3' RDF and XML

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3. RDF and XML

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• 1. Basic of RDF
• 2. Example of RDF
• 3. How XML Namespaces Work
• 4. The Abbreviated RDF Syntax
• 5. RDF Resource Collections
• 6. Reified Statements

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Basic of RDF

• The Triples of RDF
• resource, property, and property value.
• Literals
• Constants (String values)
• A relationship between a resource and a literal
  is expressed by a statement, which connects a
  subject with an object via a predicate

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Basic of RDF

• Ex
• The author of this book is Johan Hjelm

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Basic of RDF

• The difference from traditional predicate logic
• The syntax of RDF is declared in the RDF Schema
• The RDF Schema
• define the set of resource and literal values
• creates the structure which you later fill with
  your description.

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Example of RDF

• Simple RDF graph
• http//www.wireless-information.net -gt Creator -gt
  Johan Hjelm
• Creator of http//www.wireless-information.net is
  Johan Hjelm

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Example of RDF

• We might also want to make clear that we meant
  creator in some special, well-defined sense, by
  using a special element, dcCreator (as opposed
  to biblicalCreator, for instance).
• Of course, then we would have to declare where
  the definition of that special element came from.

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Example of RDF

• lt?xml version1.0?gt
• ltrdfRDF xmlnsrdfhttp//www.w3.org/1999/02/2
  -rdf-syntax-ns xmlnsdchttp//purl.org/dc/elem
  ent/1.1/gt
• ltrdfDescription abouthttp//www.wireless-i
  nformation.net/gt
• ltdccreator rdfresourcehttp//johanhjel
  m.com/whois/ /gt
• lt/rdfDescriptiongt
• lt/rdfRDFgt

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Example of RDF

• lt?xml version1.0gt
• The XML declaration shows that this is XML, and
  declares which version it is.
• ltrdfRDF xmlnsrdfhttp//www.w3.org..
• The rdf element begins here It encapsulates the
  actual statement and is followed by the namespace
  declaration. It also the RDF namespace (i.e.,
  where to find out what all tags prefixed with
  rdf mean)

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Example of RDF

• Xmlnsdc http//purl.org/dc/elements...
• The second namespace declaration declares the
  Dublin Core namespace, here prefixed with dc .
  Both namespace declarations are attributes on the
  rdf element.
• ltrdfDescription about http//www.wireless...
  gt
• The rdfDescription elements declares which
  resource the statement is about.

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Example of RDF

• ltdccreator rdfresource http//johanhjelm.com/.
  . /gt
• The dcCreator element contains the value of the
  dcCreator property.
• There can be more properties for the same
  resource.

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How XML Namespaces Work

• XML namespaces
• identify a set of element names by using prefixes
  in the RDF code
• declaring the namespace in the head of the
  document.
• Namespaces
• declared using a family of reserved attributes
• Such an attributes name must either be xmlns or
  have xmlns as a prefix (these two are reserved,
  but you can use any other prefix for your
  namespaces).

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The Abbreviated RDF Syntax

• ltrdfdescriptiongt about http//www.wireless-..
  gt ltdccreatorgtjohanhjelm.comlt/dccreatorgtlt/rdf
  descriptiongt
• And the abbreviated syntax looks like this
• ltdescription about http//www.wireless-..
  dccreator johanhjelm.com /gt

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RDF Resource Collections

• Bag
• Unordered list of resource (URIs) or atomic
  values (literals, i.e., strings).
• Used to declare that a property has multiple
  values
• The order of the values does not matter
• There can be duplicates of resources and
  duplicate values in literals.

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RDF Resource Collections

• Sequence
• Ordered list of resources or atomic values
  (literals)
• The order is significant
• Possible to have duplicates of the same value
• Used to declare that a property has multiple
  values.

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RDF Resource Collections

• Alternative
• List of resources or values that represent
  possible alternatives for the (single) value of a
  property
• Choose any one of the items in the list

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Reified Statements

• There is a mechanism in RDF to make statements
  about statements (he said that she said). In
  philosophy, this is called a reified statement.

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Reified Statements

• ltRDFgt xmlns http//www.w3.org/1999/...
  xmlnsrdf http//www.w3.org/1999/...
  xmlnsdc http//purl.org/metadata/dc..
  xmlnsl http//mycorp.com/schemas..
  ltDescription about http//www.Webnuts.net/Jan97.
  htmlgt ltdcsubject rdfvalue020Library
  Science lClassificationDewey Decimal
  Code/gt lt/Descriptiongtlt/RDFgt
• The page Jan97.html in the site www.Webnut.net
  has a Dublin Core Subject that is Library
  Science, 020 according to the Dewey Decimal Code

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Reified Statements

• The different namespaces make the elements
  unique, using the namespace mechanism.
• Each set of elements (the vocabulary) has to be
  described by a schema, which contains more
  information than what is available in the DTD.
• The RDF Schema language describes the parameters
  of the elements

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Reified Statements

• To further illustrate how to use aggregates,
  consider an example of a document with two
  authors specified alphabetically, a title
  specified in two different languages, and having
  two equivalent locations on the Web
• ltrdfRDF xmlnsrdf http//www.w3.org..
  xmlnsdc http//purl.org../dc..
  ltrdfDescription about http//www.foo.com/cool.h
  tmlgt ltdcCreatorgt ltrdfSeq ID
  CreatorsAhphabeticalBySurnamegt ltrdfligtMary
  Andrewlt/rdfligt ltrdfligtJacky
  Crystallt/rdfligt lt/rdfSeqgt lt/dcCreatorgt

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Reified Statements

• ltdcIdentifiergt ltrdfBag IDMirroredSitesgt
  ltrdfli rdfresource http//www.foo.com.au
  /cool.html/gt ltrdfli rdfresource
  http//www.foo.com.it/cool.html/gt
  lt/rdfbaggt lt/dcIdentifiergt ltdcTitlegt
  ltrdfAltgt ltrdfli xmllangengtThe Coolest
  Web Pagelt/rdfligt ltrdfli xmllangitgtI1
  Pagio di Web Fubalt/rdfligt lt/rdfAltgt
  lt/dcTitlegt lt/rdfDescriptiongtlt/rdfRDFgt