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Copyright © 2007-2011 W3C ® ( MIT , ERCIM , Keio ), All Rights Reserved. W3C liability , trademark and document use rules apply.
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) 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.
RDFa is a way of expressing RDF -style relationships using simple attributes in existing markup languages such as HTML. RDF is fully internationalized, and permits the use of Internationalized Resource Identifiers, or IRIs. You will see the term 'IRI' used throughout this specification. Even if you are not familiar with the term IRI, you probably have seen the term 'URI' or 'URL'. IRIs are an extension of URIs that permits the use of characters outside those of plain ASCII. RDF allows the use of these characters, and so does RDFa. This specification has been careful to use the correct term, IRI, to make it clear that this is the case.
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
version
reflects
changes
made
as
a
result
of
comments
received
during
the
first
a
second
Last
Call
period.
That
document
was
widely
reviewed
and
the
Working
Group
has
made
a
number
of
significant
improvements
and
clarifications
as
a
result.
The
Working
Group
believes
that
all
comments
received
during
the
last
review
call
have
been
addressed.
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
an
Editor's
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
21
April
2011.
All
feedback
is
welcome.
Publication
as
a
Working
Editor's
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
is
a
Last
Call
Working
Draft
and
thus
the
Working
Group
has
determined
that
this
document
has
satisfied
the
relevant
technical
requirements
and
is
sufficiently
stable
to
advance
through
the
Technical
Recommendation
process.
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 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 ].
In
RDF,
it
is
common
for
people
to
shorten
vocabulary
terms
via
abbreviated
URIs
IRIs
that
use
a
'prefix'
and
a
'reference'.
This
mechanism
is
explained
in
detail
in
the
section
titled
Compact
URI
Expressions
.
The
examples
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/ |
dbp-owl: | http://dbpedia.org/ontology/ |
dbr: | http://dbpedia.org/resource/ |
dc: | 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# |
In
some
of
the
examples
below
we
have
used
URIs
IRIs
with
fragment
identifiers
that
are
local
to
the
document
containing
the
RDFa
fragment
identifiers
shown
(e.g.,
'
').
This
idiom,
which
is
also
used
in
RDF/XML
[
RDF-SYNTAX-GRAMMAR
]
and
other
RDF
serializations,
gives
a
simple
way
to
'mint'
new
about="#me"
about="#me"
URIs
IRIs
for
entities
described
by
RDFa
and
therefore
contributes
considerably
to
the
expressive
power
of
RDFa.
However,
Unfortunately,
this
practice
is
not
at
present
covered
by
the
media
type
registrations
that
govern
the
meaning
of
fragment
identifiers
(see
section
3.5
of
the
URI
specification
[
RFC3986
],
[
RFC3023
],
and
[
RFC2854
])
have
not
yet
caught
up
with
this
practice.
]).
For
more
information
about
fragment
identifier
semantics,
see
[
WEBARCH
]
section
3.2.1.
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
there
is
no
clear
way
to
add
new
@rel
values;
RDFa
sets
out
to
explicitly
solve
this
problem,
and
does
so
by
allowing
URIs
IRIs
as
values.
It
also
introduces
the
concepts
of
terms
and
'
compact
URI
expressions
'
—
referred
to
as
CURIEs
in
this
document
—
which
allow
a
full
URI
IRI
value
to
be
expressed
succinctly.
For
a
complete
list
of
RDFa
attribute
names
and
syntax,
see
Attributes
and
Syntax
.
In
HTML,
authors
can
include
metadata
and
relationships
concerning
the
current
document
by
using
the
meta
and
link
elements.
For
example,
the
author
of
the
page
along
with
the
pages
preceding
and
following
the
current
page
can
be
expressed
using
the
link
and
meta
elements:
<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
full
URIs:
IRIs:
<html xmlns="http://www.w3.org/1999/xhtml"> <head> <title>My home-page</title><meta property="" content="Mark Birbeck" /> <link rel="" href="http://www.example.com/#us" /><meta property="http://purl.org/dc/terms/creator" content="Mark Birbeck" /> <link rel="http://xmlns.com/foaf/0.1/topic" href="http://www.example.com/#us" /> </head> <body>...</body> </html>
Because
using
full
URIs
IRIs
like
those
above
can
be
cumbersome,
RDFa
also
permits
the
use
of
compact
URI
expressions
so
an
author
can
use
a
shorthand
to
reference
terms
in
multiple
vocabularies:
<html xmlns="http://www.w3.org/1999/xhtml" prefix="foaf: http://xmlns.com/foaf/0.1/ dc: http://purl.org/dc/terms/" > <head> <title>My home-page</title><meta property="" content="Mark Birbeck" /> <link rel="" href="http://www.example.com/#us" /><meta property="dc:creator" content="Mark Birbeck" /> <link rel="foaf:topic" href="http://www.example.com/#us" /> </head> <body>...</body> </html>
RDFa supports the use of @rel and @rev on any element. This is even more useful with the addition of support for different vocabularies:
This document is licensed under the <a prefix="cc: http://creativecommons.org/ns#" rel="cc:license" href="http://creativecommons.org/licenses/by-nc-nd/3.0/"> Creative Commons By-NC-ND License </a>.
Not
only
can
URIs
IRIs
in
the
document
be
re-used
to
provide
metadata,
but
so
can
inline
text
when
used
with
@property
:
<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 ><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, a more precise value can be added, which can optionally include @datatype :
<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"><span property="cal:summary"> one last summer Barbecue </span>, on<span ><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 using @typeof :
<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 ><p typeof="cal:Vevent"> I'm holding<span property="cal:summary"><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"><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/" > <head> <title>Books by Marco Pierre White</title> </head> <body> I think White's book'<span >'<span about="urn:ISBN:0091808189" typeof="bibo:Book" property="dc: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 ><span about="urn:ISBN:1596913614" typeof="bibo:Book" property="dc:description"> White's autobiography </span>. </body> </html>
When
dealing
with
small
amounts
of
markup,
it
is
sometimes
easier
to
use
full
URIs,
IRIs,
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 '<spanabout="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 <spanabout="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
using
terms
from
a
specific
vocabulary
is
to
use
@vocab
to
define
a
default
vocabulary
URI.
IRI.
For
example,
to
use
FOAF
terms:
<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 example above will produce the following triples, expressed here in Turtle syntax:
@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.
RDFa
is
short
for
"RDF
"RDF
in
Attributes".
Attributes".
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 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 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', '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
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
IRI
references
.
URIs
IRIs
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
IRI
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 in the German Empire. <http://dbpedia.org/resource/Albert_Einstein> has a picture at http://en.wikipedia.org/wiki/Image:Albert_Einstein_Head.jpg.
URI
IRI
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,
an
IRI,
but
we
could
also
use
a
URI
an
IRI
to
uniquely
identify
the
country
'German
Empire'.
At
the
same
time
we'll
indicate
that
the
name
and
date
of
birth
really
are
literals
(and
not
URIs),
IRIs),
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
IRI
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"."Albert Einstein". <http://dbpedia.org/resource/Albert_Einstein> <http://dbpedia.org/property/dateOfBirth>"March 14, 1879"."March 14, 1879". <http://dbpedia.org/resource/Albert_Einstein> <http://dbpedia.org/property/birthPlace> <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
IRI
resources
are
always
used
for
subjects
and
predicates,
the
object
part
of
a
triple
can
be
either
a
URI
an
IRI
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"
"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
an
IRI
to
the
end
of
a
plain
literal
,
and
this
URI
IRI
indicates
the
literal's
datatype.
This
URI
IRI
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""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,
IRIs,
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
IRIs
to
be
abbreviated
by
using
a
URI
an
IRI
mapping,
which
can
be
used
to
express
a
compact
URI
IRI
expression
as
follows:
@prefix dbp: <http://dbpedia.org/property/> . @prefix foaf: <http://xmlns.com/foaf/0.1/> . <http://dbpedia.org/resource/Albert_Einstein>"Albert Einstein" .foaf:name "Albert Einstein" . <http://dbpedia.org/resource/Albert_Einstein> dbp:birthPlace <http://dbpedia.org/resource/German_Empire> .
Here
'dbp:'
has
been
mapped
to
the
URI
IRI
for
DBPedia
and
'foaf:'
has
been
mapped
to
the
URI
IRI
for
the
'Friend
of
a
Friend'
taxonomy.
Any
URI
IRI
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
IRI
in
the
Turtle
representation:
<>
This
indicates
the
'current
document',
i.e.,
the
document
being
processed.
In
reality
there
would
always
be
a
full
URI
IRI
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.
IRIs.
A
collection
of
triples
is
called
a
graph
.
All
of
the
triples
that
are
defined
by
this
specification
are
contained
in
the
default
output
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
IRI
reference
s
into
a
form
called
a
'compact
URI
expression',
or
CURIE
.
A
detailed
discussion
of
this
mechanism
is
in
the
section
CURIE
and
URI
IRI
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
IRI
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 .)
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
IRI
reference
or
a
blank
node
(or
bnode)
,
the
predicate
is
always
a
URI
IRI
reference
,
and
the
object
of
a
statement
can
be
a
URI
IRI
reference
,
a
literal
,
or
a
bnode
.
In
RDFa,
a
subject
URI
IRI
reference
is
generally
indicated
using
@about
or
@src
,
and
predicates
are
represented
using
one
of
@property
,
@rel
,
or
@rev
.
Objects
which
are
URI
IRI
reference
s
are
represented
using
@resource
,
@src
,
or
@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
output
graph
to
mean
all
of
the
triples
asserted
by
a
document
according
to
the
Processing
Model
section.
The
term
processor
graph
is
used
to
denote
the
collection
of
all
informational,
warning,
and
error
triples
that
may
be
generated
by
the
RDFa
Processor
to
report
its
status
.
The
default
output
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
output
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 has made changes to the white space in a document before that document ever reaches the RDFa Processor (e.g., [ XMLSCHEMA-1 ] processors are permitted to 'normalize' white space in attribute values - see section 3.1.4). To ensure maximum consistency between processing environments, authors should remove any unnecessary white space in their plain and XML Literal content.
A
conforming
RDFa
Processor
must
examine
the
media
type
of
a
document
it
is
processing
to
determine
the
document's
Host
Language.
If
the
RDFa
Processor
is
unable
to
determine
the
media
type,
or
does
not
support
the
media
type,
the
RDFa
Processor
must
process
the
document
as
if
it
were
media
type
application/xml
.
See
XML+RDFa
Document
Conformance
.
A
Host
Language
may
specify
additional
announcement
mechanisms.
A conforming RDFa Processor may use additional mechanisms (e.g., the DOCTYPE, a file extension, the root element) to attempt to determine the Host Language if the media type is unavailable. These mechanisms are unspecified.
Host Languages that incorporate RDFa must adhere to the following:
<myml:myElement
property="next">
property="next">
).
When
a
Host
Language
does
not
use
the
attributes
in
'no
namespace',
they
must
be
referenced
via
the
XHTML
Namespace
(
http://www.w3.org/1999/xhtml
).
This
specification
does
not
define
a
stand-alone
document
type.
The
attributes
herein
are
intended
to
be
integrated
into
other
host
languages
(e.g.,
HTML+RDFa
or
XHTML+RDFa).
However,
this
specification
does
define
processing
rules
for
generic
XML
documents
-
that
is,
those
documents
delivered
as
media
types
text/xml
or
application/xml
.
Such
documents
must
meet
all
of
the
following
criteria:
http://www.w3.org/1999/xhtml
).
When
an
RDFa
Processor
processes
an
XML+RDFa
document,
it
does
so
in
via
the
following
context:
initial
context
:
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
rel
or
property
attribute
on
the
same
element
is
to
be
added
to
the
list
for
that
predicate.
Causes
a
list
to
be
created
if
it
does
not
already
exist
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,
IRI,
but
these
are
usually
long
and
unwieldy.
RDFa
therefore
supports
a
mechanism
by
which
URIs
IRIs
can
be
abbreviated,
called
'compact
URI
expressions'
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
'no
prefix'
mapping.
RDFa
Host
Languages
must
not
define
a
'no
prefix'
mapping.
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
an
IRI:
An
IRI
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.
IRI.
The
rules
for
determining
that
URI
IRI
are:
prefix
and
a
reference
,
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
reference
.
prefix
,
then
the
value
is
not
a
CURIE.
See General Use of Terms in Attributes for the way items with no colon can be interpreted in some datatypes by RDFa Processors.
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
IRIs
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
is
not
a
valid
QName
simply
because
"0321154991"
"0321154991"
is
not
a
valid
element
name.
Yet,
in
the
example
given,
we
don't
really
want
to
define
a
valid
element
name
anyway.
The
whole
reason
for
using
a
QName
was
to
reference
an
item
in
a
private
scope
-
that
of
ISBNs.
Moreover,
in
this
example,
we
want
the
names
within
that
scope
to
map
to
a
URI
an
IRI
that
will
reveal
the
meaning
of
that
ISBN.
As
you
can
see,
the
definition
of
QNames
and
this
(relatively
common)
use
case
are
in
conflict
with
one
another.
This specification addresses the problem by defining CURIEs. Syntactically, CURIEs are a superset of QNames.
Note
that
this
specification
is
targeted
at
language
designers,
not
document
authors.
Any
language
designer
considering
the
use
of
QNames
as
a
way
to
represent
URIs
IRIs
or
unique
tokens
should
consider
instead
using
CURIEs:
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
output
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
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" 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
('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
the
German
Empire,
which
has
a
long
name
of
"the
"the
German
Empire").
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, at some point in his life, a residence both in the German Empire and in Switzerland:
<div about="http://dbpedia.org/resource/Albert_Einstein"> <div rel="dbp-owl:residence" resource="http://dbpedia.org/resource/German_Empire"></div> <div rel="dbp-owl:residence" resource="http://dbpedia.org/resource/Switzerland"></div> </div>
Now, we show the same information, but this time we create an incomplete triple from the residence 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-owl:residence"> <span about="http://dbpedia.org/resource/German_Empire"></span> <span about="http://dbpedia.org/resource/Switzerland"></span> </div>
In this example, the incomplete triple actually gets completed twice, once for the German Empire and once for Switzerland, giving exactly the same information as we had in the earlier example:
<http://dbpedia.org/resource/Albert_Einstein> dbp-owl:residence <http://dbpedia.org/resource/German_Empire> . <http://dbpedia.org/resource/Albert_Einstein> dbp-owl:residence <http://dbpedia.org/resource/Switzerland> .
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-owl:residence"> <span about="http://dbpedia.org/resource/German_Empire"></span> <span about="http://dbpedia.org/resource/Switzerland"></span> </div> </div>
Since
RDFa
is
ultimately
a
means
for
transporting
RDF,
a
key
concept
is
the
resource
and
its
manifestation
as
a
URI.
IRI.
RDF
deals
with
complete
URIs
IRIs
(not
relative
paths);
when
converting
RDFa
to
triples,
any
relative
URIs
IRIs
must
be
resolved
relative
to
the
base
URI,
IRI,
using
the
algorithm
defined
in
section
5
6.5
of
RFC
3986
3987
[
URI
RFC3987
],
Reference
Resolution
.
The
values
of
RDFa
attributes
that
refer
to
URIs
IRIs
use
three
different
datatypes:
URI
,
SafeCURIEorCURIEorURI
SafeCURIEorCURIEorIRI
,
or
TERMorCURIEorAbsURI
TERMorCURIEorAbsIRI
.
All
these
attributes
are
mapped,
after
processing,
to
URIs.
IRIs.
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
IRI
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.
IRI.
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
an
IRI
as
follows:
<div prefix="db: http://dbpedia.org/"> <div about="db:resource/Albert_Einstein"> ... </div> </div>
The
author
is
free
to
split
the
URI
IRI
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=""><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
IRIs
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 > <div about=""><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
IRI
by
this
specification.
An
example
of
an
attribute
that
can
contain
a
CURIEorURI
CURIEorIRI
is
@about
.
To
express
a
URI
an
IRI
directly,
an
author
might
do
this:
<div about="http://dbpedia.org/resource/Albert_Einstein">
...
</div>
whilst
to
express
the
URI
IRI
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:
IRI:
<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
an
IRI
using
the
following
logic:
term
matches
an
item
in
the
list
of
local
term
mappings
.
First
compare
against
the
list
case-sensitively
,
and
if
there
is
no
match
then
compare
case-insensitively
.
If
there
is
a
match,
use
the
associated
term
.
term
has
no
associated
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.
IRI.
For
example
in
[
XHTML-RDFA
]
the
following
examples:
<link rel="next" href="http://example.org/page2.html" /> <linkrel="rel=" xhv:next" href="http://example.org/page2.html"" 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
IRIs
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
defines
processing
rules
for
optional
attributes
that
may
not
be
present
in
all
Host
Languages
(e.g.,
@xmlns:
).
If
these
attributes
are
not
supported
in
the
Host
Language,
then
the
corresponding
processing
rules
are
not
relevant
for
that
language.
The processing rules are:
If
no
URI
IRI
is
provided
by
a
resource
attribute,
then
the
first
match
from
the
following
rules
will
apply:
If
no
URI
IRI
is
provided
then
the
first
match
from
the
following
rules
will
apply:
Then
the
current
object
resource
is
set
to
the
URI
IRI
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.
IRI.
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,
IRI,
as
described
in
[
RDF-CONCEPTS
],
which
will
have
been
obtained
according
to
the
section
on
CURIE
and
URI
IRI
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 XML namespace declarations (if any) declared on the serialized element. Since the child element node could also declare new 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 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 possible 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 rdfa:Info, rdfa:Warning, or rdfa: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
output
graph
as
long
as
it
can
be
determined
if
a
generated
triple
belongs
in
the
processor
graph
or
the
default
output
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
output
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
output
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
URI.
IRI.
For
this
class
of
RDFa
Processor,
the
software
must
allow
the
caller
to
specify
if
they
would
like
to
retrieve
the
default
output
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
output
graph
must
be
returned.
To ensure interoperability, a core hierarchy of classes is defined for the content of the processor graph. Separate errors or warnings are resources (typically blank nodes) of a specific type, with additional properties giving more details on the error condition or the warning. This specification defines only the top level classes and the ones referring to the error and warning conditions defined explicitly by this document. Other, implementation-specific subclasses may be defined by the RDFa Processor.
The
top
level
classes
are
rdfa:Error
,
rdfa:Warning
,
and
rdfa:Info
,
defined
as
part
of
the
RDFa
Vocabulary
.
Furthermore,
a
single
property
is
defined
on
those
classes,
namely
rdfa:context
,
that
provides
an
extra
context
for
the
error,
e.g.,
http
response,
an
XPath
information,
or
simply
the
URI
IRI
to
the
RDFa
resource.
Usage
of
this
property
is
optional,
and
more
than
one
triple
can
be
used
with
this
predicate
on
the
same
subject.
Finally,
error
and
warning
instances
should
use
the
dc:description
and
dc:date
properties.
dc:description
should
provide
a
short,
human
readable
but
implementation
dependent
description
of
the
error.
dc:date
should
give
the
time
when
the
error
was
found
and
it
is
advised
to
be
as
precise
as
possible
to
allow
the
detection
of,
for
example,
possible
network
errors.
The
example
below
shows
the
triples
that
should
be
minimally
present
in
the
processor
graph
as
a
result
of
an
error
(the
content
of
the
literal
for
the
dc:description
predicate
is
implementation
dependent):
@prefix rdfa: <http://www.w3.org/ns/rdfa#> . @prefix xsd: <http://www.w3.org/2001/XMLSchema#> .@prefix dcterms: <http://purl.org/dc/terms/> . [] a rdfa:ProfileReferenceError ; dcterms:description "The @profile value could not be deferenced" ; dcterms:date "2010-06-30T13:40:23"^^xsd:dateTime@prefix dc: <http://purl.org/dc/terms/> . [] a rdfa:DocumentError ; dc:description "The document could not be parsed due to parsing errors" ; dc:date "2010-06-30T13:40:23"^^xsd:dateTime .
A
slightly
more
elaborate
example
makes
use
of
the
rdfa:context
property
to
provide
further
information,
using
external
vocabularies
to
represent
HTTP
headers
or
XPointer
information:
information
(note
that
a
processor
may
not
have
these
information
in
all
cases,
i.e.,
these
rdfa:context
information
are
not
required):
@prefix rdfa: <http://www.w3.org/ns/rdfa#> . @prefix xsd: <http://www.w3.org/2001/XMLSchema#> .@prefix dcterms: <http://purl.org/dc/terms/> .@prefix dc: <http://purl.org/dc/terms/> . @prefix ptr: <http://www.w3.org/2009/pointers#> . @prefix http: <http://www.w3.org/2006/http#> .[] a rdfa:ProfileReferenceError ; dcterms:description "The @profile value could not be deferenced" ; dcterms:date "2010-06-30T13:40:23"^^xsd:dateTime ; rdfa:context <http://www.example.org/profile> ;[] a rdfa:DocumentError ; dc:description "The document could not be parsed due to parsing errors" ; dc:date "2010-06-30T13:40:23"^^xsd:dateTime ; rdfa:context <http://www.example.org/doc> ; rdfa:context [ a ptr:Pointer ; # detailed xpointer/xpath information provided here to locate the# DOM node with the @profile attribute# error ] ; rdfa:context [ a http:Response ; http:responseCode <http://www.w3.org/2006/http#404># Get the HTTP response headers on the request for the profile file.# Get the HTTP response headers on the request for the source file. ].
This section is non-normative.
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.
There are many examples in this section. The examples are all written using XHTML+RDFa. However, the explanations are relevant regardless of the Host Language.
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
IRI
of
the
document
being
parsed,
or
a
value
as
set
by
a
Host
Language-provided
mechanism
(e.g.,
the
base
element
in
(X)HTML).
This
means
that
by
default
any
metadata
found
in
the
document
will
concern
the
document
itself:
<html> <head> <title>Jo's Friends and Family Blog</title><link rel="foaf:primaryTopic" href="#bbq" /> <meta property="dc:creator" content="Jo" /> </head> <body> ... </body> </html>
This would generate the following triples:
<> foaf:primaryTopic <#bbq> . <>dcterms:creator "Jo"dc:creator "Jo" .
It is possible for the data to appear elsewhere in the document:
<html> <head> <title>Jo's Blog</title> </head> <body><h1>'s blog</h1><h1><span property="dc:creator">Jo</span>'s blog</h1> <p> Welcome to my blog. </p> </body> </html>
which would still generate the triple:
<>dcterms:creator "Jo"dc:creator "Jo" .
In
(X)HTML
the
value
of
base
may
change
the
initial
value
of
current
subject
:
<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="dcterms:creator" content="Jo" /><link rel="foaf:primaryTopic" href="#bbq" /> <meta property="dc:creator" content="Jo" /> </head> <body> ... </body> </html>
An
RDFa
Processor
should
now
generate
the
following
triples,
regardless
of
the
URI
IRI
from
which
the
document
is
served:
<http://www.example.org/jo/blog> foaf:primaryTopic <#bbq> . <http://www.example.org/jo/blog>dcterms:creator "Jo"dc:creator "Jo" .
As
processing
progresses,
any
@about
attributes
will
change
the
current
subject
.
The
value
of
@about
is
a
URI
an
IRI
or
a
CURIE.
If
it
is
a
relative
URI
IRI
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 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="dcterms:creator" content="Jo" /><link rel="foaf:primaryTopic" href="#bbq" /> <meta property="dc:creator" content="Jo" /> </head> <body><p ><p about="#bbq" typeof="cal:Vevent"> I'm holding<span ><span property="cal:summary"> one last summer barbecue </span>, on<span content="2015-09-16T16:00:00-05:00" datatype="xsd:dateTime"><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" . <#bbq> rdf:type cal:Vevent . <#bbq> cal:summary "one last summer barbecue" . <#bbq> cal:dtstart"2015-09-16T16:00:00-05:00"^^xsd:dateTime"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 rel="foaf:knows" href="mailto:jim@example.org">Jim</a>.<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="dcterms:creator">Mark Birbeck</span><span property="dc:creator">Mark Birbeck</span> </div>
which should generate the following triples:
<photo1.jpg>dcterms:creator "Mark Birbeck" . 8.1.1.3 Using @src 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 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/" /> Since normal chaining rules will apply, the image URI can also be used to complete hanging triples: <div about="http://www.blogger.com/profile/1109404" rel="foaf:img"> <img rel="license" resource="http://creativecommons.org/licenses/by/2.0/" 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> dcterms:creator "Mark Birbeck"dc: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""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 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/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
the
German
Empire
were
elided
connected
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/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" 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 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/German_Empire"> <span property="dbp:conventionalLongName">the German Empire</span> <span rel="dbp-owl: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/German_Empire> .
whilst the second group refer to the @resource that contains them:
<http://dbpedia.org/resource/German_Empire> dbp:conventionalLongName "the German Empire" . <http://dbpedia.org/resource/German_Empire> dbp-owl: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
connect
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-owl: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-owl:influenced <http://dbpedia.org/resource/Albert_Einstein> .<http://dbpedia.org/resource/Albert_Einstein> foaf:name "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"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-owl:influenced"> <div><span property="foaf:name">Albert Einstein</span> <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span><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
IRI
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-owl:influenced _:a . _:a foaf:name "Albert Einstein" . _:a dbp:dateOfBirth"1879-03-14"^^xsd:date"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-owl: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-owl: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 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 this country is the German Empire :
<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/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 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/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 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>
dbp:birthPlace
<http://dbpedia.org/resource/German_Empire>
.
and second it is used to generate its own triple:
<http://dbpedia.org/resource/German_Empire> dbp:conventionalLongName"the"the GermanEmpire"Empire" .
Note that each occurrence of @about will complete any incomplete triples. For example, to mark up the fact that Albert Einstein had a residence both in the German Empire and Switzerland, 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-owl:residence"> <span about="http://dbpedia.org/resource/German_Empire" /> <span about="http://dbpedia.org/resource/Switzerland" /> </div>
In this example there is one incomplete triple:
<http://dbpedia.org/resource/Albert_Einstein>
dbp-owl:residence
?
.
When the processor meets each of the @about values, this triple is completed, giving:
<http://dbpedia.org/resource/Albert_Einstein> dbp-owl:residence <http://dbpedia.org/resource/German_Empire> . <http://dbpedia.org/resource/Albert_Einstein> dbp-owl:residence <http://dbpedia.org/resource/Switzerland> .
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-owl: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 > <span property="foaf:name">Arthur Schopenhauer</span> <span property="dbp:dateOfBirth" datatype="xsd:date">1788-02-22</span><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-owl: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-owl:influenced _:a . _:a rdf:type foaf:Person .foaf:name "Albert Einstein" . dbp:dateOfBirth "1879-03-14"^^xsd:date . <http://dbpedia.org/resource/Baruch_Spinoza"> dbp-owl:influenced ._:a foaf:name "Albert Einstein" . _:a dbp:dateOfBirth "1879-03-14"^^xsd:date . <http://dbpedia.org/resource/Baruch_Spinoza"> dbp-owl:influenced _:b . _:b rdf:type foaf:Person .foaf:name "Arthur Schopenhauer" ._:b foaf:name "Arthur Schopenhauer" . _:b dbp:dateOfBirth"1788-02-22"^^xsd:date"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-owl:influenced"> <span property="foaf:name">Albert Einstein</span> <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span> <div rel="dbp-owl:residence"> <span about="http://dbpedia.org/resource/German_Empire" /> <span about="http://dbpedia.org/resource/Switzerland" /> </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-owl:influenced
?
.
Next,
the
RDFa
Processor
processes
the
predicate
values
for
foaf:name
,
dbp:dateOfBirth
and
dbp-owl:residence
,
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-owl:influenced _:a . _:a foaf:name"Albert Einstein""Albert Einstein" .
but
processing
dbp:dateOfBirth
generates
only
one:
_:a dbp:dateOfBirth"1879-03-14"^^xsd:date"1879-03-14"^^xsd:date .
Processing
dbp-owl:residence
also
uses
the
same
bnode
,
but
note
that
it
also
generates
its
own
'incomplete
triple':
_:a
dbp-owl:residence
?
.
As before, the two occurrences of @about complete the 'incomplete triple', once each:
_:a dbp-owl:residence <http://dbpedia.org/resource/German_Empire> . _:a dbp-owl:residence <http://dbpedia.org/resource/Switzerland> .
The entire set of triples that an RDFa Processor should generate are as follows:
<http://dbpedia.org/resource/Baruch_Spinoza> dbp-owl:influenced _:a . _:a foaf:name "Albert Einstein" . _:a dbp:dateOfBirth "1879-03-14"^^xsd:date . _:a dbp-owl:residence <http://dbpedia.org/resource/German_Empire> . _:a dbp-owl:residence <http://dbpedia.org/resource/Switzerland> .
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
IRI
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
IRI
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.
Literal object resolution
@content can be used to indicate a plain literal , as follows:
<meta about="http://internet-apps.blogspot.com/" property="dc:creator"content="Mark Birbeck"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"> Mark Birbeck </span>
Both of these examples give the following triple:
<http://internet-apps.blogspot.com/>dcterms:creator "Mark Birbeck"dc: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"content="Mark Birbeck"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"^^"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" content="E = mc<sup>2</sup>: The Most Urgent Problem of Our Time" /> </head>
RDFa therefore supports the use of normal markup to express XML literals, by using @datatype :
<h2 property="dc: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:
<>dcterms:title "Edc:title "E = mc<sup>2</sup>: The Most Urgent Problem of OurTime"^^rdf:XMLLiteralTime"^^rdf:XMLLiteral .
This
requires
that
a
URI
an
IRI
mapping
for
the
prefix
rdf
has
been
defined.
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
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"
"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
IRI
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> <head> <title>On Crime and Punishment</title><base href="http://www.example.com/candp.xhtml" /><base href="http://www.example.com/candp.xhtml" /> </head> <body><blockquote about="#q1" rel="dcterms:source" > <p id="q1"><blockquote about="#q1" rel="dc: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
or
@src
are
next
in
priority
order,
order
for
setting
the
object.
When
a
predicate
has
been
expressed
using
@rel
,
the
@href
or
@src
on
the
[RDFa
statement]'s
RDFa
statement's
element
is
used
to
identify
the
object
with
a
[URI
reference].
Its
type
is
a
URI:
IRI
reference
.
Their
types
are
an
IRI:
<link about="mailto:john@example.org"
rel="foaf:knows"
href="mailto:sue@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 about="http://www.blogger.com/profile/1109404" src="photo1.jpg" rev="dc:creator" rel="foaf:img" />
which then yields two triples:
<photo1.jpg> dc: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' .
<http://www.example.com> <http://www.example.com/predicate> "first object", "second object" ;
@prefix bibo: <http://purl.org/ontology/bibo/> . @prefix dc: <http://purl.org/dc/terms/> . @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> . [ a bibo:Chapter ; dc:title "Semantic Annotation and Retrieval" ; dc:creator [ rdf:first "Ben Adida" ; rdf:rest [ rdf:first "Mark Birbeck" ; rdf:rest [ rdf:first "Ivan Herman" ; rdf:rest rdf:nil . ] . ] . ] . ... ]
@prefix bibo: <http://purl.org/ontology/bibo/> . @prefix dc: <http://purl.org/dc/terms/> . @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> . [ a bibo:Chapter ; dc:title "Semantic Annotation and Retrieval" ; dc:creator ( "Ben Adida" "Mark Birbeck" "Ivan Herman" ) . ... ]
It
would
of
course
be
possible
to
reproduce
the
same
structure
in
RDFa,
using
the
RDF
predicates
rdf:first
,
rdf:rest
,
as
well
as
the
special
resource
rdf:nil
.
However,
to
make
this
easier,
RDFa
provides
the
@inlist
.
What
this
attributes
signals
is
that
the
object
generated
on
that
element
should
be
put
on
a
list;
the
list
is
used
with
the
common
predicate
with
the
common
subject.
Here
is
how
the
previous
structure
could
look
like
in
RDFa:
<p prefix="bibo: http://purl.org/ontology/bibo/ dc: http://purl.org/dc/terms/ typeof="bibo:Chapter"> “<span property="dc:title">Semantic Annotation and Retrieval</span>”, by <span inlist="" property="dc:creator">Ben Adida</span>, <span inlist="" property="dc:creator">Mark Birbeck</span>, and <span inlist="" property="dc:creator">Ivan Herman</span>. </p>
Note that the order in the list is determined by the document order. (The value of the @inlist is not relevant, only its presence is.)
<p prefix="bibo: http://purl.org/ontology/bibo/ dc: http://purl.org/dc/terms/ typeof="bibo:Chapter"> “<span property="dc:title">Semantic Annotation and Retrieval</span>”, by <span inlist="" rel="dc:creator" resource="http://ben.adida.net/#me">Ben Adida</span>, <span inlist="" property="dc:creator">Mark Birbeck</span>, and <span inlist="" rel="dc:creator" resource="http://www.ivan-herman.net/foaf#me">Ivan Herman</span>. </p>
@prefix bibo: <http://purl.org/ontology/bibo/> . @prefix dc: <http://purl.org/dc/terms/> . @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> . [ a bibo:Chapter ; dc:title "Semantic Annotation and Retrieval" ; dc:creator ( <http://ben.adida.net/#me> "Mark Birbeck" <http://www.ivan-herman.net/foaf#me> ) . ... ]
Incomplete Triples can also be used in conjunction with lists when all list elements are resources and not literal. For example, the previous example, this time with all three authors referring to their FOAF profile, could have been written as:
<p prefix="bibo: http://purl.org/ontology/bibo/ dc: http://purl.org/dc/terms/ typeof="bibo:Chapter"> “<span property="dc:title">Semantic Annotation and Retrieval</span>”, by <span rel="dc:creator" inlist=""> <a href="http://ben.adida.net/#me">Ben Adida</a>, <a href="http://internet-apps.blogspot.com/2008/03/my-profile.html#me">Mark Birbeck</a>, and <a href="http://www.ivan-herman.net/foaf#me">Ivan Herman</a>. </span> </p>
@prefix bibo: <http://purl.org/ontology/bibo/> . @prefix dc: <http://purl.org/dc/terms/> . @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> . [ a bibo:Chapter ; dc:title "Semantic Annotation and Retrieval" ; dc:creator ( <http://ben.adida.net/#me> <http://internet-apps.blogspot.com/2008/03/my-profile.html#me> <http://www.ivan-herman.net/foaf#me> ) . ... ]
Note that it is also possible to generate an empty list easily, without @inlist , using:
<span rel="prop" resource="rdf:nil"/>
RDFa
Profiles
are
collections
permits
Host
Languages
to
define
an
initial
context
.
Such
a
context
is
a
collection
of
terms,
prefix
mappings,
and/or
default
vocabulary
declarations.
A
profile
An
initial
context
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
XML+RDFa
and
XHTML+RDFa
[
XHTML-RDFA
]).
They
may
also
be
defined
in
other
formats
(e.g.,
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
initial
context
document
includes
@profile
,
its
value
is
processed
from
beginning
to
end,
with
each
separate
URI
processed,
it
is
evaluated
as
follows:
rdfa:prefix
and
rdfa:uri
,
create
a
key-value
mapping
from
the
rdfa:prefix
object
literal
(the
key)
to
the
rdfa:uri
object
literal
(the
value).
Add
rdfa:term
and
rdfa:uri
,
create
a
key-value
mapping
from
the
rdfa:term
object
literal
(the
key)
to
the
rdfa:uri
object
literal
(the
value).
Add
rdfa:vocabulary
,
rdfa:vocabulary
predicate.
When
an
RDFa
Profile
Initial
Context
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
.
Profiles
referenced
on
the
same
element
are
processed
from
beginning
to
end
of
the
value
of
@profile
.
If
any
conflict
arises
between
two
RDFa
Profiles
associated
with
URIs
in
the
@profile
value,
the
declaration
from
the
RDFa
Profile
associated
with
the
right-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.
context.
Caching
of
the
relevant
triples
retrieved
via
this
mechanism
is
recommended
.
Embedding
definitions
for
well
known,
stable
RDFa
Profiles
Initial
Contexts
in
the
implementation
is
recommended
.
The
object
literal
for
the
rdfa:uri
predicate
must
be
an
absolute
URI.
IRI.
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.
IRI.
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
create
the
associated
mapping.
Web
authors
utilizing
Since
RDFa
Profiles
should
be
aware
that
if
a
profile
that
they
list
is
not
available
for
any
reason,
based
on
RDF,
the
semantics
of
RDF
vocabularies
can
be
used
to
gain
more
knowledge
about
data.
Vocabularies,
properties
and
classes
are
identified
by
IRIs,
which
enables
them
to
be
discoverable.
RDF
data
published
at
the
location
of
these
IRIs
can
be
retrieved,
and
descriptions
of
the
properties
and
classes
using
specified
semantics
can
be
applied.
RDFa
Processor
has
not
previously
cached
Vocabulary
Expansion
is
an
optional
processing
step
which
may
be
added
once
the
profile,
that
all
normal
processing
steps
described
in
Processing
Model
are
complete.
Vocabulary
expansion
relies
on
a
sub-set
of
RDFS
entailment
[
RDF-MT
]
to
add
triples
that
should
to
the
output
graph
based
on
rules
and
property/class
relationships
described
in
referenced
vocabularies.
Vocabulary
expansion
may
be
generated
performed
as
part
of
a
result
larger
RDF
toolset
including
an
RDFS
reasoner.
Alternatively,
using
vocabulary
data
added
to
the
output
graph
in
processing
step
2
of
Sequence
,
expansion
may
also
be
done
using
a
separate
reasoner
after
the
profile
will
not
output
graph
has
been
generated,
or
as
the
last
processing
step
by
an
RDFa
processor.
It
can
be
generated.
In
addition,
any
other
triples
very
useful
to
make
generalized
data
available
for
subsequent
usage
of
RDFa-embedded
data
by
expanding
inferred
statements
entailed
by
these
semantics.
This
provides
for
existing
vocabularies
that
exist
in
a
subtree
extend
well-known
vocabularies
to
have
those
properties
added
to
the
output
graph
automatically.
For
example,
the
namespace
document
of
the
DOM
will
not
Creative
Commons
vocabulary,
i.e.,
http://creativecommons.org/ns
,
defines
cc:license
to
be
generated
either
since
processing
halts
for
a
subtree
in
sub-property
of
dc:license
.
By
using
the
DOM
when
@vocab
attribute,
one
can
describe
a
profile
cannot
licensing
information
as
follows:
This document is licensed under the <a vocab="http://creativecommons.org/ns#" rel="license" href="http://creativecommons.org/licenses/by-nc-nd/3.0/"> Creative Commons By-NC-ND License </a>.
which results in the following output graph :
@prefix cc: <http://creativecommons.org/ns#> . @prefix rdfa: <http://www.w3.org/ns/rdfa#> . <> cc:license <http://creativecommons.org/licenses/by-nc-nd/3.0/> ; rdfa:hasVocabulary <http://creativecommons.org/ns#> .
After vocabulary expansion, the output graph would now contain:
@prefix cc: <http://creativecommons.org/ns#> . @prefix rdfa: <http://www.w3.org/ns/rdfa#> . @prefix dc: <http://purl.org/dc/terms/> . <> cc:license <http://creativecommons.org/licenses/by-nc-nd/3.0/>; dc:license <http://creativecommons.org/licenses/by-nc-nd/3.0/> ; rdfa:hasVocabulary <http://creativecommons.org/ns#> .
Other
vocabularies,
specifically
intended
to
provide
relations
to
multiple
vocabularies,
could
also
be
fetched.
defined
by
publishers,
allowing
use
of
terms
in
a
single
namespace
which
result
in
properties
and/or
classes
from
other
primary
vocabularies
being
imported.
This
benefits
publishers
as
data
is
now
more
widely
searchable
and
encourages
the
practice
of
referencing
well-known
vocabularies.
This section is non-normative.
Web
authors
utilizing
both
Once
the
output
graph
is
generated
following
the
processing
steps
defined
in
Sequence
,
processors
may
perform
the
following
processing
steps
on
the
output
graph.
It
must
do
so
only
if
the
user
of
the
processor
explicitly
asks
for
it,
as
prescribed
in
Vocabulary
Expansion
Control
of
RDFa
Profiles
and
Processors
.
A
vocabulary
graph
is
created
as
follows:
For
each
IRI
being
the
object
of
a
JavaScript-based
triple
in
the
output
graph
with
the
subject
being
the
current
document
(
base
)
IRI
and
the
property
being
rdfa:hasVocabulary
,
that
IRI
is
dereferenced.
If
the
dereferencing
yields
the
serialization
of
an
RDF
graph,
that
serialization
is
parsed
and
the
resulting
graph
is
merged
with
the
vocabulary
graph.
(An
RDFa
processor
capable
of
vocabulary
expansion
must
accept
an
RDF
graph
serialized
in
RDFa,
and
should
be
aware
that
same-origin
security
protections
are
enforced
for
RDFa
processors,
just
like
any
accept
other
JavaScript
code.
This
means
standard
serialization
formats
of
RDF
such
as
RDF/XML
[
RDF-SYNTAX-GRAMMAR
]
and
Turtle
[
TURTLE
].)
Note
that
if
if,
in
the
second
step,
a
profile
particular
vocabulary
is
serialized
in
RDFa,
that
particular
graph
is
not
served
up
expected
to
undergo
any
vocabulary
expansion
on
its
own.
Vocabulary expansion is then performed as follows:
For
the
purpose
of
vocabulary
processing,
RDFa
defines
a
restricted
subset
of
the
full
RDFS
entailment
(
section
4.4
of
[
CORS
RDF-MT
]
or
]).
This
entailment
is
based
on
a
similar
technology
that
restriction
of
the
processor
may
fail
to
retrieve
vocabulary
used
by
the
profile
RDFS
Interpretation
(
section
4.1
of
[
RDF-MT
]).
RDFa
vocabulary
entailment
uses
only
the
following
terms:
and
thus
the
expected
RDFa
vocabulary
entailment
does
not
use
any
of
the
axiomatic
triples
defined
in
sections
3
or
7
of
[
RDF-MT
].
While
the
subtree
formal
definition
of
the
DOM
where
RDFa
Entailment
is
defined
in
terms
of
RDF
Semantics,
practical
implementations
may
rely
on
the
@profile
(informative)
entailment
rules
published
in
[
RDF-MT
].
In
particular,
the
relevant
rules
are
(using
the
rule
identification
in
section
7
of
[
RDF-MT
]):
rdfs5
,
rdfs7
,
rdfs9
,
and
rdfs11
.
The
entailment
described
in
this
section
is
specified
will
the
minimum
useful
level
for
RDFa.
Processors
may,
of
course,
choose
to
follow
more
powerful
entailment
regimes,
like
RDFS
[
RDF-MT
],
or
OWL
[
OWL2-OVERVIEW
].
Using
those
entailments
applications
may
perform
datatype
validation
by
checking
rdfs:range
of
a
property,
or
use
the
facilities
offered
by,
e.g.,
OWL’s
owl:equivalentProperty
to
interlink
vocabularies.
Conforming RDFa processors are NOT required to provide vocabulary expansion.
If
an
RDFa
processor
provides
vocabulary
expansion,
it
must
not
be
generated.
performed
by
default.
Instead,
the
processor
must
provide
an
option,
vocab_expansion
,
which,
when
used,
instructs
the
RDFa
processor
to
perform
a
vocabulary
expansion
before
returning
the
output
graph.
Although vocabulary expansion is described in terms of a vocabulary graph and RDFS entailment rules, processors are free to use any process which obtains equivalent results.
This section is non-normative.
For RDFa Processors caching the relevant graphs retrieved via this mechanism is recommended . Caching is usually based on HTTP response headers like expiration time, cache control, etc.
For publishers of vocabularies, the URI for the vocabularies should be dereferencable, and should return an RDF graph with the vocabulary description. This vocabulary description should be available encoded in RDFa, and may also be available in other RDF serialization syntaxes (using content negotiation to choose among the different formats). If possible, vocabulary descriptions should include subproperty and subclass statements linking the vocabulary terms to other, well-known vocabularies. Finally, HTTP responses should include fields usable for cache control, e.g., expiration date.
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"
"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:schemaxmlns: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"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: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 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: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 name="SafeCURIEs"> <xs:list itemType="xh11d:SafeCURIE"/> </xs:simpleType><xs:simpleType name="TERM"> <xs:list itemType="xs:NCName"/><xs:simpleType name="TERM"> <xs:list itemType="xs:NCName"/> </xs:simpleType><xs:simpleType name="CURIEorURI"> <xs:union memberTypes="xh11d:CURIE xs:anyURI" /><xs:simpleType name="CURIEorIRI"> <xs:union memberTypes="xh11d:CURIE xs:anyURI" /> </xs:simpleType><xs:simpleType name="CURIEorURIs"> <xs:list itemType="xh11d:CURIEorURI"/><xs:simpleType name="CURIEorIRIs"> <xs:list itemType="xh11d:CURIEorIRI"/> </xs:simpleType><xs:simpleType name="SafeCURIEorCURIEorURI"> <xs:union memberTypes="xh11d:SafeCURIE xh11d:CURIE xs:anyURI" /><xs:simpleType name="SafeCURIEorCURIEorIRI"> <xs:union memberTypes="xh11d:SafeCURIE xh11d:CURIE xs:anyURI" /> </xs:simpleType><xs:simpleType name="SafeCURIEorCURIEorURIs"> <xs:list itemType="xh11d:SafeCURIEorCURIEorURI"/><xs:simpleType name="SafeCURIEorCURIEorIRIs"> <xs:list itemType="xh11d:SafeCURIEorCURIEorIRI"/> </xs:simpleType><xs:simpleType name='AbsURI'><xs:simpleType name='AbsIRI'> <xs:restriction base='xs:string'><xs:pattern value="[\i-[:]][\c-[:]]+:.+" /><xs:pattern value="[\i-[:]][\c-[:]]+:.+" /> </xs:restriction> </xs:simpleType><xs:simpleType name="TERMorCURIEorAbsURI"> <xs:union memberTypes="xh11d:TERM xh11d:CURIE xh11d:AbsURI" /><xs:simpleType name="TERMorCURIEorAbsIRI"> <xs:union memberTypes="xh11d:TERM xh11d:CURIE xh11d:AbsIRI" /> </xs:simpleType><xs:simpleType name="TERMorCURIEorAbsURIs"> <xs:list itemType="xh11d:SafeCURIEorCURIEorAbsURI"/><xs:simpleType name="TERMorCURIEorAbsIRIs"> <xs:list itemType="xh11d:SafeCURIEorCURIEorAbsIRI"/> </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 % CURIEorIRI.datatype "CDATA" > <!ENTITY % CURIEorIRIs.datatype "CDATA" > <!ENTITY % SafeCURIEorCURIEorIRI.datatype "CDATA" > <!ENTITY % SafeCURIEorCURIEorIRIs.datatype "CDATA" > <!ENTITY % TERMorCURIEorAbsIRI.datatype "CDATA" > <!ENTITY %TERMorCURIEorAbsURIs.datatype "CDATA"TERMorCURIEorAbsIRIs.datatype "CDATA" >
The
RDFa
Vocabulary
has
two
three
roles:
it
contains
the
predicates
to
define
the
terms
and
prefixes
in
profile
initial
context
documents,
and
it
contains
the
classes
and
predicates
for
the
messages
that
a
processor
graph
may
contain.
contain
and,
finally,
it
contains
the
predicate
necessary
for
vocabulary
processing.
The
URI
IRI
of
the
vocabulary
is
http://www.w3.org/ns/rdfa#
;
the
usual
prefix
used
in
this
document
is
rdfa
.
This vocabulary specification is available in XHTML+RDFa 1.1 , Turtle , and in RDF/XML formats.
The RDFa Vocabulary includes the following triples (shown here in Turtle [ TURTLE ] format):
@prefix dc: <http://purl.org/dc/terms/> . @prefix owl: <http://www.w3.org/2002/07/owl#> . @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> . @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> . @prefix rdfa: <http://www.w3.org/ns/rdfa#> . @prefix foaf: <http://xmlns.com/foaf/0.1/> . <http://www.w3.org/ns/rdfa#> a owl:Ontology . rdfa:PrefixOrTermMapping a rdfs:Class, owl:Class ;dcterms:description "is the top level class for prefix or term mappings" . rdfa:PrefixMapping dcterms:description "is the class for prefix mappings" .dc:description "is the top level class for prefix or term mappings" . rdfa:PrefixMapping dc:description "is the class for prefix mappings" . rdfs:subClassOf rdfa:PrefixOrTermMapping .rdfa:TermMapping dcterms:description "is the class for term mappings" .rdfa:TermMapping dc:description "is the class for term mappings" . rdfs:subClassOf rdfa:PrefixOrTermMapping . rdfa:hasVocabulary a rdf:Property, owl:ObjectProperty ; rdfs:subPropertyOf foaf:primaryTopic; dc:description "identifies an IRI used as the value of a @vocab attribute." . rdfa:prefix a rdf:Property, owl:DatatypeProperty ; rdfs:domain rdfa:PrefixMapping ;dcterms:description "defines a prefix mapping for a URI; the value is supposed to be a NMTOKEN" .dc:description "defines a prefix mapping for an IRI; the value is supposed to be a NMTOKEN" . rdfa:term a rdf:Property, owl:DatatypeProperty ; rdfs:domain rdfa:TermMapping ;dcterms:description "defines a term mapping for a URI; the value is supposed to be a NMTOKEN" .dc:description "defines a term mapping for an IRI; the value is supposed to be a NMTOKEN" . rdfa:uri a rdf:Property, owl:DatatypeProperty ; rdfs:domain rdfa:PrefixOrTermMapping ;dcterms:description "defines the URI for either a prefix or a term mapping; the value is supposed to be an absolute URI" .dc:description """defines the IRI for either a prefix or a term mapping; the value is supposed to be an absolute IRI""" . rdfa:vocabulary a rdf:Property, owl:DatatypeProperty ;dcterms:description "defines a URI to be used as a default vocabulary; the value is can be any string; for documentation purposes it is advised to usedc:description """defines an IRI to be used as a default vocabulary; the value is can be any string; for documentation purposes it is advised to use the string ‘true’ or‘True’."‘True’.""" .
These
predicates
can
be
used
to
'pair'
URI
IRI
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""foaf" .
The Vocabulary includes the following term definitions (shown here in Turtle [ TURTLE ] format):
@prefix dc: <http://purl.org/dc/terms/> . @prefix owl: <http://www.w3.org/2002/07/owl#> . @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> . @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> . @prefix rdfa: <http://www.w3.org/ns/rdfa#> . rdfa:PGClass a rdfs:Class, owl:Class;dcterms:description "is the top level class of the hierarchy" .dc:description "is the top level class of the hierarchy" .rdfa:Error dcterms:description "is the class for all error conditions";rdfa:Error dcterms:description "is the class for all error conditions"; rdfs:subClassOf rdfa:PGClass .rdfa:Warning dcterms:description "is the class for all warnings";rdfa:Warning dcterms:description "is the class for all warnings"; rdfs:subClassOf rdfa:PGClass .rdfa:Info dcterms:description "is the class for all informations";rdfa:Info dcterms:description "is the class for all informations"; rdfs:subClassOf rdfa:PGClass .rdfa:DocumentError dcterms:description "error condition; to be used when the document fails to be fully processed as a result of non-conformant host language markup";rdfa:DocumentError dc:description "error condition; to be used when the document fails to be fully processed as a result of non-conformant host language markup"; rdfs:subClassOf rdfa:Error .rdfa:ProfileReferenceError dcterms:description "error condition; to be used when an RDFa Profile document fails to be retrieved and thus, a portion of the document fails to be processed";rdfa:ProfileReferenceError dc:description "error condition; to be used when a default RDFa Profile document fails to be retrieved and thus the document fails to be processed"; rdfs:subClassOf rdfa:Error .rdfa:UnresolvedTerm dcterms:description "warning; to be used when a Term fails to be resolved";rdfa:VocabReferenceError dc:description "warning; to be used when the value of a @vocab attribute cannot be dereferenced, hence the vocabulary expansion cannot be completed."; rdfs:subClassOf rdfa:Warning .rdfa:UnresolvedCURIE dcterms:description "warning; to be used when a CURIE prefix fails to be resolved";rdfa:UnresolvedTerm dc:description "warning; to be used when a Term fails to be resolved"; rdfs:subClassOf rdfa:Warning .rdfa:UnresolvedCURIE dc:description "warning; to be used when a CURIE prefix fails to be resolved"; rdfs:subClassOf rdfa:Warning . rdfa:context a owl:ObjectProperty, rdf:Property;dcterms:description "provides extra context for the error, e.g., http response, an XPointer/XPath information, or simply the URI that created the error";dc:description "provides extra context for the error, e.g., http response, an XPointer/XPath information, or simply the IRI that created the error"; rdfs:domain rdfa:PGClass .
The Vocabulary includes the following term definitions (shown here in Turtle [ TURTLE ] format):
@prefix dc: <http://purl.org/dc/terms/> . @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#> . rdfa:hasVocabulary a owl:ObjectProperty, rdf:Property; dc:description "provides a relationship between the host document and a vocabulary defined using the @vocab facility of RDFa1.1" .
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.
While this specification strives to be as backward compatible as possible with [ RDFA-SYNTAX ], the changes above mean that there are some circumstances where it is possible for different RDF triples to be output for the same document when processed by an RDFa 1.0 processor vs. an RDFa 1.1 processor. In order to minimize these differences, a document author can do the following:
XHTML+RDFa
1.0
on
the
html
element.
datatype='rdf:XMLLiteral'
.
datatype=''
.
When producing XHTML+RDFa 1.1 documents, it is possible to reduce the incompatibilities with RDFa 1.0 conforming processors by doing the following:
XHTML+RDFa
1.0
on
the
html
element.
datatype='rdf:XMLLiteral'
.
datatype=''
.
This section is non-normative.
At the time of publication, the active members of the RDFa Working Group were: