Copyright ©1998,1999 W3C (MIT, INRIA, Keio) , All Rights Reserved. W3C liability, trademark, document use and software licensing rules apply.
This document is a Proposed Recommendation of the World Wide Web Consortium. Review comments on this specification should be sent by 2359Z April 06, 1999 to <www-rdf-comments@w3.org>. The archive of public comments is available at http://w3.org/Archives/Public/www-rdf-comments. W3C Members may send their formal comments, visible only to W3C staff, to <w3c-rdf-review@w3.org>.
This specification is a revision of the last-call working draft dated 1998-10-30 incorporating suggestions received in review comments and further deliberations of the W3C RDF Schema Working Group. The detailed differences are available for reviewers to compare.
The Working Group anticipates no further substantial changes to this specification. We encourage active implementation to test this specification during the Proposed Recommendation review period.
Publication as a Proposed Recommendation does not imply endorsement by the W3C membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite W3C Drafts as other than "work in progress".
The Resource Description Framework is part of the W3C Metadata Activity. The goal of this activity, and of RDF specifically, is to produce a language for the exchange of machine-understandable descriptions of resources on the Web. A separate specification describes the data model and syntax for the interchange of metadata using RDF.
Note: The HTML source of this document contains embedded RDF and will therefore not validate against the HTML4.0 DTD. A solution for those requiring DTD-style validation services may come from future W3C work.
The Resource Description Framework (RDF) is a foundation for processing metadata; it provides interoperability between applications that exchange machine-understandable information on the Web. RDF emphasizes facilities to enable automated processing of Web resources. RDF can be used in a variety of application areas; for example: in resource discovery to provide better search engine capabilities, in cataloging for describing the content and content relationships available at a particular Web site, page, or digital library, by intelligent software agents to facilitate knowledge sharing and exchange, in content rating, in describing collections of pages that represent a single logical "document", for describing intellectual property rights of Web pages, and for expressing the privacy preferences of a user as well as the privacy policies of a Web site. RDF with digital signatures will be key to building the "Web of Trust" for electronic commerce, collaboration, and other applications.
Descriptions used by these applications can be modelled as relationships among Web resources. The RDF data model, as specified in [RDFMS], defines a simple model for describing interrelationships among resources in terms of named properties and values. RDF properties may be thought of as attributes of resources and in this sense correspond to traditional attribute-value pairs. RDF properties also represent relationships between resources. As such, the RDF data model can therefore resemble an entity-relationship diagram. The RDF data model, however, provides no mechanisms for declaring these properties, nor does it provide any mechanisms for defining the relationships between these properties and other resources. That is the role of RDF Schema.
Resource description communities require the ability to say certain things about certain kinds of resources. For describing bibliographic resources, for example, descriptive attributes including "author", "title", and "subject" are common. For digital certification, attributes such as "checksum" and "authorization" are often required. The declaration of these properties (attributes) and their corresponding semantics are defined in the context of RDF as an RDF schema. A schema defines not only the properties of the resource (Title, Author, Subject, Size, Color, etc.) but may also define the kinds of resources being described (books, Web pages, people, companies, etc.).
This document does not specify a vocabulary of descriptive elements such as "author". Instead, it specifies the mechanisms needed to define such elements, to define the classes of resources they may be used with, to restrict possible combinations of classes and relationships, and to detect violations of those restrictions. Thus, this document defines a schema specification language. More succinctly, the RDF Schema mechanism provides a basic type system for use in RDF models. It defines resources and properties such as Class and subClassOf that are used in specifying application-specific schemas.
The typing system is specified in terms of the basic RDF data model - as resources and properties. Thus, the resources constituting this typing system become part of the RDF model of any description that uses them. The schema specification language is a declarative representation language influenced by ideas from knowledge representation (e.g. semantic nets, frames, predicate logic) as well as database schema specification languages (e.g. NIAM) and graph data models. The RDF schema specification language is less expressive, but much simpler to implement, than full predicate calculus languages such as CycL[CycL] and KIF [KIF].
RDF and the RDF Schema language were also based on metadata research in the the Digital Library community. In particular, RDF adopts a modular approach to metadata along the lines of the Warwick Framework [WF]. RDF represents an evolution of the Warwick Framework model in that the Warwick Framework allowed each metadata vocabulary to be represented in a different syntax. In RDF, all vocabularies are expressed within a single well defined model and syntax. This allows for a finer grained mixing of machine-processable vocabularies, and addresses the need [EXTWEB] to create metadata in which statements can draw upon multiple vocabularies that are managed in a decentralised fashion by various communities of expertise.
RDF Schemas might be contrasted with XML Document Type Definitions (DTDs). Unlike an XML DTD, which gives specific constraints on the structure of a document, an RDF Schema provides information about the interpretation of the statements given in an RDF data model. The RDF/XML syntax itself provides considerable flexibility in the syntactic expression of the data model. A syntactic schema alone is not sufficient for RDF purposes. RDF Schemas may also specify consistency constraints that should be followed by these data models.
The RDF Schema specification is not aimed at theoretical issues, but at solving a small number of immediate problems. Its creators expect that other problems (some of which are illustrated in the examples below) will share similar characteristics and that they also may be able to use the basic classes described in this specification.
The RDF Schema specification was directly influenced by consideration of the following problems:
The RDF Model and Syntax is adequate to represent PICS labels [PICS], however it does not provide a general-purpose mapping from PICS rating systems into an RDF representation. One such mapping is presented in section 7.
One obvious application for RDF is in the description of Web pages. This is one of the basic functions of the Dublin Core [DC] initiative. The Dublin Core is a set of 15 elements believed to be broadly applicable to describing Web resources to enable their discovery. The Dublin Core has been a major influence on the development of RDF. An important consideration in the development of the Dublin Core was to not only allow simple descriptions, but also to provide the ability to qualify descriptions in order to provide both domain specific elaboration and descriptive precision.
The RDF Schema specification provides a machine-understandable system for defining 'schemas' for descriptive vocabularies like the Dublin Core. It allows designers to specify classes of Resource types and properties to convey descriptions of those classes, and constraints on the allowed combinations of classes, properties, and values.
An initial schema for the simple Dublin Core is provided in Appendix B. This schema defines the 15 elements as properties, and gives a description of their purpose. Despite the simplicity of the definition, it is believed that this schema serves as the foundation for more elaborate definitions. Future extensions to the Dublin Core are likely to specify the structure of the values of the properties, which will involve defining classes, the properties that apply to those classes, and some constraints on the property values. In order for browsers and authoring tools to understand and enforce these constraints, this information should be machine understandable. This document provides a machine understandable schema language for expressing such definitions and constraints.
A sitemap is a hierarchical description of a Web site. A subject taxonomy is a classification system that might be used by content creators or trusted third parties to organise or classify Web resources. The RDF Schema specification provides a mechanism for defining the vocabularies needed for such applications.
Thesauri and library classification schemes are well known examples of hierarchical systems for representing subject taxonomies in terms of the relationships between named concepts. The RDF Schema specification provides sufficient resources for creating RDF models that represent the logical structure of thesauri (and other library classification systems).
The W3C Platform for Privacy Preferences Project (P3P) requires a grammar for constructing statements about a site's data collection practices and personal preferences as exercised over those practices, as well as a syntax for exchanging structured data. The ability to provide third party assurances (signed statements) regarding P3P practices is also important. For instance, entities may wish to certify that P3P practice statements were properly generated in accordance with industry guidelines, have been audited, or are compliant with the relevant privacy regulations.
Although personal data collection practices could be described using an application-specific XML tagset, there are benefits to using a general metadata model for this data. Using a metadata schema to describe the formal structure of privacy practice descriptions will permit privacy practice data to be used along with other metadata in a query during resource discovery, and will permit a generic software agent to act on privacy metadata using the same techniques as used for other descriptive metadata.
An RDF Schema can be expressed by the data model described in the RDF Model and Syntax [RDFMS] specification. The schema description language is simply a set of resources and properties defined by this specification and implicitly part of every RDF model using this schema machinery.
This document specifies the RDF Schema mechanism as a set of RDF resources (including properties), and constraints on their relationships.
The Resource Description Framework is intended to be flexible and easily extensible; this suggests that a great variety of schemas will be created and that new and improved versions of these schemas will be a common occurence on the Web. Since changing the logical structure of a schema risks breaking other RDF models which depend on that schema, this specification recommends that a new URI is used whenever an RDF schema is changed.
In effect, changing the RDF statements which constitute a schema creates a new one; new schema namespaces should have their own URI to avoid ambiguity. Since an RDF Schema URI unambiguously identifies a single version of a schema, software that uses or manages RDF (eg. caches) should be able to safely store copies of RDF schema models for an indefinite period. The problems of RDF schema evolution share many characteristics with XML DTD version management and the general problem of Web resource versioning. A general approach to these issues is beyond the scope of this specification.
Since each RDF schema has its own unchanging URI, these can be used to construct unique URI references for the resources defined in a schema. This is achieved by combining the local identifier for a resource with the URI associated with that schema namespace. The XML representation of RDF uses the XML namespace mechanism for associating elements and attributes with URI references for each vocabulary item used.
Note that the formal URI corresponding to the RDF Schema namespace has not yet been assigned. For the purposes of this document we use as a temporary URI the identifier for the working draft of the RDF Schema specification. The same approach is adopted here for the URI of the RDF Model and Syntax namespace. Consequently, these URIs will change in the final version of this specification.
In accord with Appendix C.1 of the RDF Model and Syntax Specification we adopt the following convention concerning capitalization of property and class names:
rdfs:Resource. Additional words in
the class name are also capitalized, e.g.,
rdfs:ConstraintProperty. rdfs:domain.
However, additional words in
the name are capitalized,
e.g., rdfs:subClassOf. In this section we describe the core RDF Schema. We first define the type system and then introduce the core classes and properties.
The RDF Schema defined in this specification is a collection of RDF resources
that can be used to describe properties of other RDF resources (including properties)
which define application-specific RDF vocabularies.
The core schema vocabulary is defined in a namespace informally called 'rdfs'
here, and which will be given a formal URI in the final version of this specification.
As described in the RDF Model and Syntax specification, resources may be instances of
one or more classes; this is indicated with the rdf:type
property. Classes themselves are often organised in a hierarchical
fashion, for example the class 'dog' might be considered a sub-class of
'animal' which is a sub-class of 'organism' etc., meaning that any
resource which is of rdf:type 'dog' is ipso facto also of
rdf:type 'animal' and so on. This specification describes a
property, rdfs:subClassOf, to denote such relationships
between classes.
In addition to the rdfs:subClassOf property, this
specification introduces a number of
other resources for making statements about constraints on the consistent use of
properties and classes in RDF data. For example, an RDF schema might describe
limitations on the types of values that are valid for some property, or on the classes
to which it makes sense to ascribe such properties.
This specification gives a mechanism for describing such constraints, but does not
say whether or how an application must process the constraint information.
For example, while an RDF schema may express that a 'Book' may have an 'author' property,
it does not say whether or how an application should act in processing
that information. We expect that different applications will use these
constraints in different ways. e.g., a validator will look for errors,
an interactive editor might suggest legal values.
We anticipate the development of a set of classes corresponding to a
set of "datatypes." This specification does not define datatypes, but does
note that datatypes may be used as the value of the
rdfs:range property.
rdf:type property of that resource whose value is the resource defining the
containing class. (These properties are shown as arcs in the directed labelled
graph representation in figure 2). The RDF resources depicted in figure 1
are described either in the remainder of this specification, or in the RDF
Model and Syntax specification.
Figure 1: Classes and Resources as Sets and Elements
Figure 2 shows the same information about the class hierarchy
as in figure 1, but does so using a "nodes and arcs" graph representation of
the RDF data model.
If a class is a subset of another, then there is an rdfs:subClassOf
arc from the node
representing the first class to the node representing the second. Similarly,
if a Resource was an instance of a Class, then there is an rdf:type
arc from the resource to the node representing the class. (Not all such arcs are shown.
We only show the arc to the most tightly encompassing class, and rely on the transitivity
of the rdfs:subClassOf relation to provide the rest).
Figure 2: Class Hierarchy for the RDF Schema
The following resources are core classes that are defined as part of the RDF Schema machinery. Every RDF model that draws upon the RDF Schema vocabulary namespace (implicitly) includes these.
All things being described by RDF expressions are called resources, and are
considered to be instances of the class rdfs:Resource.
The RDF class rdfs:Resource
represents the set called 'Resources' in the formal model for RDF presented in section 5 of
the Model and Syntax specification [RDFSMS].
This roughly corresponds to the concept of Object in Java.
Note that the class rdfs:Resource
is unrelated to the syntactic attribute 'resource' used in the RDF/XML
serialization syntax (just as the resource 'Description' defined by
the Dublin Core element set is unconnected to the syntactic element
'Description' used in the RDF/XML syntax).
The class rdf:Property represents the subset of RDF resources that are properties,
i.e., all the elements of the set introduced as 'Properties' in section 5 of the Model
and Syntax specification [RDFSMS].
This corresponds to the generic concept of a Type or
Category, similar to the notion of a Class in object-oriented
programming languages such as Java.
When a schema defines a new class, the resource representing that class must
have an rdf:type property whose value is the resource rdfs:Class.
RDF classes can be defined to represent almost anything, such as Web pages, people, document types,
databases or abstract concepts.
Every RDF model which uses the schema mechanism
also (implicitly) includes the following core properties.
These are instances of the rdf:Property class and provide
a mechanism for expressing relationships between classes and their instances
or superclasses.
This indicates that a resource is a member of a class, and thus has all
the characteristics that are to be expected of a member of that class.
When a resource has an rdf:type property whose value is
some specific class, we say
that the resource is an instance of the specified class. The value of an
rdf:type property for some resource is
another resource which must be an instance of rdfs:Class.
The resource known as rdfs:Class is itself a resource of rdf:type
rdfs:Class. Individual classes (for example, 'Dog') will always have an
rdf:type property whose value is rdfs:Class (or some sub-class
of rdfs:Class, as described in section 2.3.2). A resource may be an instance
of more than one class.
This property specifies a subset/superset relation between classes.
The rdfs:subClassOf property is transitive.
If class A is a sub-class of some broader class B, and B is a sub-class of
C, then A is also implicitly a sub-class of C. Consequently, resources
that are instances of class A will also be instances of C, since A is a
sub-set of both B and C. Only instances of rdfs:Class can have the
rdfs:subClassOf property and the property value is always of
rdf:type rdfs:Class. A class may be a
sub-class of more than one class.
A class can never be declared to be a sub-class of itself, nor of any of its own sub-classes. Note that this constraint is not expressible using the RDF Schema constraint facilities provided below, and so does not appear in the RDF version of this specification given in Appendix A.
This is a very simple example that expresses the following class hierarchy. We first define a class 'MotorVehicle'. We then define 3 subclasses of 'MotorVehicle', namely 'PassengerVehicle', 'Truck' and 'Van'. We then define a class 'Minivan' which is a sub-class of both 'Van' and 'PassengerVehicle'.
The RDF/XML shown here uses the basic RDF syntax defined in section 2.2.1
of the Model and Syntax specification [RDFMS]. The RDF
syntax also provides (in section 2.2.2) a more compact mechanism for
making statements about the rdf:type of a resource, by
allowing the identifier for a class (eg. 'Van' in the examples below) to
be used directly as an XML element name. For example, <Van
ID="v_323"/> tells us both that there is an resource known as
'v_323' and that it is an instance of the class Van.
Similarly, <rdfs:Class ID="MiniVan"/> might be used
to state that the resource known as MiniVan is itself a
class. For clarity,
these initial examples do not take advantage of the rdf:type
abbreviation mechanism provided by the RDF serialisation syntax. In
general, the XML representation of an RDF schema can make full use of all
the syntactic flexibility of RDF.
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An instance of rdf:Property that is used to specify that one
property is a specialization of another. A property may have
zero, one, or more properties that it is a
specialization of. If some property P2 is a subPropertyOf another more
general property P1, and if a resource A has a P2 property with a value B,
this implies that the resource A also has a P1 property with
value B.
A property can never be declared to be a sub-property of itself, nor of any of its own sub-properties. Note that this constraint is not expressible using the RDF Schema constraint facilities provided below, and so does not appear in the RDF version of this specification given in Appendix A.
If the property biologicalFather is a
sub-property of the broader property biologicalParent, and if Fred is the
biologicalFather of John, then it is implied that Fred is also the
biologicalParent of John.
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The property rdfs:seeAlso
specifies a resource that contains information about the subject resource. This
property may be specialized using rdfs:subPropertyOf to more precisely
indicate the nature of the information the object resource has about the
subject resource. The object and the subject resources are constrained only to be instances of the class
rdfs:Resource.
The property rdfs:isDefinedBy is a sub-property of
rdfs:seeAlso, and indicates the resource
defining the subject resource. As with rdf:seeAlso, this
property can be applied to any instance of rdfs:Resource and may have
as its value any rdfs:Resource.
The most common anticipated usage is to identify the RDF schema given a name for one of the properties or classes defined by that schema. Although XML namespace declarations will typically provide the URI where RDF vocabulary resources are defined, there are cases where additional information is required.
For example, constructs such as
<rdfs:subPropertyOf
rdf:resource="http://purl.org/dc/elements/1.0/Creator"/>
do not
indicate the URI of the schema that includes the vocabulary item
Creator (ie., http://purl.org/dc/elements/1.0/).
In such cases, the rdfs:isDefinedBy property can be used to explicitly
represent that
information. This approach will also work when the URIs of the namespace and its
components have no obvious relationship, as would be the case if they
were identified using schemes such as GUIDs or MD-5 hashes.
The RDF Schema type system is similar to the type systems of
object-oriented programming languages such as Java.
However, RDF differs from many such systems in that instead of defining
a class in terms of the properties its instances may have, an RDF schema
will define properties in terms of the classes of resource that they apply
to.
This is the role of the rdfs:domain and
rdfs:range constraints described below. For example, we
could define the "author" property to
have a domain of "Book" and a range of "Literal", whereas a classical
OO system might typically define a class "Book" with a property called
"author" of type "Literal". One benefit of this property-centric
approach is that it is very easy for anyone to say anything they want
about existing resources, which is one of the axioms of the Web.
An RDF schema can declare constraints associated with classes and properties. In particular, the concepts of domain and range are used in RDF schemas to make statements about the contexts in which certain properties "make sense".
Although the RDF data model does not allow for explicit properties (such
as an rdf:type property) to be ascribed to Literals (atomic values),
we nevertheless consider these entities to be members of classes (e.g.
the string "John Smith" is considered to be a member of the class
rdfs:Literal.)
We expect future work in RDF and XML data-typing to provide
clarifications in this area.
An RDF model that violates any of the consistency
constraints specified in this document is said to be
an inconsistent model. The following constraints are specified:
rdfs:domain and rdfs:range
constraints on property usage, the rule that rdfs:subPropertyOf and
rdfs:subClassOf properties should not form loops, plus any
further consistency constraints defined using the
rdfs:ConstraintResource extensibility mechanism.
Different applications may exhibit different behaviours in the
face of an inconsistent model.
Some examples of constraints include:
RDF schemas can express constraints that relate vocabulary items from multiple independently developed schemas. Since URI references are used to identify classes and properties, it is possible to create new properties whose domain or range is constrained to be a class defined in another namespace.
The RDF Schema uses the constraint properties to constrain how its own properties can be used. These constraints are shown below in figure 3. Nodes with bold outlines are instances of rdfs:Class.
Figure 3: Constraints in the RDF Schema
This resource defines a sub-class of rdfs:Resource whose
instances are RDF schema constructs involved in the expression of
constraints. The purpose of this class is to provide a mechanism
that allows RDF processors to assess their ability to check the
consistency of an RDF model. Since this specification does not provide a
mechanism for the dynamic discovery of new forms of constraint, an RDF 1.0
processor encountering previously unknown instances of
rdfs:ConstraintResource can be sure that it is
unqualified to judge the consistency of resources using properties
defined using that resource.
This resource defines a sub-class of rdf:Property, all of whose instances
are properties used to specify constraints. This class is a subclass
of rdfs:ConstraintResource and corresponds to the subset of
that class representing properties. Both rdfs:domain and
rdfs:range are instances of
rdfs:ConstraintProperty.
An instance of ConstraintProperty that is used to constrain
property values. The value of a range property is always a
Class. The value of a property whose range is A is constrained to
be an instance of class A. A property can have at most one
range property. It is possible for it to have no range, in
which case the class of the property value is unconstrained.
Although it is not possible to express two or more range constraints on a property,
a similar outcome can be achieved by defining a common superclass for any classes
that represent appropriate values for some property. For example, to express the
constraint that a property xyz:drivesMotorVehicle can have
values which are Vans, Trucks or PassengerVehicles, we assert that
xyz:drivesMotorVehicle has a rdfs:range of
MotorVehicle. If Van, Truck and
PassengerVehicle are known to be sub-classes of
MotorVehicle, then all these types of resource are acceptable
values for
xyz:drivesMotorVehicle. In cases where a common super-class
does not exist, one can be defined in a schema in the normal manner.
As mentioned earlier, we anticipate the development of a set of classes corresponding to a
set of "datatypes." Once available, these datatypes may be used as the value of the
rdfs:range constraint property.
An instance of ConstraintProperty that is used to specify a
class on which a property may be used. A property may have
zero, one, or more than one class as its domain. If there is no domain
property, it may be used with any resource. If there is exactly one domain
property, it may only be used on instances of that class (which is the
value of the domain property).
If there is more than one domain property, the constrained property
can be used with instances of any of the classes (that are values of
those domain properties).
Continuing with our earlier example of MotorVehicle, in this example, we define two properties : 'registeredTo' and 'rearSeatLegRoom'. The 'registeredTo' property is applicable to any MotorVehicle and its value is a Person (defined in the examples below). For the sake of this example, 'rearSeatLegRoom' only applies to Minivans and PassengerVehicles. The value is a 'Number' (we anticipate that some concept like this will be provided by the work on data types), which is the number of centimetres of rear seat legroom.
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The RDF Schema specification builds upon the foundations provided by XML and by the RDF Model and Syntax. It provides some additional facilities to support the evolution both of individual RDF vocabularies, and of the core RDF Schema specification vocabulary introduced in this document.
The phrase 'RDF vocabulary' is used here to refer to those resources which evolve over time; 'RDF schema' is used to denote those resources which constitute the particular (unchanging) versions of an RDF vocabulary at any point in time. Thus we might talk about the evolution of the Dublin Core vocabulary. Each version of the Dublin Core vocabulary would be a different RDF schema, and would have a corresponding RDF model and concrete syntactic representation.
The resources defined in RDF schemas are themselves Web resources, and
can be be described in other RDF schemas. This principle provides the
basic mechanism for RDF vocabulary evolution. This specification does not
attempt to provide a full framework for expressing mappings between
schemas; it does however provide the rdfs:subClassOf and
rdfs:subPropertyOf properties. The ability to express
specialization relationships between classes (subClassOf) and
between properties (subPropertyOf) provides a simple
mechanism for making statements about how such resources map to
their predecessors.
There are many scenarios for which these simple mechanisms are not adequate; a more general schema mapping mechanism for RDF may be developed in future W3C Activity.
A schema representing version 1.0 of some vocabulary might define
classes corresponding to a number of vehicle types. The schema for version
2.0 of this vocabulary constitutes a different Web resource. If the new
schema defines for example a class 'Van' whose members are a subset of the
members of the class 'Van' in version 1.0, the
rdfs:subClassOf property can be used to state that all
instances of V2:Van are also instances of
V1:Van.
Where the vocabulary defines properties, the same approach can be taken,
using rdfs:subPropertyOf to make statements about
relationships between properties defined in successive versions of an RDF
vocabulary.
This specification defines a sub-class of resources known as 'constraint
resources' (section 3.1). This is provided to allow for the addition of
new ways of expressing RDF consistency constaints.
Future extensions to the Resource Description Framework may
introduce new resources that are instances of the
rdfs:ConstraintResource class. It is
necessary to anticipate RDF content which draws upon properties or
classes defined using constraints other than those
available in this version of RDF. This specification introduces the notion
of consistent and inconsistent RDF models. As yet
unknown constraints may contribute to a more expressive
framework for specifying whether an RDF model is or is not
considered consistent.
RDF agents unfamiliar with the semantics of unknown instances of
rdfs:ConstraintResource may therefore lack the
knowledge to evaluate models for consistency when vocabulary items are
defined using those unknown constraints.
Since RDF itself may not represent declaratively the full
meaning of these constraint resources, the acquisition of RDF
statements about a new ConstraintResource may not
provide enough information to enable its use in consistency checking.
For example, when encountering a previously unknown constraint property
type called RDF3:mysteryConstraint we may learn
mechnically that it has a range of rdf:Class and a domain of
rdf:Property. The range and domain constraints if
encountered alone would be enough to tell us how to legally use
RDF3:mysteryConstraint, but they
do not tell us anything about the nature of the constraint expressed when
it is used in that fashion.
The rdfs:ConstraintResource
construct is provided here as a simple future-proofing mechanism, and
addresses some of the issues discussed at greater length in the Extensible
Web Languages W3C NOTE [EXTWEB]. By flagging new forms
of constraint as members of this class, we indicate that they are intended to express Schema
language contraints whose semantics must be understood for
consistency checking to be possible. Membership of the class
suggests, but does not imply, that those semantics may be inexpressible
in a declarative form. Since the expressive facilties available within RDF
for doing so are also likely to evolve, this distinction itself presents
a moving target. All RDF agents will have implicit knowledge of certain
constraints (for example, this specification declares that subClassOf
properties must not form a loop in an RDF graph) which may or may not be capable of
representation within (some version of) RDF. It may be the case that some future RDF
specification provides facilties which will allow RDF agents to
comprehend declarative specifications for as-yet uninvented
constraint properties. In such a case, these agents could safely
comprehend (some) previously unencountered forms of constraint. By
providing the basic rdfs:ConstraintResource class, we
anticipate such developments. All RDF agents written to this specification
will appreciate their ignorance of the meaning of unknown instances
of that class, since this specification provides no mechanism for learning
about such constraints through the interpretation of RDF statements.
Future specifications, should they offer such facilities, could also
define sub-classes of ConstraintProperty to classify new constructs
according to whether or not they had inexpressible semantics.
The following properties are provided to support simple documentation
and user-interface related annotations within RDF schemas. Multilingual
documentation of schemas is supported at the syntactic level through
use of the xml:lang language tagging facility. Since RDF schemas are
expressed within the RDF data model, vocabularies defined in other namespaces may be used
to provide richer documentation.
This is used to provide a human-readable natural language description of a resource.
This is used to provide a human-readable version of a resource name.
The RDF Model and Syntax specification [RDFMS]
introduces certain concepts. A number of these
are defined formally in another RDF Schema whose namespace URI
reference is http://www.w3.org/1999/02/22-rdf-syntax-ns#. In
addition, some further concepts are introduced in the RDF Model and Syntax
specification but do not appear in the RDF Model and Syntax
schema. These formally belong in the Schema namespace (for example,
rdfs:Literal and rdfs:Resource). In cases where
an RDF resource belongs to the http://www.w3.org/1999/02/22-rdf-syntax-ns#
namespace, this document can provide only a convenience copy of
that resource's definition.
Appendix A provides an RDF/XML schema for the RDF resources
defined in this document, including RDF Model concepts such as
Literal and Resource.
The RDF/XML Schema in Appendix A also makes RDF statements about resources
defined in the RDF Model and Syntax namespace. These have the status of
annotations rather than definitions. This is indicated
through the use of the <rdf:Description about="...">
syntax, as against the <rdf:Description ID="...">
construct used for the definitions of RDF Schema resources such as
domain and range.
This corresponds to the set called the 'Literals' in the formal model for RDF presented in section 5 of the Model and Syntax specification [RDFMS]. Atomic values such as textual strings are examples of RDF literals.
This corresponds to the set called the 'Statement' in the formal model for RDF presented in section 5 of the Model and Syntax specification [RDFMS].
This corresponds to the property called the 'subject' in the formal model for RDF
presented in section 5 of the Model and Syntax
specification [RDFMS]. Its rdfs:domain
is rdf:Statement and rdfs:range is
rdfs:Resource. This is used to specify the resource
described by a reified statement.
This corresponds to the property called the 'predicate' in the formal model for RDF
presented in section 5 of the Model and Syntax
specification [RDFMS]. Its rdfs:domain
is rdf:Statement and rdfs:range is
rdf:Property. This is used to identify the property used
in the modelled statement.
This corresponds to the property called the 'object' in the formal model
for RDF presented in section 5 of the Model and Syntax
specification [RDFMS]. Its rdfs:domain is rdf:Statement.
This is used to identify the property value in the modelled statement.
This class is used to hold the Container classes described in section 3
of the Model and Syntax specification [RDFMS]. It is
an instance of rdfs:Class and rdfs:subClassOf of
rdfs:Resource.
This corresponds to the class called 'Bag' in the formal model for RDF presented
in section 5 of the Model and Syntax specification [RDFMS]. It
is an instance of rdfs:Class and
rdfs:subClassOf rdfs:Container.
This corresponds to the class called 'Sequence' in the formal model for RDF
presented in section 5 of the Model and Syntax specification [RDFMS]. It is an instance of rdfs:Class and
rdfs:subClassOf rdfs:Container.
This corresponds to the class called 'Alternative' in the formal model for RDF
presented in section 5 of the Model and Syntax
specification [RDFMS]. It is an instance of rdfs:Class and
rdfs:subClassOf rdfs:Container.
This class has as members the properties _1, _2, _3 ... used
to indicate container membership, as described in section 3
of the Model and Syntax specification [RDFMS]. This is a
rdfs:subClassOf rdf:Property.
This corresponds to the 'value' property described in section 2.3 of the Model and Syntax specification [RDFMS].
This section gives some brief examples of using the RDF Schema machinery to define classes and properties for some possible applications. Note that some of these examples use the abbreviated RDF syntax (mentioned in 2.3.2.1 above) to express class membership.
In this example, Person is a class with a corresponding
human-readable description of "The class of people".
A Person is a sub-class of Animal. A
Person may have an age property. The value of age is an integer. A Person may
also have an ssn ("Social Security Number") property. The value of ssn is an
integer. A Person's marital status is one of {Married, Divorced,
Single, Widowed}. This is achieved through use of the range constraint:
we define both a 'maritalStatus' property and a class 'MaritalStatus'
(adopting the convention of using lower case letters to begin the names of
properties, and capitals for classes). We then use rdfs:range to state
that a maritalStatus property only 'makes sense' when it has a value which
is an instance of the class MaritalStatus. The schema then defines a
number of instances of this class. Whether resources declared to
be of type MaritalStatus in another graph are trusted is an application
level decision.
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In this example we sketch an outline of an RDF vocabulary for
use with searchable Internet services. SearchQuery
is declared to be a class. Every SearchQuery can have both a
queryString whose value is an rdfs:Literal and a
queryService whose value is a SearchService.
A SearchService is a sub-class of InternetService (which is
defined elsewhere). A SearchQuery has some number of result
properties (whose value is SearchResult). Each SearchResult has
a title (value is a literal), a rating (value is
between 0 and 1) and of course, the page itself.
The modularity of RDF allows other vocabularies to be combined with simple schemas such as this to characterise more fully the properties of networked resources. For example, Dublin Core or a library-based classification vocabulary might be used to describe the subject coverage or collections-level properties for each SearchService, while an independently managed "search protocols" vocabulary could be used to describe connection details for (say) LDAP, WHOIS++ or Z39.50 search interfaces offered by the service. By allowing the creation of statements which draw upon specialised schemas from various domains, RDF makes it possible for communities of expertise to contribute to a decentralised web of machine-readable vocabularies.
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Status: this is work-in-progess illustrating how a PICS schema could be expressed in RDF.
There are three parts:
The vocabulary that appears in a PICS-1.1 label is a mixture of descriptions of the service providing the rating, data having to do with the mechanics of transporting the ratings, descriptions of the ratings themselves, and descriptions of the target resource. The vocabulary associated with the transport mechanics (error, for) are not included here. The generic keyword is handled by the RDF aboutURIPrefix mechanism. Two deprecated PICS-1.1 options (signature-RSA-MD5 and MIC-MD5) are omitted from this vocabulary.
The syntax presented in the RDF/XML fragments below assumes that the RDF Model and Syntax
namespace is available using the prefix rdf:, and that the RDF Schema
namespace is the current default XML namespace.
The PICS 'at' option is a higher-order relation between the document being rated and the rating statement. As such, it is modelled as a property of the (reified) rating statement.
<rdf:Property ID="at"
comment="The last modification date of the
subject resource at the time the rating was assigned">
<domain rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Statement"/>
</rdf:Property>
The rating service URI identifies the source of the labels. In PICS-1.1 the rating system (rating vocabulary) is identified within the service description at the service URI. In PICS/RDF the rating vocabulary should be identified with its own schema (and namespace name). The ratingService property therefore serves only to identify the source of the statements. It is an open question of semantics as to what responsibility the ratingService has for the content of the statements.
<rdf:Property ID="ratingService"
comment="The service from whom this rating was received.">
<domain rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Statement"/>
</rdf:Property>
The remainder of the label options identify additional properties of the rating statement itself.
<rdf:Property ID="by"
comment="An identifier for the person or entity within the rating service
who was responsible for creating this particular rating.">
<domain rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Statement"/>
</rdf:Property>
<rdf:Property ID="on"
comment="The date on which this rating was issued.">
<domain rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Statement"/>
</rdf:Property>
The PICS 'until' label option can also be written as 'exp'. We
could use subPropertyOf to represent this synonym; instead,
we eliminate it altogether in this respresentation of PICS.
<rdf:Property ID="until"
comment="The date on which this rating expires.">
<domain rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Statement"/>
</rdf:Property>
The collection of statements may have the following two properties.
<rdf:Property ID="comment"
comment="Information for humans who may see the label; no associated semantics.">
<domain
rdf:resource="http://www.w3.org/TR/1999/PR-rdf-schema-19990303#Container"/>
</rdf:Property>
The 'completeLabel' property will appear only on a container that has not (yet) been replaced by the contents of the complete label resource. When the complete label is used, the statements in the original collection are withdrawn from the database and the statements in the complete label resource are added.
<rdf:Property ID="completeLabel"
comment="A complete label that can be used in place of the current one.">
<domain
rdf:resource="http://www.w3.org/TR/1999/PR-rdf-schema-19990303#Container"/>
</rdf:Property>
A PICS Rating Service Description is essentially a schema. Some of the vocabulary used in PICS-1.1 translates directly to the RDF Schema vocabulary. In this RDF representation of PICS the rating system is specified separately from the rating service, rather than as a part of the rating service.
<rdf:Property ID="ratingService"
comment="A document describing the rating service in human-readable terms."/>
<rdf:Property ID="icon"
comment="A graphic associated with the rating service or with a
specific rating property."/>
The following properties are used within the rating system schema to describe the schema and the properties within the schema.
<rdf:Property ID="name"
comment="The name of the rating system."/>
<rdf:Property ID="description"
comment="A human-readable description of the rating system."/>
<rdf:Property ID="minimum"
comment="The minimum numeric value permitted."/>
<rdf:Property ID="maximum"
comment="The maximum numeric value permitted."/>
<Class rdf:ID="Integer"
comment="Integer numbers."/>
<Class rdf:ID="Encoding"
comment="A value with a label"/>
<Class rdf:ID="Multivalue">
<subClassOf resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Bag"/>
<!-- used as the range of properties that can take either a singleton or a Bag as their value -->
</Class>
<Class rdf:ID="Multiordered">
<subClassOf resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Seq"/>
<!-- used as the range of properties that can take either a singleton or a Seq as their value -->
</Class>
'Categories' and the transmit-as property are simply the property name. The category name is the RDF label property.
The "Good Clean Fun" rating system example can be written in RDF/XML as follows:
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Note: This document was prepared and approved for publication by the W3C RDF Schema Working Group (WG). WG approval of this document does not necessarily imply that all WG members voted for its approval.
David Singer of IBM is the chair of the group; we thank David for his efforts and thank IBM for supporting him and us in this endeavor. Particular thanks are also due to Andrew Layman for his editorial work on earlier versions of this specification.
The working group membership has included:
Nick Arnett (Verity), Dan Brickley (ILRT / University of Bristol), Walter Chang (Adobe), Sailesh Chutani (Oracle), Ron Daniel (DATAFUSION), Charles Frankston (Microsoft), Joe Lapp (webMethods Inc.), Patrick Gannon (CommerceNet), RV Guha (Netscape), Tom Hill (Apple Computer), Renato Iannella (DSTC), Sandeep Jain (Oracle), Kevin Jones, (InterMind), Emiko Kezuka (Digital Vision Laboratories), Ora Lassila (Nokia Research Center), Andrew Layman (Microsoft), John McCarthy (Lawrence Berkeley National Laboratory), Michael Mealling (Network Solutions), Norbert Mikula (DataChannel), Eric Miller (OCLC), Frank Olken (Lawrence Berkeley National Laboratory), Sri Raghavan (Digital/Compaq), Lisa Rein (webMethods Inc.), Tsuyoshi Sakata (Digital Vision Laboratories), Leon Shklar (Pencom Web Works), David Singer (IBM), Wei (William) Song (SISU), Neel Sundaresan (IBM), Ralph Swick (W3C), Naohiko Uramoto (IBM), Charles Wicksteed (Reuters Ltd.), Misha Wolf (Reuters Ltd.)
Not all of the people listed above have been members throughout the entire duration of the working group, but all have contributed to the evolution of this document.
The RDF specification of the above is given here in the serialization syntax. Please note that the namespace URIs listed are examples only; formal identifiers have not yet been assigned for these schemas. This schema includes annotations describing RDF resources defined formally in the RDF Model and Syntax specification, as well as definitions for new resources belonging to the RDF Schema namespace.
Note that there are some constraints (such as those given in 2.3.2 above) on certain RDF Schema resources which are themselves not fully expressible using the RDF Schema specification. For example, the RDF below does not tell us that subClassOf arcs should not (to use terminology from the nodes and arcs representation) form loops in any RDF model.
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The following is the same RDF data expressed using the more compact XML serialisation syntax. This representation is suitable for embedding in XML documents (such as this specification) intended for display, since the literal string values are encoded as the values of XML attributes, and as such are not displayed by HTML-oriented applications. The source code for this specification contains another copy of the following XML/RDF, making the document both human and machine readable.
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The following represents an initial schema for the simple Dublin Core Element Set [DC]. The following schema is for illustration purposes only; the authoritative Dublin Core Schema will be made available by the Dublin Core Initiative.
This schema is provided as the foundation for the Dublin Core semantics. It is believed to be all that is needed to serve as the foundation for future, more elaborate definitions. Future extensions are likely to specify the structure of the values of the properties, which will involve defining classes, the properties that apply to those classes, and some constraints on the property values.
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