RDF Schema

RDF Schema (Resource Description Framework Schema, variously abbreviated as RDFS, RDF(S), RDF-S, or RDF/S) is a set of classes with certain properties using the RDF extensible knowledge representation data model, providing basic elements for the description of ontologies, otherwise called RDF vocabularies, intended to structure RDF resources. These resources can be saved in a triplestore to reach them with the query language SPARQL.

The first version^[1][2] was published by the World-Wide Web Consortium (W3C) in April 1998, and the final^[3] W3C recommendation was released in February 2004. Many RDFS components are included in the more expressive Web Ontology Language (OWL).

Main RDFS constructs

RDFS constructs are the RDFS classes, associated properties and utility properties built on the limited vocabulary of RDF.

Classes

• rdfs:Resource is the class of everything. All things described by RDF are resources.
• rdfs:Class declares a resource as a class for other resources.

A typical example of an rdfs:Class is foaf:Person in the Friend of a Friend (FOAF) vocabulary.^[4] An instance of foaf:Person is a resource that is linked to the class foaf:Person using the rdf:type property, such as in the following formal expression of the natural-language sentence : 'John is a Person'.

ex:John rdf:type foaf:Person

The definition of rdfs:Class is recursive: rdfs:Class is the class of classes, and so it is an instance of itself.

rdfs:Class rdf:type rdfs:Class

The other classes described by the RDF and RDFS specifications are:

• rdfs:Literal – literal values such as strings and integers. Property values such as textual strings are examples of RDF literals. Literals may be plain or typed.
• rdfs:Datatype – the class of datatypes. rdfs:Datatype is both an instance of and a subclass of rdfs:Class. Each instance of rdfs:Datatype is a subclass of rdfs:Literal.
• rdf:XMLLiteral – the class of XML literal values. rdf:XMLLiteral is an instance of rdfs:Datatype (and thus a subclass of rdfs:Literal).
• rdf:Property – the class of properties.

Properties

Properties are instances of the class rdf:Property and describe a relation between subject resources and object resources. When used as such a property is a predicate (see also RDF: reification).

• rdfs:domain of an rdf:Property declares the class of the subject in a triple whose predicate is that property.
• rdfs:range of an rdf:Property declares the class or datatype of the object in a triple whose predicate is that property.

For example, the following declarations are used to express that the property ex:employer relates a subject, which is of type foaf:Person, to an object, which is of type foaf:Organization:

ex:employer rdfs:domain foaf:Person

ex:employer rdfs:range foaf:Organization

Given the previous two declarations, the following triple requires that ex:John is necessarily a foaf:Person, and ex:CompanyX is necessarily a foaf:Organization:

ex:John ex:employer ex:CompanyX

• rdf:type is a property used to state that a resource is an instance of a class. A commonly accepted qname for this property is "a".^[5]
• rdfs:subClassOf allows to declare hierarchies of classes.^[6]

For example, the following declares that 'Every Person is an Agent':

foaf:Person rdfs:subClassOf foaf:Agent

Hierarchies of classes support inheritance of a property domain and range (see definitions in next section) from a class to its subclasses.

• rdfs:subPropertyOf is an instance of rdf:Property that is used to state that all resources related by one property are also related by another.
• rdfs:label is an instance of rdf:Property that may be used to provide a human-readable version of a resource's name.
• rdfs:comment is an instance of rdf:Property that may be used to provide a human-readable description of a resource.

Utility properties

• rdfs:seeAlso is an instance of rdf:Property that is used to indicate a resource that might provide additional information about the subject resource.
• rdfs:isDefinedBy is an instance of rdf:Property that is used to indicate a resource defining the subject resource. This property may be used to indicate an RDF vocabulary in which a resource is described.

Examples of RDF Vocabularies

Popular RDF vocabularies represented in RDFS include:^[6]

• FOAF: the source of the page http://xmlns.com/foaf/spec/ is RDFS written in the RDFa syntax.
• Dublin Core: RDFS source is available in several syntaxes^[7]
• Schema.org: the source of the page http://schema.org/docs/schema_org_rdfa.html is RDFS written in the RDFa syntax.
• Simple Knowledge Organization System (SKOS): http://www.w3.org/TR/skos-reference/skos-xl.rdf is an OWL ontology for the SKOS vocabulary that uses the OWL RDF/XML syntax and hence makes use of a number of classes and properties from RDFS.

RDFS entailment

An entailment regime defines by RDFs (,OWL, etc.) not only which entailment relation is used, but also which queries and graphs are well-formed for the regime. The RDFS entailment is a standard entailment relations in the semantic web.

For example, the following declares that 'Dog1 is an animal','Cat1 is a cat', 'Zoos host animals' and 'Zoo1 hosts the Cat2'  :

ex:dog1		rdf:type		ex:animal
ex:cat1		rdf:type		ex:cat
zoo:host	rdfs:range		ex:animal
ex:zoo1		zoo:host		ex:cat2

But this graph is not well formed because the system can not guess that a cat is an animal. We have to add 'Cats are animals' to do a well-formed graph with :

ex:cat		rdfs:subClassOf		ex:animal

Voila, the correct example:

In English	The graph
Dog1 is an animal Cat1 is a cat Cats are animals Zoos host animals Zoo1 hosts the Cat2
RDF/turtle
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> . @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> . @prefix ex: <http://example.org/> . @prefix zoo: <http://example.org/zoo/> . ex:dog1 rdf:type ex:animal . ex:cat1 rdf:type ex:cat . ex:cat rdfs:subClassOf ex:animal . zoo:host rdfs:range ex:animal . ex:zoo1 zoo:host ex:cat2 .

If your triplestore (or RDF database) implements the regime entailment of RDF and RDFS, the SPARQL query as follows (the keyword "a" is equivalent to rdf:type in SPARQL):

PREFIX  ex: <http://example.org/>
SELECT ?animal
WHERE
  { ?animal a ex:animal . }

Gives the following result with cat1 in it because the Cat's type inherits of Animal's type:

See also

• SPARQL Query Language for RDF
• Platform for Internet Content Selection (PICS)

References

External links

• W3C RDFS Specification
• W3C RDF Primer
• W3C SPARQL 1.1 Entailment Regimes Specification
• W3C RDFS Entailment
• SPARQL2XQuery Map OWL-RDF/S ontologies and XML Schemas. Transform XML Schema to OWL.