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The current Web is primarily made up of an enormous number of documents that have been created using HTML. These documents contain significant amounts of structured data, which is largely unavailable to tools and applications. When publishers can express this data more completely, and when tools can read it, a new world of user functionality becomes available, letting users transfer structured data between applications and web sites, and allowing browsing applications to improve the user experience: an event on a web page can be directly imported into a user's desktop calendar; a license on a document can be detected so that users can be informed of their rights automatically; a photo's creator, camera setting information, resolution, location and topic can be published as easily as the original photo itself, enabling structured search and sharing.
RDFa
Core
is
a
specification
for
attributes
to
express
structured
data
in
any
markup
language.
The
embedded
data
already
available
in
the
markup
language
(e.g.,
XHTML)
is
can
often
be
reused
by
the
RDFa
markup,
so
that
publishers
don't
need
to
repeat
significant
data
in
the
document
content.
The
underlying
abstract
representation
is
RDF
[
RDF-PRIMER
],
which
lets
publishers
build
their
own
vocabulary,
extend
others,
and
evolve
their
vocabulary
with
maximal
interoperability
over
time.
The
expressed
structure
is
closely
tied
to
the
data,
so
that
rendered
data
can
be
copied
and
pasted
along
with
its
relevant
structure.
The rules for interpreting the data are generic, so that there is no need for different rules for different formats; this allows authors and publishers of data to define their own formats without having to update software, register formats via a central authority, or worry that two formats may interfere with each other.
RDFa shares some of the same goals with microformats [ MICROFORMATS ]. Whereas microformats specify both a syntax for embedding structured data into HTML documents and a vocabulary of specific terms for each microformat, RDFa specifies only a syntax and relies on independent specification of terms (often called vocabularies or taxonomies) by others. RDFa allows terms from multiple independently-developed vocabularies to be freely intermixed and is designed such that the language can be parsed without knowledge of the specific vocabulary being used.
This document is a detailed syntax specification for RDFa, aimed at:
For those looking for an introduction to the use of RDFa and some real-world examples, please consult the RDFa Primer .
First, if you are not familiar with either RDFa or RDF, and simply want to add RDFa to your documents, then you may find the RDFa Primer [ RDFA-PRIMER ] to be a better introduction.
If you are already familiar with RDFa, and you want to examine the processing rules — perhaps to create an RDFa Processor — then you'll find the Processing Model section of most interest. It contains an overview of each of the processing steps, followed by more detailed sections, one for each rule.
If you are not familiar with RDFa, but you are familiar with RDF, then you might find reading the Syntax Overview useful, before looking at the Processing Model since it gives a range of examples of markup that use RDFa. Seeing some examples first should make reading the processing rules easier.
If you are not familiar with RDF, then you might want to take a look at the section on RDF Terminology before trying to do too much with RDFa. Although RDFa is designed to be easy to author — and authors don't need to understand RDF to use it — anyone writing applications that consume RDFa will need to understand RDF. There is a lot of material about RDF on the web, and a growing range of tools that support RDFa, this document only contains enough background on RDF to make the goals of RDFa more clear.
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.
This is a revision of RDFa Syntax 1.0 [ RDFA-SYNTAX ]. Once development is complete, if accepted by the W3C membership, this document will supersede the previous Recommendation . There are a number of substantive differences between this version and its predecessor, including:
A sample test harness is available. This set of tests is not intended to be exhaustive. Users may find the tests to be useful examples of RDFa usage.
This document was published by the RDFa Working Group as a Last Call Working Draft. This document is intended to become a W3C Recommendation. If you wish to make comments regarding this document, please send them to public-rdfa-wg@w3.org ( subscribe , archives ). The Last Call period ends 06 December 2010. All feedback is welcome.
Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
This document was produced by a group operating under the 5 February 2004 W3C Patent Policy . W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy .
This section is non-normative.
RDF/XML [ RDF-SYNTAX ] provides sufficient flexibility to represent all of the abstract concepts in RDF [ RDF-CONCEPTS ]. However, it presents a number of challenges; first it is difficult or impossible to validate documents that contain RDF/XML using XML Schemas or DTDs, which therefore makes it difficult to import RDF/XML into other markup languages. Whilst newer schema languages such as RELAX NG [ RELAXNG-SCHEMA ] do provide a way to validate documents that contain arbitrary RDF/XML, it will be a while before they gain wide support.
Second, even if one could add RDF/XML directly into an XML dialect like XHTML, there would be significant data duplication between the rendered data and the RDF/XML structured data. It would be far better to add RDF to a document without repeating the document's existing data. For example, an XHTML document that explicitly renders its author's name in the text—perhaps as a byline on a news site—should not need to repeat this name for the RDF expression of the same concept: it should be possible to supplement the existing markup in such a way that it can also be interpreted as RDF.
Another reason for aligning the rendered data with the structured data is that it is highly beneficial to express the web data's structure 'in context'; as users often want to transfer structured data from one application to another, sometimes to or from a non-web-based application, the user experience can be enhanced. For example, information about specific rendered data could be presented to the user via 'right-clicks' on an item of interest.
In the past, many attributes were 'hard-wired' directly into the markup language to represent specific concepts. For example, in XHTML 1.1 [ XHTML11 ] and HTML [ HTML401 ] there is @cite ; the attribute allows an author to add information to a document which is used to indicate the origin of a quote.
However,
these
'hard-wired'
attributes
make
it
difficult
to
define
a
generic
process
for
extracting
metadata
from
any
document
since
an
RDFa
Processor
would
need
to
know
about
each
of
the
special
attributes.
One
motivation
for
RDFa
has
been
to
devise
a
means
by
which
documents
can
be
augmented
with
metadata
in
a
general,
rather
than
hard-wired,
manner.
This
has
been
achieved
by
creating
a
fixed
set
of
attributes
and
parsing
rules,
but
allowing
those
attributes
to
contain
properties
from
any
of
a
number
of
the
growing
range
of
available
RDF
vocabularies.
In
most
cases
the
values
of
those
properties
are
the
information
that
is
already
in
an
author's
XHTML
document.
RDFa alleviates the pressure on markup language designers to anticipate all the structural requirements users of their language might have, by outlining a new syntax for RDF that relies only on attributes. By adhering to the concepts and rules in this specification, language designers can import RDFa into their environment with a minimum of hassle and be confident that semantic data will be extractable from their documents by conforming processors.
This section is non-normative.
The following examples are intended to help readers who are not familiar with RDFa to quickly get a sense of how it works. For a more thorough introduction, please read the RDFa Primer [ RDFA-PRIMER ].
For brevity, in the following examples and throughout this document, assume that the following vocabulary prefixes have been defined:
bibo: | http://purl.org/ontology/bibo/ |
cc: | http://creativecommons.org/ns# |
dbp: | http://dbpedia.org/property/ |
dbr: | http://dbpedia.org/resource/ |
|
http://purl.org/dc/terms/ |
ex: | http://example.org/ |
foaf: | http://xmlns.com/foaf/0.1/ |
rdf: | http://www.w3.org/1999/02/22-rdf-syntax-ns# |
rdfa: | http://www.w3.org/ns/rdfa# |
rdfs: | http://www.w3.org/2000/01/rdf-schema# |
xhv: | http://www.w3.org/1999/xhtml/vocab# |
xsd: | http://www.w3.org/2001/XMLSchema# |
RDFa
makes
use
of
a
number
of
commonly
found
attributes,
as
well
as
providing
a
few
new
ones.
Attributes
that
already
exist
in
widely
deployed
languages
(e.g.,
HTML)
have
the
same
meaning
they
always
did,
although
their
syntax
has
been
slightly
modified
in
some
cases.
For
example,
in
(X)HTML,
@rel
already
defines
the
relationship
between
one
document
and
another.
However,
in
(X)HTML
there
is
no
clear
way
to
add
new
values;
RDFa
sets
out
to
explicitly
solve
this
problem,
and
does
so
by
allowing
URIs
as
values.
It
also
introduces
the
idea
concepts
of
'compact
URIs'
terms
and
'
compact
URIs
'
—
referred
to
as
CURIEs
in
this
document
—
which
allow
a
full
URI
value
to
be
expressed
succinctly.
For
a
complete
list
of
RDFa
attribute
names
and
syntax,
see
Attributes
and
Syntax
.
As
an
(X)HTML
author
you
will
already
be
familiar
with
using
meta
and
link
to
add
additional
information
to
your
documents:
<html xmlns="http://www.w3.org/1999/xhtml"> <head> <title>Page 7</title> <meta name="author" content="Mark Birbeck" /> <link rel="prev" href="page6.html" /> <link rel="next" href="page8.html" /> </head> <body>...</body> </html>
RDFa makes use of this concept, enhancing it with the ability to make use of other vocabularies by using compact URIs:
<html xmlns="http://www.w3.org/1999/xhtml" prefix="foaf: http://xmlns.com/foaf/0.1/dcterms: http://purl.org/dc/terms/" > <head> <title>My home-page</title><meta property="" content="Mark Birbeck" /><meta property="dcterms:creator" content="Mark Birbeck" /> <link rel="foaf:topic" href="http://www.formsPlayer.com/#us" /> </head> <body>...</body> </html>
RDFa supports the use of @rel and @rev on any element. This is even more useful when with the addition of support for different vocabularies:
This document is licensed under a <a prefix="cc: http://creativecommons.org/ns#" rel="cc:license" href="http://creativecommons.org/licenses/by-nc-nd/3.0/"> Creative Commons License </a>.
Not only can URLs in the document be re-used to provide metadata, but so can inline text:
<html xmlns="http://www.w3.org/1999/xhtml" prefix="cal: http://www.w3.org/2002/12/cal/ical#" > <head><title>Jo's Friends and Family Blog</title></head> <body> <p> I'm holding <span property="cal:summary"> one last summer Barbecue </span>, on September 16th at 4pm. </p> </body> </html>
If some displayed text is different to the actual 'value' it represents, more precise values can be added, which can optionally include datatypes:
<html xmlns="http://www.w3.org/1999/xhtml" prefix="cal: http://www.w3.org/2002/12/cal/ical# xsd: http://www.w3.org/2001/XMLSchema" > <head><title>Jo's Friends and Family Blog</title></head> <body> <p> I'm holding <span property="cal:summary"> one last summer Barbecue </span>, on <span property="cal:dtstart" content="2015-09-16T16:00:00-05:00" datatype="xsd:dateTime"> September 16th at 4pm </span>. </p> </body> </html>
In many cases a block of markup will contain a number of properties that relate to the same item; it's possible with RDFa to indicate the type of that item:
<html
xmlns="http://www.w3.org/1999/xhtml"
prefix="cal: http://www.w3.org/2002/12/cal/ical#
xsd: http://www.w3.org/2001/XMLSchema"
>
<head><title>Jo's Friends and Family Blog</title></head>
<body>
<p typeof="cal:Vevent">
I'm holding
<span property="cal:summary">
one last summer Barbecue
</span>,
on
<span property="cal:dtstart" content="2015-09-16T16:00:00-05:00"
datatype="xsd:dateTime">
September 16th at 4pm
</span>.
</p>
</body>
</html>
RDFa allows the document to contain metadata information about other documents and resources:
<html xmlns="http://www.w3.org/1999/xhtml" prefix="bibo: http://purl.org/ontology/bibo/dc: http://purl.org/dc/terms/"dcterms: http://purl.org/dc/terms/" > <head> <title>Books by Marco Pierre White</title> </head> <body> I think White's book '<span about="urn:ISBN:0091808189" typeof="bibo:Book">property="dcterms:title"> Canteen Cuisine </span>' is well worth getting since although it's quite advanced stuff, he makes it pretty easy to follow. You might also like <span about="urn:ISBN:1596913614" typeof="bibo:Book">property="dcterms:description"> White's autobiography </span>. </body> </html>
When dealing with small amounts of markup, it is sometimes easier to use full URIs, rather than CURIEs. The previous example can also be written as follows:
<html xmlns="http://www.w3.org/1999/xhtml"> <head> <title>Books by Marco Pierre White</title> </head> <body> I think White's book '<span about="urn:ISBN:0091808189" typeof="http://purl.org/ontology/bibo/Book" property="http://purl.org/dc/terms/title" >Canteen Cuisine</span>' is well worth getting since although it's quite advanced stuff, he makes it pretty easy to follow. You might also like <span about="urn:ISBN:1596913614" typeof="http://purl.org/ontology/bibo/Book" property="http://purl.org/dc/terms/description" >White's autobiography</span>. </body> </html>
A simple way of defining a portion of a document to use FOAF terms is to use @vocab to define a default vocabulary URI:
<div vocab="http://xmlns.com/foaf/0.1/" about="#me"> My name is <span property="name">John Doe</span> and my blog is called <a rel="homepage" href="http://example.org/blog/">Understanding Semantics</a>. </div>
the following triples will be generated:
@prefix foaf: <http://xmlns.com/foaf/0.1/> . <#me> foaf:name "John Doe" ; foaf:homepage <http://example.org/blog/> .
RDFa
also
permits
external
definition
of
collections
of
prefixes.
prefixes
.
The
following
(mythical)
example
RDFa
Profile
document,
residing
at
with
a
URI
of
http://www.example.org/vocab-rdf-dc.html
,
defines
the
standard
RDF
prefixes
as
well
as
the
FOAF
and
Dublin
Core
vocabulary
prefix
prefixes
in
RDFa.
<html xmlns="http://www.w3.org/1999/xhtml" prefix="rdfa: http://www.w3.org/ns/rdfa#"> <head> ... </head> <body> <p>This is an example to defining the standard RDF and Dublin Core prefixes </p> <p typeof=""> The "<span property="rdfa:prefix">rdf</span>" prefix can be used for the URI: "<span property="rdfa:uri">http://www.w3.org/1999/02/22-rdf-syntax-ns#</span>".</p> <p typeof=""> The "<span property="rdfa:prefix">rdfs</span>" prefix can be used for the URI: "<span property="rdfa:uri">http://www.w3.org/2000/01/rdf-schema#</span>".</p> <p typeof="">The "<span >dc</span>" prefix canThe "<span property="rdfa:prefix">dcterms</span>" prefix can be used for the URI: "<span property="rdfa:uri">http://purl.org/dc/terms/</span>".</p> <p typeof=""> The "<span property="rdfa:prefix">foaf</span>" prefix can be used for the URI: "<span property="rdfa:uri">http://xmlns.com/foaf/0.1/</span>".</p> </html>
Using @profile , the following RDFa snippet:
<p about="http://www.example.org/doc" profile="http://www.example.org/vocab-rdf-dc"><span property="dc:title">title of the document</span><span property="dcterms:title">title of the document</span> <span property="rdfs:comment">and this is a longer comment on the same document</span> </p>
yields
would
yield
the
following
triples:
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .@prefix dc: <http://purl.org/dc/terms/> .@prefix dcterms: <http://purl.org/dc/terms/> . <http://www.example.org/doc>dc:title "title of the document" ;dcterms:title "title of the document" ; rdfs:comment "and this is a longer comment on the same document" .
It
is
also
possible
to
define
terms.
terms
.
Given
the
following
RDFa
Profile
document
at
http://www.example.org/vocab-foaf-terms.html
:
<html xmlns="http://www.w3.org/1999/xhtml" prefix="rdfa: http://www.w3.org/ns/rdfa#"> <head> <title>Example RDFa Vocabulary</title> </head> <body> <p> This is an example RDFa vocabulary that makes it easier to use the foaf:name and foaf:homepage terms. </p> <p typeof=""> The "<span property="rdfa:term">name</span>" term can be used for the URI: "<span property="rdfa:uri">http://xmlns.com/foaf/0.1/name</span>".</p> <p typeof=""> The "<span property="rdfa:term">homepage</span>" term can be used for the URI: "<span property="rdfa:uri">http://xmlns.com/foaf/0.1/homepage</span>".</p> </body> </html>
and the following HTML markup:
<div profile="http://www.example.org/vocab-foaf-terms" about="#me"> My name is <span property="name">John Doe</span> and my blog is called <a rel="homepage" href="http://example.org/blog/">Understanding Semantics</a>. </div>
the following triples will be generated:
@prefix foaf: <http://xmlns.com/foaf/0.1/> . <#me> foaf:name "John Doe" ; foaf:homepage <http://example.org/blog/> .
This section is non-normative.
The previous section gave examples of typical markup in order to illustrate the structure of RDFa markup. However, what RDFa represents is RDF. In order to author RDFa you do not need to understand RDF, although it would certainly help. However, if you are building a system that consumes the RDF output of a language that supports RDFa you will almost certainly need to understand RDF. This section introduces the basic concepts and terminology of RDF. For a more thorough explanation of RDF, please refer to the RDF Concepts document [ RDF-CONCEPTS ] and the RDF Syntax Document [ RDF-SYNTAX ].
The structured data that RDFa provides access to is a collection of statements . A statement is a basic unit of information that has been constructed in a specific format to make it easier to process. In turn, by breaking large sets of information down into a collection of statements, even very complex metadata can be processed using simple rules.
To illustrate, suppose we have the following set of facts:
Albert was born on March 14, 1879, in Germany. There is a picture of him atAlbert was born on March 14, 1879, in the German Empire. There is a picture of him at the web address, http://en.wikipedia.org/wiki/Image:Albert_Einstein_Head.jpg.
This would be quite difficult for a machine to interpret, and it is certainly not in a format that could be passed from one data application to another. However, if we convert the information to a set of statements it begins to be more manageable. The same information could therefore be represented by the following shorter 'statements':
Albert was born on March 14, 1879.Albert was born in Germany.Albert was born in the German Empire. Albert has a picture at http://en.wikipedia.org/wiki/Image:Albert_Einstein_Head.jpg.
To make this information machine-processable, RDF defines a structure for these statements. A statement is formally called a triple , meaning that it is made up of three components. The first is the subject of the triple, and is what we are making our statements about . In all of these examples the subject is 'Albert'.
The second part of a triple is the property of the subject that we want to define. In the examples here, the properties would be 'was born on', 'was born in', and 'has a picture at'. These are more usually called predicates in RDF.
The
final
part
of
a
triple
is
called
the
object
.
In
the
examples
here
the
three
objects
have
the
values
'March
14,
1879',
'Germany',
'the
German
Empire',
and
'http://en.wikipedia.org/wiki/Image:Albert_Einstein_Head.jpg'.
Breaking
complex
information
into
manageable
units
helps
us
be
specific
about
our
data,
but
there
is
still
some
ambiguity.
For
example,
which
'Albert'
are
we
talking
about?
If
another
system
has
more
facts
about
'Albert',
how
could
we
know
whether
they
are
about
the
same
person,
and
so
add
them
to
the
list
of
things
we
know
about
that
person?
If
we
wanted
to
find
people
born
in
Germany,
the
German
Empire,
how
could
we
know
that
the
predicate
'was
born
in'
has
the
same
purpose
as
the
predicate
'birthplace'
that
might
exist
in
some
other
system?
RDF
solves
this
problem
by
replacing
our
vague
terms
with
URI
references
.
URIs are most commonly used to identify web pages, but RDF makes use of them as a way to provide unique identifiers for concepts. For example, we could identify the subject of all of our statements (the first part of each triple) by using the DBPedia [ http://dbpedia.org ] URI for Albert Einstein, instead of the ambiguous string 'Albert':
<http://dbpedia.org/resource/Albert_Einstein> has the name Albert Einstein. <http://dbpedia.org/resource/Albert_Einstein> was born on March 14, 1879. <http://dbpedia.org/resource/Albert_Einstein> was born inGermany.the German Empire. <http://dbpedia.org/resource/Albert_Einstein> has a picture at http://en.wikipedia.org/wiki/Image:Albert_Einstein_Head.jpg.
URI
references
are
also
used
to
uniquely
identify
the
objects
in
metadata
statements
(the
third
part
of
each
triple).
The
picture
of
Einstein
is
already
a
URI,
but
we
could
also
use
a
URI
to
uniquely
identify
the
country
Germany.
'German
Empire'.
At
the
same
time
we'll
indicate
that
the
name
and
date
of
birth
really
are
literals
(and
not
URIs),
by
putting
quotes
around
them:
<http://dbpedia.org/resource/Albert_Einstein> has the name "Albert Einstein". <http://dbpedia.org/resource/Albert_Einstein> was born on "March 14, 1879". <http://dbpedia.org/resource/Albert_Einstein> was born in.<http://dbpedia.org/resource/German_Empire>. <http://dbpedia.org/resource/Albert_Einstein> has a picture at < http://en.wikipedia.org/wiki/Image:Albert_Einstein_Head.jpg > .
URI references are also used to ensure that predicates are unambiguous; now we can be sure that 'birthplace', 'place of birth', 'Lieu de naissance' and so on, all mean the same thing:
<http://dbpedia.org/resource/Albert_Einstein> <http://xmlns.com/foaf/0.1/name> "Albert Einstein". <http://dbpedia.org/resource/Albert_Einstein> <http://dbpedia.org/property/dateOfBirth> "March 14, 1879". <http://dbpedia.org/resource/Albert_Einstein> <http://dbpedia.org/property/birthPlace><http://dbpedia.org/resource/Germany>.<http://dbpedia.org/resource/German_Empire>. <http://dbpedia.org/resource/Albert_Einstein> <http://xmlns.com/foaf/0.1/depiction> <http://en.wikipedia.org/wiki/Image:Albert_Einstein_Head.jpg>.
Although URI resources are always used for subjects and predicates, the object part of a triple can be either a URI or a literal . In the example triples, Einstein's name is represented by a plain literal , which means that it is a basic string with no type or language information:
<http://dbpedia.org/resource/Albert_Einstein>
<http://xmlns.com/foaf/0.1/name>
"Albert
Einstein"
.
Some
literals,
such
as
dates
and
numbers,
have
very
specific
meanings,
so
RDF
provides
a
mechanism
for
indicating
the
type
of
a
literal.
A
typed
literal
is
indicated
by
attaching
a
URI
to
the
end
of
a
plain
literal
,
and
this
URI
indicates
the
literal's
datatype.
This
URI
is
usually
based
on
datatypes
defined
in
the
XML
Schema
Datatypes
specification
[
XMLSCHEMA-2
].
The
following
syntax
would
be
used
to
unambiguously
express
Einstein's
date
of
birth
as
a
literal
of
type
http://www.w3.org/2001/XMLSchema#date
:
<http://dbpedia.org/resource/Albert_Einstein>
<http://dbpedia.org/property/dateOfBirth>
"1879-03-14"
^^<http://www.w3.org/2001/XMLSchema#date>
.
RDF itself does not have one set way to express triples, since the key ideas of RDF are the triple and the use of URIs, and not any particular syntax. However, there are a number of mechanisms for expressing triples, such as RDF/XML [ RDF-SYNTAX-GRAMMAR ], Turtle [ TURTLE ], and of course RDFa. Many discussions of RDF make use of the Turtle syntax to explain their ideas, since it is quite compact. The examples we have just seen are already using this syntax, and we'll continue to use it throughout this document when we need to talk about the RDF that could be generated from some RDFa. Turtle allows long URIs to be abbreviated by using a URI mapping, which can be used to express a compact URI as follows:
@prefix dbp: <http://dbpedia.org/property/> . @prefix foaf: <http://xmlns.com/foaf/0.1/> . <http://dbpedia.org/resource/Albert_Einstein> foaf:name "Albert Einstein" . <http://dbpedia.org/resource/Albert_Einstein> dbp:birthPlace<http://dbpedia.org/resource/Germany><http://dbpedia.org/resource/German_Empire> .
Here 'dbp:' has been mapped to the URI for DBPedia and 'foaf:' has been mapped to the URI for the 'Friend of a Friend' taxonomy.
Any URI in Turtle could be abbreviated in this way. This means that we could also have used the same technique to abbreviate the identifier for Einstein, as well as the datatype indicator:
@prefix dbp: <http://dbpedia.org/property/> . @prefix dbr: <http://dbpedia.org/resource/> . @prefix foaf: <http://xmlns.com/foaf/0.1/> . @prefix xsd: <http://www.w3.org/2001/XMLSchema#> . dbr:Albert_Einstein dbp:dateOfBirth "1879-03-14"^^xsd:date . dbr:Albert_Einstein foaf:depiction <http://en.wikipedia.org/wiki/Image:Albert_Einstein_Head.jpg> .
When writing examples, you will often see the following URI in the Turtle representation:
<>
This indicates the 'current document', i.e., the document being processed. In reality there would always be a full URI based on the document's location, but this abbreviation serves to make examples more compact. Note in particular that the whole technique of abbreviation is merely a way to make examples more compact, and the actual triples generated would always use the full URIs.
A collection of triples is called a graph . All of the triples that are defined by this specification are contained in the default graph by an RDFa Processor. For more information on graphs and other RDF concepts, see [ RDF-CONCEPTS ].
In order to allow for the compact expression of RDF statements, RDFa allows the contraction of most URI reference s into a form called a 'compact URI', or CURIE . A detailed discussion of this mechanism is in the section CURIE and URI Processing .
Note that CURIEs are only used in the markup and Turtle examples, and will never appear in the generated triple s, which are defined by RDF to use URI reference s.
Full details on how CURIEs are processed are in the section titled CURIE Processing .
A
growing
use
of
embedded
metadata
is
to
take
fragments
of
markup
and
move
them
from
one
document
to
another.
This
may
happen
through
the
use
of
tools,
such
as
drag-and-drop
in
a
browser,
or
through
snippets
of
code
provided
to
authors
for
inclusion
in
their
documents.
(A
good
example
of
the
latter
is
the
licensing
fragment
provided
by
Creative
Commons.)
Commons
.)
However, those involved in creating fragments (either by building tools, or authoring snippets), should be aware that this specification does not say how fragments are processed. Specifically, the processing of a fragment 'outside' of a complete document is undefined because RDFa processing is largely about context. Future versions of this or related specifications may do more to define this behavior.
Developers of tools that process fragments, or authors of fragments for manual inclusion, should also bear in mind what will happen to their fragment once it is included in a complete document. They should carefully consider the amount of 'context' information that will be needed in order to ensure a correct interpretation of their fragment.
The following is a brief description of RDFa in terms of the RDF terminology introduced here. It may be useful to readers with an RDF background:
The aim of RDFa is to allow a single RDF graph to be carried in various types of document markup. An RDF graph comprises node s linked by relationships. The basic unit of an RDF graph is a triple , in which a subject node is linked to an object node via a predicate . The subject node is always either a URI reference or a blank node (or bnode) , the predicate is always a URI reference , and the object of a statement can be a URI reference , a literal , or a bnode .
In
RDFa,
a
subject
URI
reference
is
generally
indicated
using
@about
or
@src
,
and
predicates
are
represented
using
one
of
@property
,
@rel
,
or
@rev
.
Objects
which
are
URI
reference
s
are
represented
using
@href
,
@resource
,
or
@src
@href
,
whilst
objects
that
are
literal
s
are
represented
either
with
@content
or
the
content
of
the
element
in
question
(with
an
optional
datatype
expressed
using
@datatype
,
and
an
optional
language
expressed
using
a
Host
Language-defined
mechanism
such
as
@xml:lang
).
As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.
The key words must , must not , required , should , should not , recommended , may , and optional in this specification are to be interpreted as described in [ RFC2119 ].
A conforming RDFa Processor must make available to a consuming application a single RDF graph containing all possible triples generated by using the rules in the Processing Model section. This specification uses the term default graph to mean all of the triples asserted by a document according to the Processing Model section. The processor graph term is used to denote the collection of all informational, warning, and error triples that are generated by the RDFa Processor as a result of processing the document. The default graph and the processor graph are separate graphs and must not be stored in the same graph by the RDFa Processor.
A conforming RDFa Processor may make available additional triples that have been generated using rules not described here, but these triples must not be made available in the default graph . (Whether these additional triples are made available in one or more additional RDF graph s is implementation-specific, and therefore not defined here.)
A conforming RDFa Processor must preserve white space in both plain literal s and XML literals . However, it may be the case that the architecture in which a processor operates does not make all white space available. It is therefore advisable for authors who would like to make their documents consumable across different processors, to remove any unnecessary white space in their markup.
Host Languages that incorporate RDFa must adhere to the following:
<myml:myElement
property="next">
).
Host Languages are required to change the URI of their default profile if items are added or removed from the default profile document. The URI change is required to accomodate RDFa Processors that statically embed the terms defined in the profile. Host Languages are expected to change their profiles very rarely.
This specification defines a number of attributes and the way in which the values of those attributes are to be interpreted when generating RDF triples. This section defines the attributes and the syntax of their values.
CDATA
string,
for
supplying
machine-readable
content
for
a
literal
(a
'plain
literal
object',
in
RDF
terminology);
NCName ':' ' '+ xs:anyURI
Many attributes accept a white space separated list of tokens. This specification defines white space as:
whitespace ::= (#x20 | #x9 | #xD | #xA)+
When attributes accept a white space separated list of tokens, an RDFa Processor must ignore any leading or trailing white space.
This definition is consistent with the definition found in [ XML10 ].
The key component of RDF is the URI, but these are usually long and unwieldy. RDFa therefore supports a mechanism by which URIs can be abbreviated, called 'compact URIs' or simply, CURIEs.
A
CURIE
is
comprised
of
two
components,
a
prefix
and
a
reference
.
The
prefix
is
separated
from
the
reference
by
a
colon
(
:
).
In
general
use
it
is
possible
to
omit
the
prefix,
and
so
create
a
CURIE
that
makes
use
of
the
'default
prefix'
mapping;
in
RDFa
the
'default
prefix'
mapping
is
http://www.w3.org/1999/xhtml/vocab#
.
It's
also
possible
to
omit
both
the
prefix
and
the
colon,
and
so
create
a
CURIE
that
contains
just
a
reference
which
makes
use
of
the
'no
prefix'
mapping.
This
specification
does
not
define
a
default
'no
prefix'
mapping.
However,
Host
Languages
may
define
a
default.
This
mapping
may
be
changed
via
@vocab
.
The RDFa 'default prefix' should not be confused with the 'default namespace' as defined in [ XML-NAMES ]. An RDFa Processor must not treat an XML-NAMES 'default namespace' declaration as if it were setting the 'default prefix'.
The general syntax of a CURIE can be summarized as follows:
prefix ::= NCName])reference ::= irelative-ref (as defined in [RFC3987]) curie ::= [ [ prefix ] ':' ] reference safe_curie ::= '[' [ [ prefix ] ':' ] reference ']'
The
production
safe_curie
is
not
required,
even
in
situations
where
an
attribute
value
is
permitted
to
be
a
CURIE
or
a
URI:
A
URI
that
uses
a
scheme
that
is
not
an
in-scope
mapping
cannot
be
confused
with
a
CURIE.
The
concept
of
a
safe_curie
is
retained
for
backward
compatibility.
In normal evaluation of CURIEs the following context information would need to be provided:
:p
);
p
);
_:p
).
In RDFa these values are defined as follows:
A CURIE is a representation of a full URI. The rules for determining that URI are:
prefix
and
a
reference
,
the
URI
is
obtained
by
taking
the
current
default
prefix
mapping
and
concatenating
it
with
the
reference
.
If
there
is
no
current
default
prefix
mapping,
then
this
is
not
a
valid
CURIE
and
must
be
ignored.
prefix
and
reference
,
and
if
there
is
an
in-scope
mapping
for
prefix
,
(when
compared
case-insensitively),
then
the
URI
is
created
by
using
that
mapping,
and
concatenating
it
with
the
reference
.
prefix
,
then
the
value
is
not
a
CURIE.
This section is non-normative.
In many cases, language designers have attempted to use QNames for an extension mechanism [ XMLSCHEMA-2 ]. QNames do permit independent management of the name collection, and can map the names to a resource. Unfortunately, QNames are unsuitable in most cases because 1) the use of QName as identifiers in attribute values and element content is problematic as discussed in [ QNAMES ] and 2) the syntax of QNames is overly restrictive and does not allow all possible URIs to be expressed.
A
specific
example
of
the
problem
this
causes
comes
from
attempting
to
define
the
name
collection
for
books.
In
a
QName,
the
part
after
the
colon
must
be
a
valid
element
name,
making
an
example
such
as
the
following
invalid
:
isbn:0321154991
This section looks at a generic set of processing rules for creating a set of triples that represent the structured data present in an RDFa document. Processing need not follow the DOM traversal technique outlined here, although the effect of following some other manner of processing must be the same as if the processing outlined here were followed. The processing model is explained using the idea of DOM traversal which makes it easier to describe (particularly in relation to the evaluation context ).
Evaluating a document for RDFa triples is carried out by starting at the document object, and then visiting each of its child elements in turn, in document order, applying processing rules. Processing is recursive in that for each child element the processor also visits each of its child elements, and applies the same processing rules.
In some environments there will be little difference between starting at the root element of the document, and starting at the document object itself. It is defined this way because in some environments important information is present at the document object level which is not present on the root element.
As processing continues, rules are applied which may generate triples, and may also change the evaluation context information that will then be used when processing descendant elements.
This specification does not say anything about what should happen to the triples generated, or whether more triples might be generated during processing than are outlined here. However, to be conforming, an RDFa Processor must act as if at a minimum the rules in this section are applied, and a single RDF graph produced. As described in the RDFa Processor Conformance section, any additional triples generated must not appear in the default graph .
During processing, each rule is applied using information provided by an evaluation context . An initial context is created when processing begins. That context has the following members:
base
element.
The
important
thing
is
that
it
establishes
a
URL
against
which
relative
paths
can
be
resolved.
During the course of processing, new evaluation context s are created which are passed to each child element. The rules described below will determine the values of the items in the context. Additionally, some rules will cause new triples to be created by combining information provided by an element with information from the evaluation context .
During the course of processing a number of locally scoped values are needed, as follows:
Statement chaining is an RDFa feature that allows the author to link RDF statements together while avoiding unnecessary repetitive markup. For example, if an author were to add statements as children of an object that was a resource, these statements should be interpreted as being about that resource:
<div about="http://dbpedia.org/resource/Albert_Einstein"> <span property="foaf:name">Albert Einstein</span> <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span><div rel="dbp:birthPlace" ><div rel="dbp:birthPlace" resource="http://dbpedia.org/resource/German_Empire"> <span property="dbp:conventionalLongName">the German Empire</span> </div> </div>
In
this
example
we
can
see
that
an
object
resource
('Germany'),
('German_Empire'),
has
become
the
subject
for
nested
statements.
This
markup
also
illustrates
the
basic
chaining
pattern
of
'A
has
a
B
has
a
C'
(i.e.,
Einstein
has
a
birth
place
of
Germany,
the
German
Empire,
which
has
a
long
name
of
"Federal
Republic
of
Germany").
"the
German
Empire").
It's also possible for the subject of nested statements to provide the object for containing statements — essentially the reverse of the example we have just seen. To illustrate, we'll take an example of the type of chaining just described, and show how it could be marked up more efficiently. To start, we mark up the fact that Albert Einstein had both German and American citizenship:
<div about="http://dbpedia.org/resource/Albert_Einstein"><div rel="dbp:citizenship" resource="http://dbpedia.org/resource/German_Empire"></div> <div rel="dbp:citizenship" resource="http://dbpedia.org/resource/United_States"></div> </div>
Now, we show the same information, but this time we create an incomplete triple from the citizenship part, and then use any number of further subjects to 'complete' that triple, as follows:
<div about="http://dbpedia.org/resource/Albert_Einstein" rel="dbp:citizenship"><span about="http://dbpedia.org/resource/German_Empire"></span> <span about="http://dbpedia.org/resource/United_States"></span> </div>
In
this
example,
the
incomplete
triple
actually
gets
completed
twice,
once
for
Germany
the
German
Empire
and
once
for
the
USA,
giving
exactly
the
same
information
as
we
had
in
the
earlier
example:
<http://dbpedia.org/resource/Albert_Einstein>dbp:citizenship <http://dbpedia.org/resource/Germany> .dbp:citizenship <http://dbpedia.org/resource/German_Empire> . <http://dbpedia.org/resource/Albert_Einstein> dbp:citizenship <http://dbpedia.org/resource/United_States> .
Chaining
can
sometimes
involve
elements
containing
relatively
minimal
markup,
for
example
showing
only
one
resource,
or
only
one
predicate.
Here
the
img
element
is
used
to
carry
a
picture
of
Einstein:
<div about="http://dbpedia.org/resource/Albert_Einstein">
<div rel="foaf:depiction">
<img src="http://en.wikipedia.org/wiki/Image:Albert_Einstein_Head.jpg" />
</div>
</div>
When such minimal markup is used, any of the resource-related attributes could act as a subject or an object in the chaining:
<div about="http://dbpedia.org/resource/Albert_Einstein"> <div rel="dbp:citizenship"><span ></span><span about="http://dbpedia.org/resource/German_Empire"></span> <span about="http://dbpedia.org/resource/United_States"></span> </div> </div>
Since RDFa is ultimately a means for transporting RDF, a key concept is the resource and its manifestation as a URI. RDF deals with complete URIs (not relative paths); when converting RDFa to triples, any relative URIs must be resolved relative to the base URI, using the algorithm defined in section 5 of RFC 3986 [ URI ], Reference Resolution . The values of RDFa attributes that refer to URIs use three different datatypes: URI , SafeCURIEorCURIEorURI , or TERMorCURIEorAbsURI . All these attributes are mapped, after processing, to URIs. The handling of these attributes is as follows:
Note that it is possible for all values in an attribute to be ignored. When that happens, the attribute must be treated as if it were empty.
For example, the full URI for Albert Einstein on DBPedia is:
http://dbpedia.org/resource/Albert_Einstein
This can be shortened by authors to make the information easier to manage, using a CURIE. The first step is for the author to create a prefix mapping that links a prefix to some leading segment of the URI. In RDFa these mappings are expressed using the XML namespace syntax:
<div prefix="db: http://dbpedia.org/">
...
</div>
Once the prefix has been established, an author can then use it to shorten a URI as follows:
<div prefix="db: http://dbpedia.org/">
<div about="db:resource/Albert_Einstein">
...
</div>
</div>
The author is free to split the URI at any point, as long as it begins at the left end. However, since a common use of CURIEs is to make available libraries of terms and values, the prefix will usually be mapped to some common segment that provides the most re-use, often provided by those who manage the library of terms. For example, since DBPedia contains an enormous list of resources, it is more efficient to create a prefix mapping that uses the base location of the resources:
<div prefix="dbr: http://dbpedia.org/resource/"> <div about="dbr:Albert_Einstein"> ... </div> <div about="dbr:Baruch_Spinoza"> ... </div> </div>
CURIE prefix mappings are defined on the current element and its descendants. The inner-most mapping for a given prefix takes precedence. For example, the URIs expressed by the following two CURIEs are different, despite the common prefix, because the prefix mappings are locally scoped:
<div prefix="dbr: http://dbpedia.org/resource/"> <div about="dbr:Albert_Einstein"> ... </div> </div> <div prefix="dbr: http://someotherdb.org/resource/"> <div about="dbr:Albert_Einstein"> ... </div> </div>
There are a number of ways that attributes make use of CURIEs, and they need to be dealt with differently. These are:
An
empty
attribute
value
(e.g.,
typeof=''
)
is
still
a
CURIE,
and
is
processed
as
such.
The
rules
for
this
processing
are
defined
in
Sequence
.
Specifically,
however,
an
empty
attribute
value
is
never
treated
as
a
relative
URI
by
this
specification.
An example of an attribute that can contain a CURIEorURI is @about . To express a URI directly, an author might do this:
<div about="http://dbpedia.org/resource/Albert_Einstein">
...
</div>
whilst to express the URL above as a CURIE they would do this:
<div about="dbr:Albert_Einstein">
...
</div>
The author could also use a safe CURIE, as follows:
<div about="[dbr:Albert_Einstein]">
...
</div>
Since non-CURIE values must be ignored, the following value in @about would not set a new subject, since @about does not permit the use of TERM s, and the CURIE has no prefix separator.
<div about="[Albert_Einstein]">
...
</div>
However, this markup would set a subject, since it is not a CURIE, but a valid relative URI:
<div about="Albert_Einstein">
...
</div>
Note that several RDFa attributes are able to also take TERMS as their value. This is discussed in the next section.
Some RDFa attributes have a datatype that permits a term to be referenced. RDFa defines the syntax of a term as:
term ::= NCName
When an RDFa attribute permits the use of a term, and the value being evaluated matches the production for term above, it is transformed to a URI using the following logic:
term
matches
an
item
in
the
list
of
local
term
mappings
.
First
compare
against
the
list
term
.
term
has
no
associated
URI
and
must
be
ignored.
The general rules discussed in the previous sections apply to the RDFa attributes in the following ways:
Any value that matches a defined term must be expanded into a reference to the corresponding URI. For example in [ XHTML-RDFA ] the following examples:
<link rel="next" href="http://example.org/page2.html" /> <link rel=" xhv:next " href="http://example.org/page2.html" />
would each generate the following triple:
<> <http://www.w3.org/1999/xhtml/vocab#next> <http://example.org/page2.html> .
In RDFa, it is possible to establish relationships using various types of resource references, including bnode s. If a subject or object is defined using a CURIE, and that CURIE explicitly names a bnode , then a Conforming Processor must create the bnode when it is encountered during parsing. The RDFa Processor must also ensure that no bnode created automatically (as a result of chaining ) has a name that collides with a bnode that is defined by explicit reference in a CURIE.
Consider the following example:
<link about="_:john" rel="foaf:mbox" href="mailto:john@example.org" /> <link about="_:sue" rel="foaf:mbox" href="mailto:sue@example.org" /> <link about="_:john" rel="foaf:knows" resource="_:sue" />
In the above fragment, two bnodes are explicitly created as the subject of triples. Those bnodes are then referenced to demonstrate the relationship between the parties. After processing, the following triples will be generated:
_:john foaf:mbox <mailto:john@example.org> . _:sue foaf:mbox <mailto:sue@example.org> . _:john foaf:knows _:sue .
RDFa Processors use, internally, implementation-dependent identifiers for bnodes. When triples are retrieved , new bnode indentifiers are used, which usually bear no relation to the original identifiers. However, implementations do ensure that these generated bnode identifiers are consistent: each bnode will have its own identifier, all references to a particular bnode will use the same identifier, and different bnodes will have different identifiers.
As
a
special
case,
_:
is
also
a
valid
reference
for
one
specific
bnode
.
Processing
would
normally
begin
after
the
document
to
be
parsed
has
been
completely
loaded.
However,
there
is
no
requirement
for
this
to
be
the
case,
and
it
is
certainly
possible
to
use
a
stream-based
approach,
such
as
SAX
[
SAX
]
to
extract
the
RDFa
information.
However,
if
some
approach
other
than
the
DOM
traversal
technique
defined
here
is
used,
it
is
important
to
ensure
that
Host
Language-specific
processing
rules
are
applied
(e.g.,
XHTML+RDFa
[
XHTML-RDFA
]
indicates
the
base
element
can
be
used,
and
base
will
affect
the
interpretation
of
URIs
in
meta
or
link
elements
even
if
those
elements
are
before
the
base
element
in
the
stream).
At the beginning of processing, an initial evaluation context is created, as follows:
base
element);
Processing begins by applying the processing rules below to the document object, in the context of this initial evaluation context . All elements in the tree are also processed according to the rules described below, depth-first, although the evaluation context used for each set of rules will be based on previous rules that may have been applied.
This
specification
assumes
defines
processing
rules
for
optional
attributes
that
certain
elements
are
may
not
be
present
in
the
all
Host
Language
Languages
(e.g.,
).
If
these
head
@href
elements
attributes
are
not
supported
in
the
Host
Language,
then
the
corresponding
processing
rules
are
not
relevant
for
that
language.
The working group as not reached consensus as to whether to include the optional attributes in this specification, or whether to have them defined in the relevant Host Language specifications.
The processing rules are:
If no URI is provided by a resource attribute, then the first match from the following rules will apply:
If no URI is provided then the first match from the following rules will apply:
Then the current object resource is set to the URI obtained from the first match from the following rules:
Note that final value of the current object resource will either be null (from initialization) or a full URI.
XMLLiteral
in
the
vocabulary
http://www.w3.org/1999/02/22-rdf-syntax-ns#
.
The actual literal is either the value of @content (if present) or a string created by concatenating the value of all descendant text nodes, of the current element in turn. The final string includes the datatype URI, as described in [ RDF-CONCEPTS ], which will have been obtained according to the section on CURIE and URI Processing .
XMLLiteral
in
the
vocabulary
http://www.w3.org/1999/02/22-rdf-syntax-ns#
.
The
value
of
the
XML
literal
is
a
string
created
by
serializing
to
text,
all
nodes
that
are
descendants
of
the
current
element
,
i.e.,
not
including
the
element
itself,
and
giving
it
a
datatype
of
XMLLiteral
in
the
vocabulary
http://www.w3.org/1999/02/22-rdf-syntax-ns#
.
The
format
of
the
resulting
serialized
content
is
as
defined
in
Exclusive
XML
Canonicalization
Version
[
XML-EXC-C14N
].
In order to maintain maximum portability of this literal, any children of the current node that are elements must have the current in scope profiles, default vocabulary, prefix mappings, and XML namespace declarations (if any) declared on the serialized element using their respective attributes. Since the child element node could also declare new prefix mappings or XML namespaces, the RDFa Processor must be careful to merge these together when generating the serialized element definition. For avoidance of doubt, any re-declarations on the child node must take precedence over declarations that were active on the current node.
Additionally, if there is a value for current language then the value of the plain literal should include this language information, as described in [ RDF-CONCEPTS ]. The actual literal is either the value of @content (if present) or a string created by concatenating the text content of each of the descendant elements of the current element in document order.
The current object literal is then used with each predicate to generate a triple as follows:
The processing rules covered in the previous section are designed to extract as many triples as possible from a document. The RDFa Processor is designed to continue processing, even in the event of errors. For example, failing to resolve a prefix mapping or term would result in the RDFa Processor skipping the generation of a triple and continuing with document processing. There are cases where knowing each RDFa Processor warning or error would be beneficial to authors. The processor graph is designed as a mechanism to capture all informational, warning, and error messages as triples from the RDFa Processor. These status triples may be retrieved and used to aid RDFa authoring or automated error detection.
If an RDFa Processor supports the generation of a processor graph, then it must generate a set of triples when the following processing issues occur:
Other implementation-specific INFORMATIONAL, WARNING, or ERROR triples may be generated by the RDFa Processor.
Accessing the processor graph may be accomplished in a variety of ways and is dependent on the type of RDFa Processor and access method that the developer is utilizing.
SAX-based processors or processors that utilize function or method callbacks to report the generation of triples are classified as event-based RDFa Processor s. For Event-based RDFa Processors, the software must allow the developer to register a function or callback that is called when a triple is generated for the processor graph . The callback may be the same as the one that is used for the default graph as long as it can be determined if a generated triple belongs in the processor graph or the default graph .
A whole-graph RDFa Processor is defined as any RDFa Processor that processes the entire document and only allows developer access to the triples after processing has completed. RDFa Processors that typically fall into this category express their output via a single call using RDF/XML, N3, TURTLE, or N-Triples notation. For whole-graph RDFa Processors, the software must allow the developer to specify if they would like to retrieve the default graph , the processor graph , or both graphs as a single, combined graph from the RDFa Processor. If the graph preference is not specified, the default graph must be returned.
An
web
service
RDFa
Processor
is
defined
as
any
RDFa
Processor
that
is
capable
of
processing
a
document
by
performing
an
HTTP
GET,
POST
or
similar
action
on
an
RDFa
Processor
URL.
For
this
class
of
RDFa
Processor,
the
software
must
allow
the
caller
to
specify
if
they
would
like
to
retrieve
the
default
graph
,
the
processor
graph
,
or
both
graphs
as
a
single,
combined
graph
from
the
web
service.
The
rdfagraph
query
parameter
must
be
used
to
specify
the
value.
The
allowable
values
are
default
,
processor
or
both
values,
in
any
order,
separated
by
a
comma
character.
If
the
graph
preference
is
not
specified,
the
default
graph
must
be
returned.
This section provides an in-depth examination of the processing steps described in the previous section. It also includes examples which may help clarify some of the steps involved.
The key to processing is that a triple is generated whenever a predicate/object combination is detected. The actual triple generated will include a subject that may have been set previously, so this is tracked in the current evaluation context and is called the parent subject . Since the subject will default to the current document if it hasn't been set explicitly, then a predicate/object combination is always enough to generate one or more triples.
The attributes for setting a predicate are @rel , @rev and @property , whilst the attributes for setting an object are @resource , @href , @content , and @src . @typeof is unique in that it sets both a predicate and an object at the same time (and also a subject when it appears in the absence of other attributes that would set a subject). Inline content might also set an object, if @content is not present, but @property is present.
When triples are created they will always be in relation to a subject resource which is provided either by new subject (if there are rules on the current element that have set a subject) or parent subject , as passed in via the evaluation context . This section looks at the specific ways in which these values are set. Note that it doesn't matter how the subject is set, so in this section we use the idea of the current subject which may be either new subject or parent subject .
When
parsing
begins,
the
current
subject
will
be
the
URI
of
the
document
being
parsed,
or
a
value
as
set
by
a
Host
Language-provided
mechanism
such
as
(e.g.,
the
base
element
in
(X)HTML.
(X)HTML).
This
means
that
by
default
any
metadata
found
in
the
head
of
the
document
will
concern
the
document
itself:
<html><html profile="http://www.example.org/vocab-rdf-dc.html"> <head> <title>Jo's Friends and Family Blog</title> <link rel="foaf:primaryTopic" href="#bbq" /><meta property="dcterms:creator" content="Jo" /> </head> <body> ... </body> </html>
This would generate the following triples:
<> foaf:primaryTopic <#bbq> . <>dc:creatordcterms:creator "Jo" .
It is possible for the data to appear elsewhere in the document:
<html><html profile="http://www.example.org/vocab-rdf-dc.html"> <head> <title>Jo's Blog</title> </head> <body><h1>'s blog</h1><h1><span property="dcterms:creator">Jo</span>'s blog</h1> <p> Welcome to my blog. </p> </body> </html>
which would still generate the triple:
<>dc:creatordcterms:creator "Jo" .
In
(X)HTML
the
value
of
base
may
change
the
initial
value
of
current
subject
:
<html><html profile="http://www.example.org/vocab-rdf-dc.html"> <head> <base href="http://www.example.org/jo/blog" /> <title>Jo's Friends and Family Blog</title> <link rel="foaf:primaryTopic" href="#bbq" /><meta property="dc:creator" content="Jo" /><meta property="dcterms:creator" content="Jo" /> </head> <body> ... </body> </html>
An RDFa Processor should now generate the following triples, regardless of the URL from which the document is served:
<http://www.example.org/jo/blog> foaf:primaryTopic <#bbq> . <http://www.example.org/jo/blog>dc:creatordcterms:creator "Jo" .
As
processing
progresses,
any
@about
attributes
will
change
the
current
subject
.
The
value
of
@about
is
a
URI
or
a
CURIE.
If
it
is
a
relative
URI
then
it
needs
to
be
resolved
against
the
current
base
value.
To
illustrate
how
this
affects
the
statements,
note
in
this
markup
how
the
properties
inside
the
(X)HTML
body
element
become
part
of
a
new
calendar
event
object,
rather
than
referring
to
the
document
as
they
do
in
the
head
of
the
document:
<html><html profile="http://www.example.org/vocab-rdf-dc.html" prefix="cal: http://www.w3.org/2002/12/cal/ical#"> <head> <title>Jo's Friends and Family Blog</title> <link rel="foaf:primaryTopic" href="#bbq" /><meta property="dc:creator" content="Jo" /><meta property="dcterms:creator" content="Jo" /> </head> <body> <p about="#bbq" typeof="cal:Vevent"> I'm holding <span property="cal:summary"> one last summer barbecue </span>, on <span property="cal:dtstart" content="2015-09-16T16:00:00-05:00" datatype="xsd:dateTime"> September 16th at 4pm </span>. </p> </body> </html>
With this markup an RDFa Processor will generate the following triples:
<> foaf:primaryTopic <#bbq> .<> dc:creator "Jo" .<> dcterms:creator "Jo" . <#bbq> rdf:type cal:Vevent . <#bbq> cal:summary "one last summer barbecue" . <#bbq> cal:dtstart "2015-09-16T16:00:00-05:00"^^xsd:dateTime .
Other kinds of resources can be used to set the current subject , not just references to web-pages. Although not advised, email addresses might be used to represent a person:
John knows <a about="mailto:john@example.org" rel="foaf:knows" href="mailto:sue@example.org">Sue</a>. Sue knows <a about="mailto:sue@example.org" rel="foaf:knows" href="mailto:jim@example.org">Jim</a>.
This should generate the following triples:
<mailto:john@example.org> foaf:knows <mailto:sue@example.org> . <mailto:sue@example.org> foaf:knows <mailto:jim@example.org> .
Similarly, authors may make statements about images:
<div about="photo1.jpg"> this photo was taken by<span property="dc:creator">Mark Birbeck</span><span property="dcterms:creator">Mark Birbeck</span> </div>
which should generate the following triples:
<photo1.jpg>dc:creatordcterms:creator "Mark Birbeck" .
If @about is not present, then @src is next in priority order, for setting the subject of a statement. A typical use would be to indicate the licensing type of an image:
<img src="photo1.jpg" rel="license"
resource="http://creativecommons.org/licenses/by/2.0/"
/>
Since there is no difference between @src and @about , then the information expressed in the last example in the section on @about (the creator of an image), could be expressed as follows:
<img src="photo1.jpg" rel="license" resource="http://creativecommons.org/licenses/by/2.0/"property="dcterms:creator" content="Mark Birbeck" />
Since normal chaining rules will apply, the image URL can also be used to complete hanging triples:
<div about="http://www.blogger.com/profile/1109404" rel="foaf:img"> <img src="photo1.jpg" rel="license" resource="http://creativecommons.org/licenses/by/2.0/"property="dc:creator" content="Mark Birbeck"property="dcterms:creator" content="Mark Birbeck" /> </div>
The complete markup yields three triples:
<http://www.blogger.com/profile/1109404> foaf:img <photo1.jpg> . <photo1.jpg> xhv:license <http://creativecommons.org/licenses/by/2.0/> . <photo1.jpg>dc:creatordcterms:creator "Mark Birbeck" .
Whilst
@about
explicitly
creates
a
new
context
for
statements,
@typeof
does
so
implicitly.
@typeof
works
differently
to
other
ways
of
setting
a
predicate
since
the
predicate
is
always
rdf:type
,
which
means
that
the
processor
only
requires
one
attribute,
the
value
of
the
type.
Since @typeof is setting the type of an item, this means that if no item exists one should automatically be created. This involves generating a new bnode , and is examined in more detail below; it is mentioned here because the bnode used by the new item will become the subject for further statements.
For example, an author may wish to create markup for a person using the FOAF vocabulary, but without having a clear identifier for the item:
<div typeof="foaf:Person">
<span property="foaf:name">Albert Einstein</span>
<span property="foaf:givenName">Albert</span>
</div>
This
markup
would
cause
a
bnode
to
be
created
which
has
a
'type'
of
foaf:Person
,
as
well
as
name
and
given
name
properties:
_:a rdf:type foaf:Person . _:a foaf:name "Albert Einstein" . _:a foaf:givenName "Albert" .
_:a
as
being
distinct
from
_:b
.
But
by
not
exposing
these
values
to
any
external
software,
it
is
possible
to
have
complete
control
over
the
identifier,
as
well
as
preventing
further
statements
being
made
about
the
item.
As described in the previous two sections, @about will always take precedence and mark a new subject, but if no @about value is available then @typeof will do the same job, although using an implied identifier, i.e., a bnode .
But if neither @about or @typeof are present, there are a number of ways that the subject could be arrived at. One of these is to 'inherit' the subject from the containing statement, with the value to be inherited set either explicitly, or implicitly.
The
most
usual
way
that
an
inherited
subject
might
get
set
would
be
when
the
parent
statement
has
an
object
that
is
a
resource.
Returning
to
the
earlier
example,
in
which
the
long
name
for
Germany
the
German_Empire
was
added,
the
following
markup
was
used:
<div about="http://dbpedia.org/resource/Albert_Einstein"> <span property="foaf:name">Albert Einstein</span> <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span><div rel="dbp:birthPlace" resource="http://dbpedia.org/resource/Germany" /> <span about="http://dbpedia.org/resource/Germany" property="dbp:conventionalLongName">Federal Republic of Germany</span><div rel="dbp:birthPlace" resource="http://dbpedia.org/resource/German_Empire" /> <span about="http://dbpedia.org/resource/German_Empire" property="dbp:conventionalLongName">the German Empire</span> </div>
In
an
earlier
illustration
the
subject
and
object
for
Germany
the
German
Empire
were
elided
by
removing
the
@resource
,
relying
on
the
@about
to
set
the
object:
<div about="http://dbpedia.org/resource/Albert_Einstein"> <span property="foaf:name">Albert Einstein</span> <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span> <div rel="dbp:birthPlace"><span about="http://dbpedia.org/resource/Germany" property="dbp:conventionalLongName">Federal Republic of Germany</span><span about="http://dbpedia.org/resource/German_Empire" property="dbp:conventionalLongName">the German Empire</span> </div> </div>
but it is also possible for authors to achieve the same effect by removing the @about and leaving the @resource :
<div about="http://dbpedia.org/resource/Albert_Einstein"> <span property="foaf:name">Albert Einstein</span> <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span><div rel="dbp:birthPlace" > <span property="dbp:conventionalLongName">Federal Republic of Germany</span><div rel="dbp:birthPlace" resource="http://dbpedia.org/resource/German_Empire"> <span property="dbp:conventionalLongName">the German Empire</span> </div> </div>
In
this
situation,
all
statements
that
are
'contained'
by
the
object
resource
representing
Germany
the
German
Empire
(the
value
in
@resource
)
will
have
the
same
subject,
making
it
easy
for
authors
to
add
additional
statements:
<div about="http://dbpedia.org/resource/Albert_Einstein"> <span property="foaf:name">Albert Einstein</span> <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span><div rel="dbp:birthPlace" resource="http://dbpedia.org/resource/Germany"> <span property="dbp:conventionalLongName">Federal Republic of Germany</span><div rel="dbp:birthPlace" resource="http://dbpedia.org/resource/German_Empire"> <span property="dbp:conventionalLongName">the German Empire</span> <span rel="dbp:capital" resource="http://dbpedia.org/resource/Berlin" /> </div> </div>
Looking at the triples that an RDFa Processor would generate, we can see that we actually have two groups of statements; the first group are set to refer to the @about that contains them:
<http://dbpedia.org/resource/Albert_Einstein> foaf:name "Albert Einstein" . <http://dbpedia.org/resource/Albert_Einstein> dbp:dateOfBirth "1879-03-14"^^xsd:date . <http://dbpedia.org/resource/Albert_Einstein> dbp:birthPlace<http://dbpedia.org/resource/Germany><http://dbpedia.org/resource/German_Empire> .
whilst the second group refer to the @resource that contains them:
<http://dbpedia.org/resource/Germany> dbp:conventionalLongName "Federal Republic of Germany" . <http://dbpedia.org/resource/Germany><http://dbpedia.org/resource/German_Empire> dbp:conventionalLongName "the German Empire" . <http://dbpedia.org/resource/German_Empire> dbp:capital <http://dbpedia.org/resource/Berlin> .
Note also that the same principle described here applies to @src and @href .
There will be occasions when the author wants to elide the subject and object as shown above, but is not concerned to name the resource that is common to the two statements (i.e., the object of the first statement, which is the subject of the second). For example, to indicate that Einstein was influenced by Spinoza the following markup could well be used:
<div about="http://dbpedia.org/resource/Baruch_Spinoza" rel="dbp:influenced"> <div about="http://dbpedia.org/resource/Albert_Einstein"> <span property="foaf:name">Albert Einstein</span> <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span> </div> </div>
An RDFa Processor will generate the following triples:
<http://dbpedia.org/resource/Baruch_Spinoza> dbp:influenced <http://dbpedia.org/resource/Albert_Einstein> . <http://dbpedia.org/resource/Albert_Einstein> foaf:name "Albert Einstein" . <http://dbpedia.org/resource/Albert_Einstein> dbp:dateOfBirth "1879-03-14"^^xsd:date .
However, an author could just as easily say that Spinoza influenced something by the name of Albert Einstein, that was born on March 14th, 1879 :
<div about="http://dbpedia.org/resource/Baruch_Spinoza" rel="dbp:influenced"> <div> <span property="foaf:name">Albert Einstein</span> <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span> </div> </div>
In RDF terms, the item that 'represents' Einstein is anonymous , since it has no URI to identify it. However, the item is given an automatically generated bnode , and it is onto this identifier that all child statements are attached:
An RDFa Processor will generate the following triples:
<http://dbpedia.org/resource/Baruch_Spinoza> dbp:influenced _:a . _:a foaf:name "Albert Einstein" . _:a dbp:dateOfBirth "1879-03-14"^^xsd:date .
Note
that
the
div
is
superfluous,
and
an
RDFa
Processor
will
create
the
intermediate
object
even
if
the
element
is
removed:
<div about="http://dbpedia.org/resource/Baruch_Spinoza" rel="dbp:influenced"> <span property="foaf:name">Albert Einstein</span> <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span> </div>
An
alternative
pattern
is
to
keep
the
div
and
move
the
@rel
onto
it:
<div about="http://dbpedia.org/resource/Baruch_Spinoza">
<div rel="dbp:influenced">
<span property="foaf:name">Albert Einstein</span>
<span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span>
</div>
</div>
From the point of view of the markup, this latter layout is to be preferred, since it draws attention to the 'hanging rel'. But from the point of view of an RDFa Processor, all of these permutations need to be supported.
When
a
new
subject
is
calculated,
it
is
also
used
to
complete
any
incomplete
triples
that
are
pending.
This
situation
arises
when
the
author
wants
to
'chain'
a
number
of
statements
together.
For
example,
an
author
could
have
a
statement
that
Albert
Einstein
was
born
in
Germany:
the
German
Empire:
<div about="http://dbpedia.org/resource/Albert_Einstein"> <span property="foaf:name">Albert Einstein</span> <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span><div rel="dbp:birthPlace" resource="http://dbpedia.org/resource/German_Empire" /> </div>
and
then
a
further
statement
that
the
'long
name'
for
Germany
this
country
is
the
Federal
Republic
of
Germany
the
German
Empire
:
<span about="http://dbpedia.org/resource/Germany" property="dbp:conventionalLongName">Federal Republic of Germany</span><span about="http://dbpedia.org/resource/German_Empire" property="dbp:conventionalLongName">the German Empire</span>
RDFa allows authors to insert this statement as a self-contained unit into other contexts:
<div about="http://dbpedia.org/resource/Albert_Einstein"> <span property="foaf:name">Albert Einstein</span> <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span><div rel="dbp:birthPlace" resource="http://dbpedia.org/resource/Germany" /> <span about="http://dbpedia.org/resource/Germany" property="dbp:conventionalLongName">Federal Republic of Germany</span><div rel="dbp:birthPlace" resource="http://dbpedia.org/resource/German_Empire" /> <span about="http://dbpedia.org/resource/German_Empire" property="dbp:conventionalLongName">the German Empire</span> </div>
But
it
also
allows
authors
to
avoid
unnecessary
repetition
and
to
'normalize'
out
duplicate
identifiers,
in
this
case
the
one
for
Germany:
the
German
Empire:
<div about="http://dbpedia.org/resource/Albert_Einstein"> <span property="foaf:name">Albert Einstein</span> <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span> <div rel="dbp:birthPlace"><span about="http://dbpedia.org/resource/Germany" property="dbp:conventionalLongName">Federal Republic of Germany</span><span about="http://dbpedia.org/resource/German_Empire" property="dbp:conventionalLongName">the German Empire</span> </div> </div>
When this happens the @rel for 'birth place' is regarded as a 'hanging rel' because it has not yet generated any triples, but these 'incomplete triples' are completed by the @about that appears on the next line. The first step is therefore to store the two parts of the triple that the RDFa Processor does have, but without an object:
<http://dbpedia.org/resource/Albert_Einstein>
dbp:birthPlace
?
.
Then
as
processing
continues,
the
RDFa
Processor
encounters
the
subject
of
the
statement
about
the
long
name
for
Germany,
the
German
Empire,
and
this
is
used
in
two
ways.
First
it
is
used
to
complete
the
'incomplete
triple':
<http://dbpedia.org/resource/Albert_Einstein><http://dbpedia.org/resource/Albert_Einstein> dbp:birthPlace<http://dbpedia.org/resource/Germany><http://dbpedia.org/resource/German_Empire> .
and second it is used to generate its own triple:
<http://dbpedia.org/resource/Germany><http://dbpedia.org/resource/German_Empire> dbp:conventionalLongName"Federal Republic of Germany""the German Empire" .
Note that each occurrence of @about will complete any incomplete triples. For example, to mark up the fact that Albert Einstein had both German and American citizenship, an author need only specify one @rel value that is then used with multiple @about values:
<div about="http://dbpedia.org/resource/Albert_Einstein" rel="dbp:citizenship"><span about="http://dbpedia.org/resource/German_Empire" /> <span about="http://dbpedia.org/resource/United_States" /> </div>
In this example there is one incomplete triple:
<http://dbpedia.org/resource/Albert_Einstein>
dbp:citizenship
?
.
When the processor meets each of the @about values, this triple is completed, giving:
<http://dbpedia.org/resource/Albert_Einstein>dbp:citizenship .dbp:citizenship <http://dbpedia.org/resource/German_Empire> . <http://dbpedia.org/resource/Albert_Einstein> dbp:citizenship <http://dbpedia.org/resource/United_States> .
These examples show how @about completes triples, but there are other situations that can have the same effect. For example, when @typeof creates a new bnode (as described above), that will be used to complete any 'incomplete triples'. To illustrate, to indicate that Spinoza influenced both Einstein and Schopenhauer, the following markup could be used:
<div about="http://dbpedia.org/resource/Baruch_Spinoza"> <div rel="dbp:influenced"> <div typeof="foaf:Person"> <span property="foaf:name">Albert Einstein</span> <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span> </div> <div typeof="foaf:Person"> <span property="foaf:name">Arthur Schopenhauer</span> <span property="dbp:dateOfBirth" datatype="xsd:date">1788-02-22</span> </div> </div> </div>
First the following incomplete triple is stored:
<http://dbpedia.org/resource/Baruch_Spinoza>
dbp:influenced
?
.
Then when the RDFa Processor processes the two occurrences of @typeof , each generates a bnode , which is used to both complete the 'incomplete triple', and to set the subject for further statements:
<http://dbpedia.org/resource/Baruch_Spinoza"> dbp:influenced _:a . _:a rdf:type foaf:Person . _:a foaf:name "Albert Einstein" . _:a dbp:dateOfBirth "1879-03-14"^^xsd:date . <http://dbpedia.org/resource/Baruch_Spinoza"> dbp:influenced _:b . _:b rdf:type foaf:Person . _:b foaf:name "Arthur Schopenhauer" . _:b dbp:dateOfBirth "1788-02-22"^^xsd:date .
Triples are also 'completed' if any one of @property , @rel or @rev are present. However, unlike the situation when @about or @typeof are present, all predicates are attached to one bnode :
<div about="http://dbpedia.org/resource/Baruch_Spinoza" rel="dbp:influenced"> <span property="foaf:name">Albert Einstein</span> <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span> <div rel="dbp:citizenship"><span about="http://dbpedia.org/resource/Germany" /><span about="http://dbpedia.org/resource/German_Empire" /> <span about="http://dbpedia.org/resource/United_States" /> </div> </div>
This example has two 'hanging rels', and so two situations when 'incomplete triples' will be created. Processing would proceed as follows; first an incomplete triple is stored:
<http://dbpedia.org/resource/Baruch_Spinoza>
dbp:influenced
?
.
Next,
the
RDFa
Processor
processes
the
predicate
values
for
foaf:name
,
dbp:dateOfBirth
and
dbp:citizenship
,
but
note
that
only
the
first
needs
to
'complete'
the
'hanging
rel'.
So
processing
foaf:name
generates
two
triples:
<http://dbpedia.org/resource/Baruch_Spinoza> dbp:influenced _:a . _:a foaf:name "Albert Einstein" .
but
processing
dbp:dateOfBirth
generates
only
one:
_:a
dbp:dateOfBirth
"1879-03-14"^^xsd:date
.
Processing
dbp:citizenship
also
uses
the
same
bnode
,
but
note
that
it
also
generates
its
own
'incomplete
triple':
_:a
dbp:citizenship
?
.
As before, the two occurrences of @about complete the 'incomplete triple', once each:
_:a dbp:citizenship ._:a dbp:citizenship <http://dbpedia.org/resource/German_Empire> . _:a dbp:citizenship <http://dbpedia.org/resource/United_States> .
The entire set of triples that an RDFa Processor should generate are as follows:
<http://dbpedia.org/resource/Baruch_Spinoza> dbp:influenced _:a . _:a foaf:name "Albert Einstein" . _:a dbp:dateOfBirth "1879-03-14"^^xsd:date .dbp:citizenship <http://dbpedia.org/resource/Germany> ._:a dbp:citizenship <http://dbpedia.org/resource/German_Empire> . _:a dbp:citizenship <http://dbpedia.org/resource/United_States> .
Although objects have been discussed in the previous sections, as part of the explanation of subject resolution, chaining, evaluation contexts, and so on, this section will look at objects in more detail.
There are two types of object, URI resource s and literal s.
A literal object can be set by using @property to express a predicate , and then using either @content , or the inline text of the element that @property is on. Note that the use of @content prohibits the inclusion of rich markup in your literal. If the inline content of an element accurately represents the object, then documents should rely upon that rather than duplicating that data using the @content .
A URI resource object can be set using one of @rel or @rev to express a predicate , and then either using one of @href , @resource or @src to provide an object resource explicitly, or using the chaining techniques described above to obtain an object from a nested subject, or from a bnode .
An object literal will be generated when @property is present. @property provides the predicate, and the following sections describe how the actual literal to be generated is determined.
@content can be used to indicate a plain literal , as follows:
<meta about="http://internet-apps.blogspot.com/"property="dc:creator"property="dcterms:creator" content="Mark Birbeck" />
The plain literal can also be specified by using the content of the element:
<span about="http://internet-apps.blogspot.com/"property="dc:creator">property="dcterms:creator"> Mark Birbeck </span>
Both of these examples give the following triple:
<http://internet-apps.blogspot.com/>dc:creatordcterms:creator "Mark Birbeck" .
The value of @content is given precedence over any element content, so the following would give exactly the same triple as shown above:
<span about="http://internet-apps.blogspot.com/"property="dc:creator"property="dcterms:creator" content="Mark Birbeck" >John Doe</span>
Literals can be given a data type using @datatype .
This can be represented in RDFa as follows:
<span property="cal:dtstart" content="2015-09-16T16:00:00-05:00"
datatype="xsd:dateTime">
September 16th at 4pm
</span>.
The triples that this markup generates include the datatype after the literal:
<>
cal:dtstart
"2015-09-16T16:00:00-05:00"^^
xsd:dateTime
.
XML documents cannot contain XML markup in their attributes, which means it is not possible to represent XML within @content (the following would cause an XML parser to generate an error):
<head><meta property="dc:title"<meta property="dcterms:title" content="E = mc<sup>2</sup>: The Most Urgent Problem of Our Time" /> </head>
It
does
not
help
to
escape
the
content,
since
the
output
would
simply
be
a
string
of
text
containing
numerous
ampersands:
<>
dc:title
"E
=
mc
<sup>2&lt;/sup>
:
The
Most
Urgent
Problem
of
Our
Time"
.
RDFa
therefore
supports
the
use
of
normal
markup
to
express
XML
literals,
by
using
@datatype
:
<h2 property="dc:title" datatype="rdf:XMLLiteral"><h2 property="dcterms:title" datatype="rdf:XMLLiteral"> E = mc<sup>2</sup>: The Most Urgent Problem of Our Time </h2>
This would generate the following triple, with the XML preserved in the literal:
<>dc:titledcterms:title "E = mc<sup>2</sup>: The Most Urgent Problem of Our Time"^^rdf:XMLLiteral .
This
requires
that
a
URI
mapping
for
the
prefix
rdf
has
been
defined.
To
make
authoring
easier,
if
there
are
child
elements
and
no
@datatype
attribute,
then
the
effect
is
the
same
as
if
@datatype
have
been
explicitly
set
to
rdf:XMLLiteral
:
In
the
examples
given
here
the
sup
element
is
actually
part
of
the
meaning
of
the
literal,
but
there
will
be
situations
where
the
extra
markup
means
nothing,
and
can
therefore
be
ignored.
In
this
situation
omitting
the
@datatype
attribute
or
specifying
an
empty
@datatype
value
can
be
used
to
override
the
XML
literal
behaviour:
create
a
plain
literal:
<p>You searched for <strong>Einstein</strong>:</p>
<p about="http://dbpedia.org/resource/Albert_Einstein">
<span property="foaf:name" datatype="">Albert <strong>Einstein</strong></span>
(b. March 14, 1879, d. April 18, 1955) was a German-born theoretical physicist.
</p>
Although the rendering of this page has highlighted the term the user searched for, setting @datatype to nothing ensures that the data is interpreted as a plain literal, giving the following triples:
<http://dbpedia.org/resource/Albert_Einstein>
foaf:name
"Albert
Einstein"
.
The value of this XML Literal is the exclusive canonicalization [ XML-EXC-C14N ] of the RDFa element's value.
Most of the rules governing the processing of objects that are resources are to be found in the processing descriptions given above, since they are important for establishing the subject. This section aims to highlight general concepts, and anything that might have been missed.
One or more URI object s are needed when @rel or @rev is present. Each attribute will cause triples to be generated when used with @href , @resource or @src , or with the subject value of any nested statement if none of these attributes are present.
@rel and @rev are essentially the inverse of each other; whilst @rel establishes a relationship between the current subject as subject, and the current object resource as the object, @rev does the exact opposite, and uses the current object resource as the subject, and the current subject as the object.
RDFa provides the @resource attribute as a way to set the object of statements. This is particularly useful when referring to resources that are not themselves navigable links:
<html><html profile='http://www.example.org/vocab-rdf-dc.html'> <head> <title>On Crime and Punishment</title> <base href="http://www.example.com/candp.xhtml" /> </head> <body><blockquote about="#q1" rel="dc:source" ><blockquote about="#q1" rel="dcterms:source" resource="urn:ISBN:0140449132" > <p id="q1"> Rodion Romanovitch! My dear friend! If you go on in this way you will go mad, I am positive! Drink, pray, if only a few drops! </p> </blockquote> </body> </html>
The
blockquote
element
generates
the
following
triple:
<http://www.example.com/candp.xhtml#q1> <http://purl.org/dc/terms/source> <urn:ISBN:0140449132> .
If no @resource is present, then @href is next in priority order, for setting the object.
When a predicate has been expressed using @rel , the @href on the [RDFa statement]'s element is used to identify the object with a [URI reference]. Its type is a URI:
<link about="mailto:john@example.org"
rel="foaf:knows"
href="mailto:sue@example.org"
/>
It's also possible to use both @rel and @rev at the same time on an element. This is particularly useful when two things stand in two different relationships with each other, for example when a picture is taken by Mark, but that picture also depicts him:
<img src="photo1.jpg"<img src="photo1.jpg" rel="dcterms:creator" rev="foaf:img" href="http://www.blogger.com/profile/1109404" />
which then yields two triples:
<photo1.jpg>dc:creator <http://www.blogger.com/profile/1109404> .dcterms:creator <http://www.blogger.com/profile/1109404> . <http://www.blogger.com/profile/1109404> foaf:img <photo1.jpg> .
When a triple predicate has been expressed using @rel or @rev , but no @href , @src , or @resource exists on the same element, there is a 'hanging rel'. This causes the current subject and all possible predicates (with an indicator of whether they are 'forwards, i.e., @rel values, or not, i.e., @rev values), to be stored as 'incomplete triples' pending discovery of a subject that could be used to 'complete' those triples.
This process is described in more detail in Completing 'Incomplete Triples' .
RDFa
Profiles
are
optional
external
documents
that
define
collections
of
terms
and/or
terms,
prefix
mappings.
mappings,
and/or
default
vocabulary
declarations.
A
profile
is
either
intrinsically
known
to
the
parser,
or
it
is
loaded
as
an
external
document
and
processed.
These
documents
must
be
defined
in
an
approved
RDFa
Host
Language
(currently
XHTML+RDFa
[
XHTML-RDFA
]).
They
may
also
be
defined
in
other
RDF
serializations
as
well
formats
(e.g.,
JSON-LD
[
JSON-LD
],
RDF/XML
[
RDF-SYNTAX-GRAMMAR
]
],
or
Turtle
[
TURTLE
]).
RDFa
Profiles
are
referenced
via
@profile
,
and
can
be
used
by
document
authors
to
simplify
the
task
of
adding
semantic
markup.
When
an
RDFa
document
includes
@profile
,
each
URI
in
the
value
is
processed
as
follows:
When
a
profile
is
not
retrievable,
an
RDFa
Processor
will
not
generate
triples
from
the
value.
element
the
profile
is
referenced
from,
nor
from
any
of
its
children.
Consequently,
any
further
processing
of
the
triples
would
be
effectively
ignored.
rdfa:prefix
and
an
rdfa:uri
predicate,
create
a
mapping
from
the
object
literal
of
the
rdfa:prefix
predicate
to
the
object
literal
of
the
rdfa:uri
predicate.
Add
or
update
this
mapping
in
the
local
list
of
URI
mappings
after
transforming
the
'prefix'
component
to
lower-case.
rdfa:term
and
an
rdfa:uri
predicate,
create
a
mapping
from
the
object
literal
of
the
rdfa:term
predicate
to
the
object
literal
of
the
rdfa:uri
predicate.
Add
or
update
this
mapping
in
the
local
term
mappings
.
rdfa:vocabulary
,
update
the
default
vocabulary
to
be
the
object
literal
of
the
rdfa:vocabulary
predicate.
When an RDFa Profile is defined using an RDF serialization, it must use the vocabulary terms above to declare the components of the profile.
Once all the URIs in the @profile value have been processed, continue with the normal processing of the current element .
If any conflict arises between two RDFa Profiles associated with URIs in the @profile value, the declaration from the RDFa Profile associated with the left-most URI takes precedence.
It is possible that a referenced RDFa document will in turn reference other documents via @profile . Regardless of the depth to which such references might go, only the triples in the top level document effect current processing.
RDFa
Processor
developers
are
permitted
and
encouraged
to
cache
Caching
of
the
relevant
triples
retrieved
via
this
mechanism,
including
embedding
mechanism
is
recommended
.
Embedding
definitions
for
well
known
vocabularies
known,
stable
RDFa
Profiles
in
the
implementation
if
appropriate.
is
recommended
.
The
object
literal
for
the
rdfa:uri
predicate
must
be
an
absolute
URI.
The
object
literal
for
the
rdfa:term
predicate
must
match
the
production
for
term
.
The
object
literal
for
the
rdfa:prefix
predicate
must
match
the
production
for
prefix
.
The
object
literal
for
the
rdfa:vocabulary
predicate
must
be
an
absolute
URI.
If
one
of
the
objects
is
not
a
Literal,
does
not
match
its
associated
production,
if
there
is
more
than
one
rdfa:vocabulary
predicate,
or
if
there
are
additional
rdfa:uri
or
rdfa:term
predicates
sharing
the
same
subject,
an
RDFa
Processor
must
not
update
the
associated
mapping.
In
order
to
facilitate
the
use
of
CURIEs
in
markup
languages,
this
specification
defines
some
additional
datatypes
in
the
XHTML
datatype
space
(
http://www.w3.org/1999/xhtml/datatypes/
).
Markup
languages
that
want
to
import
these
definitions
can
find
them
in
the
"datatypes"
file
for
their
schema
grammar:
Specifically, the following datatypes are defined:
This section is non-normative.
The following informative XML Schema definition for these datatypes is included as an example:
<?xml version="1.0" encoding="UTF-8"?> <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns="http://www.w3.org/1999/xhtml/datatypes/" xmlns:xh11d="http://www.w3.org/1999/xhtml/datatypes/" targetNamespace="http://www.w3.org/1999/xhtml/datatypes/" elementFormDefault="qualified" > <xs:simpleType name="CURIE"> <xs:restriction base="xs:string"> <xs:pattern value="(([\i-[:]][\c-[:]]*)?:)?.+" /> <xs:minLength value="1"/> </xs:restriction> </xs:simpleType> <xs:simpleType name="CURIEs"> <xs:list itemType="xh11d:CURIE"/> </xs:simpleType> <xs:simpleType name="SafeCURIE"> <xs:restriction base="xs:string"> <xs:pattern value="\[(([\i-[:]][\c-[:]]*)?:)?.+\]" /> <xs:minLength value="3"/> </xs:restriction> </xs:simpleType> <xs:simpleType name="SafeCURIEs"> <xs:list itemType="xh11d:SafeCURIE"/> </xs:simpleType> <xs:simpleType name="TERM"> <xs:list itemType="xs:NCName"/> <xs:simpleType> <xs:simpleType name="CURIEorURI"> <xs:union memberTypes="xh11d:CURIE xs:anyURI" /> </xs:simpleType> <xs:simpleType name="CURIEorURIs"> <xs:list itemType="xh11d:CURIEorURI"/> </xs:simpleType> <xs:simpleType name="SafeCURIEorCURIEorURI"> <xs:union memberTypes="xh11d:SafeCURIE xh11d:CURIE xs:anyURI" /> </xs:simpleType> <xs:simpleType name="SafeCURIEorCURIEorURIs"> <xs:list itemType="xh11d:SafeCURIEorCURIEorURI"/> </xs:simpleType> <xs:simpleType name='AbsURI'> <xs:restriction base='xs:string'> <xs:pattern value="[\i-[:]][\c-[:]]+:.+" /> </xs:restriction> </xs:simpleType> <xs:simpleType name="TERMorCURIEorAbsURI"> <xs:union memberTypes="xh11d:TERM xh11d:CURIE xh11d:AbsURI" /> </xs:simpleType> <xs:simpleType name="TERMorCURIEorAbsURIs"> <xs:list itemType="xh11d:SafeCURIEorCURIEorAbsURI"/> </xs:simpleType> </xs:schema>
This section is non-normative.
The following informative XML DTD definition for these datatypes is included as an example:
<!ENTITY % CURIE.datatype "CDATA" > <!ENTITY % CURIEs.datatype "CDATA" > <!ENTITY % CURIEorURI.datatype "CDATA" > <!ENTITY % CURIEorURIs.datatype "CDATA" > <!ENTITY % SafeCURIEorCURIEorURI.datatype "CDATA" > <!ENTITY % SafeCURIEorCURIEorURIs.datatype "CDATA" > <!ENTITY % TERMorCURIEorAbsURI.datatype "CDATA" > <!ENTITY % TERMorCURIEorAbsURIs.datatype "CDATA" >
The
RDFa
Vocabulary
is
used
to
modify
RDFa
processing
behavior
and
to
define
the
terms
usable
in
the
processor
graph
.
Its
URI
is
http://www.w3.org/ns/rdfa#
.
The Vocabulary includes the following term definitions (shown here in Turtle [ TURTLE ] format):
@prefix dcterms: <http://purl.org/dc/terms/> . @prefix dist: <http://www.w3.org/2007/08/pyRdfa/distiller#> . @prefix foaf: <http://xmlns.com/foaf/0.1/> . @prefix owl: <http://www.w3.org/2002/07/owl#> . @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> . @prefix rdfa: <http://www.w3.org/ns/rdfa#> . @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> . @prefix xhv: <http://www.w3.org/1999/xhtml/vocab#> . @prefix xml: <http://www.w3.org/XML/1998/namespace> . @prefix xsd: <http://www.w3.org/2001/XMLSchema#> . rdfa:PrefixMapping a rdfs:Class ;dc:description "used in conjunction with the definition of prefixes as the domain of the rdfa:prefix property"@en ;dcterms:description "used in conjunction with the definition of prefixes as the domain of the rdfa:prefix property"@en ; rdfs:subClassOf rdfa:VocabularyMapping . rdfa:TermMapping a rdfs:Class ;dc:description "used in conjunction with the definition of terms as the domain of the rdfa:term property"@en ;dcterms:description "used in conjunction with the definition of terms as the domain of the rdfa:term property"@en ; rdfs:subClassOf rdfa:VocabularyMapping . rdfa:VocabularyMapping a rdfs:Class . <http://www.w3.org/ns/rdfa#> a owl:Ontology ;dc:creator <http://www.ivan-herman.net/foaf#me> ; dc:date "2010-07-23"@en ; dc:description "This document describes the RDFa Vocabulary for Term and Prefix Assignment. The Vocabulary is used to modify RDFa1.1 processing behavior."@en ; dc:title "RDFa Vocabulary for Term and Prefix Assignment"@en ;dcterms:creator <http://www.ivan-herman.net/foaf#me> ; dcterms:date "2010-07-23"@en ; dcterms:description "This document describes the RDFa Vocabulary for Term and Prefix Assignment. The Vocabulary is used to modify RDFa1.1 processing behavior."@en ; dcterms:title "RDFa Vocabulary for Term and Prefix Assignment"@en ; rdfs:isDefinedBy <http://www.w3.org/TR/rdfa-core/> ; rdfs:seeAlso <http://www.w3.org/TR/rdfa-core/> ;owl:versionInfo "$Date: 2010/10/26 14:10:34 $"@en .owl:versionInfo "$Date: 2010/10/26 14:10:34 $"@en . rdfa:prefix a rdf:Property, owl:DatatypeProperty, owl:FunctionalProperty ;dc:description "defines a prefix"@en ;dcterms:description "defines a prefix"@en ; rdfs:domain rdfa:PrefixMapping ; rdfs:range xsd:NMTOKEN . rdfa:term a rdf:Property, owl:DatatypeProperty, owl:FunctionalProperty ;dc:description "defines a term"@en ;dcterms:description "defines a term"@en ; rdfs:domain rdfa:TermMapping ; rdfs:range xsd:NMTOKEN . rdfa:uri a rdf:Property, owl:DatatypeProperty, owl:FunctionalProperty ;dc:description "defines a uri string to be used either with a term or a prefix definition"@en ;dcterms:description "defines a uri string to be used either with a term or a prefix definition"@en ; rdfs:domain rdfa:VocabularyMapping ; rdfs:range xsd:anyURI . rdfa:vocabulary a rdf:Property, owl:DatatypeProperty ;dc:description "defines a uri string to be used as a default vocabulary"@en ;dcterms:description "defines a uri string to be used as a default vocabulary"@en ; rdfs:range xsd:anyURI . <http://www.w3.org/ns/rdfa.html> xhv:stylesheet <http://www.w3.org/StyleSheets/TR/base.css> . <http://www.ivan-herman.net/foaf#me> a foaf:Person ; rdfs:seeAlso <http://www.ivan-herman.net/foaf> ; foaf:mbox <mailto:ivan@w3.org> ; foaf:name "Ivan Herman"@en ; foaf:title "Semantic Web Activity Lead"@en ; foaf:workplaceHomepage <http://www.w3.org> . [ a owl:AllDisjointClasses ;owl:members ( rdfa:PrefixMapping rdfa:TermMapping )].owl:members ( rdfa:PrefixMapping rdfa:TermMapping )].
This vocabulary is also available in an separate file in Turtle format and in RDF/XML format .
These predicates can be used to 'pair' URI strings and their usage in the form of a prefix and/or a term as part of, for example, a blank node. An example can be as follows:
[] rdfa:uri "http://xmlns.com/foaf/0.1/name" ; rdfa:prefix "foaf" .
which defines a prefix for the foaf URI.
This section is non-normative.
This specification introduces a number of new features, and extends the behavior of some features from the previous version. The following summary may be helpful to RDFa Processor developers, but is not meant to be comprehensive.
2010-07-26: Added the 'vocabulary' term to the RDFa Profile handling.
2010-02-25: Split into RDFa Core and XHTML+RDFa.
2010-01-01: Applied changes to start production of version 1.1. This includes the re-integration of datatype CURIEorURI.
This section is non-normative.
At the time of publication, the members of the RDFa Working Group were: