The Semantic Web

The Semantic Web

• The Semantic Web Vision
• Enabling technologies
• Finding semantics
• Example applications
• Problems and challenges

The Vision

The Semantic Web provides a common framework that allows data to be shared and reused across application, enterprise, and community boundaries. http://www.w3.org/2001/sw/

"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." -- Tim Berners-Lee, James Hendler, Ora Lassila, The Semantic Web, Scientific American, May 2001

Where is the Semantic Web

• Grass Roots efforts

  • RSS: the blogging world
  • FOAF: friend of a friend
  • DOAP: description of a project

• Large scale projects

  • LSID: Life Sciences Identifier
  • XMP: Adobe document metadata
  • CS AKTive Space: UK Computer Science

Enabling Technologies

XML

  <slidesinfo>
    <title>The Semantic Web</title>
    <author>
      <firstname>Steve</firstname>
      <surname>Cassidy</surname>
    </author>
    <affiliation>Centre for 
       Language Technology</affiliation>
    <affiliation>Macquarie University
    </affiliation>
    <affiliation>Sydney</affiliation>
  </slidesinfo>
    

Names for Things

• Steve Cassidy
• Steve Cassidy, Macquarie University
• Steve Cassidy, Macquarie University, extn 9581
• Steve.Cassidy@mq.edu.au
• http://www.ics.mq.edu.au/~cassidy/
• http://www.ics.mq.edu.au/people#cassidy

Describing Things

• Things have properties
• Things are related to other things

<http://www.ics.mq.edu.au/people#cassidy>
  name: "Steve Cassidy"
  email: Steve.Cassidy@mq.edu.au
  office: E6A381
  webpage: http://www.ics.mq.edu.au/~cassidy/
  teaches: http://www.mq.edu.au/units#comp348
  reading: isbn:1-55860-534-7
  http://schema.peoplesdns.com/menow/#mood: Happy

Resource Description Framework

A formal framework for describing resources (things) based on assertions of triples (subject, predicate, object)

RDF/XML

<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">

    <rdf:Description rdf:about="http://www.ics.mq.edu.au/people#cassidy">
        <name xmlns="file:/tmp/tmp.n3#">Steve Cassidy</name>
        <email xmlns="file:/tmp/tmp.n3#">Steve.Cassidy@mq.edu.au</email>
        <office xmlns="file:/tmp/tmp.n3#">E6A381</office>
        <webpage xmlns="file:/tmp/tmp.n3#"
            rdf:resource="http://www.ics.mq.edu.au/~cassidy/"/>
    </rdf:Description>
</rdf:RDF>
    

Where's the Semantics?

• RDF and XML provide a syntactic solution to the data representation and interchange problem
• There's little more meaning than if I'd written a regular web page.
• Problems:
  • What does 'name' mean?
  • What do I call the person who wrote a book?
• Answer: ontology, vocabularies

Ontology

• Some core machinery: Class, Resource, Property

  :employs a rdf:Property;
     rdfs:domain :company;
     rdfs:range foaf:person .
  

• Agreed vocabularies
• Classes, properties, relations in a domain.
• Rules encoding domain knowledge: the enemy of my enemy is my friend
• Formalised in RDF Schema and OWL (Web Ontology Language)

Ontology: OWL

• OWL is the W3C Ontology definition language
• Provides a way of describing classes, properties and their relations
• OWL formal semantics specifies how to derive logical consequences from an ontology
• OWL provides an encoding of knowledge representations
• Three flavours:
  • OWL Lite: primarily classification hierarchies
  • OWL DL: maximum expressiveness, completeness, decidability
  • OWL Full: maximum expressiveness, no guarantees

Ontology: FOAF

• http://www.foaf-project.org/ [http://www.foaf-project.org/]
• FOAF Explorer [http://xml.mfd-consult.dk/foaf/explorer/?foaf=http://www.ics.mq.edu.au/~cassidy/foaf.rdf]
• Foaf-a-matic [http://www.ldodds.com/foaf/foaf-a-matic.html]
• FOAFBot: IRC chat agent on #foaf [irc://irc.freenode.net/foaf] :

  <edd> foafbot, edd's name
  <foafbot> edd's name is 'Edd Dumbill', according
    to Dan Brickley, Anon35, Niel Bornstein, Jo Walsh, 
    Dave Beckett, Edd Dumbill, Matt Biddulph, Paul Ford
  	

Other Vocabularies and Ontologies

• Dublin Core: creator, title, date of resources
• iCalendar: derived from iCal standard, rdf version in development
• vCard: address book entries
• DOAP: Description of a Project

Inference

• RDF statements are a form of Description Logic
  • a decidabile fragment of FOL
• Relationship to Prolog is obvious
• Write rules to infer new things from known facts:

  { ?x g:husband ?y } => { ?y g:wife ?x }.
  { ?y g:wife ?x } => { ?y a g:Man. ?x a g:Woman }.
  { ?ch g:father ?him; g:mother ?her.} 
            => { ?him g:wife ?her }.
      

• Closed World Assumption: We know everything that's true.
• Open World: there are things we don't know.

Applications

• Norman Walsh [http://norman.walsh.name/2004/06/07/images/20040531-145424.html]
• LSID: Life Sciences Identifier
• XMP: Adobe document metadata
• CS AKTive Space: UK Computer Science

Problems and Challenges

• Originating RDF
• Merging Ontologies
• Will it scale?
• Security, provenance, trust