3.1 General Approach
The main idea behind the syntax for RDF/A is that existing
data should be easy to update to convey RDF triples. Thus,
the bulk of RDF/A can be expressed using only attributes
applied to existing elements within the XML document,
specifically about, rel, rev, property,
and content. In addition, it should be possible
to represent a [triple] using only one XML element. In
this way it owes more to 'flat' syntaxes like RDF/N3
[N3-PRIMER] than it does to RDF/XML
[RDF-SYNTAX], despite its use of XML.
For example, given an XHTML chunk as follows:
This photo was taken by
<span class="author">Mark Birbeck</span>.
a simple attribute augmentation can yield an RDF triple:
This photo was taken by
<span class="author" about="photo1.jpg" property="dc:creator">Mark Birbeck</span>.
which yields:
<photo1.jpg> dc:creator "Mark Birbeck"^^rdf:XMLLiteral .
Note that, in the above example, had "Mark Birbeck" not been
enclosed in an existing span element, a new one could
have simply been used with about
and property as its only two attributes.
Similarly, links can be augmented to express RDF triples. Consider an XHTML chunk:
This photo was taken by
<a href="http://www.blogger.com/profile/1109404">Mark Birbeck</a>.
When the RDF object is a URI, the RDF predicate is designated using rel:
This photo was taken by
<a about="photo1.jpg" rel="dc:creator"
href="http://www.blogger.com/profile/1109404">Mark Birbeck</a>.
which yields:
<photo1.jpg> dc:creator <http://www.blogger.com/profile/1109404> .
It's important to note that the various RDF/A attributes can be used
on any existing element of the XML dialect. Note also that one can
express a reverse relationship using the rev
attribute. For example, if the photo in question is actually a
depiction of Mark, one could write:
This photo was taken by
<a about="photo1.jpg" rev="foaf:img"
href="http://www.blogger.com/profile/1109404">Mark Birbeck</a>.
which would yield:
<http://www.blogger.com/profile/1109404> foaf:img <photo1.jpg> .
Both relations can be expressed simultaneously:
This photo was taken by
<a about="photo1.jpg" rel="dc:creator" rev="foaf:img"
href="http://www.blogger.com/profile/1109404">Mark Birbeck</a>.
which then yields both triples:
<photo1.jpg> dc:creator <http://www.blogger.com/profile/1109404> .
<http://www.blogger.com/profile/1109404> foaf:img <photo1.jpg> .
And it's also possible to go further and add the attributes used for
denoting statements in which the object is a [literal]:
This photo was taken by
<a about="photo1.jpg" property="dc:title"
content="Portrait of Mark" rel="dc:creator"
rev="foaf:img" href="http://www.blogger.com/profile/1109404">Mark Birbeck</a>.
which would then yield:
<photo1.jpg> dc:creator <http://www.blogger.com/profile/1109404> .
<http://www.blogger.com/profile/1109404> foaf:img <photo1.jpg> .
<photo1.jpg> dc:title "Portrait of Mark" .
It's possible to do all of this without ambiguity, since
the property attribute always denotes a predicate in a
statement in which the object is a [literal], whilst
the rel and rev attributes always denote a
predicate in a statement in which the object is a [URI reference]. Put
a different way, property always works
with content, whilst rel
and rev work with
href.
Of course, the more natural way to express the three
above triples is to strive to make all metadata literals
and URIs meaningful within the host XML
dialect. Specifically, in the case of XHTML2, it makes
sense to render as much of the useful metadata as
possible and use RDF/A to mark up this rendered data. The
following XHTML thus generates the same triples shown
above.
This photo, entitled
<span about="photo1.jpg" property="dc:title">Portrait of Mark</span>
was taken by
<a about="photo1.jpg" rel="dc:creator" rev="foaf:img"
href="http://www.blogger.com/profile/1109404">Mark himself</a>.
As expected, an empty string in the about attribute
designates the current URI. In addition, the value of
the about attribute is inherited from parent
elements. The following XHTML thus generates the very same triples as
the XHTML above.
<div about="photo1.jpg">
This photo, entitled
<span property="dc:title">Portrait of Mark</span>
was taken by
<a rel="dc:creator" rev="foaf:img"
href="http://www.blogger.com/profile/1109404">Mark himself</a>.
</div>
3.2 Qualifying document components
A second feature of RDF/A is that it is possible to use
parts of the host document to provide the [subject] of a
[triple]. This marks RDF/A from other approaches to
serialising RDF, in that the the same syntax can now be
used to make statements about parts of a document, and
external documents.
It is possible to make such statements using the syntax
introduced in the examples above:
<html xmlns:dc="http://purl.org/dc/elements/1.1/">
<head>
<title>On <em>Crime and Punishment</em></title>
</head>
<body>
<blockquote id="q1" about="#q1" rel="dc:source" href="urn:isbn:0140449132" >
<p>
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>
However, two problems arise: the href causes
the entire quotation to become a clickable link, which may
not be the desired visual effect, and only one triple can
be expressed. Thus, RDF/A provides another mechanism,
using the special link or meta
element without a specified [subject]. In such cases, the
[triple] concerns the parent element. This allows the
example above to be recast as follows:
<html xmlns:dc="http://purl.org/dc/elements/1.1/">
<head>
<title>On <em>Crime and Punishment</em></title>
</head>
<body>
<blockquote id="q1">
<link rel="dc:source" href="urn:isbn:0140449132" />
<p>
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>
This syntax (omitting the about attribute to
refer to the parent element) only applies to the
elements link and meta, which means
that even without an id attribute, the
following statement is still 'about'
the blockquote element, and not the document as
a whole:
<blockquote>
<link rel="dc:source" href="urn:isbn:0140449132" />
<p>
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>
If more than one piece of metadata needs to be attached to the same
element, then additional link or meta
elements can be added:
<blockquote>
<link rel="dc:source" href="urn:isbn:0140449132" />
<meta property="dc:creator" content="Fyodor Dostoevsky" />
<p>
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>
Now we have attached two pieces of metadata to
the blockquote element -- the source of the
quote, and its author.
Note:
We say nothing here about how this metadata is used. In
the previous example, the information may be of use to
an RDF/A-aware browser, and it could be made available
to the user accessing the page via a mechanism such as
tooltips. But it may also be the case that the document
is parsed by some external processor and the output
stored as a set of [triple]s. In the latter case the
[triple]s generated by the previous example would have a
[unique anonymous ID] as the subject of each statement,
as follows:
_:a dc:source <urn:isbn:0140449132> .
_:a dc:creator "Fyodor Dostoevsky" .
If one wishes a non-anonymous node to represent the blockquote,
one only needs to add an additional attribute to
the blockquote, namely the usual XML id
attribute:
<blockquote id="q1">
<link rel="dc:source" href="urn:isbn:0140449132" />
<meta property="dc:creator" content="Fyodor Dostoevsky" />
<p>
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>
which then yields:
<#q1> dc:source <urn:isbn:0140449132> .
<#q1> dc:creator "Fyodor Dostoevsky" .
3.3 Relating document components
Using qualifying statements, RDF/A allows a single XML dialect
document to include multiple RDF entities. Relations between the
various entities of a given page can also be defined using RDF/A
notation.
Consider the following XHTML, which defines two RDF entities of
type taxo:topic, two RDF entities of
type biblio:Publication, metadata pertinent to each
publication, including dc:title
and dc:creator, and relations of
type taxo:topics between the publications and tags:
<html xmlns:dc="http://purl.org/dc/elements/1.1/">
<head>
<title>Mark's Publications</title>
</head>
<body>
<h2>Tags</h2>
<div id="tag_standards">
<link rel="rdf:type" href="[taxo:topic]" />
Standards
</div>
<div id="tag_xforms">
<link rel="rdf:type" href="[taxo:topic]" />
XForms
</div>
<h2>Publications</h2>
<div id="publication_1">
<link rel="rdf:type" href="biblio:Publication" />
<link rel="dc:creator" href="http://www.blogger.com/profile/1109404" />
<meta property="dc:title">A Standards-Based Virtual Machine</meta>
<link rel="taxo:topics" href="#tag_standards" />
</div>
<div id="publication_2">
<link rel="rdf:type" href="biblio:Publication" />
<link rel="dc:creator" href="http://www.blogger.com/profile/1109404" />
<meta property="dc:title">XForms and Internet Applications</meta>
<link rel="taxo:topics" href="#tag_standards" />
<link rel="taxo:topics" href="#tag_xforms" />
</div>
</body>
</html>
This yields the expected triples:
<#tag_standards> rdf:type taxo:topic .
<#tag_xforms> rdf:type taxo:topic .
<#publication_1> rdf:type biblio:Publication .
<#publication_1> dc:creator <http://www.blogger.com/profile/1109404>
<#publication_1> dc:title "A Standards-Based Virtual Machine" .
<#publication_1> taxo:topics <#tag_standards> .
<#publication_2> rdf:type biblio:Publication .
<#publication_2> dc:creator <http://www.blogger.com/profile/1109404>
<#publication_2> dc:title "XForms and Internet Applications" .
<#publication_2> taxo:topics <#tag_standards> .
<#publication_2> taxo:topics <#tag_xforms> .
Beyond this theoretical example, this application of RDF/A is
particularly useful for formats like FOAF. (See examples.)