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RDFS: Vocabularies for RDF

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RDF vocabularies can be defined at different levels of formality:

  • Informal: A web page listing terms and human-readable descriptions. Only humans can interpret them.

  • Semi-formal (RDFS): The vocabulary is defined as RDF triples themselves, using constructs like rdfs:domain and rdfs:range. An RDFS-aware processor can reason over these.

  • Formal (OWL): Same as RDFS but with richer constraints (cardinality, disjointness, equivalence). An OWL-aware processor has more built-in inference rules.

RDFS (RDF Schema) is the semi-formal level. It provides constructs for defining the meaning of terms used in RDF graphs, sitting between raw RDF (no semantics beyond triples) and OWL (full ontology language).

Constructs ​

The meanings of these constructs are hardcoded into RDFS-aware processors.

Vocabulary authors use them to describe their own IRIs, and the processor applies its built-in inference rules accordingly.

Basic Notions ​

Everything is a resource classified by rdf:type.

  • A class is a category (subjects and objects are instances of classes).

  • A property is a relationship (used as the predicate). A property is just an instance of the class rdf:Property, the same way Bob is an instance of Person:

txt
<Bob> rdf:type <Person> .              (Bob is an instance of Person)
<Person> rdf:type rdfs:Class .         (Person is an instance of Class)
<foaf:knows> rdf:type rdf:Property .   (knows is an instance of Property)
<rdf:type> rdf:type rdf:Property .     (type is itself an instance of Property)

Similarly, rdf:Property is a class whose instances are properties:

txt
<foaf:knows> rdf:type rdf:Property .  (knows is a Property)

Constructs ​

Construct Kind Triple form Purpose
rdfs:ClassA class C rdf:type rdfs:ClassC (a resource) is an RDF class
rdf:PropertyA class P rdf:type rdf:PropertyP (a resource) is an RDF property
rdf:typeA property I rdf:type CI (a resource) is an instance of C (a class)
rdfs:subClassOfA property C1 rdfs:subClassOf C2C1 (a class) is a subclass of C2 (a class)
rdfs:subPropertyOfA property P1 rdfs:subPropertyOf P2P1 (a property) is a sub-property of P2 (a property)
rdfs:domainA property P rdfs:domain CDomain of P (a property) is C (a class)
rdfs:rangeA property P rdfs:range CRange of P (a property) is C (a class)

The two different prefixes (rdf: and rdfs:) are a historical artefact preserved for backward compatibility.

Example:

txt
<Person> <type> <Class> .
<is a friend of> <type> <Property> .
<is a friend of> <domain> <Person> .
<is a friend of> <range> <Person> .
<is a good friend of> <subPropertyOf> <is a friend of> .

Note that properties are themselves resources. <is a friend of> is typically used as a predicate, but here it appears as the subject of triples (assigning type, domain, range) and as the object of a triple (describing <is a good friend of>).

Entailment ​

RDFS enables entailment: Deriving new triples from existing ones through logical inference.

Example: If foaf:knows has rdfs:domain foaf:Person and rdfs:range foaf:Person, then from <Bob> foaf:knows <Alice> a system can infer that both Bob and Alice are of type foaf:Person.

For a formal specification of RDFS semantics, see the RDF Semantics document. For more comprehensive semantic modeling, see OWL, which is itself an RDF vocabulary and can be used in combination with RDFS.

Vocabularies get their value from reuse: The more a vocabulary's IRIs are reused by others, the more valuable it becomes (network effect). Prefer reusing existing IRIs over inventing new ones.

  • FOAF: One of the first worldwide RDF vocabularies, for describing people and social networks.

  • Dublin Core (dcterms): General-purpose metadata (creator, publisher, title).

  • schema.org: Vocabulary developed by major search providers for marking up web pages so search engines understand the content.

  • SKOS: W3C recommendation (since 2009) for publishing classification schemes, terminologies, and thesauri. The Library of Congress published its Subject Headings as SKOS.