Chapter 7: Resource Description Framework (RDF)

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• Chapter 7Resource Description Framework (RDF)

Service-Oriented Computing Semantics, Processes,
Agents Munindar P. Singh and Michael N. Huhns,
Wiley, 2005
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RDF

• Provides a basis for knowledge representation
• Supports inferencing
• Simple language to capture assertions
  (statements), which help capture knowledge, e.g.,
  about resources
• Combines old KR ideas (frames, OO modeling) but
  uses the Web to enhance their range and avoid
  some longstanding problems

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Why RDF?

• XML
• Gives us a document tree
• Doesnt identify the content represented by a
  document, where content means
• Concepts the document is about
• Relationships among them
• Enables multiple representations for the same
  content
• RDF expresses the content itself

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Resources and Literals

• RDF captures descriptions of resources
• A resource is an addressable object
• Identified via a URI
• Of which a description can be given (and which is
  worth talking about)
• A literal is something simpler
• A value, e.g., string or integer
• Cannot be given a description

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Statements or Triples

• RDF is based on a simple grammar
• An RDF document is just a set of statements or
  triples
• Each statement consists of
• Subject a resource
• Object a resource or a literal
• Predicate a resource
• Comes with RDFS, a vocabulary to create
  vocabularies

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Rendering RDF

• RDF is not about the surface syntax but about the
  underlying content
• Using the XML serialization of RDF
• RDF is not tied to XML
• Standard XML namespace syntax
• Namespaces defined by the RDF standard
• Typically abbreviated rdf and rdfs

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Example in XML (Using Dublin Core)

• lt?xml version'1.0' encoding'UTF-8'?gt
• ltrdfRDF
• xmlnsrdf"http//www.w3.org/1999/02/22-rdf-syn
  tax-ns"
• xmlnsdc"http//purl.org/dc/elements/1.1/"gt
• ltrdfDescription rdfabout"http//www.wiley.co
  m/SOC"gt
• ltdctitlegtService-Oriented
  Computinglt/dctitlegt
• ltdccreatorgtMunindarlt/dccreatorgt
• ltdccreatorgtMichaellt/dccreatorgt
• ltdcpublishergtWileylt/dcpublishergt
• lt/rdfDescriptiongt
• lt/rdfRDFgt
• rdfDescription gathers statements about one
  subject

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Example in N-Triples Notation

• lthttp//www.wiley.com/SOCgt
• lthttp//purl.org/dc/elements/1.1/titlegt
• "Service-Oriented Computing" .
• lthttp//www.wiley.com/SOCgt
• lthttp//purl.org/dc/elements/1.1/creatorgt
• "Munindar" .
• lthttp//www.wiley.com/SOCgt
• lthttp//purl.org/dc/elements/1.1/creatorgt
• "Michael" .
• lthttp//www.wiley.com/SOCgt
• lthttp//purl.org/dc/elements/1.1/publishergt
• "Wiley" .
• Could also write individual statements in the XML
  syntax, but the rdfDescription element
  simplifies the notation

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Exercise

• Graphs represent binary relationships naturally
• The vendor ships SKU-99
• Express a three-party relationship
• The vendor ships SKU-99 quickly
• Hint think of gerunds from natural language
  grammar

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Reification of Statements

• Reify to make referenceable, essential for
  quoting statements to
• Agree or disagree with them
• Assert modalities possible, desirable,
• Make a statement into a resource then talk about
  it
• rdfStatement is the class whose rdftype the
  given statement (object) is additional
  properties such as rdfsubject, rdfobject, and
  rdfpredicate

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RDF Schema

• Analogous to an object-oriented type system built
  on top of RDF. Defines
• rdfsClass, rdfssubClassOf
• rdfsResource, rdfsLiteral
• rdfsProperty, rdfssubPropertyOf
• rdfsrange, rdfsdomain
• rdfslabel, rdfscomment, rdfsseeAlso
• Applications of RDF Schema deferred to OWL, which
  greatly enhances the above

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RDF Schema versus XML Schema

• Both help define custom vocabularies
• An XML Schema document gives us syntactic details
• An RDF Schema document gives us (part of) the
  meaning of a vocabulary
• An OWL document (next chapter) captures richer
  meaning

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Collections

• Function as containers
• rdfBag
• rdfSequence
• rdfAlt (choice)
• Accompanied by properties to extract elements
• Schematically represented as rdf_1, and so on

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Chapter 7 Summary

• RDF captures deeper structure than XML
• RDF captures graphs in general
• Meaning depends on the graph, not the document
  that represents a graph
• RDF is based on an simple linguistic
  representation subject, predicate, object
• But webified via URIs
• RDF comes with RDF Schema
• In essence, an object-oriented type system a
  vocabulary to create new vocabularies
• Used for important vocabularies (FOAF, DC,
  Mozilla extensions)
• Provides a basis for OWL