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Concepts in RDF


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We can distinguish three kinds of concepts in the RDF:

A D V E R T I S E M E N T
The fundamental concepts, The schema-definition concepts (useful for defining a new vocabularies) and The utility concepts (are the concepts which are not absolutely necessary, but are likely to be useful in any of the application domain).


The Fundamental concepts

RDF Resource

RDF is about describing the resources; according to the "resources are always named by URIs" and "anything can have URI". So an RDF can theoretically be used to describe anything. Yet it mainly designed to handle "network retrievable" resources.

[] underlines that "the resource is conceptual mapping to an entity (...), not necessary that the entity which corresponds to this mapping at any particular instance of time". However most of the time we are interested in the entities themselves. It is therefore important to note that meta-data we express about resources may require the different levels of interpretation, that may be valid in certain context only.

For example, URI http://www.w3.org/Icons/WWW/w3c_main returns W3C logo in a PNG or GIF format, depending on the browser being used. Another example is daily weather report, whose URL would return a different page each day.

It follows that interpretation of the resources (and therefore of the RDF triples) is highly contextual. We can define the notion of a stable resource as follows: The stability for the resource is a property of being the same in any context, from the user (or community of users) point of view. This definition is still very contextual: it is dependant on the users we are considering, more precisely on the task which they need to accomplish.

Consider for an example, from a standard reader point of view of, the W3C logo is stable, since GIF and PNG versions look the same, but a weather report is not stable. On the other hand, someone interested in image formats only may consider W3C logo unstable and weather report stable -- assuming a weather report as always generating images in a same format. However, in most of the applications, task is less likely to change than retrieving context, thus the stability assumption might be valuable.

RDF Property

Properties are the resources used as predicate of the triples; semantics of a triple clearly depends on a property used as predicate. Two things are very important with this concept of property:

Firstly, The RDF considers properties as the first class object, unlike most of the object modeling languages, where the properties are the attributes of a class. Even if the concept of the class exists in the RDF, properties can be defined and used independently of the classes. .

Secondly, as it has been mentioned, the fact that properties are the resources allows to describe them with the RDF itself. This will be widely used by the following concepts.

RDF Statement

A statement is a resource reifying the triple. Such a resource must have atleast three properties : the RDF:subject, the RDF :object and RDF:predicate, valued by corresponding resources.

The reification of the triples may seem a utility concept rather than the fundamental concept. Nevertheless it is defined as the part of model in a W3C recommendation. This will support the will to use the RDF as its own meta-system, to make every element of the RDF describable in RDF itself.


The Schema definition concepts

In the schemas, new resources can be defined as specialization of the old ones, thus allowing to infer implicit triples. Schemas also constrains the context in which the defined resources can be used, inducing the notion of the schema validity. We will see that those two notions can be seen as one, in a point of view based on the first-order logic. They all can be expressed as the rules allowing to infer new facts (basically, the new triples or negations of the triples). In these rules, the 3-ary logical predicate will be used to represent the believed triple.

RDFS:subPropertyOf

Any of the property denotes the relation between resources (the set of a resource couples linked by the arc labeled with a property). RDFS:subPropertyOf applies to the properties and must be interpreted as a subset relation between the relation which they denote. Thus the following rule does stand:

For example, if the "mother" is sub-property of "parent", any triple having the "mother" as predicate should also be considered as having "parent" as the predicate. This property is very important in the schema definitions for the interoperability between the RDF agents. In the example above, the agent not knowing the semantics of "mother" could at least treat it as the "parent" (assuming it knows semantics of the "parent"). Since the RDFS:subPropertyOf denotes a subset relation, the transitivity rule also do stand:

Note that it is considered invalid to have the cycles in RDFS:subPropertyOf, though it does not define the way to express this constraint in the RDF4. Anyway, the corresponding logical rule is the following (since any of the cycle would result, with the transitivity, in a property being its own sub-property):

Note that there is no standard URI for an universal property (super-property of any of the property).

RDFS:Class, rdf:type and the RDFS:subClassOf

Classes are the resources denoting a set of the resources, by the means of the property RDF:type (instances that have the property RDF:type valued by a class). Since all the sets of resources presented in this section are resources (they have an URI), by definition they do have the property RDF:type valued by the RDFS:Class. On the other hand, all the properties (defined in the W3C recommendation or in any schema) have RDF:type valued by RDF:Property.

Classes are structured in the same way as the properties, in a subset hierarchy denoted by the property RDFS:subClassOf. As for RDFS:subPropertyOf, cycles should not exist though it can be used to express the equivalence, but contrary to property hierarchy, the class hierarchy has the maximum element: it is of course the RDF:Resource (so any class can implicitly has the RDFS:subClassOf valued by the RDFS:Resource). The following rules, similar to the rules related to the RDFS:subPropertyOf, do stand:

RDFS:domain and RDFS:range

These properties do apply to the properties and must be valued by the classes. They are used to restrict a set of resources that may have the given property (the property's domain) and the set of valid values for the property (its range). A property may have as many values for the RDFS:domain as needed, but no more than one value for the RDFS:range

For the triple to be valid, object must match the range (if any) of predicate (that is, it should have the RDF:type valued by corresponding class or one of its subclasses), and the subject should match at least one of the domains (if any) of the predicate (Note that if the predicate has the super-properties, this should also be checked recursively for all of them). This can logically bne expressed by:

It is worth noting that, though this two rules are intended to be used for the validity checking only, and the first one (RDFS:domain rule) can actually be used only this way (it cannot be used to perform the inference since its consequence is existentially qualified), the second one (RDFS:range rule) has different interpretations depending on the hypothesizing closed or open world. In a closed world hypothesis, any missing triple is considered negated, so a RDFS:range rule has only to be verified. But in the open world hypothesis, missing triples are not necessarily to be false, so the rule can be used to perform the inference instead. Since the "natural" field of the RDF is web, where the information is by essence distributed and incomplete, the open world hypothesis seems to be much more reasonable.

RDFS:Literal

A resource rdfs:Literal, denoting the set of literals, declared as the class (though literals are not the resources, according to recommendation!). Its intended use is to be a range of properties.


The Utility concepts

These concepts might have been defined in the external schemas, but since they are of common use, they have been defined once for all in a core schema.

RDFS:Container

Containers are the collection of resources. They are modeled by the instance of one of the three subclasses of RDFS:Container: the RDF:Bag ( unordered collection ), RDF:Seq ( ordered collection ) or RDF:Alt ( an alternative ). Membership is modeled by the automatically generated properties RDF:_ 1, RDF:_ 2, etc. These properties are the instances of RDFS:ContainerMembershipProperty, a subclass of RDF:Property5.

RDFS:ConstraintResource and RDFS:ConstraintProperty

It can be interesting for the RDF agent to be informed of an unknown resource (or more specifically the property) is defining the validity constraint. The set of such resources is RDFS:ConstraintResource. Its subclass RDFS:ConstraintProperty is of course the subclass of RDF:Property too. The Properties RDFS:domain and RDFS:range defined above are the instances of RDFS:ConstraintProperty.

RDFS:seeAlso and RDFS:isDefinedBy

A given resource might have be described in more than one place over the internet. The RDFS:seeAlso property can be used to point out the alternative descriptions of the subject resource. Its sub-property RDFS:isDefinedBy more specifically points to the original or authoritative description.

RDFS:label and RDFS:comment

It could be useful to describe the resource with human readable text in addition to the "pure" RDF properties; this is the role of RDFS:label and RDFS:comment. The former is used to give the human-readable name of a resource, the latter, to give the longer description. Note that they can have the multiple values for internationalization needs.



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Keywords: rdf model, resource description framework xml, rdf statements, rdf schema

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