The Resource Description Framework (RDF) is a framework for
representing information in the Web.
This document defines an abstract syntax on which RDF is based,
and which serves to link its concrete syntax to its formal
semantics. It also includes discussion of design goals, meaning of
RDF documents, key concepts, datatyping, character normalization
and handling of URI references.
This is a W3C RDF
Core Working Group Working Draft produced as part of the W3C Semantic Web Activity (Activity
Statement).
This document is being released for review by W3C Members and
other interested parties to encourage feedback and comments,
especially with regard to the sections on datatyping and how the
changes affect existing implementations and content.
This is a public W3C Working Draft and may be updated, replaced,
or obsoleted by other documents at any time. It is inappropriate to
use W3C Working Drafts as reference material or to cite as other
than "work in progress". A list of current W3C Recommendations and
other technical documents can be found at http://www.w3.org/TR/.
In conformance with W3C policy
requirements, known patent and IPR constraints associated
with this Working Draft are detailed on the RDF Core Working Group Patent Disclosure page.
Comments on this document are invited and should be sent to the
public mailing list www-rdf-comments@w3.org. An
archive of comments is available at http://lists.w3.org/Archives/Public/www-rdf-comments/.
The Resource Description Framework (RDF) is a framework for
representing information in the Web.
This document defines an abstract syntax on which RDF is based,
and which serves to link its concrete syntax to its formal
semantics. It also includes discussion of design goals, meaning of
RDF documents, key concepts, datatyping, character normalization
and handling of URI references.
Normative documentation of the RDF core falls into the following
areas:
The framework is designed so that vocabularies can be layered on
top of this core. RDF vocabulary definition language (RDF schema)
[RDF-VOCABULARY] is the first
such vocabulary. Others (cf. OWL [OWL] and
the applications in the primer [RDF-PRIMER]) are in development.
In section 2, some background to
the design goals and rationale of RDF is presented. There is also
some discussion of the intended implications of publishing an RDF
document (section 2.4).
RDF's abstract syntax is a graph, which can be serialized using
XML (but which is quite distinct from XML's tree-based infoset [XML-INFOSET]). The abstract syntax
captures the fundamental structure of RDF, independently of any
concrete syntax used for serialization. The formal semantics of RDF
are defined in terms of the abstract syntax. XML content of
literals is described in section
3, and the abstract syntax is defined in section 4 of this document.
Section 5 discusses character
normalization and fragment identifier use.
RDF draws upon ideas from knowledge representation, artificial
intelligence and data management, including from Conceptual Graphs,
logic-based knowledge representation, frames, and relational
databases. Some possible sources of background information are [Sowa] [CG] [KIF] [Hayes] [Luger] [Gray].
RDF has an abstract syntax that reflects a simple graph-based
data model, and formal semantics with a rigorously defined notion
of entailment providing a basis for well founded deductions in RDF
data.
The development of RDF has been motivated by the following uses,
among others:
- Web metadata: providing information about Web resources and
the systems that use them (e.g. content rating, capability
descriptions, privacy preferences, etc.)
- Applications that require open rather than constrained
information models (e.g. scheduling activities, describing
organizational processes, annotation of Web resources, etc.)
- To do for machine processable information (application data)
what the World Wide Web has done for hypertext: to allow data to
be processed outside the particular environment in which it was
created, in a fashion that can work at Internet scale.
- Interworking among applications: combining data from several
applications to arrive at new information.
- Automated processing of Web information by software agents:
the Web is moving from having just human-readable information to
being a world-wide network of cooperating processes. RDF provides
a world-wide lingua franca for these processes.
RDF is designed to represent information in a minimally
constraining, flexible way. It can be used in isolated
applications, where individually designed formats may be more
perspicuous, but RDF's generality offers greater value from
sharing. The value of information thus increases as it becomes
accessible to more applications across the entire Internet.
The design of RDF is intended to meet the following goals:
- A simple data model
- Formal semantics and provable inference
- Extensible URI-based vocabulary
- XML-based syntax
- Support use of XML schema datatypes
- Anyone can make simple assertions about anything
- Universal expression of simple facts
- A basis for legally binding agreements
RDF has a simple data model that is easy for applications to
process and manipulate. The data model is independent of any
specific serialization syntax.
NOTE: the term "model" used here in "data model" has a
completely different sense to its use in the term "model theory".
See the RDF model theory specification [RDF-SEMANTICS] or a textbook on logical
semantics (e.g., [HUNTER] [DAVIS]) for more information about "model
theory" as used in the literature of mathematics and logic.
RDF has a formal semantics which provides a dependable basis for
reasoning about the meaning of an RDF expression. In particular, it
supports rigorously defined notions of entailment which provide a
basis for defining reliable rules of inference in RDF data.
The vocabulary is fully extensible, being based on URIs with
optional fragment identifiers (URI references, or
URIrefs). URI references are used for naming all kinds
of things in RDF.
The other kind of value that appears in RDF data is a
literal.
RDF has a recommended XML serialization form [RDF-SYNTAX], which can be used to encode the
data model for exchange of information among applications.
RDF can use values represented according to XML schema datatypes
[XML-SCHEMA2], thus assisting the
exchange of information between RDF and other XML applications.
To facilitate operation at Internet scale, RDF is an open-world
framework that allows anyone to make simple assertions about
anything. In general, it is not assumed that all information about
any topic is available. A consequence of this is that RDF cannot
prevent anyone from making assertions that are nonsensical or
inconsistent with the world as people see it, and applications that
build upon RDF must find ways to deal with incomplete and
conflicting sources of information. (This is where RDF departs from
more prescriptive approaches to representing data in XML, which aim
to present information that is well-formed and complete for an
application's needs.)
RDF can represent arbitrary information that can be expressed
simple facts. (What constitutes a simple fact is discussed later,
in section 2.3.6)
RDF is intended to convey assertions that are meaningful to the
extent that they may, in appropriate contexts, be used to express
the terms of binding agreements.
This goal is explored further in section 2.4 below.
RDF uses the following key concepts:
- Graph data model
- URI-based vocabulary
- Datatypes
- Literals
- XML serialization syntax
- Information as representation of simple facts
- Entailment
The underlying structure of any expression in RDF can be viewed
as a directed labelled graph, which consists of nodes and labelled
directed arcs that link pairs of nodes (these notions are defined
more formally in section 4).
The RDF graph is a set of triples:
Each property arc represents a statement of a relationship
between the nodes that it links, having three parts:
- a property that describes some
relationship (also called a predicate),
- a value that is the subject of the statement, and
- a value that is the object of the statement.
The direction of the arc is significant: it always points toward
the object of a statement.
The meaning of an RDF graph is the conjunction (i.e. logical
AND) of all the statements that it contains.
Nodes in an RDF graph are URIs with
optional fragment identifiers (URI references, or URIrefs), literals, or
blank (having no separate form of identification). Arcs are
labelled with URI references. (See [URIS], section 4, for a description of URI
reference forms, noting that relative URIs are not used in an RDF
graph. See also section
4.1.)
The URI reference or literal on a node identifies what that node
represents. The label on an arc identifies the relationship between
the nodes connected by the arc. The arc label may also be a node in
the graph.
A blank
node is an RDF graph node that is not a URI reference or
a literal. In the RDF abstract syntax, a blank node is just a
unique node that can be used in one or more RDF statements, and has
no globally distinguishing identity.
A convention used by some linear representations of an RDF graph
to allow several statements to reference the same blank node is to
use a blank
node identifier, which is a local identifier that can be
distinguished from all URIs and literals. When graphs are merged,
their blank nodes must be kept distinct if meaning is to be
preserved; this may call for re-allocation of blank node
identifiers.
Note that blank node identifiers are not part of the
RDF abstract syntax, and the representation of statements that use
blank nodes is entirely dependent on the particular concrete syntax
used.
Datatypes are used by RDF in the representation of values such
as integers, floating point numbers and dates.
RDF uses the datatype abstraction defined by XML Schema Part 2:
Datatypes [XML-SCHEMA2]. A datatype
consists of a lexical space, a value space and a datatype
mapping.
A datatype mapping is a set of pairs
whose first element belongs to the lexical space of the datatype,
and the second element belongs to the value space of the datatype:
- Each member of the lexical space is paired with
(maps to) exactly one member of the value
space.
- Each member of the value space is paired with at
least one value in the lexical space (a lexical
representation for that value).
With one exception, the datatypes used in RDF have a
lexical space consisting of a set of
strings. The exception is rdfs:XMLLiteral, whose
lexical space also includes pairs of strings and language
identifiers. The value obtained through its datatype mapping may
depend on the language identifier.
For example, the datatype mapping for the XML Schema datatype
xsd:boolean, where each member of the value space
(represented here as 'T' and 'F') has two lexical representations,
is as follows:
| Value Space |
{T, F} |
| Lexical Space |
{"0", "1", "true", "false"} |
| Datatype Mapping |
{<"true", T>, <"1", T>, <"0", F>,
<"false", F>} |
RDF predefines just one datatype rdfs:XMLLiteral, used for
embedding XML in RDF (see section
3).
There is no built-in concept of numbers or dates or other common
values. Rather, RDF defers to datatypes that are defined
separately, and identified with URIs.The predefined XML Schema
datatypes [XML-SCHEMA2] are expected
to be widely used for this purpose. The defining authority of a URI
which identifies a datatype is responsible for specifying the
datatype's lexical space, value space and datatype mapping.
RDF provides no mechanism for defining new datatypes. XML Schema
Datatypes [XML-SCHEMA2] provides an
extensibility framework suitable for defining new datatypes for use
in RDF.
Literals are used to identify values such as numbers and dates
by means of a lexical representation. Anything represented by a
literal could also be represented by a URI, but it is often more
convenient or intuitive to use literals.
A literal may be the object of an RDF statement, but not the
subject or the arc.
Literals may be plain or typed :
- A plain literal is a string combined
with an optional language identifier. This should be used for
plain text in a natural language. As recommended in the RDF
formal semantics [RDF-SEMANTICS], these plain literals are
self-denoting.
- A typed literal is a string, a
datatype URI and an optional language identifier. It denotes the
member of the identified datatype's value space obtained by
applying the datatype mapping to the literal string.
Continuing the example from section
2.3.3, the typed literals which can be defined using the XML
Schema datatype xsd:boolean are:
| Typed Literal |
Datatype Mapping |
Value |
| <xsd:boolean, "true"> |
<"true", T> |
T |
| <xsd:boolean, "1"> |
<"1", T> |
T |
| <xsd:boolean, "false"> |
<"false", F> |
F |
| <xsd:boolean, "0"> |
<"0", F> |
F |
An RDF graph, as described by the RDF abstract syntax, can be
represented in various ways, using different concrete syntaxes but
each conveying a common RDF meaning.
Only the XML syntax [RDF-SYNTAX]
is normatively specified and recommended for use to exchange
information between applications.
Roughly, a "simple fact" is the kind of information that can be
stored in one row of a relational database, possibly about any
nameable thing or concept.
The basic building block of RDF is a statement, which is a
binary relational assertion. For example, the expression "floats(oil,water)" is a binary relational
assertion expressing that oil floats on water. The term "floats" names a relationship that holds
between "oil" and "water". An RDF statement can also contain a
variable; e.g., as in "floats(?x,water)" expressing that there is
something that floats on water, where "?x" stands for the something, without saying
what it is.
Conjunction (logical-AND) of statements can be used to express
more complex facts, such as "floats(oil,water) AND burns(oil,air)". Using
the same variable in several different statements of a conjunction
can say more than one might immediately expect, e.g. "type(?x,fluid) AND floats(?x,water)" says there
is a fluid that floats on water.
Relationships involving more than two things can be expressed as
a conjunction of binary relations, so "boilsAt(water,100C,1atm)" could be expressed as
the existence of a boiling event, say "?b", such that "boils(?b,water) AND temp(?b,100C) AND
press(?b,1atm)".
The expressive power of RDF corresponds to the
existential-conjunctive (EC) subset of first order logic [Sowa]. It does not provide means to express
negation (NOT) or disjunction (OR). RDF is unusual, for a first
order logic subset, in that it allows statements to be made about
the relation terms themselves, e.g. "type(floats,physical-relationship) and
floats(oil,water)". This kind of expression is more commonly
associated with higher order logics, but the use allowed by RDF has
first-order semantics [RDF-SEMANTICS] [[[cite
section when MT is stabilized]]].
Through its use of extensible URI-based vocabularies, RDF
provides for expression of facts about arbitrary subjects; i.e.
assertions of named properties about specific named things. A URI
can be constructed for any thing that can be named, so RDF facts
can be about any such things.
In an RDF graph, the function of variables in the examples above
is provided by blank nodes.
The ideas on meaning and inference in RDF are underpinned by the
concept of entailment. An RDF expression A is said to
entail another
RDF expression B if every possible arrangement of things in
the world that makes A true also makes B true. On this basis, if we
presume or demonstrate the truth of A then we can also infer the
truth of B. Entailment is discussed at greater length in the RDF
formal semantics document [RDF-SEMANTICS].
This idea of entailment sets RDF apart from many other network
data formats. What obligations does an entailment place on a
processor of RDF data? The answer is: none. RDF applications are
not required to find all facts that can be inferred on the basis of
allowed entailments. (Further, it may be not possible to find all
such facts.) But useful applications may infer some such facts, and
treat those facts as if they were part of the supplied input
data.
The RDF test cases described by [RDF-TESTS] contain some entailment and
non-entailment tests (positive entailment tests and negative
entailment tests), covering both RDF-entailment and
RDFS-entailment. A positive entailment test indicates that the
indicated conclusion can be inferred from the given antecedents;
RDF applications are allowed to perform such inferences, but not
required to do so. A negative entailment test indicates that an RDF
application is not entitled by the rules of RDF alone to infer the
indicated conclusion from the corresponding antecedent. A
non-entailment does not mean that the conclusion is necessarily
false: it may be true for reasons unrelated to the antecedent
facts.
So we have the situation that a positive entailment does not
mean that an RDF application must infer the conclusion, and a
negative entailment does not mean the conclusion is necessarily
false. How are we to judge whether a given RDF application is truly
playing by the rules of RDF? The answer lies in the existence of a
proof. An application that validly infers a conclusion
from some antecedent facts must do so in a series of steps that can
be directly traced to allowable entailments, which series
constitutes a proof. The allowable entailments and
corresponding proof steps sanctioned by the RDF specification are
set out in the RDF formal semantics document [RDF-SEMANTICS].
There are two aspects to the meaning of an RDF graph. There is
the formal meaning as determined by the RDF model theory [RDF-SEMANTICS]. This determines, with
mathematical precision, all the conclusions that can be
legitimately drawn from an RDF graph. There is also the social
meaning of the graph. It is the social meaning that affects what it
means to people and how it interacts with human social institutions
such as our systems of law.
RDF/XML expressions, i.e. encodings of RDF graphs, can be used
to make claims or assertions about the 'real' world. Such
expressions are said to be asserted.
But not every RDF/XML expression is asserted. While the formal
semantics of an RDF graph is that of an assertion, some may convey
meaning that is partly determined by the circumstances in which
they are used. For example, in English, a statement "I don't
believe that George is a clown" contains the words "George is a
clown", which, considered in isolation, has the form of an
assertion that George exhibits certain comic qualities. However,
considering the whole sentence, no such assertion is considered to
be made.
When an RDF graph is asserted in the Web, its publisher is
saying something about their view of the world. Such an assertion
should be understood to carry the same social import and
responsibilities as an assertion in any other format. A combination
of social (e.g. legal) and technical machinery (protocols, file
formats, publication frameworks) provide the contexts that fix the
intended meanings of the vocabulary of some piece of RDF, and which
distinguish assertions from other uses (e.g. citations, denials or
illustrations).
The technical machinery includes protocols for transferring
information (e.g. HTTP, SMTP) and file formats for encapsulating
and labelling information (e.g. MIME, XML). A media type, application/rdf+xml [RDF-MIME-TYPE] indicates the use of
RDF/XML as distinct from some other XML that happens to look like
RDF. Issuing an HTTP GET request and obtaining data with a
"200 OK" response code is a technical indication that the received
data was published at the request URI; but data received with a
"404 Not found" response cannot be considered to be similarly
published information.
The social machinery includes the form of publication:
publishing some unqualified statements on one's World Wide Web home
page would generally be taken as an assertion of those statements.
But publishing the same statements with a qualification, such as
"here are some common myths", or as part of a rebuttal, would
likely not be construed as an assertion of the truth of those
statements. Similar considerations apply to the publication of
assertions expressed in RDF.
When a user invokes an application that uses RDF, there is also
a social and technical context of invocation that determines some
set of RDF assertions that will be assumed to be true: the
application itself, and any RDF files that are passed to it.
Garbage-in, garbage-out applies: if the initial assumed facts are
wrong or meaningless, the results will have little value. No
specific mechanisms for deciding or evaluating the validity of any
such assertions are defined here.
[[[This whole section needs more work: the issues
still are not being conveyed clearly]]]
Using RDF, 'received meaning' can be characterized as the social
meaning of any logical consequences. If you publish a graph G and G
logically entails G', and we interpret G' using the same social
conventions that everyone agrees could be reasonably used to
interpret G, then you are asserting that content of G' as well.
Human publishers of RDF content commit themselves to the
mechanically-inferred social obligations. The machines doing the
inferences are not expected to know about all these social
conventions and obligations.
The social conventions surrounding use of RDF include the idea
that each URI 'belongs to' somebody who has authority and
responsibility for defining its meaning. The social conventions are
rooted in the URI specification [RFC2396]
and registration procedures [RFC2717]. A
URI scheme registration refers to a specification of the detailed
syntax and interpretation for that scheme, from which the defining
authority for a given URI may be deduced. In the case of
http: URIs, the defining specification is the HTTP
protocol specification [RFC2616], which obtains a resource
representation from the host named in the URI; thus, the owner of
the host's DNS domain controls (observable aspects of) the URI's
meaning.
Imagine three websites each publishing some RDF:
(A) http://insult.example.com/lexicon#
asserts the following, and this is all that one can
find on the website about that term: |
| A:Clown |
rdf:type |
rdfs:Class . |
| A:Clown |
rdfs:Comment |
"A foolish person, whose
pronouncements are probably ill-considered and not to be taken
seriously" . |
(B) http://AngloSaxon.example.org/lexicon#
asserts: |
| B:Comic |
rdf:subClassOf |
<http://insult.example.com/lexicon#Clown> . |
|
(C) http://skunk.example.org/
asserts the following, assuming that C:JohnSmith is understood to refer to
some particular person:
|
| C:JohnSmith |
rdf:type |
<http://AngloSaxon.example.org/lexicon#Comic> . |
Now, it follows by the formal RDF model theory that these three
together entail:
| C:JohnSmith |
rdf:type |
<http://insult.example.com/lexicon#Clown> . |
which the person identified as C:JohnSmith might reasonably consider an
insult. Why? Not because of the RDF model theory, which merely says
he is in some class about which nothing can be formally
inferred. However, the rdfs:comment
associated with that class name by the owner of that name provides
the insulting content, in the social context of Web
publication, even though it cannot be formally inferred via
the RDF inference rules.
But who has insulted the identified person? A merely defined the
term; B does not mention him in particular, so even A and B
together do not constitute a personal insult. And C might argue
that although he refers to the person, he only asserts that he is a
comic, which is not in itself grounds for a libel suit. However,
one could reasonably claim that C is to blame, since C uses not a
generic term 'Comic', but a particular URI reference which is
defined by its owner (B) in a way which is clearly insulting, since
B in turn explicitly refers to, and uses, the term defined by A.
Thus, C's use of a B-defined term suggests a clear intent by C to
convey a meaning defined by B, by virtue of a definition by A,
which is insulting.
By using the specific name http://AngloSaxon.example.org/lexicon#Comic
instead of some term defined in, say, a glossary of job
descriptions, B has explicitly removed his use of the term 'Clown'
from any formal connection with people who are entertainers. In
order to succeed in his probable intent of making a generic slander
against these people, B should have used a term that was defined by
someone else, such as:
<http://entertainers.example.com/glossary#Comic>
rdfs:subClassOf
<http://insult.example.com/lexicon#Clown> . |
and then if C had also used this first URI reference, then in
spite of a similar formal inference chain generating the insulting
conclusion about C:JohnSmith, there
would be nobody to sue, since now C would indeed have simply made a
harmless observation about his occupation, and B's assertion, while
indeed arguably offensive, makes no reference to him in
particular.
The point of this example is to emphasize that publication of
RDF, when considered as a social act, constitutes a publication of
some content that is defined by whatever normal social conditions are used by the publishers of
any terms in the RDF to define the meanings of those terms, even if
those meanings and definitions are not accessible to the formal
semantics of RDF; and, moreover, those meanings are preserved under
any formally sanctioned inference processes. In a nutshell, the
formal entailments of social meanings are themselves part of the
social meaning.
RDF assumes that for any URI some individual or organization has
the authority to define the meaning of that URI. An RDF predicate
is defined by the individual or organization with such the
authority with respect to the its URI, and misuse by others should
not be permitted to undermine that authority.
RDF uses URIs to identify resources and properties. Certain URIs
are reserved for use by RDF, and may not be used for any purpose
not sanctioned the RDF specifications. Specifically, URIs with the
following leading substrings are reserved for RDF core
vocabulary:
- http://www.w3.org/1999/02/22-rdf-syntax-ns#
(conventionally associated with namespace prefix rdf:)
- http://www.w3.org/2000/01/rdf-schema#
(conventionally associated with namespace prefix rdfs:)
Used with the RDF/XML serialization, these URI prefix strings
correspond to XML namespaces [XML-NS]
associated with the RDF core vocabulary terms.
NOTE: these namespace URIs are the same as those used in
earlier RDF documents [RDF-MS] [RDF-SCHEMA].
[[[NOTE FOR REVIEWERS: Some terms in
these namespaces have been deprecated, some have been added, and
some RDF schema terms have had their meaning changed. We invite
community feedback regarding the relative costs of adopting these
changes under the old namespace URIs vs creating new URIs for
this revision of RDF.]]]
Vocabulary terms in the rdf: namespace
are listed in section
3.4 of the RDF syntax specification [RDF-SYNTAX].
Vocabulary terms defined in the rdfs:
namespace are defined [[[where?]]] in the
RDF schema vocabulary specification [RDF-VOCABULARY].
RDF provides for XML content as a possible literal value. This
typically originates from the use of
rdf:parseType="Literal" in the RDF/XML Syntax [RDF-SYNTAX].
Such content is indicated in an RDF graph using a typed literal
whose datatype is a special built-in datatype,
rdfs:XMLLiteral.
As part of the definition of this datatype, we use an ancillary
definition.
The XML document corresponding to a pair ( str,
lang ) is formed as follows:
Concatenate the five strings:
- "<rdf-wrapper xml:lang='"
- lang
- "'>"
- str
- "</rdf-wrapper>"
Encode the resulting Unicode string in UTF-8 to form the
corresponding XML document.
No escaping is applied. The choice of rdf-wrapper
is fixed but arbitrary.
The XML document corresponding to a string
str is formed as the XML document corresponding to the
pair (str, "").
Using this, the datatype rdfs:XMLLiteral is
defined as follows.
- The datatype URI
- is
http://www.w3.org/2000/01/rdf-schema#XMLLiteral.
- The value space
-
is the set of all XML documents that:
- Have root element tag:
<rdf-wrapper>
- Have no attributes on the root element other than
xml:lang
- are
Canonical XML [XML-C14N]
(with comments).
- The lexical space
- contains all pairs ( string, lang ) where
lang is any language identifier [RFC-3066] in lowercase, and
string is well-balanced, self-contained XML element
content [XML], for which the XML document
corresponding to the pair is a well-formed
XML document [XML] that also conforms
to XML
Namespaces [XML-NS].
- also contains all strings
string which are
well-balanced, self-contained XML element content [XML], and for which the corresponding XML document
is a well-formed
XML document [XML] that also conforms
to XML
Namespaces [XML-NS].
- The mapping
- is defined as the function that maps a pair or string to the
canonical
form [XML-C14N] (with comments)
of the corresponding XML document.
REMINDER: All other datatypes have a lexical space
being a set of strings, and a mapping which maps strings to
values.
NOTE: Not all values of this datatype are compliant
with XML 1.1 [XML 1.1]. If compliance
with XML 1.1 is desired, then only those values that are fully
normalized according to XML 1.1 should be used.
[[[This section has had a substantial rewrite since
the last Working Group review. ]]]
This section defines the RDF abstract syntax. The RDF abstract
syntax is a set of triples, called the RDF graph.
This section also defines equality between RDF graphs. A
definition of equality is needed to support the RDF Test Cases [RDF-TESTS] specification.
An RDF
triple contains three components, called:
The subject may not be an RDF
literal.
Note: subjects and objects are otherwise unrestricted,
since anything that is neither an RDF literal nor an RDF URI reference. is treated as a
blank node.
An RDF triple is conventionally written in the order subject,
predicate, object.
An RDF
graph is a set of RDF triples.
The nodes of an RDF graph is the set of subjects and objects of
triples in the graph.
The blank nodes of an RDF graph are those nodes that are not RDF literals or RDF URI references.
Two RDF graphs G and G' are equal if there
is a bijection I between the nodes of the two graphs,
such that:
- I(lit)=lit for all RDF literals lit which
are nodes of either graph.
- I(uri)=uri for all RDF URI references uri
which are nodes of either graph.
- The triple ( s, p, o ) is in G if and
only if the triple ( I(s), p, I(o) ) is in
G'
[[[This text should be reviewed in light of the IRI
section in the namespaces 1.1 Last Call WD and comments made on it;
the editor had one attempt but it failed.]]]
A URI reference within an RDF graph (an RDF URI reference) is a
Unicode string [UNICODE] that:
- is in Normal Form C [NFC] and
-
would produce a US-ASCII string that is an absolute URI
reference in the form defined by [URIS], as modified by [RFC-2732], when:
- encoded in UTF-8, and
- with disallowed characters escaped according to the
percent escaping algorithm below
The disallowed characters that must be %-escaped include all
non-ASCII characters, the excluded characters listed in Section 2.4
of [URIS], except for the number sign (#)
and percent sign (%) characters and the square bracket characters
re-allowed in [RFC-2732].
Disallowed characters must be escaped as follows:
- Each disallowed character is converted to UTF-8 [RFC-2279] as one or more bytes.
- Any bytes corresponding to a disallowed character are escaped
with the URI escaping mechanism (that is, converted to
%HH, where HH is the hexadecimal notation
of the byte value).
- The original character is replaced by the resulting character
sequence.
Two RDF URI references are equal if and only if they compare as
equal, character by character, as Unicode strings.
Note: RDF URI references are compatible with the
anyURI datatype as defined by XML schema datatypes [XML-SCHEMA2], constrained to be an
absolute rather than a relative URI reference, and constrained to
be in Unicode Normal Form C [NFC] (for
compatibility with [CHARMOD]).
Note: RDF URI references are compatible with International Resource
Identifiers as defined by [XML
Namespaces 1.1].
Note: The restriction to absolute URI references is
found in this abstract syntax. When there is a well-defined base
URI, concrete syntaxes, such as RDF/XML, may permit relative URIs
as a shorthand for such absolute URI references,
A literal in an RDF graph contains three components called:
The lexical form is present in all RDF literals; the language
identifier and the datatype URI may be absent from an RDF
literal.
A plain literal is one in which the datatype URI is absent.
A typed literal is one in which the datatype URI is present.
Note: Literals in which the lexical form begins with a
composing character (as defined by [CHARMOD]) are allowed however they may cause
interoperability problems, particularly with XML version 1.1 [XML 1.1].
Note: When using the language identifier, care must be
taken not to confuse language with locale. The language
identifier only relates to human language text. Presentational
issues, how to best represent typed data to the end-user, should
be addressed in end-user applications.
Two literals are equal if and only if all of the following
hold:
- The strings of the two lexical forms compare equal, character
by character.
- Either both or neither have language identifiers.
- The language identifiers of the two lexical forms compare
equal.
- Either both or neither have datatype URIs.
- The two datatype URIs, if any, compare equal, character by
character.
Note: RDF Literals are distinct and distinguishable
from RDF URI references; e.g. http://example.org as an RDF
Literal (untyped, without a language identifier) is not equal to
http://example.org as an RDF URI reference.
The datatype URI refers to a datatype. For XML Schema
built-in datatypes, URIs such as
http://www.w3.org/2001/XMLSchema#int are used. The URI
of the datatype rdfs:XMLLiteral may be used.
There may be other, implementation dependent, mechanisms by which
URIs refer to datatypes.
The value associated with a typed literal is found by
applying the datatype mapping associated with the datatype URI to
the lexical form. This mapping fails if the lexical form is not in
the lexical space of the datatype associated with the datatype URI.
Exceptionally, if the datatype is rdfs:XMLLiteral and the
literal has a language identifier, then the datatype mapping is
applied to the pair form by the lexical form and the language
identifier.
A typed literal for which the datatype does not map the lexical
form to a value is not syntacticly ill-formed.
[[[Review interaction with model theory concerning
typed values.]]]
[[[This subsection will be deleted at the next
draft of this document. This subsection normatively depends on
CHARMOD, currently a Last Call Working Draft. The editors are
unwilling to progress to Last Call with such a dependency.]]]
For the processing of character data that can be represented in
different ways, RDF processors are required to conform to Early
Uniform Normalization, as described by Character Model for
the World Wide Web 1.0 [CHARMOD].
RDF uses an RDF URI Reference, which may include a fragment
identifier, as a context free identifier for a resource. RFC 2396
[URIS] states that the meaning of a
fragment identifier depends on the MIME content-type of a document,
i.e. is context dependent.
These apparently conflicting views are reconciled by considering
that, in an RDF graph, any RDF URI reference consisting of an
absolute URI and a fragment identifier identifies the same thing as
the fragment identifier does in an application/rdf+xml [RDF-MIME-TYPE] representation of the
resource identified by the absolute URI component. Thus:
- we assume that the URI part (i.e. excluding fragment
identifier) indicates a Web resource with an RDF representation.
So when someurl#frag is used in an RDF
document, someurl is presumed to
designate an RDF document.
- someurl#frag means the thing that
is indicated, according to the rules of the application/rdf+xml MIME content-type as a
"fragment" or "view" of the RDF document at someurl. If the document does not exist, or cannot
be retrieved, then exactly what that view may be is somewhat
undetermined, but that does not prevent use of RDF to say things
about it.
- the RDF interpretation of a fragment identifier allows it to
indicate a thing that is entirely external to the document, or
even to the "shared information space" known as the Web. That is,
it can be an abstract idea, like my car or a mythical
Unicorn.
- thus, an application/rdf+xml
document acts as an intermediary between some Web retrievable
documents (itself, at least, also any other Web retrievable URIs
that it may use, including schema URIs and references to other
RDF documents), and some set of abstract or non-Web entities that
the RDF may describe.
This provides a handling of URI references and their denotation
that is consistent with the RDF model theory and usage, and also
with conventional Web behavior.
This document contains a significant contribution from Pat
Hayes, Sergey Melnik and Patrick Stickler, under whose leadership
was developed the framework described in the RDF family of
specifications for representing datatyped values, such as integers
and dates.
The editors acknowledge valuable contributions from the
following:
- Frank Manola
- Pat Hayes
- Dan Brickley
- Jos de Roo
- Dave Beckett
- Patrick Stickler
- Peter F. Patel-Schneider
- Jerome Euzenat
- Massimo Marchiori
- Tim Berners-Lee
- Dave Reynolds
- Dan Connolly
- [[[Other contributors]]]
Jeremy Carroll thanks Oreste
Signore, his host at the W3C
Office in Italy and Istituto di Scienza e Tecnologie dell'Informazione
"Alessandro Faedo", part of the Consiglio Nazionale delle Ricerche,
where Jeremy is a visiting researcher.
This document is a product of extended deliberations by the
RDFcore Working Group, whose members have included:
- Art Barstow (W3C)
- Dave Beckett (ILRT)
- Dan Brickley (ILRT)
- Dan Connolly (W3C)
- Jeremy Carroll (Hewlett Packard)
- Ron Daniel (Interwoven Inc)
- Bill dehOra (InterX)
- Jos De Roo (AGFA)
- Jan Grant (ILRT)
- Graham Klyne (Nine by Nine)
- Frank Manola (MITRE Corporation)
- Brian McBride (Hewlett Packard)
- Eric Miller (W3C)
- Stephen Petschulat (IBM)
- Patrick Stickler (Nokia)
- Aaron Swartz (HWG)
- Mike Dean (BBN Technologies / Verizon)
- R. V. Guha (Alpiri Inc)
- Pat Hayes (IHMC)
- Sergey Melnik (Stanford University)
- Martyn Horner (Profium Ltd)
This specification also draws upon an earlier RDF Model and
Syntax document edited by Ora Lassilla and Ralph Swick, and RDF
Schema edited by Dan Brickley and R. V. Guha. RDF and RDF Schema
Working Group members who contributed to this earlier work are:
- Nick Arnett (Verity)
- Tim Berners-Lee (W3C)
- Tim Bray (Textuality)
- Dan Brickley (ILRT / University of Bristol)
- Walter Chang (Adobe)
- Sailesh Chutani (Oracle)
- Dan Connolly (W3C)
- Ron Daniel (DATAFUSION)
- Charles Frankston (Microsoft)
- Patrick Gannon (CommerceNet)
- R. V. Guha (Epinions, previously of Netscape
Communications)
- Tom Hill (Apple Computer)
- Arthur van Hoff (Marimba)
- Renato Iannella (DSTC)
- Sandeep Jain (Oracle)
- Kevin Jones, (InterMind)
- Emiko Kezuka (Digital Vision Laboratories)
- Joe Lapp (webMethods Inc.)
- Ora Lassila (Nokia Research Center)
- Andrew Layman (Microsoft)
- Ralph LeVan (OCLC)
- John McCarthy (Lawrence Berkeley National Laboratory)
- Chris McConnell (Microsoft)
- Murray Maloney (Grif)
- Michael Mealling (Network Solutions)
- Norbert Mikula (DataChannel)
- Eric Miller (OCLC)
- Jim Miller (W3C, emeritus)
- Frank Olken (Lawrence Berkeley National Laboratory)
- Jean Paoli (Microsoft)
- Sri Raghavan (Digital/Compaq)
- Lisa Rein (webMethods Inc.)
- Paul Resnick (University of Michigan)
- Bill Roberts (KnowledgeCite)
- Tsuyoshi Sakata (Digital Vision Laboratories)
- Bob Schloss (IBM)
- 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.)
- Lauren Wood (SoftQuad)
7.1 Normative References
- [RDF-SYNTAX]
- RDF/XML
Syntax Specification (Revised), Dave Beckett, World Wide
Web Consortium, 6 November 2002 (work in progress). This version
of the RDF/XML Syntax Specification (Revised) is
http://www.w3.org/TR/2002/WD-rdf-syntax-grammar-20021108/. The
latest version is at http://www.w3.org/TR/rdf-syntax-grammar/.
- [RDF-SEMANTICS]
- RDF Model
Theory, P. Hayes, Editor. Work in progress. World Wide
Web Consortium, 29 April 2002. This version of the RDF Model
Theory is http://www.w3.org/TR/2002/WD-rdf-mt-20020429/. The latest version of the RDF
Model Theory is at http://www.w3.org/TR/rdf-mt/.
- [RDF-VOCABULARY]
- RDF Vocabulary
Description Language 1.0: RDF Schema, Dan Brickley, R.V.
Guha, World Wide Web Consortium, April 2002 (work in progress).
The latest version is at http://www.w3.org/TR/rdf-schema/.
- [RDF-MIME-TYPE]
-
Application/rdf+xml Media Type Registration, A. Swartz,
IETF Internet Draft, March 2002 (work in progress). Version
available at
http://www.ietf.org/internet-drafts/draft-swartz-rdfcore-rdfxml-mediatype-01.txt.
- [RDF-TESTS]
- RDF Test
Cases, Jan Grant and Dave Beckett, Editors. Work in
progress. World Wide Web Consortium, 29 April 2002. This version
of the RDF Test Cases is
http://www.w3.org/TR/2002/WD-rdf-testcases-20020429/.
The latest version of the RDF Test Cases is at http://www.w3.org/TR/rdf-testcases/.
- [XML]
- Extensible Markup
Language (XML) 1.0, Second Edition, T. Bray, J. Paoli,
C.M. Sperberg-McQueen and E. Maler, Editors. World Wide Web
Consortium. 6 October 2000. This version is
http://www.w3.org/TR/2000/REC-xml-20001006. The
latest version of XML is available at http://www.w3.org/TR/REC-xml.
- [XML-NS]
- Namespaces in
XML, T. Bray, D. Hollander and A. Layman, Editors.
World Wide Web Consortium. 14 January 1999. This version is
http://www.w3.org/TR/1999/REC-xml-names-19990114/.
The latest version
of Namespaces in XML is available at
http://www.w3.org/TR/REC-xml-names/.
- [URIS]
- RFC
2396 - Uniform Resource Identifiers (URI): Generic
Syntax, T. Berners-Lee, R. Fielding and L. Masinter,
IETF, August 1998. This document is
http://www.isi.edu/in-notes/rfc2396.txt.
- [URI-REG]
- RFC
2717 - Registration Procedures for URL Scheme Names,
R. Petke and I. King, IETF, November 1999. This document is
http://www.isi.edu/in-notes/rfc2717.txt.
- [RFC-2732]
- RFC 2732
- Format for Literal IPv6 Addresses in URL's, R. Hinden,
B. Carpenter and L. Masinter, IETF, December 1999. This document
is http://www.isi.edu/in-notes/rfc2732.txt.
- [RFC-2279]
- RFC 2279
- UTF-8, a transformation format of ISO 10646, F.
Yergeau, IETF, January 1998. This document is
http://www.isi.edu/in-notes/rfc2279.txt.
- [UNICODE]
- The Unicode Standard, Version 3, The Unicode
Consortium, Addison-Wesley, 2000. ISBN 0-201-61633-5, as updated
from time to time by the publication of new versions. (See http://www.unicode.org/unicode/standard/versions/
for the latest version and additional information on versions of
the standard and of the Unicode Character Database).
- [NFC]
- Unicode
Normalization Forms, Unicode Standard Annex #15, Mark
Davis, Martin Dürst. (See http://www.unicode.org/unicode/reports/tr15/
for the latest version).
- [CHARMOD]
- Character Model
for the World Wide Web 1.0, M. Dürst, F. Yergeau,
R. Ishida, M. Wolf, A. Freytag, T Texin, Editors, World Wide Web
Consortium Working Draft, work in progress, 20 February 2002.
This version of the Character Model is
http://www.w3.org/TR/2002/WD-charmod-20020220/. The latest version of the Character
Model is at http://www.w3.org/TR/charmod/.
- [RFC-3066]
- RFC 3066
- Tags for the Identification of Languages, H.
Alvestrand, IETF, January 2001. This document is
http://www.isi.edu/in-notes/rfc3066.txt.
- XML-C14N
- Canonical
XML. J. Boyer. W3C Recommendation, March 2001.
- Available at http://www.w3.org/TR/2001/REC-xml-c14n-20010315
- Available at http://www.ietf.org/rfc/rfc3076.txt
- [KEYWORDS]
- RFC 2119
- Key words for use in RFCs to Indicate Requirement
Levels, S. Bradner, IETF. March 1997. This document is
http://www.ietf.org/rfc/rfc2119.txt. [[[Is
this used?]]]
- [RFC-3023]
- RFC 3032
- XML Media Types, M. Murata, S. St.Laurent, D.Kohn,
IETF, January 2001. This document is
http://www.ietf.org/rfc/rfc3023.txt.
7.2 Informational
References
- [RDF-PRIMER]
- RDF
Primer, F. Manola, E. Miller, Editors, World Wide Web
Consortium W3C Working Draft, work in progress, 26 April 2002.
This version of the RDF Primer is
http://www.w3.org/TR/2002/WD-rdf-primer-20020426/. The latest version of the RDF
Primer is at http://www.w3.org/TR/rdf-primer/.
- [XML-1.1]
- Extensible Markup
Language (XML) 1.1, John Cowan, Editor. World Wide Web
Consortium Working Draft 25 April 2002. (Work in
progress)
- [XML-NAMESPACES-1.1]
- Namespaces
in XML 1.1, Tim Bray, Dave Hollander, Andrew Layman,
Richard Tobin, Editors. World Wide Web Consortium Working Draft 5
September 2002. (Work in progress)
- [XML-INFOSET]
- XML
Information Set, John Cowan and Richard Tobin, W3C
Recommendation, 24 October 2001. This document is
http://www.w3.org/TR/xml-infoset/.
- [XML-SCHEMA0]
- XML Schema
Part 0: Primer - W3C Recommendation, World Wide Web
Consortium, 2 May 2001.
- [XML-SCHEMA1]
- XML Schema
Part 1: Structures - W3C Recommendation, World Wide
Web Consortium, 2 May 2001.
- [XML-SCHEMA2]
- XML Schema
Part 2: Datatypes - W3C Recommendation, World Wide Web
Consortium, 2 May 2001.
- [OWL]
- OWL Web
Ontology Language 1.0 Reference, Mike Dean, Dan Connolly,
Frank van Harmelen, James Hendler, Ian Horrocks, Deborah L.
McGuinness, Peter F. Patel-Schneider, and Lynn Andrea Stein. W3C
Working Draft 29 July 2002. Latest version is available at http://www.w3.org/TR/owl-ref/.
- [SOWA]
- Knowledge Representation: Logical, Philosophical and
Computational Foundations, John F. Sowa, Brookes/Cole, 2000.
ISBN 0-534-94965-7.
- [SOWA2]
- Mathematical
Background, John F. Sowa, (an extended version of
appendix A from Conceptual Structures: Information Processing
in Mind and Machine, 1984).
- [CG]
- Conceptual Graphs, John F. Sowa, ISO working document
ISO/JTC1/SC 32/WG2 N 000, 2 April 2001 (work in progress).
Available at http://users.bestweb.net/~sowa/cg/cgstand.htm.
- [KIF]
- Knowledge Interchange Format, Michael R. Genesereth,
draft proposed American National Standard NCITS.T2/98-004.
Available at http://logic.stanford.edu/kif/dpans.html.
- [LUGER]
- Artificial Intelligence: Structures and Strategies for
Complex Problem Solving (3rd ed.), George F. Luger and
William A. Stubblefield, Addison Wesley Longman, 1998. ISBN
0-805-31196-3.
- [HAYES]
- In Defense of Logic, Patrick J. Hayes, Proceedings
from the International Joint Conference on Artificial
Intelligence, 1975, San Francisco. Morgan Kaufmann Inc., 1977.
Also in Computation and Intelligence: Collected Readings,
George F. Luger (ed), AAAI press/MIT press, 1995. ISBN
0-262-62101-0.
- [GRAY]
- Logic, Algebra and Databases, Peter Gray, Ellis
Horwood Ltd., 1984. ISBN 0-85312-709-3, 0-85312-803-0,
0-470-20103-7, 0-470-20259-9.
- [HUNTER]
- Metalogic: An Introduction to the Metatheory of Standard
First Order Logic, Geoffrey Hunter, University of California
Press, 1971. ISBN 0-520-02356-0.
- [DAVIS]
- Truth, Deduction and Computation: logic and semantics for
computer science, Ruth E. Davis, Computer Science Press,
1989. ISBN 0-7167-8201-4.
- [QUINE]
- Philosophy of Logic (2nd ed.), W. V. Quine, Harvard
University Press 1986, ISBN 0-674-66563-5.
- [NOTATION3]
- Tim Berners-Lee, DesignIssues note on N3, ...
- [RDF-MS]
- Resource
Description Framework (RDF) Model and Syntax
Specification, O. Lassila and R. Swick, Editors. World
Wide Web Consortium. 22 February 1999. This version is
http://www.w3.org/TR/1999/REC-rdf-syntax-19990222/. The latest version of RDF
M&S is available at
http://www.w3.org/TR/REC-rdf-syntax/.
- [RDF-SCHEMA]
- Resource
Description Framework (RDF) Schema Specification 1.0, Dan
Brickley and R. V. Guha, W3C Candidate Recommendation, 27 March
2000. This document is http://www.w3.org/TR/rdf-schema/.
