RDF Schema


RDF Schema is a set of classes with certain properties using the RDF extensible knowledge representation data model, providing basic elements for the description of ontologies. It uses various forms of RDF vocabularies, intended to structure RDF resources. RDF and RDFS can be saved in a triplestore, then one can extract some knowledge from them using a query language, like SPARQL.
The first version was published by the World-Wide Web Consortium in April 1998, and the final W3C recommendation was released in February 2014. Many RDFS components are included in the more expressive Web Ontology Language.
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Terminology

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

Classes

A typical example of an rdfs:Class is in the Friend of a Friend vocabulary. An instance of is a resource that is linked to the class using the 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 is recursive: 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:

Properties

Properties are instances of the class and describe a relation between subject resources and object resources. When used as such a [|property] is a predicate.
For example, the following declarations are used to express that the property relates a subject, which is of type, to an object, which is of type :

ex:employer rdfs:domain foaf:Person
ex:employer rdfs:range foaf:Organization

Given the previous two declarations, from the triple:

ex:John ex:employer ex:CompanyX

can be inferred that is a, and is a.
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 from a class to its subclasses.

RDFS entailment

An entailment regime defines whether the triples in a graph are logically contradictory or not. RDFS entailment is not very restrictive, i.e. it does not contain a large amount of rules limiting what kind of statements are valid in the graph. On the other hand it is also not very expressive, meaning that the semantics that can be represented in a machine-interpretable way with the graph is quite limited.
Below in a simple example of the capabilities and limits of RDFS entailment, we start with a graph containing the following explicit triples:

foo:SomeGiraffe rdf:type bar:Animal.
foo:SomeElephant rdf:type bar:Elephant.
foo:SomeZoo rdf:type bar:Zoo.
bar:livesInZoo rdfs:domain bar:Animal.
bar:livesInZoo rdfs:range bar:Zoo.
foo:SomeElephant bar:livesInZoo foo:SomeZoo.

Without enabling inferencing with RDFS entailment, the data we have does not tell us whether foo:SomeElephant is a bar:Animal. When we do RDFS-based inferencing, we will get the following extra triple:

foo:SomeElephant rdf:type bar:Animal.

The rdfs:domain statement dictates that any subject in triples where bar:livesInZoo is the predicate is of type bar:Animal. What RDFS entailment is not able to tell us is the relationship between bar:Animal and bar:Elephant. Due to inferencing we now know that foo:SomeElephant is both bar:Animal and bar:Elephant so these classes do intersect but there is no information to deduce whether they merely intersect, are equal or have a subclass relationship.
In RDFS 1.1, the domain and range statements do not carry any formal meaning and their interpretation is left up to the implementer. On the other hand in the 1.2 Working draft they are used as entailment rules for inferencing the types of individuals. Nevertheless in both versions, it is very clearly stated that the expected functionality of range is "the values of a property are instances of one or more classes" and domain "any resource that has a given property is an instance of one or more classes".
The example above demonstrated some of the limits and capabilities of RDFS entailment, but did not show an example of a logical inconsistency, meaning that the statements the triples make are in conflict and try to express contradictory states of affairs. An example of this in RDFS would be having conflicting datatypes for objects.

Examples of RDF vocabularies

RDF vocabularies represented in RDFS include: