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The current Web is primarily made up of an enormous number of documents that have been created using HTML. These documents contain significant amounts of structured data, which is largely unavailable to tools and applications. When publishers can express this data more completely, and when tools can read it, a new world of user functionality becomes available, letting users transfer structured data between applications and web sites, and allowing browsing applications to improve the user experience: an event on a web page can be directly imported into a user's desktop calendar; a license on a document can be detected so that users can be informed of their rights automatically; a photo's creator, camera setting information, resolution, location and topic can be published as easily as the original photo itself, enabling structured search and sharing.
RDFa Core is a specification for attributes to express structured data in any markup language. The embedded data already available in the markup language (e.g., XHTML) 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 a second Last Call period. That document was widely reviewed and the Working Group has made significant improvements and clarifications as a result. The Working Group believes that all comments received during the last 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 Working Draft. This document is intended to become a W3C Recommendation. If you wish to make comments regarding this document, please send them to public-rdfa-wg@w3.org ( subscribe , archives ). All feedback is welcome.
Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
This document was produced by a group operating under the 5 February 2004 W3C Patent Policy . W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy .
This section is non-normative.
RDF/XML [ RDF-SYNTAX ] provides sufficient flexibility to represent all of the abstract concepts in RDF [ RDF-CONCEPTS ]. However, it presents a number of challenges; first it is difficult or impossible to validate documents that contain RDF/XML using XML Schemas or DTDs, which therefore makes it difficult to import RDF/XML into other markup languages. Whilst newer schema languages such as RELAX NG [ RELAXNG-SCHEMA ] do provide a way to validate documents that contain arbitrary RDF/XML, it will be a while before they gain wide support.
Second, even if one could add RDF/XML directly into an XML dialect like XHTML, there would be significant data duplication between the rendered data and the RDF/XML structured data. It would be far better to add RDF to a document without repeating the document's existing data. For example, an XHTML document that explicitly renders its author's name in the text—perhaps as a byline on a news site—should not need to repeat this name for the RDF expression of the same concept: it should be possible to supplement the existing markup in such a way that it can also be interpreted as RDF.
Another reason for aligning the rendered data with the structured data is that it is highly beneficial to express the web data's structure 'in context'; as users often want to transfer structured data from one application to another, sometimes to or from a non-web-based application, the user experience can be enhanced. For example, information about specific rendered data could be presented to the user via 'right-clicks' on an item of interest.
In the past, many attributes were 'hard-wired' directly into the markup language to represent specific concepts. For example, in XHTML 1.1 [ XHTML11 ] and HTML [ HTML401 ] there is @cite ; the attribute allows an author to add information to a document which is used to indicate the origin of a quote.
However, these 'hard-wired' attributes make it difficult to define a generic process for extracting metadata from any document since an RDFa Processor would need to know about each of the special attributes. One motivation for RDFa has been to devise a means by which documents can be augmented with metadata in a general, rather than hard-wired, manner. This has been achieved by creating a fixed set of attributes and parsing rules, but allowing those attributes to contain properties from any of a number of the growing range of available RDF vocabularies. In most cases the values of those properties are the information that is already in an author's 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 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
IRIs
with
fragment
identifiers
that
are
local
to
the
document
containing
the
RDFa
fragment
identifiers
shown
(e.g.,
'
about="#me"
').
This
idiom,
which
is
also
used
in
RDF/XML
[
RDF-SYNTAX-GRAMMAR
]
and
other
RDF
serializations,
gives
a
simple
way
to
'mint'
new
IRIs
for
entities
described
by
RDFa
and
therefore
contributes
considerably
to
the
expressive
power
of
RDFa.
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
]).
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 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 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 IRIs:
<html xmlns="http://www.w3.org/1999/xhtml"> <head> <title>My home-page</title> <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 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="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 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 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"> one last summer Barbecue </span>, on <span property="cal:dtstart" content="2015-09-16T16:00:00-05:00" datatype="xsd:dateTime"> September 16th at 4pm </span>. </p> </body> </html>
In many cases a block of markup will contain a number of properties that relate to the same item; it's possible with RDFa to indicate the type of that item 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 typeof="cal:Vevent"> I'm holding <span property="cal:summary"> one last summer Barbecue </span>, on <span property="cal:dtstart" content="2015-09-16T16:00:00-05:00" datatype="xsd:dateTime"> September 16th at 4pm </span>. </p> </body> </html>
RDFa allows the document to contain metadata information about other documents and resources:
<html xmlns="http://www.w3.org/1999/xhtml" prefix="bibo: http://purl.org/ontology/bibo/ dc: http://purl.org/dc/terms/" > <head> <title>Books by Marco Pierre White</title> </head> <body> I think White's book '<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 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 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 '<span about="urn:ISBN:0091808189" typeof="http://purl.org/ontology/bibo/Book" property="http://purl.org/dc/terms/title" >Canteen Cuisine</span>' is well worth getting since although it's quite advanced stuff, he makes it pretty easy to follow. You might also like <span about="urn:ISBN:1596913614" typeof="http://purl.org/ontology/bibo/Book" property="http://purl.org/dc/terms/description" >White's autobiography</span>. </body> </html>
A simple way of defining a portion of a document using terms from a specific vocabulary is to use @vocab to define a default vocabulary 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/> .
In simple cases the @property property can also be used in place of @rel . Indeed, in case when the element does not contain @rel , @datatype , of @content , but there is, for example, a @href , the effect of @property is analogous to the role of @rel . For example, the previous example could have been written:
<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 property="homepage" href="http://example.org/blog/">Understanding Semantics</a>. </div>
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 in 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 IRI references.
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 ] 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.
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 an IRI, but we could also use 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 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 > .
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". <http://dbpedia.org/resource/Albert_Einstein> <http://dbpedia.org/property/dateOfBirth> "March 14, 1879". <http://dbpedia.org/resource/Albert_Einstein> <http://dbpedia.org/property/birthPlace> <http://dbpedia.org/resource/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 IRI resources are always used for subjects and predicates, the object part of a triple can be either 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"
.
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
an
IRI
to
the
end
of
a
plain
literal
,
and
this
IRI
indicates
the
literal's
datatype.
This
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" ^^<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 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 IRIs to be abbreviated by using an IRI mapping, which can be used to express a compact IRI expression as follows:
@prefix dbp: <http://dbpedia.org/property/> . @prefix foaf: <http://xmlns.com/foaf/0.1/> . <http://dbpedia.org/resource/Albert_Einstein> foaf:name "Albert Einstein" . <http://dbpedia.org/resource/Albert_Einstein> dbp:birthPlace <http://dbpedia.org/resource/German_Empire> .
Here 'dbp:' has been mapped to the IRI for DBPedia and 'foaf:' has been mapped to the IRI for the 'Friend of a Friend' taxonomy.
Any 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 IRI in the Turtle representation:
<>
This indicates the 'current document', i.e., the document being processed. In reality there would always be a full 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 IRIs.
A collection of triples is called a graph . All of the triples that are defined by this specification are contained in the 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 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 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 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 IRI reference or a blank node (or bnode) , the predicate is always a IRI reference , and the object of a statement can be a IRI reference , a literal , or a bnode .
In RDFa, a subject IRI reference is generally indicated using @about and predicates are represented using one of @property , @rel , or @rev . Objects which are 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 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 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 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">
).
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 via the following initial context :
describedby
,
license
,
and
role
),
defined
in
http://www.w3.org/2011/rdfa-context/rdfa-1.1
.
dc
),
defined
in
http://www.w3.org/2011/rdfa-context/rdfa-1.1
.
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 IRI, but these are usually long and unwieldy. RDFa therefore supports a mechanism by which IRIs can be abbreviated, called 'compact URI expressions' or simply, CURIEs .
When expanded, the resulting IRI must be a syntactically valid IRI [ RFC3987 ]. For a more detailed explanation see CURIE and IRI Processing . The lexical space of a CURIE is as defined in curie below. The value space is the set of IRIs.
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
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 IRI. The rules for determining that IRI are:
prefix
and
a
reference
,
the
IRI
is
obtained
by
taking
the
current
default
prefix
mapping
and
concatenating
it
with
the
reference
.
If
there
is
no
current
default
prefix
mapping,
then
this
is
not
a
valid
CURIE
and
must
be
ignored.
prefix
and
reference
,
and
if
there
is
an
in-scope
mapping
for
prefix
(when
compared
case-insensitively),
then
the
IRI
is
created
by
using
that
mapping,
and
concatenating
it
with
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 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" 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 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 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 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
an
IRI
against
which
relative
paths
can
be
resolved.
During the course of processing, new evaluation context s are created which are passed to each child element. The rules described below will determine the values of the items in the context. Additionally, some rules will cause new triples to be created by combining information provided by an element with information from the evaluation context .
During the course of processing a number of locally scoped values are needed, as follows:
rdf:type
relationships
to
types
specified
in
@datatype
.
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 German Empire").
It's also possible for the subject of nested statements to provide the object for containing statements — essentially the reverse of the example we have just seen. To illustrate, we'll take an example of the type of chaining just described, and show how it could be marked up more efficiently. To start, we mark up the fact that Albert Einstein had, 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>
Note that, as noted above, in many situations the @property and @rel are interchangeable. This is not true for chaining. Taking the first example, if that example was used as follows:
<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 property="dbp:birthPlace" resource="http://dbpedia.org/resource/German_Empire"> <span property="dbp:conventionalLongName">the German Empire</span> </div> </div>
The subject for the 'German Empire' would remain Albert Einstein (and that would, of course, be an error). This is the main difference between @property and @rel : the latter induces chaining, whereas the former, usually, does not.
Since RDFa is ultimately a means for transporting RDF, a key concept is the resource and its manifestation as a IRI. RDF deals with complete IRIs (not relative paths); when converting RDFa to triples, any relative IRIs must be resolved relative to the base IRI, using the algorithm defined in section 6.5 of RFC 3987 [ RFC3987 ], Reference Resolution . The values of RDFa attributes that refer to IRIs use three different datatypes: URI , SafeCURIEorCURIEorIRI , or TERMorCURIEorAbsIRI . All these attributes are mapped, after processing, to 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 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 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 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 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="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 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 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
IRI
by
this
specification.
An example of an attribute that can contain a CURIEorIRI is @about . To express an IRI directly, an author might do this:
<div about="http://dbpedia.org/resource/Albert_Einstein">
...
</div>
whilst to express the 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 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 ::= NCNameStartChar termChar* termChar ::= ( NameChar - ':' ) | '/'
For the avoidance of doubt, this definition means a 'term' in RDFa is an XML NCName that also permits slash as a non-leading character.
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 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
IRI.
term
.
term
has
no
associated
IRI
and
must
be
ignored.
The general rules discussed in the previous sections apply to the RDFa attributes in the following ways:
Any value that matches a defined term must be expanded into a reference to the corresponding IRI. For example in [ XHTML-RDFA ] the following examples:
<link rel="next" href="http://example.org/page2.html" /> <link rel=" xhv:next " href="http://example.org/page2.html" />
would each generate the following triple:
<> <http://www.w3.org/1999/xhtml/vocab#next> <http://example.org/page2.html> .
In RDFa, it is possible to establish relationships using various types of resource references, including bnode s. If a subject or object is defined using a CURIE, and that CURIE explicitly names a bnode , then a Conforming Processor must create the bnode when it is encountered during parsing. The RDFa Processor must also ensure that no bnode created automatically (as a result of chaining ) has a name that collides with a bnode that is defined by explicit reference in a CURIE.
Consider the following example:
<link about="_:john" rel="foaf:mbox" href="mailto:john@example.org" /> <link about="_:sue" rel="foaf:mbox" href="mailto:sue@example.org" /> <link about="_:john" rel="foaf:knows" resource="_:sue" />
In the above fragment, two bnodes are explicitly created as the subject of triples. Those bnodes are then referenced to demonstrate the relationship between the parties. After processing, the following triples will be generated:
_:john foaf:mbox <mailto:john@example.org> . _:sue foaf:mbox <mailto:sue@example.org> . _:john foaf:knows _:sue .
RDFa Processors use, internally, implementation-dependent identifiers for bnodes. When triples are retrieved , new bnode indentifiers are used, which usually bear no relation to the original identifiers. However, implementations do ensure that these generated bnode identifiers are consistent: each bnode will have its own identifier, all references to a particular bnode will use the same identifier, and different bnodes will have different identifiers.
As
a
special
case,
_:
is
also
a
valid
reference
for
one
specific
bnode
.
Processing
would
normally
begin
after
the
document
to
be
parsed
has
been
completely
loaded.
However,
there
is
no
requirement
for
this
to
be
the
case,
and
it
is
certainly
possible
to
use
a
stream-based
approach,
such
as
SAX
[
SAX
]
to
extract
the
RDFa
information.
However,
if
some
approach
other
than
the
DOM
traversal
technique
defined
here
is
used,
it
is
important
to
ensure
that
Host
Language-specific
processing
rules
are
applied
(e.g.,
XHTML+RDFa
[
XHTML-RDFA
]
indicates
the
base
element
can
be
used,
and
base
will
affect
the
interpretation
of
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 IRI is provided by a resource attribute, then the first match from the following rules will apply:
if the element contains a @about , @href , @src , or @resource attribute, new subject is set to the IRI obtained from the first value from this set of attributes, obtained according to the section on CURIE and IRI Processing .
If no IRI is provided by a resource attribute, then the first match from the following rules will apply:
if @typeof is present, set the typed resource to the value of new subject .
if the @typeof attribute is present, set typed resource to new subject .
If no IRI is provided then the first match from the following rules will apply:
Then the current object resource is set to the IRI obtained from the first match from the following rules:
If @typeof is present and @about is not, set typed resource to current object resource .
Note that final value of the current object resource will either be null (from initialization) or a full 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 IRI, as described in [ RDF-CONCEPTS ], which will have been obtained according to the section on CURIE and 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 property value 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 output graph as long as it can be determined if a generated triple belongs in the processor graph or the 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 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 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
IRI.
For
this
class
of
RDFa
Processor,
the
software
must
allow
the
caller
to
specify
if
they
would
like
to
retrieve
the
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
output
,
processor
or
both
values,
in
any
order,
separated
by
a
comma
character.
If
the
graph
preference
is
not
specified,
the
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
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 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
(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 dc: <http://purl.org/dc/terms/> . @prefix ptr: <http://www.w3.org/2009/pointers#> . @prefix http: <http://www.w3.org/2006/http#> . [] 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 # 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 source file. ].
Processors may perform limited RDFS entailment rules to perform vocabulary expansion, as described in RDFa Vocabulary Expansion .
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
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> . <> 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><span property="dc:creator">Jo</span>'s blog</h1> <p> Welcome to my blog. </p> </body> </html>
which would still generate the triple:
<> 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="dc:creator" content="Jo" /> </head> <body> ... </body> </html>
An RDFa Processor should now generate the following triples, regardless of the IRI from which the document is served:
<http://www.example.org/jo/blog> foaf:primaryTopic <#bbq> . <http://www.example.org/jo/blog> dc:creator "Jo" .
As
processing
progresses,
any
@about
attributes
will
change
the
current
subject
.
The
value
of
@about
is
an
IRI
or
a
CURIE.
If
it
is
a
relative
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="dc:creator" content="Jo" /> </head> <body> <p about="#bbq" typeof="cal:Vevent"> I'm holding <span property="cal:summary"> one last summer barbecue </span>, on <span property="cal:dtstart" content="2015-09-16T16:00:00-05:00" datatype="xsd:dateTime"> September 16th at 4pm </span>. </p> </body> </html>
With this markup an RDFa Processor will generate the following triples:
<> foaf:primaryTopic <#bbq> . <> dc:creator "Jo" . <#bbq> rdf:type cal:Vevent . <#bbq> cal:summary "one last summer barbecue" . <#bbq> cal:dtstart "2015-09-16T16:00:00-05:00"^^xsd:dateTime .
Other kinds of resources can be used to set the current subject , not just references to web-pages. Although not advised, email addresses might be used to represent a person:
John knows <a about="mailto:john@example.org" rel="foaf:knows" href="mailto:sue@example.org">Sue</a>. Sue knows <a about="mailto:sue@example.org" rel="foaf:knows" href="mailto:jim@example.org">Jim</a>.
This should generate the following triples:
<mailto:john@example.org> foaf:knows <mailto:sue@example.org> . <mailto:sue@example.org> foaf:knows <mailto:jim@example.org> .
Similarly, authors may make statements about images:
<div about="photo1.jpg"> this photo was taken by <span property="dc:creator">Mark Birbeck</span> </div>
which should generate the following triples:
<photo1.jpg> dc:creator "Mark Birbeck" .
@typeof
defines
typing
triples.
@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.
The
question
is:
which
resource
gets
these
typing
information?
If the element has an @about , which creates a new context for statements, the typing relationships are defined on that resource. For example, the following:
<div about="http://dbpedia.org/resource/Albert_Einstein" typeof="foaf:Person"> <span property="foaf:name">Albert Einstein</span> <span property="foaf:givenName">Albert</span> </div>
<http://dbpedia.org/resource/Albert_Einstein> rdf:type foaf:Person .
The @about attribute is the main source for typing; if it is present on an element, it determines the effect of @typeof with the highest priority. If @about is not present, but the element is used only to define possible subject resources via, e.g., @resource (i.e., there is no @rel , @rev , or @property present), then that resource is used for the typed resource, just like @about .
If an @rel is present (and still no @about ) then the explicit object of the triples defined by @rel is typed. For example, in the case of:
<div about="http://dbpedia.org/resource/Albert_Einstein"> <div rel="dbp:birthPlace" resource="http://dbpedia.org/resource/German_Empire" typeof="http://schema.org/Country"> </div> </div>
the generated triples also include:
<http://dbpedia.org/resource/German_Empire> rdf:type <http://schema.org/Country> .
Finally, @typeof also has the additional feature of creating a new context for statements, in case no other attributes define any . This involves generating a new bnode (see below for more about bnodes). 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" .
This usage of @typeof is a shorthand for:
<div about="_:a" typeof="foaf:Person"> <span property="foaf:name">Albert Einstein</span> <span property="foaf:givenName">Albert</span> </div>
<div about="http://dbpedia.org/resource/Albert_Einstein"> <div rel="dbp:birthPlace" typeof="http://schema.org/Country"> <span property="dbp:conventionalLongName">the German Empire</span> </div> </div>
<http://dbpedia.org/resource/Albert_Einstein"> dbp:birthPlace _:b . _:b dbp:conventionalLongName "the German Empire" .
_: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 emphasized in the section on chaining , one of the main differences between @property and @rel (or @rev ) is that the former does not induce chaining. The only exception to this rule is when @typeof is also present on the element. In that case the effect of @property is identical to @rel . For example, the previous example could have been written as:
<div about="http://dbpedia.org/resource/Albert_Einstein"> <div property="dbp:birthPlace" typeof="http://schema.org/Country"> <span property="dbp:conventionalLongName">the German Empire</span> </div> </div>
generating the same triples as before.
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 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 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> dbp:dateOfBirth "1879-03-14"^^xsd:date .
However, an author could just as easily say that Spinoza influenced something by the name of Albert Einstein, that was born on March 14th, 1879 :
<div about="http://dbpedia.org/resource/Baruch_Spinoza" rel="dbp-owl:influenced"> <div> <span property="foaf:name">Albert Einstein</span> <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span> </div> </div>
In RDF terms, the item that 'represents' Einstein is anonymous , since it has no 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 .
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 German 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 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 . _: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 . _:b foaf:name "Arthur Schopenhauer" . _:b dbp:dateOfBirth "1788-02-22"^^xsd:date .
Triples are also 'completed' if any one of @property , @rel or @rev are present. However, unlike the situation when @about or @typeof are present, all predicates are attached to one bnode :
<div about="http://dbpedia.org/resource/Baruch_Spinoza" rel="dbp-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" .
but
processing
dbp:dateOfBirth
generates
only
one:
_:a dbp:dateOfBirth "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, 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 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 . Alternatively , the @property can also be used to define an IRI resource, in the presence of an @href , @resource , or @src and in the absence of @rel , @rev , @datatype , or @content .
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" />
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/> 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" >John Doe</span>
Literals can be given a data type using @datatype .
This can be represented in RDFa as follows:
<span property="cal:dtstart" content="2015-09-16T16:00:00-05:00"
datatype="xsd:dateTime">
September 16th at 4pm
</span>.
The triples that this markup generates include the datatype after the literal:
<> cal:dtstart "2015-09-16T16:00:00-05:00"^^ xsd:dateTime .
XML documents cannot contain XML markup in their attributes, which means it is not possible to represent XML within @content (the following would cause an XML parser to generate an error):
<head> <meta property="dc:title" 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:
<> dc:title "E = mc<sup>2</sup>: The Most Urgent Problem of Our Time"^^rdf:XMLLiteral .
This
requires
that
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"
.
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 IRI object s are needed when @rel , @rev , or @property 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" /> </head> <body> <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> .
Note that, in the example above, @property could have been used instead of @rel , yielding the same triple.
If no @resource is present, then @href or @src are next in priority order for setting the object.
When a predicate has been expressed using @rel , the @href or @src on the RDFa statement's element is used to identify the object with a IRI reference . Their types are an IRI:
<link about="mailto:john@example.org"
rel="foaf:knows"
href="mailto:sue@example.org"
/>
It's also possible to use both @rel and @rev at the same time on an element. This is particularly useful when two things stand in two different relationships with each other, for example when a picture is taken by Mark, but that picture also depicts him:
<img 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="" property="dc:creator" resource="http://ben.adida.net/#me">Ben Adida</span>, <span inlist="" property="dc:creator">Mark Birbeck</span>, and <span inlist="" property="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> ) . ... ]
In the example above, @rel could have been used leading exactly to the same triples:
<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>
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 permits Host Languages to define an initial context . Such a context is a collection of terms, prefix mappings, and/or default vocabulary declarations. An initial context is either intrinsically known to the parser, or it is loaded as external documents 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 ]). When an initial context document is 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
this
mapping
to
the
list
of
IRI
mappings
of
the
initial
evaluation
context
,
after
transforming
the
'prefix'
component
to
lower-case.
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
this
mapping
to
the
term
mappings
of
the
initial
evaluation
context
.
rdfa:vocabulary
,
define
the
default
vocabulary
of
the
initial
evaluation
context
to
be
the
object
literal
of
the
rdfa:vocabulary
predicate.
When an RDFa Initial Context is defined using an RDF serialization, it must use the vocabulary terms above to declare the components of the context.
Caching of the relevant triples retrieved via this mechanism is recommended . Embedding definitions for well known, stable RDFa Initial Contexts in the implementation is recommended .
The
object
literal
for
the
rdfa:uri
predicate
must
be
an
absolute
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
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
create
the
associated
mapping.
Since RDFa is 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 Vocabulary Expansion is an optional processing step which may be added once the normal processing steps described in Processing Model are complete. Vocabulary expansion relies on a very small sub-set of OWL entailment [ OWL2-OVERVIEW ] to add triples to the output graph based on rules and property/class relationships described in referenced vocabularies. Vocabulary expansion may be performed as part of a larger RDF toolset including, for example, an OWL 2 RL reasoner. Alternatively, using vocabulary data added to the output graph in processing step 2 of Sequence , expansion may also be done using a separate and dedicated (e.g., rule based) reasoner after the output graph has been generated, or as the last processing step by an RDFa processor.
It
can
be
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
extend
well-known
vocabularies
to
have
those
properties
added
to
the
output
graph
automatically.
For
example,
the
namespace
document
of
the
Creative
Commons
vocabulary,
i.e.,
http://creativecommons.org/ns
,
defines
cc:license
to
be
a
sub-property
of
dc:license
.
By
using
the
@vocab
attribute,
one
can
describe
a
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:usesVocabulary <http://creativecommons.org/ns#> .
After vocabulary expansion, the output graph contains:
@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:usesVocabulary <http://creativecommons.org/ns#> .
Other vocabularies, specifically intended to provide relations to multiple vocabularies, could also be 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.
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 Processors .
A
vocabulary
graph
is
created
as
follows:
For
each
IRI
being
the
object
of
a
triple
in
the
output
graph
with
the
subject
being
the
current
document
(
base
)
IRI
and
the
property
being
rdfa:usesVocabulary
,
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
accept
other
standard
serialization
formats
of
RDF
such
as
RDF/XML
[
RDF-SYNTAX-GRAMMAR
]
and
Turtle
[
TURTLE
].)
Note that if, in the second step, a particular vocabulary is serialized in RDFa, that particular graph is not expected to undergo any vocabulary expansion on its own.
Vocabulary expansion is then performed as follows:
For the purpose of vocabulary processing, RDFa used a very restricted subset of the OWL vocabulary and is based on the RDF-Based Semantics of OWL[ OWL2-RDF-BASED-SEMANTICS ]. The RDFa Vocabulary Entailment uses the following terms:
and
it
considers
only
the
entailment
on
individuals
(i.e.,
not
on
the
relationships
that
can
be
deduced
on
the
properties
or
the
classes
themselves.)
While
the
formal
definition
of
the
RDFa
Entailment
refers
to
the
general
OWL
2
Semantics,
practical
implementations
may
rely
on
a
subset
of
the
OWL
2
RL
Profile’s
entailment
expressed
in
rules
(
section
4.3
of
[
OWL2-PROFILES
]).
In
particular,
the
relevant
rules
are
(using
the
rule
identifications
in
section
4.3
of
[
OWL2-PROFILES
]):
prp-spo1
,
prp-eqp1
,
prp-eqp2
,
cax-sco
,
cax-eqc1
,
and
cax-eqc2
.
rdfs:range
of
a
property,
or
use
the
advanced
facilities
offered
by,
e.g.,
OWL’s
property
chains
to
interlink
vocabularies
further.
Conforming RDFa processors are NOT required to provide vocabulary expansion.
If
an
RDFa
processor
provides
vocabulary
expansion,
it
must
not
be
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"
file
for
their
schema
grammar:
Specifically, the following datatypes are defined:
This section is non-normative.
The following informative XML Schema definition for these datatypes is included as an example:
<?xml version="1.0" encoding="UTF-8"?> <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns="http://www.w3.org/1999/xhtml/datatypes/" xmlns:xh11d="http://www.w3.org/1999/xhtml/datatypes/" targetNamespace="http://www.w3.org/1999/xhtml/datatypes/" elementFormDefault="qualified" > <xs:simpleType name="CURIE"> <xs:restriction base="xs:string"> <xs:pattern value="(([\i-[:]][\c-[:]]*)?:)?.+" /> <xs:minLength value="1"/> </xs:restriction> </xs:simpleType> <xs:simpleType name="CURIEs"> <xs:list itemType="xh11d:CURIE"/> </xs:simpleType> <xs:simpleType name="SafeCURIE"> <xs:restriction base="xs:string"> <xs:pattern value="\[(([\i-[:]][\c-[:]]*)?:)?.+\]" /> <xs:minLength value="3"/> </xs:restriction> </xs:simpleType> <xs:simpleType name="SafeCURIEs"> <xs:list itemType="xh11d:SafeCURIE"/> </xs:simpleType> <xs:simpleType name="TERM"> <xs:restriction base="xs:Name"> <xs:pattern value="[\i-[:]][/\c-[:]]*" /> </xs:restriction> </xs:simpleType> <xs:simpleType name="CURIEorIRI"> <xs:union memberTypes="xh11d:CURIE xs:anyURI" /> </xs:simpleType> <xs:simpleType name="CURIEorIRIs"> <xs:list itemType="xh11d:CURIEorIRI"/> </xs:simpleType> <xs:simpleType name="SafeCURIEorCURIEorIRI"> <xs:union memberTypes="xh11d:SafeCURIE xh11d:CURIE xs:anyURI" /> </xs:simpleType> <xs:simpleType name="SafeCURIEorCURIEorIRIs"> <xs:list itemType="xh11d:SafeCURIEorCURIEorIRI"/> </xs:simpleType> <xs:simpleType name='AbsIRI'> <xs:restriction base='xs:string'> <xs:pattern value="[\i-[:]][\c-[:]]+:.+" /> </xs:restriction> </xs:simpleType> <xs:simpleType name="TERMorCURIEorAbsIRI"> <xs:union memberTypes="xh11d:TERM xh11d:CURIE xh11d:AbsIRI" /> </xs:simpleType> <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 % TERMorCURIEorAbsIRIs.datatype "CDATA" >
The
RDFa
Vocabulary
has
three
roles:
it
contains
the
predicates
to
define
the
terms
and
prefixes
in
initial
context
documents,
it
contains
the
classes
and
predicates
for
the
messages
that
a
processor
graph
may
contain
and,
finally,
it
contains
the
predicate
necessary
for
vocabulary
processing.
The
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 ; 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 dc:description "is the class for term mappings" . rdfs:subClassOf rdfa:PrefixOrTermMapping . rdfa:prefix a rdf:Property, owl:DatatypeProperty ; rdfs:domain rdfa:PrefixMapping ; 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 ; 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 ; 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 ; dc: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’.""" .
These predicates can be used to define the initial context for a given Host Language.
These predicates are used to 'pair' 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" .
which defines a prefix for the foaf IRI.
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; dc:description "is the top level class of the hierarchy" . 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"; rdfs:subClassOf rdfa:PGClass . rdfa:Info dcterms:description "is the class for all informations"; rdfs:subClassOf rdfa:PGClass . 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: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: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; 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:usesVocabulary 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: