Defining RDF Vocabularies: RDF Schema

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Defining RDF Vocabularies RDF Schema
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Introduction

• RDF provides a way to express simple statements
  about resources, using named properties and
  values.
• RDF user communities also need the ability to
  define the vocabularies (terms) they intend to
  use in those statements, specifically, to
  indicate that they are describing specific kinds
  or classes of resources, and will use specific
  properties in describing those resources.
• extermsTent extermsmodel, extermsweightInKg,
  and extermspackedSize
• ex2Book or ex2MagazineArticle ex2author,
  ex2title, ex2subject
• ex3Person and ex3Company ex3age,
  ex3jobTitle, ex3stockSymbol, and
  ex3numberOfEmployees.

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Introduction (Cont.)

• RDF itself provides no means for defining such
  application-specific classes and properties.
• Instead, such classes and properties are
  described as an RDF vocabulary, using extensions
  to RDF referred to as RDF Schema.
• RDF Schema provides the facilities needed to
  describe classes and properties, and to indicate
  which classes and properties are expected to be
  used together
• In other words, RDF Schema provides a type system
  for RDF

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Introduction (Cont.)

• Vocabulary descriptions (schemas) written in the
  RDF Schema language are legal RDF graphs.
• RDF software that is not written to also process
  the additional RDF Schema vocabulary can still
  interpret a schema as a legal RDF graph
  consisting of various resources and properties,
  but will not "understand" the additional built-in
  meanings of the RDF Schema terms.
• To understand these additional meanings, RDF
  software must be written to process an extended
  language that includes not only the rdf
  vocabulary, but also the rdfs vocabulary,
  together with their built-in meanings.

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Describe Class

• A class in RDF Schema corresponds to the generic
  concept of a Type or Category
• Classes are described using the RDF Schema
  resources rdfsClass and rdfsResource, and the
  properties rdftype and rdfssubClassOf
• In RDF Schema, a class is any resource having an
  rdftype property whose value is the resource
  rdfsClass

exMotorVehicle rdftype
rdfsClass exVan rdftype rdfsClass exTruck
rdftype rdfsClass
exthingscompanyCar rdftype exMotorVehicle
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Describe Subclass

• The specialization relationship between two
  classes is described using the predefined
  rdfssubClassOf property to relate the two
  classes
• The rdfssubClassOf property is transitive
• A class may be a subclass of more than one class
• RDF Schema defines all classes as subclasses of
  class rdfsResource

exVan rdfssubClassOf exMotorVehicle
exVan rdfssubClassOf exMotorVehicle
exMiniVan rdfssubClassOf exVan
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A Vehicle Class Hierarchy
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A Vehicle Class Hierarchy (Cont.)
exMotorVehicle rdftype rdfsClass .
exPassengerVehicle rdftype rdfsClass .
exVan rdftype rdfsClass . exTruck
rdftype rdfsClass . exMiniVan rdftype
rdfsClass . exPassengerVehicle
rdfssubClassOf exMotorVehicle . exVan
rdfssubClassOf exMotorVehicle . exTruck
rdfssubClassOf exMotorVehicle . exMiniVan
rdfssubClassOf exVan . exMiniVan
rdfssubClassOf exPassengerVehicle .
ex13.xml
ex14.xml (Type Node Abbreviation)
ex15.xml (Instance)
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Describe Properties

• User communities also need to be able to describe
  specific properties that characterize those
  classes of things.
• In RDF Schema, properties are described using the
  RDF class rdfProperty, and the RDF Schema
  properties rdfsdomain, rdfsrange, and
  rdfssubPropertyOf.
• All properties in RDF are described as instances
  of class rdfProperty

extermsweightInKg rdftype rdfProperty
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rdfsrange and rdfsdomain

• RDF Schema also provides vocabulary for
  describing how properties and classes are
  intended to be used together in RDF data.
• The most important information of this kind is
  supplied by using the RDF Schema properties
  rdfsrange and rdfsdomain
• rdfsrange indicate that the values of a
  particular property are instances of a designated
  class
• rdfsdomain indicate that a particular property
  applies to a designated class

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rdfsrange and rdfsdomain (Example)
exPerson rdftype rdfsClass . exauthor
rdftype rdfProperty . exauthor
rdfsrange exPerson .
exage rdftype rdfProperty . exage
rdfsrange xsdinteger .
exBook rdftype rdfsClass . exauthor
rdftype rdfProperty . exauthor rdfsdomain
exBook .
ex16.xml
ex17.xml (instance)
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Other Schema Information

• rdfscomment property a human-readable
  description of a resource
• rdfslabel property a more human-readable
  version of a resource's name
• rdfsseeAlso a resource that might provide
  additional information about the subject
  resource.
• rdfsisDefinedBy property a subproperty of
  rdfsseeAlso, and can be used to indicate a
  resource that "defines" the subject resource.

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Richer Schema Languages

• Richer schema capabilities that have been
  identified as useful (but that are not provided
  by RDF Schema) include
• Cardinality constraints on properties, e.g., that
  a Person has exactly one biological father.
• Specifying that a given property (such as
  exhasAncestor) is transitive, e.g., that if A
  exhasAncestor B, and B exhasAncestor C, then A
  exhasAncestor C.
• Specifying that a given property is a unique
  identifier (or key) for instances of a particular
  class.
• Specifying that two different classes (having
  different URIrefs) actually represent the same
  class.

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Richer Schema Languages (Cont.)

• Richer schema capabilities (Cont.)
• Specifying that two different instances (having
  different URIrefs) actually represent the same
  individual.
• Specifying constraints on the range or
  cardinality of a property that depend on the
  class of resource to which a property is applied,
  e.g., being able to say that for a soccer team
  the exhasPlayers property has 11 values, while
  for a basketball team the same property should
  have only 5 values.
• The ability to describe new classes in terms of
  combinations (e.g., unions and intersections) of
  other classes, or to say that two classes are
  disjoint (i.e., that no resource is an instance
  of both classes).
• DAMLOIL, OWL