The Semantic Web

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The Semantic Web

• Evangelos Milios

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Why Semantic Web?

• Web content now is mostly in natural language
  with HTML annotations
• Easy for humans but difficult for computers to
  interpret
• Very hard to write programs that perform tasks
  for users based on Web content
• We further need
• Structured Web content
• Metadata (information about information)
• Standard content representations (shared)

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Example

• Book an air ticket from Air Canada
• Why is the task tedious?
• All interaction is manual
• Interaction is repetitive
• Switching between screens and menus
• Shopping is difficult
• What would it take for a computer program
  (agent) to perform the task for you?

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Interim solutions

• Screen scraping get the data off the screen into
  a more structured form (e.g. RDF)
• Form filling
• Things wont get easier until many people start
  publishing data in a more structured form.
• What does this mean?

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What is required?

• Structure XML
• Everyone can create tags
• Says nothing about what the tags mean
• Meaning
• Resource Description Framework
• (Subject, verb, object) triples distributed over
  the Web
• Ontologies
• Taxonomies
• Logic (rules, inferences) how powerful?
• Powerful enough to be useful
• Not too powerful that makes reasoning impossible

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RDF (Resource Description Framework)

• Meaning is expressed as triples of
• Things (e.g. people, objects, web pages)
• Properties (e.g. is author of)
• Values of properties (other things)
• Like subject, verb, object of a sentence
• Example (Ullman, is author of, MySQL textbook)
• Things and Properties identified by Universal
  Resource Identifiers (URI)
• Triples in RDF form webs of information about
  related things

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RDF Schema

• RDF Schema
• template for RDF documents, just like DB schemas
  are templates for databases
• itself expressed in RDF
• Example
• an RDF schema for resumes.

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RDF example(documents and their authors)
Application Level
From http//www.w3.org/TR/2002/WD-rdf-schema-20021
112/
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Ontologies

• Additional vocabulary and formal semantics
  compared to XML, RDF, RDF-S
• OWL Web Ontology Language (see Wikipedia, too)
• OWL Lite
• classification hierarchy
• simple constraints
• OWL DL
• more expressiveness
• completeness
• decidability
• OWL Full
• max expressiveness
• no computational guarantees

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OWL Lite

• Class group of individuals sharing
  propertiese.g. Deborah, Frank are members of
  class Person
• Specialization Hierarchy subClassOf
• Property relationship between individuals or
  from individuals to data valuese.g. hasChild,
  isAuthor,hasAge
• Property hierarchy subPropertyOfe.g. hasSibling
  vs. hasRelative
• Domain of a property limits individuals to which
  property appliese.g. hasChild has domain Mammal
• Range of a propertylimits individuals property
  can have as valuese.g. hasChild has range Mammal
• Individual instance of a class

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Equality

• equivalentClass e.g Car , Automobile
• equivalentProperty e.g. hasHead , hasLeader
• statement about individuals
• sameAs e.g. EMilios same as EEMilios
• differentFrom
• AllDifferent

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Property Characteristics

• inverseOf e.g. hasChild, hasParent
• TransitiveProperty e.g. hasAncestor
• SymmetricProperty e.g. isFriend
• FunctionalProperty e.g. hasPrimaryEmployer (no
  more than one for every individual)
• InverseFunctionalProperty inverse has functional
  propertye.g. hasSIN (isSINfor)

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Ontology examples

• Ontologies expressed in RDF syntax
• SchemaWeb directory of RDF schemas and OWL
  ontologies
• Standards ontology
• Ontology for instructional objects
• MINDSWAP Cancer ontology
• Magpie Semantic Web browser
• Friend of a Friend project

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Automated Reasoning

• A program called reasoner can perform inferences
  using the components of an OWL ontology
• If Frank hasChild Anna, then Frank isMammal
  (domain)
• If Sara isAncestorOf Louise and Louise
  isAncestorOf Johnthen Sara isAncestorOf John
  (transitive Property)
• No two different persons can have the same SIN
• If two instances of person have the same SIN,
  they are the same person

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Proof

• Once systems follow logic, they can prove things
• Example The law says that it is a crime for an
  American to sell weapons to hostile nations. The
  country Nono, an enemy of America, has some
  missiles, and all of its missiles were sold to it
  by Colonel West, who is American.Prove that
  Colonel West is a criminal.

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Facts and Rules
Break natural language statements into units of
individual facts and rules

• For every American X, either X does not sell
  weapons to hostile nations ORX is a criminal.
  (RULE)
• Nono is an enemy of America
• Nono has some things (MS)
• MS are missiles
• If Y is a missile, then Y is a weapon (RULE).
• Colonel West sold missiles MS to Nono
• Colonel West is American.
• Colonel West is not a criminal

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Proof by contradiction

• Combine individual facts and rules into new
  facts,
• aiming to arrive at a contradiction.
• A,H West does not sell weapons to hostile
  nations
• D, E MS are weapons
• D, E and F Colonel West sold weapons to Nono
• A,H, B Colonel West did not sell weapons to Nono
• CONTRADICTION!!!
• There are often multiple ways to arrive at a
  contradiction
• No guarantee that the search for a contradiction
  will terminate (undecidability)

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Proofs

• Difficult to create proofs
• Easy to check them
• A Web of information processors
• Follow rules and statements to draw conclusions
• Place results back on the Web
• Rules, statements
• distributed over the Web
• Accessible via URIs
• Properly designed, the Semantic Web can assist
  the evolution of human knowledge as a whole
  From The Semantic Web A new form of Web
  content that is meaningful to computers will
  unleash a revolution of new possibilities, By Tim
  Berners-Lee, James Hendler and Ora Lassila,
  Scientific American, May 17, 2001

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Summary

• Semantic Web
• RDF
• RDF schemas
• OWL
• OWL Lite
• Reasoning