002

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A Printer Ontology Products and Devices

• Products form a class.
  
• Printing and digital imaging
  devices
• form a subclass of products.
• Device

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A Printer Ontology HP Products

• y"/

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A Printer Ontology Printers and Personal
Printers

• Printers are printing and digital
  imaging
• devices.
• Printers for personal use form
• a subclass of printers.

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A Printer Ontology HP LaserJet 1100se Printers

• 1100se printers belong to the 1100
  series
• and cost 450.

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A Printer Ontology Properties

• 
  

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References

• The Semantic Web Primer, MIT Press, 2004