The Semantic Web Resource Description Framework (RDF)

1
The Semantic WebResource Description Framework
(RDF)

• Michael B. Spring
• Department of Information Science and
  Telecommunications
• University of Pittsburgh
• spring_at_imap.pitt.edu
• http//www.sis.pitt.edu/spring

2
Overview

• The Big Picture
• Pieces
• One future
• The Basics
• An RDF Description
• An RDF Schema

3
The Big Picture

• The semantic web is a transformation of the WWW
  into one that might be
• Better organized
• Traversed by agents on our behalf
• To accomplish this goal, mechanisms are needed
  that allow us to understand what objects exist
  and how they are related
• RDF with all of its component parts which dont
  exist as yet provides one means for building
  the semantic web
• The RDF Model and Syntax and The RDF Schema
  Specification are standards that define how to
  construct RDF Descriptions and RDF Schema

4
Descriptions and Schema

• RDF is one partial solution for the semantic web.
• Some have criticized it as being too complex and
  others have criticized it as being too
  simplistic.
• The fact that there are a number of pieces, many
  of which have not yet been defined confuses
  people.
• Simply put, RDF consists of
• Descriptions of resources
• Schema that define the descriptions and
  potentially relate them to other schema
• Inference engines that know how to extract
  meaning from descriptions and schema

5
Schema and Schema

• Namespaces and schema can be confusing. Keep the
  following in mind
• Under XML, a schema is used to define the model
  of a class of documents.
• Schema are powerful modeling tools in that they
  provide for inheritance, modularity, and scoping
  through the use of namespaces.
• A namespace simply defines the scope and
  qualification of XML elements from a schema
• One XML schema defines the elements that may
  appear in a documents that defines descriptions.
  The resulting document is an RDF schema.
• When an XML document is described, it will use
  elements from a particular RDF schema

6
A Mouthful of Schema

• The RDF Model and Syntax Specification says a
  description may be specified in any form, but use
  of XML is preferred.
• The RDF Schema elements are defined in an XML
  Schema
• The resource description is a set of elements
  from a given RDF Schema

RDF Schema A
XML Reference Schema
RDF Schema B
RDF Reference Schema
XML Schema
RDF Schema C
One Document Description
7
One Possible Future (Part 1)

• Consider a set of web servers at a University
• Each server has a number of documents
• Each document is described by a description
• All of the descriptions are defined in terms of a
  small set of RDF Schema
• The RDF Schema are all maintained on a schema
  server
• The descriptions of all the documents, including
  their location are stored on another server
  optimized for rapid lookups

8
The Semantic Servers
WebServer
RDF Schema
RDF Schema
RDF Schema
RDFSchemaServer
DocumentorService
DocumentorService
WebServer
DescriptionServer
WebServer
DocumentorService
DocumentorService
DocumentorService
DocumentorService
9
One Possible Future (Part 2)

• Consider an agent looking for papers on RDF by
  a particular author, Jones
• The agent begins with the attribute value pairs
• Author Jones
• Topic RDF
• The agent checks a known RDF Schema Server and
  discovers there are alternative attributes which
  mean the same thing as Author and Topic.
• The agent composes queries using the appropriate
  attributes for the various Description servers.
• The description servers return hits and their
  location on various web servers.
• The agent reports to its owner that there are x
  documents about RDF by Jones.

10
One Possible Future (Part 2)

• Consider an agent looking to make travel
  arrangements
• A known schema server is consulted with a request
  for the location of travel markets where travel
  is an accepted keyword
• The schema server returns the names of the
  description servers that provide travel services
• The agent queries these description servers for
  travel services that speak the reservation
  language interface known by the agent
• The agent then negotiates a reservation

11
Building a Description

• A simple description four property/value pairs
• ltrdRDF
• xmlnsrdhttp//www.w3.org/1999/02/22-rdf-syntax-
  ns
• xmlnsdchttp//purl.org/metadata/dublin_coregt
  
• ltrdDescription abouthttp//www.bar.org/doc.html
  gt
• ltdcTitlegtHow to Cooklt/dcTitlegt
• ltdcDategt2001-3-27lt/dcDategt
• ltdcCreatorgtMBSpringlt/dcCreatorgt
• ltdcPublishergtUniversity of Pittsburghlt/dcPublish
  ergt
• lt/rdRDFgt
• The namespace for properties is the dublin core
• The description might be replace meta tags in an
  HTML document

12
Types of Descriptions

• The syntax provides for a wide number of
  variations. Of these variations, containers and
  nesting are worthy of further elaboration
• Nesting
• A description is about a resource
• A property describes the resource via a value
  which is a literal
• If the value if a resource, descriptions are
  nested
• Containers
• When values are multiple, containers and elements
  are used for the values
• There are three types of containers bags,
  sequences and alternatives

13
Nested Description

• A nested description is used when the value of a
  property is a resource
• ltrdRDF
• xmlnsrdhttp//www.w3.org/1999/02/22-rdf-syntax-
  ns
• xmlnsdchttp//purl.org/metadata/dublin_coregt
  
• ltrdDescription abouthttp//www.bar.org/doc.html
  gt
• ltdcTitlegtHow to Cooklt/dcTitlegt
• ltdcCreatorgt
• ltrdDescription about mailtospring_at_pitt.edu
  gt
• ltdcTitlegtMichael B. Springlt/dcTitlegt
• lt/rdDescriptiongt
• lt/dcCreatorgt
• lt/rdDescriptiongt
• lt/rdRDFgt

14
Container

• A container is used for multi valued properties
• ltrdRDF
• xmlnsrdhttp//www.w3.org/1999/02/22-rdf-syntax-
  ns
• xmlnsdchttp//purl.org/metadata/dublin_coregt
  
• ltrdDescription abouthttp//www.bar.org/doc.html
  gt
• ltdcTitlegt
• ltrdAltgt
• ltrdli xmllangengtSpringlt/rdligt
• ltrdli xmllangitgtPrimaVeralt/rdligt
• lt/rdAltgt
• lt/dcTitlegt
• ltdcCreatorgt
• ltrdSeqgt
• ltrdligtMBSpringlt/rdligt
• ltrdligtJMSpringlt/rdligt
• lt/rdSeqgt
• lt/dcCreatorgt
• lt/rdDescriptiongt
• lt/rdRDFgt

15
Building a Simple Schema

• lt? xml version'1.0'?gt
• ltrdfRDF xmlnsdc"
• xmlnsrdf"http//www.w3.org/TR/REC-rdf-synta
  x
• xmlnsrdfshttp//www.w3.org/TR/WD-rdf-schema
  gt
• ltrdfDescription ID"title"gt
• ltrdftype rdfresource"http//www.w3.org/TR/R
  EC-rdf-SyntaxProperty"/gt
• ltrdfslabelgtTitlelt/rdfslabelgt
• ltrdfscommentgtThe name given to the
  resourcelt/rdfscommentgt
• ltrdfsisDefinedBy ""/gt
• lt/rdfDescriptiongt
• ltrdfDescription ID"creator"gt
• ltrdftype rdfresource"http//www.w3.org/TR/R
  EC-rdf-syntaxProperty"/gt
• ltrdfslabelgtAuthor/Creatorlt/rdfslabelgt
• ltrdfscommentgtThe person or organization who
  created itlt/rdfscommentgt
• ltrdfsisDefinedBy ""/gt
• lt/rdfDescriptiongt

16
A Schema with Types

• ltrdfRDF xmlnsrdf"http//www.w3.org/1999/02/22-
  rdf-syntax-ns"
• xmlnsrdfs"http//www.w3.org/2000/01/rdf-schema
  "gt
• ltrdfsClass rdfID"Person"gt
• ltrdfscommentgtThe class of people.lt/rdfscommentgt
  
• ltrdfssubClassOf rdfresource"http//www.w3.org/
  2000/03/example/classesAnimal"/gt
• lt/rdfsClassgt
• ltrdfProperty ID"ssn"gt
• ltrdfscommentgtSocial Security Numberlt/rdfscommen
  tgt
• ltrdfsrange rdfresource"http//www.w3.org/2000/
  03/example/classesInteger"/gt
• ltrdfsdomain rdfresource"Person"/gt
• lt/rdfPropertygt
• ltrdfProperty ID"age"gt
• ltrdfsrange rdfresource"http//www.w3.org/2000/
  03/example/classesInteger"/gt
• ltrdfsdomain rdfresource"Person"/gt
• lt/rdfPropertygt
• lt/rdfRDFgt