Resource Description Framework (RDF) Schema Specification 1.0
Resource Description Framework
(RDF) Schema Specification 1.0
W3C Candidate Recommendation 27 March 2000
This Version:
Latest Version:
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Editors:
Dan Brickley, University of Bristol
R.V. Guha, Epinions
Acknowledgments
W3C
MIT
INRIA
Keio
), All Rights Reserved. W3C
liability
trademark
document use
and
software licensing
rules apply.
Abstract
This specification describes how to use RDF to describe RDF
vocabularies. The specification also defines a basic vocabulary for this
purpose, as well as an extensibility mechanism to anticipate future
additions to RDF.
Status of this document
This document is a
Candidate Recommendation
of the
World Wide Web Consortium
Review comments on this specification should be sent by June 15, 2000 to
www-rdf-comments@w3.org
>.
The archive of public comments is available at
Private comments that you wish to be visible only to the editors,
working group chair, and W3C staff may be sent to
w3c-rdf-review@w3.org
>.
This specification is a revision of the Proposed Recommendation of
March 03
1999
, incorporating editorial suggestions
received in review
comments
. A
separate document
provides an
overview of the main changes since the last publication of this work.
With the publication of this document, the RDF Schema specification enters W3C
Candidate Recommendation
phase. W3C encourages active implementation to test this
specification during the Candidate Recommendation review period; reports
of implementation experience sent to the
review address
are especially
desired.
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.
This section describes the status of this document at the time of
its publication. Other documents may supersede this document.
Identified
Errata
in this document and the
latest
status
of this document series is maintained at the W3C.
Refer to
Appendix B
, About W3C Documents,
for a description of the W3C Technical Report publishing policy.
It is inappropriate to use W3C Candidate Recommendations as reference
material or to cite them as other than "work in progress". This is
work in progress and does not imply endorsement by, or the consensus of
W3C. Advancement of a document to Candidate Recommendation is an
explicit call for implementation and technical feedback;
it is appropriate to build implementations based on this
specification for the purposes of testing the specification and
becoming familiar with it.
A list of current W3C Recommendations and other
technical documents can be found at
Table of Contents
1. Introduction
1.1. Scope
1.1.1. Platform for Internet Content Selection (PICS)
1.1.2. Simple Web Metadata
1.1.3. Sitemaps and Concept Navigation
2. Classes and Properties
2.1. The Type System
2.1.1. Figure 1: Classes and Resources as Sets and Elements
2.1.2. Figure 2: Class hierarchy for the RDF Schema
2.2. Core Classes
2.2.1. rdfs:Resource
2.2.2. rdf:Property
2.2.3. rdfs:Class
2.3. Core Properties
2.3.1. rdf:type
2.3.2. rdfs:subClassOf
2.3.3. rdfs:subPropertyOf
2.3.4. rdfs:seeAlso
2.3.5. rdfs:isDefinedBy
3. Constraints
3.1. Core Constraints
3.1.1. rdfs:ConstraintResource
3.1.2. rdfs:ConstraintProperty
3.1.3. rdfs:range
3.1.4. rdfs:domain
3.2 Example
4. Extensibility Mechanisms
4.1 Evolvability of RDF vocabularies
4.1.1. Terminology
4.1.2. Versioning and URI references
4.1.3. Inter-Vocabulary Relationships
4.2. Evolvability of the RDF Schema Constraint Mechanism
5. Documentation
5.1. rdfs:comment
5.2. rdfs:label
6. Model and Syntax concepts
6.1. rdfs:Literal
6.2. rdf:Statement
6.3. rdf:subject
6.4. rdf:predicate
6.5. rdf:object
6.6. rdfs:Container
6.7. rdf:Bag
6.8. rdf:Seq
6.9. rdf:Alt
6.10. rdf:ContainerMembershipProperty
6.11. rdf:value
7. Examples
7.1. Example 1
7.2. Example 2
8. References
8.1. Normative References
8.2. Informational References
Acknowledgments
Appendix A: XML Serialization
RDF Schemas
1. Introduction
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 uses XML to exchange descriptions of Web resources but the
resources being described can be of any type, including XML and
non-XML resources.
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 modeled 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 (e.g., 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
rdfs:Class
and
rdfs: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 Digital Library community. In particular, RDF adopts a
modular approach to metadata that can be considered an implementation 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. 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 decentralized fashion by independent communities of
expertise.
RDF Schemas might be contrasted with XML Document Type
Definitions (DTDs) [
XML
] and XML Schemas
XMLSCHEMA
]. Unlike an XML DTD or Schema,
which gives specific constraints on the structure of an XML document, an
RDF Schema provides information about the interpretation of the statements
given in an RDF data model. While an XML Schema can be used to validate
the syntax of an RDF/XML expression, a syntactic schema alone is not
sufficient for RDF purposes. RDF Schemas may also specify constraints
that should be followed by these data models.
Future work on RDF Schema and XML Schema might enable the simple
combination of syntactic and semantic rules from both
SCHEMA-ARCH
].
This RDF Schema specification has intentionally left unspecified
a set of primitive datatypes. As RDF uses XML for its interchange
encoding, the work on data typing in XML
XMLDATATYPES
] itself should be
the foundation for such a capability.
1.1. Scope
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:
1.1.1. Platform for Internet Content Selection (PICS)
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 described in a
separate document
1.1.2. Simple Web Metadata
One obvious application for RDF is in the description of Web pages.
This is one of the basic goals of the Dublin Core [
DC
Metadata Initiative. The Dublin Core Element Set 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,
relationships between those properties and classes,
and constraints on the allowed
combinations of classes, properties, and values.
1.1.3. Sitemaps and Concept Navigation
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 organize 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).
1.1.4. P3P
The W3C Platform for Privacy Preferences Project
P3P
) has specified 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.
Although personal data collection practices have been described in P3P
using an application-specific XML tagset, there are benefits to
using a general metadata model for this data. The structure of
P3P policies can be interpreted as an RDF model. Using a metadata
schema to describe the semantics 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.
Extensions to P3P that describe the specific data elements collected
by a site could use RDF Schema to further specify how those data
elements are used.
2. Classes and Properties
An RDF Schema is 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 the RDF Schema
Specification and implicitly part of every RDF model using the RDF schema
machinery.
This document specifies the RDF Schema mechanism as a set of RDF
resources (including classes and properties), and constraints on their
relationships. The abstract RDF Schema core vocabulary can be used to make RDF
statements defining and describing application-specific vocabularies such as the
Dublin Core Element Set.
2.1. The Type System
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 identified by
the URI reference
This specification also uses the prefix '
rdf
' to refer to
the core RDF namespace
As described in the RDF Model and Syntax specification
RDFMS
], resources may be instances of
one or more classes; this is indicated with the
rdf:type
property. Classes themselves are often organized in a hierarchical
fashion, for example a class
Dog
might be considered a subclass of
Mammal
which is a subclass of
Animal
, meaning that any
resource which is of
rdf:type
Dog
is also considered to be of
rdf:type
Animal
. This specification describes a
property,
rdfs:subClassOf
, to denote such relationships
between classes.
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 to which
they apply. This is the role of the
rdfs:domain
and
rdfs:range
constraints described in
Section 3
. 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 an attribute called
author
of type
Literal
. One benefit of the RDF
property-centric approach is that it is very easy for anyone to say
anything they want about existing resources, which is one of the
architectural principles of the Web [
BERNERS-LEE98
].
This specification anticipates the development of a set of classes corresponding to a set of
datatypes. This specification does not define any specific datatypes, but
does note that datatypes may be used as the value of the
rdfs:range
property.
2.1.1. Figure 1: Classes and Resources as Sets and Elements
Figure 1
illustrates the concepts of class, subclass, and
resource. A class is depicted by a rounded rectangle; a resource is
depicted by a large dot. In the figure below, arrows are drawn from a
resource to the class it defines. A subclass is shown by having a rounded rectangle (the
subclass) completely enclosed by another (the superclass). If a resource
is inside a class, then there exists either an explicit or implicit
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
2.1.2. Figure 2: Class hierarchy for the RDF Schema
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 one 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 is 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
2.2. Core Classes
The following resources are the core classes that are defined as part of the
RDF Schema vocabulary. Every RDF model that draws upon the RDF
Schema namespace (implicitly) includes these.
2.2.1. rdfs:Resource
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 [
RDFMS
].
2.2.2. rdf:Property
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 [
RDFMS
].
2.2.3. rdfs:Class
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.
2.3. Core Properties
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.
2.3.1. rdf:type
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 subclass
of
rdfs:Class
, as described in section 2.3.2). A resource may be an instance
of more than one class.
2.3.2. rdfs:subClassOf
This property specifies a subset/superset relation between classes.
The
rdfs:subClassOf
property is transitive.
If class A is a subclass of some broader class B, and B is a subclass of
C, then A is also implicitly a subclass 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
subclass of more than one class.
A class can never be declared to be a subclass of itself, nor of any
of its own subclasses. 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.
2.3.2.1. Example
This is a very simple example that expresses the following class
hierarchy. We first define a class
MotorVehicle
. We then
define three subclasses of
MotorVehicle
, namely
PassengerVehicle
Truck
and
Van
. We then define a class
Minivan
which is a subclass 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
].
abbreviation mechanism provided by the RDF serialization syntax.
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#">
2.3.3. rdfs:subPropertyOf
The property
rdfs:subPropertyOf
is an instance of
rdf:Property
that is used to specify that one
property is a specialization of another.
A property may be a specialization of zero, one or more properties.
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 subproperty of itself, nor of
any of its own subproperties. 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.
2.3.3.1. Example
If the property
biologicalFather
is a
subproperty 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.
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#">
2.3.4. rdfs:seeAlso
The property
rdfs:seeAlso
specifies a resource that might provide additional 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
2.3.5. rdfs:isDefinedBy
The property
rdfs:isDefinedBy
is a subproperty 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 an 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
do not
indicate the URI of the schema that includes the vocabulary item
Creator
(i.e.,
).
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.
3. Constraints
This specification introduces an RDF vocabulary for making statements
about constraints on the 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.
RDF Schema provides 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 can assert that
an
author
property is used to indicate resources that are members of
the class
Person
, it does not say whether or how an application should act in processing
that class 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, and a reasoning
application might infer the class and then announce any inconsistencies.
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
constraints
reference classes defined in another namespace.
The following constraints are specified in RDF Schema 1.0:
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 constraints defined using the
rdfs:ConstraintResource
extensibility mechanism.
Different applications may exhibit different behaviors when dealing with
RDF constraints.
Some examples of constraints include:
That the value of a property should be a resource of a designated class.
This is known as a
range
constraint. For example, a range
constraint applying to the
author
property might express that the
value of an
author
property must be a resource of class
Person
That a property may be used on resources of a certain class.
This is known as a
domain
constraint.
For example, that the
author
property could only originate from a
resource that was an instance of class
Book
This specification does not attempt to enumerate every possible form of
constraint applicable to RDF vocabulary description. Instead, some basic
constraint mechanisms are defined here, accompanied by an
extension facility
to allow for the subsequent
additions of new types of constraint.
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
.)
Note: We expect future work in RDF and XML data-typing to provide
clarifications in this area.
3.1. Core Constraints
3.1.1. rdfs:ConstraintResource
This resource defines a subclass 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 use the constraint
information associated with an RDF model. Since this specification does not provide a
mechanism for the dynamic discovery of new forms of constraint, an RDF
Schema 1.0 processor encountering previously unknown instances of
rdfs:ConstraintResource
can be sure that it is
unqualified to determine the meaning of those constraints.
3.1.2. rdfs:ConstraintProperty
This resource defines a subclass 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
3.1.3. rdfs:range
An instance of
ConstraintProperty
that is used to
indicate the class(es) that the values of a property must be members of.
The value of a
range
property is always a
Class
. Range constraints are only applied to properties.
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.
Constraints on rdfs:range
The
rdfs:domain
of
rdfs:range
is the class
rdf:Property
This indicates that the
range
property applies to resources that are themselves properties.
The
rdfs:range
of
rdfs:range
is the class
rdfs:Class
This indicates that any resource that is the value of a range property will be a class.
Note: specifying multiple range classes
Although it is not permitted 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 subclasses of
MotorVehicle
, then all these types of resource are acceptable
values for
xyz:drivesMotorVehicle
. In cases where a common superclass
does not exist, one can be defined in a schema in the normal manner.
3.1.4. rdfs:domain
An instance of
ConstraintProperty
that is used to indicate
the class(es) on whose members a property can 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).
Constraints on rdfs:domain
The
rdfs:domain
of
rdfs:domain
is the class
rdf:Property
This indicates that the domain property is used on resources that are properties.
The
rdfs:range
of
rdfs:domain
is the class
rdfs:Class
This indicates that any resource that is the value of a domain property
will be a class.
Note: This specification does not constraint the number of
rdfs:domain
properties that a property may have.
If there is no
domain
property, we know nothing about the classes with which
the property is used. If there is more than one
rdfs:domain
property,
the constrained property can be used with resources that are members of
any
of the indicated classes. Note that unlike
range
this is
a very weak constraint.
Figure 4: Constraints in the RDF Schema
The RDF Schema uses the constraint properties to constrain how its
own properties can be used. These constraints are shown below in
figure 4
. Nodes with
bold
outlines are instances of
rdfs:Class
Figure 3
: Constraints in the RDF Schema
3.2. Example
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
Minivan
s and
PassengerVehicle
s. The value is a
Number
(we
anticipate that some concept like this will be provided by future work on
data types), which is the number of centimeters of rear seat legroom.
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#">
4. Extensibility Mechanisms
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.
4.1. Evolvability of RDF vocabularies
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
occurrence on the Web.
4.1.1. Terminology
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.
4.1.2. Versioning and URI references
RDF uses the XML Namespace facility [
XMLNS
] to
identify the schema in which the properties and classes are defined.
Since changing the logical structure of a schema risks
breaking other RDF models which depend on that schema, this specification
recommends that a new namespace URI should be declared
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.
4.1.3. Inter-Vocabulary Relationships
The resources defined in RDF schemas are themselves Web resources, and
can 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 comprehensive schema mapping mechanism for RDF may be developed
in future W3C Activity.
4.1.3.1. Examples
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.
4.2. Evolvability of the RDF Schema Constraint Mechanism
This specification defines a subclass of resources known as 'constraint
resources' (section
3.1
). This is provided to allow
for the addition of new ways of expressing RDF constraints.
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. As yet
unknown constraints may contribute to a more expressive
framework for specifying RDF constraints.
RDF agents unfamiliar with the semantics of unknown instances of
rdfs:ConstraintResource
may therefore lack the
knowledge to evaluate constraint satisfaction 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.
For example, when encountering a previously unknown constraint property
type called
RDF3:mysteryConstraint
we may learn
from a schema that it has a range of
rdfs: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 RDF Schema
language constraints whose semantics must be understood for
constraint checking to be possible.
Membership in the
rdfs:ConstraintResource
class suggests, but does not imply, that those semantics may be inexpressible
in a declarative form. Since the expressive facilities 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 facilities 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 solely 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 subclasses of
ConstraintProperty
to classify new constructs
according to whether or not they had inexpressible semantics.
5. Documentation
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.
5.1. rdfs:comment
This is used to provide a human-readable description of a resource.
5.2. rdfs:label
This is used to provide a human-readable version of a resource name.
6. Model and Syntax concepts
The RDF Model and Syntax specification [
RDFMS
introduces the base concepts of RDF. A number of these
are defined formally in the RDF Schema whose namespace URI
is
. 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
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
6.1. rdfs:Literal
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.
6.2. rdf:Statement
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
].
6.3. rdf:subject
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.
6.4. rdf:predicate
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 modeled statement.
6.5. rdf:object
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 modeled statement.
6.6. rdfs:Container
This class is used to represent 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
6.7. rdf:Bag
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
6.8. rdf:Seq
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
6.9. rdf:Alt
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
6.10. rdfs:ContainerMembershipProperty
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
6.11. rdf:value
This corresponds to the 'value' property described in section 2.3
of the Model and Syntax specification [
RDFMS
].
7. Examples
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.
7.1. Example 1
In this example,
Person
is a class with a corresponding
human-readable description of "The class of people".
Animal
is a class presumed to be defined in another schema.
All persons are animals, so we declare that
Person
is a subclass 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
{Single, Married, Divorced, Widowed}. This is achieved through use of the
rdfs: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.
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#">
7.2. Example 2
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
SearchService
is a subclass of
InternetService
(which is
defined elsewhere). A
SearchQuery
has some number of
result
properties (whose value is
SearchResult
). Each
SearchResult
has
title
(value is a
rdfs:Literal
), a
rating
and of course, the page itself.
The modularity of RDF allows other vocabularies to be combined with simple
schemas such as this to characterize 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 specialized schemas from various
domains, RDF makes it possible for diverse communities of expertise to
contribute to a decentralized web of machine-readable vocabularies.
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#">
8. References
8.1. Normative References
[RDFMS]
Resource Description Framework (RDF) Model and Syntax
W3C Recommendation, 22 February 1999
[XMLNS]
Namespaces in XML
; W3C Recommendation, 14 January 1999
8.2. Informational References
[NIAM]
G. M. Nijssen and Terry Halpin,
Conceptual Schema and Relational Database
Design
, (Prentice Hall, Sydney:1989)
[BERNERS-LEE98]
What the Semantic Web can represent
, Tim Berners-Lee, 1998
[CycL]
CycL: The CYC Representation Language
[KIF]
Knowledge Interchange Format (KIF)
[WF]
The Warwick Framework: A Container Architecture for Aggregating Sets
of Metadata
; Carl Lagoze, Clifford A. Lynch and Ron Daniel Jr., 1996
[EXTWEB]
Web Architecture: Extensible Languages
Tim Berners-Lee and Dan Connolly, 1998
[PICS]
Platform for Internet Content Selection
[PICS in XML/RDF]
PICS Rating Vocabularies in XML/RDF
Brickley and Swick, 2000
[DC]
Dublin Core Metadata Initiative
[SCHEMA-ARCH]
The Cambridge Communiqué
W3C NOTE, 7 October 1999, Swick and Thompson
[UML]
Unified Modeling Language (UML)
[XML]
Extensible Markup Language (XML) 1.0
W3C Recommendation, 10-February-1988, Section 3.2 Element Type Declarations
[XML-Data]
XML Data
, Layman, et. al., W3C Note, 1998
[XMLSCHEMA]
XML Schema Part 1: Structures
W3C Working Draft 25 February 2000
[XMLDATATYPES]
XML Schema Part 2: Datatypes
W3C Working Draft 25 February 2000
[XML-Link]
XML Linking Language
(XLink)
, W3C Working Draft, 21 February 2000
8. Acknowledgements
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 was the chair of the group throughout most of the
development of this specification; 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
(Epinions, previously of Netscape Communications), 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.
Appendix A: XML Serialization
An RDF specification of the core RDF Schema model is given here in RDF/XML
serialization syntax. Please note that the namespace URI for the RDF Schema Specification
will change in future versions of this specification if the schema
changes. This RDF 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.
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#">
properties that its instances can have
schema to provide constraints
Appendix B. About W3C Documents
To promote confidence and stability, W3C has instituted the following publication
policies:
Each technical report is a stable published document that has a unique identifier
(URI).
Each stable published document will always be available, unchanged, at that
URI. Retrieving (e.g., by bookmarking) the resource at that URI will always
return the same content.
A specific revision of a document is generally one of a series of related
documents (e.g., from Working Draft to Recommendation). Each series has a
unique identifier (URI) that when followed, will return the
latest
stable published document available in the series.
Each document includes a "Status" section that describes the document's
publication context
on the date of publication
. Note that since
W3C does not change stable published documents, the status section of a published
document cannot be changed, even if the document becomes obsolete at a later
date.
Each document in the series includes links to up-to-date status information
for the series.
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$Date: 2017/10/02 11:00:36 $