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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.
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.
This version reflects changes made as a result of comments received during the first Last Call period. That document was widely reviewed and the Working Group has made a number of improvements and clarifications as a result. The Working Group believes that all comments received during the last review have been addressed.
This is a revision of RDFa Syntax 1.0 [RDFA-SYNTAX]. Once development is complete, if accepted by the W3C membership, this document will supersede the previous Recommendation. There are a number of substantive differences between this version and its predecessor, including:
A sample test harness is available. This set of tests is not intended to be exhaustive. Users may find the tests to be useful examples of RDFa usage.
This document was published by the RDFa Working Group as a Last Call Working Draft. This document is intended to become a W3C Recommendation. If you wish to make comments regarding this document, please send them to public-rdfa-wg@w3.org (subscribe, archives). The Last Call period ends 21 April 2011. All feedback is welcome.
Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
This is a Last Call Working Draft and thus the Working Group has determined that this document has satisfied the relevant technical requirements and is sufficiently stable to advance through the Technical Recommendation process.
This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.
This section is non-normative.
RDF/XML [RDF-SYNTAX] provides sufficient flexibility to represent all of the abstract concepts in RDF [RDF-CONCEPTS]. However, it presents a number of challenges; first it is difficult or impossible to validate documents that contain RDF/XML using XML Schemas or DTDs, which therefore makes it difficult to import RDF/XML into other markup languages. Whilst newer schema languages such as RELAX NG [RELAXNG-SCHEMA] do provide a way to validate documents that contain arbitrary RDF/XML, it will be a while before they gain wide support.
Second, even if one could add RDF/XML directly into an XML dialect like XHTML, there would be significant data duplication between the rendered data and the RDF/XML structured data. It would be far better to add RDF to a document without repeating the document's existing data. For example, an XHTML document that explicitly renders its author's name in the text—perhaps as a byline on a news site—should not need to repeat this name for the RDF expression of the same concept: it should be possible to supplement the existing markup in such a way that it can also be interpreted as RDF.
Another reason for aligning the rendered data with the structured data is that it is highly beneficial to express the web data's structure 'in context'; as users often want to transfer structured data from one application to another, sometimes to or from a non-web-based application, the user experience can be enhanced. For example, information about specific rendered data could be presented to the user via 'right-clicks' on an item of interest.
In the past, many attributes were 'hard-wired' directly into the markup language to represent specific concepts. For example, in XHTML 1.1 [XHTML11] and HTML [HTML401] there is @cite; the attribute allows an author to add information to a document which is used to indicate the origin of a quote.
However, these 'hard-wired' attributes make it difficult to define a generic process for extracting metadata from any document since an RDFa Processor would need to know about each of the special attributes. One motivation for RDFa has been to devise a means by which documents can be augmented with metadata in a general, rather than hard-wired, manner. This has been achieved by creating a fixed set of attributes and parsing rules, but allowing those attributes to contain properties from any of a number of the growing range of available RDF vocabularies. In most cases the values of those properties are the information that is already in an author's document.
RDFa alleviates the pressure on markup language designers to anticipate all the structural requirements users of their language might have, by outlining a new syntax for RDF that relies only on attributes. By adhering to the concepts and rules in this specification, language designers can import RDFa into their environment with a minimum of hassle and be confident that semantic data will be extractable from their documents by conforming processors.
This section is non-normative.
The following examples are intended to help readers who are not familiar with RDFa to quickly get a sense of how it works. For a more thorough introduction, please read the RDFa Primer [RDFA-PRIMER].
In RDF, it is common for people to shorten vocabulary terms via abbreviated URIs 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/ |
dcterms: | http://purl.org/dc/terms/ |
ex: | http://example.org/ |
foaf: | http://xmlns.com/foaf/0.1/ |
rdf: | http://www.w3.org/1999/02/22-rdf-syntax-ns# |
rdfa: | http://www.w3.org/ns/rdfa# |
rdfs: | http://www.w3.org/2000/01/rdf-schema# |
xhv: | http://www.w3.org/1999/xhtml/vocab# |
xsd: | http://www.w3.org/2001/XMLSchema# |
In some of the examples below we have used URIs 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 URIs for
entities described by RDFa and therefore contributes considerably to
the expressive power of RDFa. However, the media type registrations
that govern the meaning of fragment identifiers (see section 3.5 of
the URI specification [RFC3986], [RFC3023], and [RFC2854]) have not yet
caught up with this practice.
For more information about fragment identifier semantics, see [WEBARCH] section 3.2.1.
RDFa makes use of a number of commonly found attributes, as well as providing a few new ones. Attributes that already exist in widely deployed languages (e.g., HTML) have the same meaning they always did, although their syntax has been slightly modified in some cases. For example, in (X)HTML there is no clear way to add new @rel values; RDFa sets out to explicitly solve this problem, and does so by allowing URIs as values. It also introduces the concepts of terms and 'compact URI expressions' — referred to as CURIEs in this document — which allow a full URI value to be expressed succinctly. For a complete list of RDFa attribute names and syntax, see Attributes and Syntax.
In HTML, authors can include metadata and relationships concerning the
current document by using the meta
and link
elements. For example, the
author of the page along with the pages preceding and following the
current page can be expressed using the link and meta elements:
<html xmlns="http://www.w3.org/1999/xhtml"> <head> <title>Page 7</title> <meta name="author" content="Mark Birbeck" /> <link rel="prev" href="page6.html" /> <link rel="next" href="page8.html" /> </head> <body>...</body> </html>
RDFa makes use of this concept, enhancing it with the ability to make use of other vocabularies by using full URIs:
<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 URIs 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/ dcterms: http://purl.org/dc/terms/" > <head> <title>My home-page</title> <meta property="dcterms:creator" content="Mark Birbeck" /> <link rel="foaf:topic" href="http://www.example.com/#us" /> </head> <body>...</body> </html>
RDFa supports the use of @rel and @rev on any element. This is even more useful with the addition of support for different vocabularies:
This document is licensed under the <a prefix="cc: http://creativecommons.org/ns#" rel="cc:license" href="http://creativecommons.org/licenses/by-nc-nd/3.0/"> Creative Commons By-NC-ND License </a>.
Not only can URIs 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/ dcterms: http://purl.org/dc/terms/" > <head> <title>Books by Marco Pierre White</title> </head> <body> I think White's book '<span about="urn:ISBN:0091808189" typeof="bibo:Book" property="dcterms:title"> Canteen Cuisine </span>' is well worth getting since although it's quite advanced stuff, he makes it pretty easy to follow. You might also like <span about="urn:ISBN:1596913614" typeof="bibo:Book" property="dcterms:description"> White's autobiography </span>. </body> </html>
When dealing with small amounts of markup, it is sometimes easier to use full URIs, rather than CURIEs. The previous example can also be written as follows:
<html xmlns="http://www.w3.org/1999/xhtml"> <head> <title>Books by Marco Pierre White</title> </head> <body> I think White's book '<span about="urn:ISBN:0091808189" typeof="http://purl.org/ontology/bibo/Book" property="http://purl.org/dc/terms/title" >Canteen Cuisine</span>' is well worth getting since although it's quite advanced stuff, he makes it pretty easy to follow. You might also like <span about="urn:ISBN:1596913614" typeof="http://purl.org/ontology/bibo/Book" property="http://purl.org/dc/terms/description" >White's autobiography</span>. </body> </html>
A simple way of defining a portion of a document using terms from a specific vocabulary is to use @vocab to define a default vocabulary URI. 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/> .
RDFa also permits external definition of collections of prefixes.
The following (mythical) example RDFa Profile document, with a URI of http://www.example.org/vocab-rdf-dc.html
, defines
some standard RDF prefixes as well as the FOAF and Dublin Core vocabulary prefixes in RDFa.
<html xmlns="http://www.w3.org/1999/xhtml" prefix="rdfa: http://www.w3.org/ns/rdfa#"> <head> ... </head> <body> <p>This is an example to defining the standard RDF and Dublin Core prefixes </p> <p typeof="rdfa:PrefixMapping"> The "<span property="rdfa:prefix">rdf</span>" prefix can be used for the URI: "<span property="rdfa:uri">http://www.w3.org/1999/02/22-rdf-syntax-ns#</span>".</p> <p typeof="rdfa:PrefixMapping"> The "<span property="rdfa:prefix">rdfs</span>" prefix can be used for the URI: "<span property="rdfa:uri">http://www.w3.org/2000/01/rdf-schema#</span>".</p> <p typeof="rdfa:PrefixMapping"> The "<span property="rdfa:prefix">dcterms</span>" prefix can be used for the URI: "<span property="rdfa:uri">http://purl.org/dc/terms/</span>".</p> <p typeof="rdfa:PrefixMapping"> The "<span property="rdfa:prefix">foaf</span>" prefix can be used for the URI: "<span property="rdfa:uri">http://xmlns.com/foaf/0.1/</span>".</p> </body> </html>
The same content in Turtle is:
@prefix rdfa: <rdfa: http://www.w3.org/ns/rdfa#> . @prefix dcterms: <http://purl.org/dc/terms/> . [] a rdfa:PrefixMapping ; rdfa:prefix "rdf" ; rdfa:uri "http://www.w3.org/1999/02/22-rdf-syntax-ns#" . [] a rdfa:PrefixMapping rdfa:prefix "rdfs" ; rdfa:uri "http://www.w3.org/2000/01/rdf-schema#" . [] a rdfa:PrefixMapping ; rdfa:prefix "dcterms" ; rdfa:uri "http://purl.org/dc/terms/" . [] a rdfa:PrefixMapping ; rdfa:prefix "foaf" ; rdfa:uri "http://xmlns.com/foaf/0.1/" .
Using @profile, the following RDFa snippet:
<p about="http://www.example.org/doc"
profile="http://www.example.org/vocab-rdf-dc.html">
<span property="dcterms:title">title of the document</span>
<span property="rdfs:comment">and this is a longer comment
on the same document</span>
</p>
would yield the following triples:
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> . @prefix dcterms: <http://purl.org/dc/terms/> . <http://www.example.org/doc> dcterms:title "title of the document" ; rdfs:comment "and this is a longer comment on the same document" .
It is also possible to define terms. Given the following RDFa Profile document at http://www.example.org/vocab-foaf-terms.html
:
<html xmlns="http://www.w3.org/1999/xhtml" prefix="rdfa: http://www.w3.org/ns/rdfa#"> <head> ... </head> <body> <p> This is an example RDFa vocabulary that makes it easier to use the foaf:name and foaf:homepage terms. </p> <p typeof="rdfa:TermMapping"> The "<span property="rdfa:term">name</span>" term can be used for the URI: "<span property="rdfa:uri">http://xmlns.com/foaf/0.1/name</span>".</p> <p typeof="rdfa:TermMapping"> The "<span property="rdfa:term">homepage</span>" term can be used for the URI: "<span property="rdfa:uri">http://xmlns.com/foaf/0.1/homepage</span>".</p> </body> </html>
and the following HTML markup:
<div profile="http://www.example.org/vocab-foaf-terms.html" about="#me"> My name is <span property="name">John Doe</span> and my blog is called <a rel="homepage" href="http://example.org/blog/">Understanding Semantics</a>. </div>
the following triples will be generated:
@prefix foaf: <http://xmlns.com/foaf/0.1/> . <#me> foaf:name "John Doe" ; foaf:homepage <http://example.org/blog/> .
This section is non-normative.
The previous section gave examples of typical markup in order to illustrate the structure of RDFa markup. 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 URI references.
URIs are most commonly used to identify web pages, but RDF makes use of them as a way to provide unique identifiers for concepts. For example, we could identify the subject of all of our statements (the first part of each triple) by using the DBPedia [http://dbpedia.org] URI for Albert Einstein, instead of the ambiguous string 'Albert':
<http://dbpedia.org/resource/Albert_Einstein> has the name Albert Einstein. <http://dbpedia.org/resource/Albert_Einstein> was born on March 14, 1879. <http://dbpedia.org/resource/Albert_Einstein> was born in the German Empire. <http://dbpedia.org/resource/Albert_Einstein> has a picture at http://en.wikipedia.org/wiki/Image:Albert_Einstein_Head.jpg.
URI references are also used to uniquely identify the objects in metadata statements (the third part of each triple). The picture of Einstein is already a URI, but we could also use a URI to uniquely identify the country 'German Empire'. At the same time we'll indicate that the name and date of birth really are literals (and not URIs), by putting quotes around them:
<http://dbpedia.org/resource/Albert_Einstein> has the name "Albert Einstein". <http://dbpedia.org/resource/Albert_Einstein> was born on "March 14, 1879". <http://dbpedia.org/resource/Albert_Einstein> was born in <http://dbpedia.org/resource/German_Empire>. <http://dbpedia.org/resource/Albert_Einstein> has a picture at <http://en.wikipedia.org/wiki/Image:Albert_Einstein_Head.jpg>.
URI references are also used to ensure that predicates are unambiguous; now we can be sure that 'birthplace', 'place of birth', 'Lieu de naissance' and so on, all mean the same thing:
<http://dbpedia.org/resource/Albert_Einstein> <http://xmlns.com/foaf/0.1/name> "Albert Einstein". <http://dbpedia.org/resource/Albert_Einstein> <http://dbpedia.org/property/dateOfBirth> "March 14, 1879". <http://dbpedia.org/resource/Albert_Einstein> <http://dbpedia.org/property/birthPlace> <http://dbpedia.org/resource/German_Empire>. <http://dbpedia.org/resource/Albert_Einstein> <http://xmlns.com/foaf/0.1/depiction> <http://en.wikipedia.org/wiki/Image:Albert_Einstein_Head.jpg>.
Although URI resources are always used for subjects and predicates, the object part of a triple can be either a URI or a literal. In the example triples, Einstein's name is represented by a plain literal, which means that it is a basic string with no type or language information:
<http://dbpedia.org/resource/Albert_Einstein>
<http://xmlns.com/foaf/0.1/name> "Albert Einstein".
Some literals, such as dates and numbers, have very specific meanings, so RDF provides a mechanism for indicating
the type of a literal. A typed literal is indicated
by attaching a URI to the end of a plain literal, and this URI indicates the literal's datatype. This URI is usually based on
datatypes defined in the
XML Schema Datatypes specification [XMLSCHEMA-2]. The following
syntax would be used to
unambiguously express Einstein's date of birth as a literal of
type http://www.w3.org/2001/XMLSchema#date
:
<http://dbpedia.org/resource/Albert_Einstein>
<http://dbpedia.org/property/dateOfBirth>
"1879-03-14"^^<http://www.w3.org/2001/XMLSchema#date>.
RDF itself does not have one set way to express triples, since the key ideas of RDF are the triple and the use of URIs, and not any particular syntax. However, there are a number of mechanisms for expressing triples, such as RDF/XML [RDF-SYNTAX-GRAMMAR], Turtle [TURTLE], and of course RDFa. Many discussions of RDF make use of the Turtle syntax to explain their ideas, since it is quite compact. The examples we have just seen are already using this syntax, and we'll continue to use it throughout this document when we need to talk about the RDF that could be generated from some RDFa. Turtle allows long URIs to be abbreviated by using a URI mapping, which can be used to express a compact URI 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 URI for DBPedia and 'foaf:' has been mapped to the URI for the 'Friend of a Friend' taxonomy.
Any URI in Turtle could be abbreviated in this way. This means that we could also have used the same technique to abbreviate the identifier for Einstein, as well as the datatype indicator:
@prefix dbp: <http://dbpedia.org/property/> . @prefix dbr: <http://dbpedia.org/resource/> . @prefix foaf: <http://xmlns.com/foaf/0.1/> . @prefix xsd: <http://www.w3.org/2001/XMLSchema#> . dbr:Albert_Einstein dbp:dateOfBirth "1879-03-14"^^xsd:date . dbr:Albert_Einstein foaf:depiction <http://en.wikipedia.org/wiki/Image:Albert_Einstein_Head.jpg> .
When writing examples, you will often see the following URI in the Turtle representation:
<>
This indicates the 'current document', i.e., the document being processed. In reality there would always be a full URI based on the document's location, but this abbreviation serves to make examples more compact. Note in particular that the whole technique of abbreviation is merely a way to make examples more compact, and the actual triples generated would always use the full URIs.
A collection of triples is called a graph. All of the triples that are defined by this specification are contained in the default graph by an RDFa Processor. For more information on graphs and other RDF concepts, see [RDF-CONCEPTS].
In order to allow for the compact expression of RDF statements, RDFa allows the contraction of most URI references into a form called a 'compact URI expression', or CURIE. A detailed discussion of this mechanism is in the section CURIE and URI Processing.
Note that CURIEs are only used in the markup and Turtle examples, and will never appear in the generated triples, which are defined by RDF to use URI references.
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 nodes linked by relationships. The basic unit of an RDF graph is a triple, in which a subject node is linked to an object node via a predicate. The subject node is always either a URI reference or a blank node (or bnode), the predicate is always a URI reference, and the object of a statement can be a URI reference, a literal, or a bnode.
In RDFa, a subject URI reference is generally indicated using @about or @src, and predicates are represented using one of @property, @rel, or @rev. Objects which are URI references are represented using @resource, or @href, whilst objects that are literals are represented either with @content or the content of the element in question (with an optional datatype expressed using @datatype, and an optional language expressed using a Host Language-defined mechanism such as xml:lang).
As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.
The key words must, must not, required, should, should not, recommended, may, and optional in this specification are to be interpreted as described in [RFC2119].
A conforming RDFa Processor must make available to a consuming application a single RDF graph containing all possible triples generated by using the rules in the Processing Model section. This specification uses the term default graph to mean all of the triples asserted by a document according to the Processing Model section. The term processor graph is used to denote the collection of all informational, warning, and error triples that may be generated by the RDFa Processor to report its status. The default graph and the processor graph are separate graphs and must not be stored in the same graph by the RDFa Processor.
A conforming RDFa Processor may make available additional triples that have been generated using rules not described here, but these triples must not be made available in the default graph. (Whether these additional triples are made available in one or more additional RDF graphs is implementation-specific, and therefore not defined here.)
A conforming RDFa Processor must preserve white space in both plain literals 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 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">
).Maintainers of Host Languages are required to change the URI of a default profile if items are removed from the default profile document. The URI change is required to accommodate RDFa Processors that statically embed the terms defined in the profile. The working group expects default RDFa Profiles to change very rarely.
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:
When an RDFa Processor processes an XML+RDFa document, it does so in the following context:
http://www.w3.org/profile/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
Many attributes accept a white space separated list of tokens. This specification defines white space as:
whitespace ::= (#x20 | #x9 | #xD | #xA)+
When attributes accept a white space separated list of tokens, an RDFa Processor must ignore any leading or trailing white space.
This definition is consistent with the definition found in [XML10].
The key component of RDF is the URI, but these are usually long and unwieldy. RDFa therefore supports a mechanism by which URIs can be abbreviated, called 'compact URI expressions' or simply, CURIEs.
When expanded, the resulting URI must be a syntactically valid URI [RFC3987]. For a more detailed explanation see CURIE and URI Processing. The lexical space of a CURIE is as defined in curie below. The value space is the set of URIs.A CURIE is comprised of two components, a prefix and a
reference. The prefix is separated from the reference by a colon
(:
). In general use it is possible to omit the prefix, and so create a CURIE that makes use of the
'default prefix' mapping; in RDFa the 'default prefix' mapping is http://www.w3.org/1999/xhtml/vocab#
.
It's also possible to omit both the prefix and the colon, and so create a CURIE that contains
just a reference which makes use of the 'no prefix' mapping. This specification
does not define a 'no prefix' mapping. RDFa Host Languages must not define a 'no prefix' mapping.
The RDFa 'default prefix' should not be confused with the 'default namespace' as defined in [XML-NAMES]. An RDFa Processor must not treat an XML-NAMES 'default namespace' declaration as if it were setting the 'default prefix'.
The general syntax of a CURIE can be summarized as follows:
prefix ::= NCName reference ::= irelative-ref (as defined in [RFC3987]) curie ::= [ [ prefix ] ':' ] reference safe_curie ::= '[' [ [ prefix ] ':' ] reference ']'
The production safe_curie
is not required, even in situations
where an attribute value is permitted to be a CURIE or a URI:
A URI that uses a scheme that is not an in-scope
mapping cannot be confused with a CURIE. The concept of a
safe_curie is retained for backward compatibility.
In normal evaluation of CURIEs the following context information would need to be provided:
:p
);p
);_:p
). In RDFa these values are defined as follows:
A CURIE is a representation of a full URI. The rules for determining that URI are:
prefix
and a reference
,
the URI is obtained by taking the current default prefix mapping and concatenating
it with the reference
. If there is no current default prefix
mapping, then this is not a valid CURIE and must be ignored.
prefix
and reference
,
and if there is an in-scope mapping for prefix
(when compared case-insensitively), then the URI is created
by using that mapping, and concatenating it with the
reference
. prefix
, then the value is not a CURIE.
See General Use of Terms in Attributes for the way items with no colon can be interpreted in some datatypes by RDFa Processors.
This section is non-normative.
In many cases, language designers have attempted to use QNames for an extension mechanism [XMLSCHEMA-2]. QNames do permit independent management of the name collection, and can map the names to a resource. Unfortunately, QNames are unsuitable in most cases because 1) the use of QName as identifiers in attribute values and element content is problematic as discussed in [QNAMES] and 2) the syntax of QNames is overly restrictive and does not allow all possible URIs 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 a URI that will reveal the meaning of that ISBN. As you can see, the definition of QNames and this (relatively common) use case are in conflict with one another.
This specification addresses the problem by defining CURIEs. Syntactically, CURIEs are a superset of QNames.
Note that this specification is targeted at language designers, not document authors. Any language designer considering the use of QNames as a way to represent URIs or unique tokens should consider instead using CURIEs:
This section looks at a generic set of processing rules for creating a set of triples that represent the structured data present in an RDFa document. Processing need not follow the DOM traversal technique outlined here, although the effect of following some other manner of processing must be the same as if the processing outlined here were followed. The processing model is explained using the idea of DOM traversal which makes it easier to describe (particularly in relation to the evaluation context).
Evaluating a document for RDFa triples is carried out by starting at the document object, and then visiting each of its child elements in turn, in document order, applying processing rules. Processing is recursive in that for each child element the processor also visits each of its child elements, and applies the same processing rules.
In some environments there will be little difference between starting at the root element of the document, and starting at the document object itself. It is defined this way because in some environments important information is present at the document object level which is not present on the root element.
As processing continues, rules are applied which may generate triples, and may also change the evaluation context information that will then be used when processing descendant elements.
This specification does not say anything about what should happen to the triples generated, or whether more triples might be generated during processing than are outlined here. However, to be conforming, an RDFa Processor must act as if at a minimum the rules in this section are applied, and a single RDF graph produced. As described in the RDFa Processor Conformance section, any additional triples generated must not appear in the default graph.
During processing, each rule is applied using information provided by an evaluation context. An initial context is created when processing begins. That context has the following members:
base
element. The important thing is that it
establishes a URI against which relative paths can be resolved.
During the course of processing, new evaluation contexts are created which are passed to each child element. The rules described below will determine the values of the items in the context. Additionally, some rules will cause new triples to be created by combining information provided by an element with information from the evaluation context.
During the course of processing a number of locally scoped values are needed, as follows:
Statement chaining is an RDFa feature that allows the author to link RDF statements together while avoiding unnecessary repetitive markup. For example, if an author were to add statements as children of an object that was a resource, these statements should be interpreted as being about that resource:
<div about="http://dbpedia.org/resource/Albert_Einstein"> <span property="foaf:name">Albert Einstein</span> <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span> <div rel="dbp:birthPlace" resource="http://dbpedia.org/resource/German_Empire"> <span property="dbp:conventionalLongName">the German Empire</span> </div> </div>
In this example we can see that an object resource ('German_Empire'), has become the subject for nested statements. This markup also illustrates the basic chaining pattern of 'A has a B has a C' (i.e., Einstein has a birth place of the German Empire, which has a long name of "the 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>
Since RDFa is ultimately a means for transporting RDF, a key concept is the resource and its manifestation as a URI. RDF deals with complete URIs (not relative paths); when converting RDFa to triples, any relative URIs must be resolved relative to the base URI, using the algorithm defined in section 5 of RFC 3986 [URI], Reference Resolution. The values of RDFa attributes that refer to URIs use three different datatypes: URI, SafeCURIEorCURIEorURI, or TERMorCURIEorAbsURI. All these attributes are mapped, after processing, to URIs. The handling of these attributes is as follows:
Note that it is possible for all values in an attribute to be ignored. When that happens, the attribute must be treated as if it were empty.
For example, the full URI for Albert Einstein on DBPedia is:
http://dbpedia.org/resource/Albert_Einstein
This can be shortened by authors to make the information easier to manage, using a CURIE. The first step is for the author to create a prefix mapping that links a prefix to some leading segment of the URI. In RDFa these mappings are expressed using the XML namespace syntax:
<div prefix="db: http://dbpedia.org/">
...
</div>
Once the prefix has been established, an author can then use it to shorten a URI as follows:
<div prefix="db: http://dbpedia.org/">
<div about="db:resource/Albert_Einstein">
...
</div>
</div>
The author is free to split the URI at any point, as long as it begins at the left end. However, since a common use of CURIEs is to make available libraries of terms and values, the prefix will usually be mapped to some common segment that provides the most re-use, often provided by those who manage the library of terms. For example, since DBPedia contains an enormous list of resources, it is more efficient to create a prefix mapping that uses the base location of the resources:
<div prefix="dbr: http://dbpedia.org/resource/"> <div about="dbr:Albert_Einstein"> ... </div> <div about="dbr:Baruch_Spinoza"> ... </div> </div>
CURIE prefix mappings are defined on the current element and its descendants. The inner-most mapping for a given prefix takes precedence. For example, the URIs expressed by the following two CURIEs are different, despite the common prefix, because the prefix mappings are locally scoped:
<div prefix="dbr: http://dbpedia.org/resource/"> <div about="dbr:Albert_Einstein"> ... </div> </div> <div prefix="dbr: http://someotherdb.org/resource/"> <div about="dbr:Albert_Einstein"> ... </div> </div>
There are a number of ways that attributes make use of CURIEs, and they need to be dealt with differently. These are:
An empty attribute value (e.g., typeof=''
)
is still a CURIE, and is processed as such. The rules for
this processing are defined in Sequence.
Specifically, however, an empty attribute value is never treated
as a relative URI by this specification.
An example of an attribute that can contain a CURIEorURI is @about. To express a URI directly, an author might do this:
<div about="http://dbpedia.org/resource/Albert_Einstein">
...
</div>
whilst to express the URI 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 TERMs, and the CURIE has no prefix separator.
<div about="[Albert_Einstein]">
...
</div>
However, this markup would set a subject, since it is not a CURIE, but a valid relative URI:
<div about="Albert_Einstein">
...
</div>
Note that several RDFa attributes are able to also take TERMS as their value. This is discussed in the next section.
Some RDFa attributes have a datatype that permits a term to be referenced. RDFa defines the syntax of a term as:
term ::= NCName
When an RDFa attribute permits the use of a term, and the value being evaluated matches the production for term above, it is transformed to a URI using the following logic:
term
matches
an item in the list of local term mappings. First compare against the list
case-sensitively, and if there is no match then compare case-insensitively. If there is a match,
use the associated URI.term
.term
has no associated URI and must be ignored.In the event that multiple term
s are defined that differ only in case (e.g., 'Agent', 'agent', and 'AGENT'), if a reference is made which DOES NOT match case-sensitively (e.g., typeof='AGENt'), the results are UNSPECIFIED.
The general rules discussed in the previous sections apply to the RDFa attributes in the following ways:
Any value that matches a defined term must be expanded into a reference to the corresponding URI. For example in [XHTML-RDFA] the following examples:
<link rel="next" href="http://example.org/page2.html" /> <link rel="xhv:next" href="http://example.org/page2.html" />
would each generate the following triple:
<> <http://www.w3.org/1999/xhtml/vocab#next> <http://example.org/page2.html> .
In RDFa, it is possible to establish relationships using various types of resource references, including bnodes. If a subject or object is defined using a CURIE, and that CURIE explicitly names a bnode, then a Conforming Processor must create the bnode when it is encountered during parsing. The RDFa Processor must also ensure that no bnode created automatically (as a result of chaining) has a name that collides with a bnode that is defined by explicit reference in a CURIE.
Consider the following example:
<link about="_:john" rel="foaf:mbox" href="mailto:john@example.org" /> <link about="_:sue" rel="foaf:mbox" href="mailto:sue@example.org" /> <link about="_:john" rel="foaf:knows" resource="_:sue" />
In the above fragment, two bnodes are explicitly created as the subject of triples. Those bnodes are then referenced to demonstrate the relationship between the parties. After processing, the following triples will be generated:
_:john foaf:mbox <mailto:john@example.org> . _:sue foaf:mbox <mailto:sue@example.org> . _:john foaf:knows _:sue .
RDFa Processors use, internally, implementation-dependent identifiers for bnodes. When triples are retrieved, new bnode indentifiers are used, which usually bear no relation to the original identifiers. However, implementations do ensure that these generated bnode identifiers are consistent: each bnode will have its own identifier, all references to a particular bnode will use the same identifier, and different bnodes will have different identifiers.
As a special case, _:
is also a valid reference for one specific bnode.
Processing would normally begin after the document to be parsed has been completely loaded. However, there is no
requirement for this to be the case, and it is certainly possible to use a stream-based approach, such as
SAX [SAX] to extract
the RDFa information. However, if some approach other than the DOM traversal technique defined here is used, it
is important to ensure that Host Language-specific processing rules are applied
(e.g., XHTML+RDFa [XHTML-RDFA] indicates the base
element can be used, and
base
will affect the interpretation of URIs in meta
or
link
elements even if those elements are before the
base
element in the stream).
At the beginning of processing, an initial evaluation context is created, as follows:
base
element);
Processing begins by applying the processing rules below to the document object, in the context of this initial evaluation context. All elements in the tree are also processed according to the rules described below, depth-first, although the evaluation context used for each set of rules will be based on previous rules that may have been applied.
This specification defines processing rules for optional
attributes that may not be present in all Host Languages (e.g., @xmlns:
).
If these attributes are not supported in
the Host Language, then the corresponding processing rules
are not relevant for that language.
The processing rules are:
If no URI is provided by a resource attribute, then the first match from the following rules will apply:
If no URI is provided then the first match from the following rules will apply:
Then the current object resource is set to the URI obtained from the first match from the following rules:
Note that final value of the current object resource will either be null (from initialization) or a full URI.
XMLLiteral
in the vocabulary http://www.w3.org/1999/02/22-rdf-syntax-ns#
.
The actual literal is either the value of @content (if present) or a string created by concatenating the value of all descendant text nodes, of the current element in turn. The final string includes the datatype URI, as described in [RDF-CONCEPTS], which will have been obtained according to the section on CURIE and URI Processing.
XMLLiteral
in the vocabulary http://www.w3.org/1999/02/22-rdf-syntax-ns#
.
The value of the XML literal is a string created by serializing to text, all nodes that are
descendants of the current element, i.e., not including the element itself, and giving it a datatype of
XMLLiteral
in the vocabulary http://www.w3.org/1999/02/22-rdf-syntax-ns#
.
The format of the resulting serialized content is as defined in
Exclusive XML Canonicalization Version
[XML-EXC-C14N].
In order to maintain maximum portability of this literal, any children of the current node that are elements must have the current XML namespace declarations (if any) declared on the serialized element. Since the child element node could also declare new XML namespaces, the RDFa Processor must be careful to merge these together when generating the serialized element definition. For avoidance of doubt, any re-declarations on the child node must take precedence over declarations that were active on the current node.
Additionally, if there is a value for current language then the value of the plain literal should include this language information, as described in [RDF-CONCEPTS]. The actual literal is either the value of @content (if present) or a string created by concatenating the text content of each of the descendant elements of the current element in document order.
The current object literal is then used with each predicate to generate a triple as follows:
The processing rules covered in the previous section are designed to extract as many triples as possible from a document. The RDFa Processor is designed to continue processing, even in the event of errors. For example, failing to resolve a prefix mapping or term would result in the RDFa Processor skipping the generation of a triple and continuing with document processing. There are cases where knowing each RDFa Processor warning or error would be beneficial to authors. The processor graph is designed as a 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 Processors. For Event-based RDFa Processors, the software must allow the developer to register a function or callback that is called when a triple is generated for the processor graph. The callback may be the same as the one that is used for the default graph as long as it can be determined if a generated triple belongs in the processor graph or the default graph.
A whole-graph RDFa Processor is defined as any RDFa Processor that processes the entire document and only allows developer access to the triples after processing has completed. RDFa Processors that typically fall into this category express their output via a single call using RDF/XML, N3, TURTLE, or N-Triples notation. For whole-graph RDFa Processors, the software must allow the developer to specify if they would like to retrieve the default graph, the processor graph, or both graphs as a single, combined graph from the RDFa Processor. If the graph preference is not specified, the default graph must be returned.
An web service RDFa Processor is defined as any RDFa
Processor that is capable of processing a document by performing an
HTTP GET, POST or similar action on an RDFa Processor URI. For this
class of RDFa Processor, the software must allow the caller to specify
if they would like to retrieve the default graph,
the processor graph, or both graphs as a single, combined
graph from the web service. The rdfagraph
query
parameter must be used to specify the value. The allowable values are
default
, processor
or both values, in any
order, separated by a comma character.
If the graph preference is not specified, the
default graph must be returned.
To ensure interoperability, a core hierarchy of classes is defined for the content of the processor graph. Separate errors or warnings are resources (typically blank nodes) of a specific type, with additional properties giving more details on the error condition or the warning. This specification defines only the top level classes and the ones referring to the error and warning conditions defined explicitly by this document. Other, implementation-specific subclasses may be defined by the RDFa Processor.
The top level classes are rdfa:Error
, rdfa:Warning
, and rdfa:Info
,
defined as part of the RDFa Vocabulary. Furthermore, a single property is defined on those classes, namely rdfa:context
, that provides an extra context for the error, e.g., http response, an XPath information, or simply the URI to the RDFa resource. Usage of this property is optional, and more than one triple can be used with this predicate on the same subject. Finally, error and warning instances should use the dc:description
and dc:date
properties. dc:description
should provide a short, human readable but implementation dependent description of the error. dc:date
should give the time when the error was found and it is advised to be as precise as possible to allow the detection of, for example, possible network errors.
The example below shows the triples that should be minimally present in the processor graph as a result of an error (the content of the literal for the dc:description
predicate is implementation dependent):
@prefix rdfa: <http://www.w3.org/ns/rdfa#> . @prefix xsd: <http://www.w3.org/2001/XMLSchema#> . @prefix dcterms: <http://purl.org/dc/terms/> . [] a rdfa:ProfileReferenceError ; dcterms:description "The @profile value could not be deferenced" ; dcterms:date "2010-06-30T13:40:23"^^xsd:dateTime .
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:
@prefix rdfa: <http://www.w3.org/ns/rdfa#> . @prefix xsd: <http://www.w3.org/2001/XMLSchema#> . @prefix dcterms: <http://purl.org/dc/terms/> . @prefix ptr: <http://www.w3.org/2009/pointers#> . @prefix http: <http://www.w3.org/2006/http#> . [] a rdfa:ProfileReferenceError ; dcterms:description "The @profile value could not be deferenced" ; dcterms:date "2010-06-30T13:40:23"^^xsd:dateTime ; rdfa:context <http://www.example.org/profile> ; rdfa:context [ a ptr:Pointer ; # detailed xpointer/xpath information provided here to locate the # DOM node with the @profile attribute ] ; rdfa:context [ a http:Response ; http:responseCode <http://www.w3.org/2006/http#404> # Get the HTTP response headers on the request for the profile file. ].
This section is non-normative.
This section provides an in-depth examination of the processing steps described in the previous section. It also includes examples which may help clarify some of the steps involved.
The key to processing is that a triple is generated whenever a predicate/object combination is detected. The actual triple generated will include a subject that may have been set previously, so this is tracked in the current evaluation context and is called the parent subject. Since the subject will default to the current document if it hasn't been set explicitly, then a predicate/object combination is always enough to generate one or more triples.
The attributes for setting a predicate are @rel, @rev and @property, whilst the attributes for setting an object are @resource, @href, @content, and @src. @typeof is unique in that it sets both a predicate and an object at the same time (and also a subject when it appears in the absence of other attributes that would set a subject). Inline content might also set an object, if @content is not present, but @property is present.
There are many examples in this section. The examples are all written using XHTML+RDFa. However, the explanations are relevant regardless of the Host Language.
When triples are created they will always be in relation to a subject resource which is provided either by new subject (if there are rules on the current element that have set a subject) or parent subject, as passed in via the evaluation context. This section looks at the specific ways in which these values are set. Note that it doesn't matter how the subject is set, so in this section we use the idea of the current subject which may be either new subject or parent subject.
When parsing begins, the current subject will be the URI 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 profile="http://www.example.org/vocab-rdf-dc.html"> <head> <title>Jo's Friends and Family Blog</title> <link rel="foaf:primaryTopic" href="#bbq" /> <meta property="dcterms:creator" content="Jo" /> </head> <body> ... </body> </html>
This would generate the following triples:
<> foaf:primaryTopic <#bbq> . <> dcterms:creator "Jo" .
It is possible for the data to appear elsewhere in the document:
<html profile="http://www.example.org/vocab-rdf-dc.html">
<head>
<title>Jo's Blog</title>
</head>
<body>
<h1><span property="dcterms:creator">Jo</span>'s blog</h1>
<p>
Welcome to my blog.
</p>
</body>
</html>
which would still generate the triple:
<> dcterms:creator "Jo" .
In (X)HTML the value of base
may change the initial value of current subject:
<html profile="http://www.example.org/vocab-rdf-dc.html">
<head>
<base href="http://www.example.org/jo/blog" />
<title>Jo's Friends and Family Blog</title>
<link rel="foaf:primaryTopic" href="#bbq" />
<meta property="dcterms:creator" content="Jo" />
</head>
<body>
...
</body>
</html>
An RDFa Processor should now generate the following triples, regardless of the URI from which the document is served:
<http://www.example.org/jo/blog> foaf:primaryTopic <#bbq> . <http://www.example.org/jo/blog> dcterms:creator "Jo" .
As processing progresses, any
@about attributes will change the current subject. The value of
@about is a URI or a CURIE. If it is a relative URI then it needs to be resolved
against the current base value. To illustrate how this affects the statements, note in this markup
how the properties inside the (X)HTML body
element become part of a new calendar event object, rather
than referring to the document as they do in the head of the document:
<html profile="http://www.example.org/vocab-rdf-dc.html" prefix="cal: http://www.w3.org/2002/12/cal/ical#"> <head> <title>Jo's Friends and Family Blog</title> <link rel="foaf:primaryTopic" href="#bbq" /> <meta property="dcterms:creator" content="Jo" /> </head> <body> <p about="#bbq" typeof="cal:Vevent"> I'm holding <span property="cal:summary"> one last summer barbecue </span>, on <span property="cal:dtstart" content="2015-09-16T16:00:00-05:00" datatype="xsd:dateTime"> September 16th at 4pm </span>. </p> </body> </html>
With this markup an RDFa Processor will generate the following triples:
<> foaf:primaryTopic <#bbq> . <> dcterms:creator "Jo" . <#bbq> rdf:type cal:Vevent . <#bbq> cal:summary "one last summer barbecue" . <#bbq> cal:dtstart "2015-09-16T16:00:00-05:00"^^xsd:dateTime .
Other kinds of resources can be used to set the current subject, not just references to web-pages. Although not advised, email addresses might be used to represent a person:
John knows <a about="mailto:john@example.org" rel="foaf:knows" href="mailto:sue@example.org">Sue</a>. Sue knows <a about="mailto:sue@example.org" rel="foaf:knows" href="mailto:jim@example.org">Jim</a>.
This should generate the following triples:
<mailto:john@example.org> foaf:knows <mailto:sue@example.org> . <mailto:sue@example.org> foaf:knows <mailto:jim@example.org> .
Similarly, authors may make statements about images:
<div about="photo1.jpg">
this photo was taken by
<span property="dcterms:creator">Mark Birbeck</span>
</div>
which should generate the following triples:
<photo1.jpg> dcterms:creator "Mark Birbeck" .
If @about is not present, then @src is next in priority order, for setting the subject of a statement. A typical use would be to indicate the licensing type of an image:
<img src="photo1.jpg" rel="license"
resource="http://creativecommons.org/licenses/by/2.0/" />
Since there is no difference between @src and @about, then the information expressed in the last example in the section on @about (the creator of an image), could be expressed as follows:
<img src="photo1.jpg"
rel="license" resource="http://creativecommons.org/licenses/by/2.0/"
property="dcterms:creator" content="Mark Birbeck"
/>
Since normal chaining rules will apply, the image URI can also be used to complete hanging triples:
<div about="http://www.blogger.com/profile/1109404" rel="foaf:img">
<img src="photo1.jpg"
rel="license" resource="http://creativecommons.org/licenses/by/2.0/"
property="dcterms:creator" content="Mark Birbeck"
/>
</div>
The complete markup yields three triples:
<http://www.blogger.com/profile/1109404> foaf:img <photo1.jpg> . <photo1.jpg> xhv:license <http://creativecommons.org/licenses/by/2.0/> . <photo1.jpg> dcterms:creator "Mark Birbeck" .
Whilst @about explicitly creates a new context for statements, @typeof does so implicitly.
@typeof works differently to other ways of setting a predicate since the predicate is always
rdf:type
, which means that the processor only requires one attribute, the value of the type.
Since @typeof is setting the type of an item, this means that if no item exists one should automatically be created. This involves generating a new bnode, and is examined in more detail below; it is mentioned here because the bnode used by the new item will become the subject for further statements.
For example, an author may wish to create markup for a person using the FOAF vocabulary, but without having a clear identifier for the item:
<div typeof="foaf:Person">
<span property="foaf:name">Albert Einstein</span>
<span property="foaf:givenName">Albert</span>
</div>
This markup would cause a bnode to be created which has a 'type' of foaf:Person
, as well as name and given
name properties:
_:a rdf:type foaf:Person . _:a foaf:name "Albert Einstein" . _:a foaf:givenName "Albert" .
_:a
as being distinct from
_:b
. But by not exposing these values to any external software, it is possible to have complete control over
the identifier, as well as preventing further statements being made about the item.
As described in the previous two sections, @about will always take precedence and mark a new subject, but if no @about value is available then @typeof will do the same job, although using an implied identifier, i.e., a bnode.
But if neither @about or @typeof are present, there are a number of ways that the subject could be arrived at. One of these is to 'inherit' the subject from the containing statement, with the value to be inherited set either explicitly, or implicitly.
The most usual way that an inherited subject might get set would be when the parent statement has an object that is a resource. Returning to the earlier example, in which the long name for the German_Empire was added, the following markup was used:
<div about="http://dbpedia.org/resource/Albert_Einstein">
<span property="foaf:name">Albert Einstein</span>
<span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span>
<div rel="dbp:birthPlace" resource="http://dbpedia.org/resource/German_Empire" />
<span about="http://dbpedia.org/resource/German_Empire"
property="dbp:conventionalLongName">the German Empire</span>
</div>
In an earlier illustration the subject and object for the German Empire were elided by removing the @resource, relying on the @about to set the object:
<div about="http://dbpedia.org/resource/Albert_Einstein">
<span property="foaf:name">Albert Einstein</span>
<span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span>
<div rel="dbp:birthPlace">
<span about="http://dbpedia.org/resource/German_Empire"
property="dbp:conventionalLongName">the German Empire</span>
</div>
</div>
but it is also possible for authors to achieve the same effect by removing the @about and leaving the @resource:
<div about="http://dbpedia.org/resource/Albert_Einstein">
<span property="foaf:name">Albert Einstein</span>
<span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span>
<div rel="dbp:birthPlace" resource="http://dbpedia.org/resource/German_Empire">
<span property="dbp:conventionalLongName">the German Empire</span>
</div>
</div>
In this situation, all statements that are 'contained' by the object resource representing the German Empire (the value in @resource) will have the same subject, making it easy for authors to add additional statements:
<div about="http://dbpedia.org/resource/Albert_Einstein">
<span property="foaf:name">Albert Einstein</span>
<span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span>
<div rel="dbp:birthPlace" resource="http://dbpedia.org/resource/German_Empire">
<span property="dbp:conventionalLongName">the German Empire</span>
<span rel="dbp-owl:capital" resource="http://dbpedia.org/resource/Berlin" />
</div>
</div>
Looking at the triples that an RDFa Processor would generate, we can see that we actually have two groups of statements; the first group are set to refer to the @about that contains them:
<http://dbpedia.org/resource/Albert_Einstein> foaf:name "Albert Einstein" . <http://dbpedia.org/resource/Albert_Einstein> dbp:dateOfBirth "1879-03-14"^^xsd:date . <http://dbpedia.org/resource/Albert_Einstein> dbp:birthPlace <http://dbpedia.org/resource/German_Empire> .
whilst the second group refer to the @resource that contains them:
<http://dbpedia.org/resource/German_Empire> dbp:conventionalLongName "the German Empire" . <http://dbpedia.org/resource/German_Empire> dbp-owl:capital <http://dbpedia.org/resource/Berlin> .
Note also that the same principle described here applies to @src and @href.
There will be occasions when the author wants to elide 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 URI to identify it. However, the item is given an automatically generated bnode, and it is onto this identifier that all child statements are attached:
An RDFa Processor will generate the following triples:
<http://dbpedia.org/resource/Baruch_Spinoza> dbp-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, URI resources and literals.
A literal object can be set by using @property to express a predicate, and then using either @content, or the inline text of the element that @property is on. Note that the use of @content prohibits the inclusion of rich markup in your literal. If the inline content of an element accurately represents the object, then documents should rely upon that rather than duplicating that data using the @content.
A URI resource object can be set using one of @rel or @rev to express a predicate, and then either using one of @href, @resource or @src to provide an object resource explicitly, or using the chaining techniques described above to obtain an object from a nested subject, or from a bnode.
An object literal will be generated when @property is present. @property provides the predicate, and the following sections describe how the actual literal to be generated is determined.
Literal object resolution
@content can be used to indicate a plain literal, as follows:
<meta about="http://internet-apps.blogspot.com/"
property="dcterms:creator" content="Mark Birbeck" />
The plain literal can also be specified by using the content of the element:
<span about="http://internet-apps.blogspot.com/"
property="dcterms:creator">Mark Birbeck</span>
Both of these examples give the following triple:
<http://internet-apps.blogspot.com/> dcterms: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="dcterms:creator" content="Mark Birbeck">John Doe</span>
Literals can be given a data type using @datatype.
This can be represented in RDFa as follows:
<span property="cal:dtstart" content="2015-09-16T16:00:00-05:00"
datatype="xsd:dateTime">
September 16th at 4pm
</span>.
The triples that this markup generates include the datatype after the literal:
<> cal:dtstart "2015-09-16T16:00:00-05:00"^^xsd:dateTime .
XML documents cannot contain XML markup in their attributes, which means it is not possible to represent XML within @content (the following would cause an XML parser to generate an error):
<head>
<meta property="dcterms: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="dcterms:title" datatype="rdf:XMLLiteral">
E = mc<sup>2</sup>: The Most Urgent Problem of Our Time
</h2>
This would generate the following triple, with the XML preserved in the literal:
<> dcterms:title "E = mc<sup>2</sup>: The Most Urgent Problem of Our Time"^^rdf:XMLLiteral .
This requires that a URI mapping for the prefix rdf
has been defined.
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 URI objects are needed when @rel or @rev is present. Each attribute will cause triples to be generated when used with @href, @resource or @src, or with the subject value of any nested statement if none of these attributes are present.
@rel and @rev are essentially the inverse of each other; whilst @rel establishes a relationship between the current subject as subject, and the current object resource as the object, @rev does the exact opposite, and uses the current object resource as the subject, and the current subject as the object.
RDFa provides the @resource attribute as a way to set the object of statements. This is particularly useful when referring to resources that are not themselves navigable links:
<html profile='http://www.example.org/vocab-rdf-dc.html'>
<head>
<title>On Crime and Punishment</title>
<base href="http://www.example.com/candp.xhtml" />
</head>
<body>
<blockquote about="#q1" rel="dcterms:source" resource="urn:ISBN:0140449132" >
<p id="q1">
Rodion Romanovitch! My dear friend! If you go on in this way
you will go mad, I am positive! Drink, pray, if only a few drops!
</p>
</blockquote>
</body>
</html>
The blockquote
element generates the following triple:
<http://www.example.com/candp.xhtml#q1> <http://purl.org/dc/terms/source> <urn:ISBN:0140449132> .
If no @resource is present, then @href is next in priority order, for setting the object.
When a predicate has been expressed using @rel, the @href on the [RDFa statement]'s element is used to identify the object with a [URI reference]. Its type is a URI:
<link about="mailto:john@example.org"
rel="foaf:knows" href="mailto:sue@example.org" />
It's also possible to use both @rel and @rev at the same time on an element. This is particularly useful when two things stand in two different relationships with each other, for example when a picture is taken by Mark, but that picture also depicts him:
<img src="photo1.jpg" rel="dcterms:creator" rev="foaf:img"
href="http://www.blogger.com/profile/1109404" />
which then yields two triples:
<photo1.jpg> dcterms:creator <http://www.blogger.com/profile/1109404> . <http://www.blogger.com/profile/1109404> foaf:img <photo1.jpg> .
When a triple predicate has been expressed using @rel or @rev, but no @href, @src, or @resource exists on the same element, there is a 'hanging rel'. This causes the current subject and all possible predicates (with an indicator of whether they are 'forwards, i.e., @rel values, or not, i.e., @rev values), to be stored as 'incomplete triples' pending discovery of a subject that could be used to 'complete' those triples.
This process is described in more detail in Completing 'Incomplete Triples'.
RDFa Profiles are collections of terms, prefix mappings, and/or default vocabulary declarations. A profile is either intrinsically known to the parser, or it is loaded as an external document and processed. These documents must be defined in an approved RDFa Host Language (currently XML+RDFa and XHTML+RDFa [XHTML-RDFA]). They may also be defined in other formats (e.g., RDF/XML [RDF-SYNTAX-GRAMMAR], or Turtle [TURTLE]). RDFa Profiles are referenced via @profile, and can be used by document authors to simplify the task of adding semantic markup. When an RDFa document includes @profile, its value is processed from beginning to end, with each separate URI evaluated as follows:
When a profile is not retrievable, an RDFa Processor will not generate triples from the element the profile is referenced from, nor from any of its children. Consequently, any further processing of the triples would be effectively ignored.
rdfa:prefix
and rdfa:uri
,
create a key-value mapping from the rdfa:prefix
object literal (the key)
to the rdfa:uri
object literal (the value).
Add or update this mapping in the
local list of URI mappings 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 or update this mapping in the
local term mappings.rdfa:vocabulary
,
update the default vocabulary to be the object literal of the rdfa:vocabulary
predicate.When an RDFa Profile is defined using an RDF serialization, it must use the vocabulary terms above to declare the components of the profile.
Once all the URIs in the @profile value have been processed, continue with the normal processing of the current element.
Profiles referenced on the same element are processed from beginning to end of the value of @profile. If any conflict arises between two RDFa Profiles associated with URIs in the @profile value, the declaration from the RDFa Profile associated with the right-most URI takes precedence.
It is possible that a referenced RDFa document will in turn reference other documents via @profile. Regardless of the depth to which such references might go, only the triples in the top level document effect current processing.
Caching of the relevant triples retrieved via this mechanism is recommended. Embedding definitions for well known, stable RDFa Profiles in the implementation is recommended.
The object literal for the rdfa:uri
predicate must
be an absolute URI. The object literal for the rdfa:term
predicate
must match the production for term.
The object literal for the
rdfa:prefix
predicate must match
the production for prefix.
The object literal for the rdfa:vocabulary
predicate must
be an absolute URI.
If one of the objects is not a Literal, does not match its
associated production, if there is more than one
rdfa:vocabulary
predicate, or if there are additional
rdfa:uri
or rdfa:term
predicates sharing the same subject, an RDFa Processor must not
update the associated mapping.
Web authors utilizing RDFa Profiles should be aware that if a profile that they list is not available for any reason, and the RDFa Processor has not previously cached the profile, that all triples that should be generated as a result of the profile will not be generated. In addition, any other triples that exist in a subtree of the DOM will not be generated either since processing halts for a subtree in the DOM when a profile cannot be fetched.
Web authors utilizing both RDFa Profiles and a JavaScript-based RDFa processor should be aware that same-origin security protections are enforced for RDFa processors, just like any other JavaScript code. This means that if a profile is not served up using [CORS] or a similar technology that the processor may fail to retrieve the profile and thus the expected triples in the subtree of the DOM where the @profile is specified will not be generated.
In order to facilitate the use of CURIEs in markup languages, this
specification defines some additional datatypes in the XHTML datatype
space (http://www.w3.org/1999/xhtml/datatypes/
).
Markup languages that want to import these
definitions can find them in the
"datatypes" file for their schema grammar:
Specifically, the following datatypes are defined:
This section is non-normative.
The following informative XML Schema definition for these datatypes is included as an example:
<?xml version="1.0" encoding="UTF-8"?> <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns="http://www.w3.org/1999/xhtml/datatypes/" xmlns:xh11d="http://www.w3.org/1999/xhtml/datatypes/" targetNamespace="http://www.w3.org/1999/xhtml/datatypes/" elementFormDefault="qualified" > <xs:simpleType name="CURIE"> <xs:restriction base="xs:string"> <xs:pattern value="(([\i-[:]][\c-[:]]*)?:)?.+" /> <xs:minLength value="1"/> </xs:restriction> </xs:simpleType> <xs:simpleType name="CURIEs"> <xs:list itemType="xh11d:CURIE"/> </xs:simpleType> <xs:simpleType name="SafeCURIE"> <xs:restriction base="xs:string"> <xs:pattern value="\[(([\i-[:]][\c-[:]]*)?:)?.+\]" /> <xs:minLength value="3"/> </xs:restriction> </xs:simpleType> <xs:simpleType name="SafeCURIEs"> <xs:list itemType="xh11d:SafeCURIE"/> </xs:simpleType> <xs:simpleType name="TERM"> <xs:list itemType="xs:NCName"/> </xs:simpleType> <xs:simpleType name="CURIEorURI"> <xs:union memberTypes="xh11d:CURIE xs:anyURI" /> </xs:simpleType> <xs:simpleType name="CURIEorURIs"> <xs:list itemType="xh11d:CURIEorURI"/> </xs:simpleType> <xs:simpleType name="SafeCURIEorCURIEorURI"> <xs:union memberTypes="xh11d:SafeCURIE xh11d:CURIE xs:anyURI" /> </xs:simpleType> <xs:simpleType name="SafeCURIEorCURIEorURIs"> <xs:list itemType="xh11d:SafeCURIEorCURIEorURI"/> </xs:simpleType> <xs:simpleType name='AbsURI'> <xs:restriction base='xs:string'> <xs:pattern value="[\i-[:]][\c-[:]]+:.+" /> </xs:restriction> </xs:simpleType> <xs:simpleType name="TERMorCURIEorAbsURI"> <xs:union memberTypes="xh11d:TERM xh11d:CURIE xh11d:AbsURI" /> </xs:simpleType> <xs:simpleType name="TERMorCURIEorAbsURIs"> <xs:list itemType="xh11d:SafeCURIEorCURIEorAbsURI"/> </xs:simpleType> </xs:schema>
This section is non-normative.
The following informative XML DTD definition for these datatypes is included as an example:
<!ENTITY % CURIE.datatype "CDATA" > <!ENTITY % CURIEs.datatype "CDATA" > <!ENTITY % CURIEorURI.datatype "CDATA" > <!ENTITY % CURIEorURIs.datatype "CDATA" > <!ENTITY % SafeCURIEorCURIEorURI.datatype "CDATA" > <!ENTITY % SafeCURIEorCURIEorURIs.datatype "CDATA" > <!ENTITY % TERMorCURIEorAbsURI.datatype "CDATA" > <!ENTITY % TERMorCURIEorAbsURIs.datatype "CDATA" >
The RDFa Vocabulary has two roles: it contains the predicates to
define the terms and prefixes in profile documents, and it contains the classes and predicates
for the messages that a processor graph may contain. The URI 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 dcterms: <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#> . <http://www.w3.org/ns/rdfa#> a owl:Ontology . rdfa:PrefixOrTermMapping a rdfs:Class, owl:Class ; dcterms:description "is the top level class for prefix or term mappings" . rdfa:PrefixMapping dcterms:description "is the class for prefix mappings" . rdfs:subClassOf rdfa:PrefixOrTermMapping . rdfa:TermMapping dcterms:description "is the class for term mappings" . rdfs:subClassOf rdfa:PrefixOrTermMapping . rdfa:prefix a rdf:Property, owl:DatatypeProperty ; rdfs:domain rdfa:PrefixMapping ; dcterms:description "defines a prefix mapping for a URI; the value is supposed to be a NMTOKEN" . rdfa:term a rdf:Property, owl:DatatypeProperty ; rdfs:domain rdfa:TermMapping ; dcterms:description "defines a term mapping for a URI; the value is supposed to be a NMTOKEN" . rdfa:uri a rdf:Property, owl:DatatypeProperty ; rdfs:domain rdfa:PrefixOrTermMapping ; dcterms:description "defines the URI for either a prefix or a term mapping; the value is supposed to be an absolute URI" . rdfa:vocabulary a rdf:Property, owl:DatatypeProperty ; dcterms:description "defines a URI to be used as a default vocabulary; the value is can be any string; for documentation purposes it is advised to use the string ‘true’ or ‘True’." .
These predicates can be used to 'pair' URI strings and their usage in the form of a prefix and/or a term as part of, for example, a blank node. An example can be as follows:
[] rdfa:uri "http://xmlns.com/foaf/0.1/name" ; rdfa:prefix "foaf" .
which defines a prefix for the foaf URI.
The Vocabulary includes the following term definitions (shown here in Turtle [TURTLE] format):
@prefix dcterms: <http://purl.org/dc/terms/> . @prefix owl: <http://www.w3.org/2002/07/owl#> . @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> . @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> . @prefix rdfa: <http://www.w3.org/ns/rdfa#> . rdfa:PGClass a rdfs:Class, owl:Class; dcterms:description "is the top level class of the hierarchy" . 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 dcterms:description "error condition; to be used when the document fails to be fully processed as a result of non-conformant host language markup"; rdfs:subClassOf rdfa:Error . rdfa:ProfileReferenceError dcterms:description "error condition; to be used when an RDFa Profile document fails to be retrieved and thus, a portion of the document fails to be processed"; rdfs:subClassOf rdfa:Error . rdfa:UnresolvedTerm dcterms:description "warning; to be used when a Term fails to be resolved"; rdfs:subClassOf rdfa:Warning . rdfa:UnresolvedCURIE dcterms:description "warning; to be used when a CURIE prefix fails to be resolved"; rdfs:subClassOf rdfa:Warning . rdfa:context a owl:ObjectProperty, rdf:Property; dcterms:description "provides extra context for the error, e.g., http response, an XPointer/XPath information, or simply the URI that created the error"; rdfs:domain rdfa:PGClass .
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=''
.2010-07-26: Added the 'vocabulary' term to the RDFa Profile handling.
2010-02-25: Split into RDFa Core and XHTML+RDFa.
2010-01-01: Applied changes to start production of version 1.1. This includes the re-integration of datatype CURIEorURI.
This section is non-normative.
At the time of publication, the active members of the RDFa Working Group were: