The last couple of years have witnessed a fascinating evolution: while the Web was initially
built predominantly for human consumption, web content is increasingly consumed by machines
which expect some amount of structured data. Sites have started to identify a page's title,
content type, and preview image to provide appropriate information in a user's newsfeed when
she clicks the "Like" button. Search engines have started to provide richer search results by
extracting fine-grained structured details from the Web pages they crawl. In turn, web
publishers are producing increasing amounts of structured data within their Web content to
improve their standing with search engines.
A key enabling technology behind these developments is the ability to add structured data to
HTML pages directly. RDFa (Resource Description Framework in Attributes) is a technique that
allows just that: it provides a set of markup attributes to augment the visual information on
the Web with machine-readable hints. In this Primer, we show how to express data using RDFa
in HTML, and in particular how to mark up existing human-readable Web page content to express
machine-readable data.
This document provides only a Primer to RDFa 1.1. The complete specification of RDFa, with
further examples, can be found in the RDFa 1.1 Core [rdfa-core], RDFa Lite [rdfa-lite],
XHTML+RDFa 1.1 [xhtml-rdfa], and the HTML5+RDFa 1.1 [html-rdfa] specifications.
Status of This Document
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 document was published by the RDFa Working Group as a Working Group Note.
If you wish to make comments regarding this document, please send them to
[email protected]
(subscribe,
archives).
All comments are welcome.
Publication as a Working Group Note 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.
The web is a rich, distributed repository of interconnected information. Until recently, it
was organized primarily for human consumption. On a typical web page, an HTML author might
specify a headline, then a smaller sub-headline, a block of italicized text, a few paragraphs
of average-size text, and, finally, a few single-word links. Web browsers will follow these
presentation instructions faithfully. However, only the human mind understands what the
headline expresses-a blog post title. The sub-headline indicates the author, the italicized
text is the article's publication date, and the single-word links are subject categories.
Computers do not understand the nuances between the information; the gap between what
programs and humans understand is large.
Figure 1: On the left, what browsers see. On the right, what
humans see. Can we bridge the gap so that browsers see more of what we see?
Fig. 1presentation vs. semantics
What if the browser, or any machine consumer such as a Web crawler, received information on
the meaning of a web page's visual elements? A dinner party announced on a blog could be
copied to the user's calendar, an author's complete contact information to the user's address
book. Users could automatically recall previously browsed articles according to
categorization labels (i.e., tags). A photo copied and pasted from a web site to a school
report would carry with it a link back to the photographer, giving him proper credit. A link
shared by a user to his social network contacts would automatically carry additional data
pulled from the original web page: a thumbnail, an author, and a specific title. When web
data meant for humans is augmented with hints meant for computer programs, these programs
become significantly more helpful, because they begin to understand the data's structure.
RDFa allows HTML authors to do just that. Using a few simple HTML attributes, authors can
mark up human-readable data with machine-readable indicators for browsers and other programs
to interpret. A web page can include markup for items as simple as the title of an article,
or as complex as a user's complete social network.
1.1
HTML vs. XHTML
Historically, RDFa 1.0 [rdfa-syntax] was specified only for XHTML. RDFa 1.1 [rdfa-core]
is the newer version and the one used in this document. RDFa 1.1 is
specified for both XHTML [xhtml-rdfa] and HTML5 [html-rdfa]. In fact, RDFa 1.1 also
works for any XML-based languages like SVG [svg11]. This document uses HTML in all of
the examples; for simplicity, we use the term "HTML" throughout this document to refer to
all of the HTML-family languages.
1.2
Validation
RDFa is based on attributes. While some of the HTML attributes (e.g., href,
src) have been re-used, other RDFa attributes are new. This is important
because some of the (X)HTML validators may not properly validate the HTML code until they
are updated to recognize the new RDFa attributes. This is rarely a problem in practice
since browsers simply ignore attributes that they do not recognize. None of the
RDFa-specific attributes have any effect on the visual display of the HTML content.
Authors do not have to worry about pages marked up with RDFa looking any different to a
human being from pages not marked up with RDFa.
2.
Using RDFa
2.1
The Basics of RDFa: RDFa Lite
We begin the introduction to RDFa by using a subset of all the possibilities called RDFa
Lite 1.1 [rdfa-lite]. The goal, when defining that subset, was to define a set of
possibilities that can be applied to most simple to moderate structured data markup
tasks, without burdening the authors with additional complexities. Many Web authors will
not need to use more than this minimal subset.
2.1.1
The First Steps: Adding Machine-Readable Hints to Web Pages
Consider Alice, a blogger who publishes a mix of professional and personal articles
at http://example.com/alice. We will construct markup examples to
illustrate how Alice can use RDFa. A more complete markup of these examples is
available on a
dedicated page.
2.1.1.1
Hints on Social Networking Sites
Alice publishes a blog and would like to provide extra structural information on
her pages like the publication date or the title. She would like to use the terms
defined in the Dublin Core vocabulary [dc11], a set of terms that are widely
used by, for example, the publishing industry or libraries. Her blog already
contain that information:
This information is, however, aimed at humans only; computers need some
sophisticated methods to extract it. But, using RDFa, she can annotate her
page to make the structured data clear:
(Notice the markup colored in red: these are the RDFa "hints".)
One useful way to visualize the structured data is:
Figure 2: A visualization of the structured data
for a blog post with a title of "The Trouble with Bob" and a creation date.
Fig. 2relationship value is text
It is worth emphasizing that RDFa uses URLs to identify just about everything.
This is why, instead of just using properties like title or
created, we use http://purl.org/dc/terms/title and
http://purl.org/dc/terms/created. The reason behind this design
decision is rooted in data portability, consistency, and information sharing.
Using URLs removes the possibility for ambiguities in terminology. Without
ensuring that there is no ambiguity, the term "title" might mean "the title of a
work", "a job title", or "the deed for real-estate property". When each
vocabulary term is a URL, a detailed explanation for the vocabulary term is just
one click away. It allows anything, humans or machines, to follow the link to
find out what a particular vocabulary term means. By using a URL to identify a
particular creation time, for example
http://purl.org/dc/terms/created, both humans and machines can
understand that the URL unambiguously refers to the "Date of creating the
resource", such as a web page.
By using URLs as identifiers, RDFa provides a solid way of disambiguating
vocabulary terms. It becomes trivial to determine whether or not vocabulary terms
used in different documents mean the same thing. If the URLs are the same, the
vocabulary terms mean the same thing. It also becomes very easy to create new
vocabulary terms and vocabulary documents. If one can publish a document to the
Web, one automatically has the power to create a new vocabulary document
containing new vocabulary terms.
2.1.1.2
Links with Flavor
The previous example demonstrated how Alice can markup text to make it machine
readable. She would also like to mark up the links in a machine-readable way, to
express the type of link being described. RDFa lets the publisher add a "flavor",
i.e., a label, to an existing clickable link that processors can understand. This
makes the same markup help both humans and machines.
In her blog's footer, Alice already declares her content to be freely reusable,
as long as she receives due credit when her articles are cited. The HTML includes
a link to a Creative Commons [cc-about] license:
A human clearly understands this sentence, in particular the meaning of
the link with respect to the current document: it indicates the document's
license, the conditions under which the page's contents are distributed.
Unfortunately, when Bob visits Alice's blog, his browser sees only a plain link
that could just as well point to one of Alice's friends or to her CV. For Bob's
browser to understand that this link actually points to the document's licensing
terms, Alice needs to add some flavor, some indication of what
kind of link this is.
She can add this flavor using again the property attribute. Indeed,
when the element contains the href (or src) attribute,
property is automatically associated with the value of this
attribute rather than the textual content of the a element. The
value of the attribute is the http://creativecommons.org/ns#license,
defined by the Creative Commons:
With this small update, Bob's browser will now understand that this link has a
flavor: it indicates the blog's license:
Figure 3: A link with flavor: the link indicates
the web page's license. We can represent web pages as nodes, the link as an
arrow connecting those nodes, and the link's flavor as the label on that
arrow.
Fig. 3two Web pages connected by a link labeled 'license' and two notes with a 'license' relationship
Alice is quite pleased that she was able to add only structured-data hints via
RDFa, never having to repeat the content of her text or the URL of her clickable
links.
2.1.1.3
Setting a Default Vocabulary
In a number of simple use cases, such as our example with Alice's blog, HTML
authors will predominantly use a single vocabulary. However, while generating
full URLs via a CMS system is not a particular problem, typing these by hand may
be error prone and tedious for humans. To alleviate this problem RDFa introduces
the vocab attribute to let the author declare a single vocabulary
for a chunk of HTML. Thus, instead of:
Note how the property values are single "terms" now; these are simply
concatenated to the URL defined via the vocab attribute. The
attribute can be placed on any HTML element (i.e., not only on the
body element like in the example) and its effect is valid for all
the elements below that point.
Default vocabularies and full URIs can be mixed at any time. I.e., Alice could
have written:
The full URL for the license term is necessary to avoid mixing vocabularies. As
an alternative, Alice could have also chosen to use the vocab
attribute again:
because the vocab in the license paragraph overrides the definition
inherited from the body of the document.
Note
The vocab attribute references structured data vocabularies, identified using URLs.
RDFa does not limit the form of these URLs or the document formats accessible by de-referencing them;
however users SHOULD aim to use widely shared, conventional values for identifying such vocabularies,
following conventions of case, spelling etc. established by their publishers.
2.1.1.4
Multiple Items per Page
Alice's blog page may contain, of course, multiple entries. Sometimes, Alice's
sister Eve guest blogs, too. The front page of the blog lists the 10 most recent
entries, each with its own title, author, and introductory paragraph. How, then,
should Alice mark up the title of each of these entries individually even though
they all appear within the same web page? RDFa provides resource, an
attribute for specifying the "context", i.e., the exact URL to which the
contained RDFa markup applies:
(Note that we used relative URLs in the example; the value of
resource could have been any URLs, i.e., relative or
absolute.) We can represent this, once again, as a diagram connecting URLs to
properties:
Figure 4: Multiple Items per Page: each blog entry
is represented by its own node, with properties attached to each.
Fig. 4two separate nodes, each with two properties
Alice can use the same technique to give her friend Bob proper credit when she
posts one of his photos:
Example 11
<div resource="/alice/posts/trouble_with_bob">
<h2 property="title">The trouble with Bob</h2>
...
The trouble with Bob is that he takes much better photos than I do:
...
<div resource="http://example.com/bob/photos/sunset.jpg">
<img src="http://example.com/bob/photos/sunset.jpg" />
<span property="title">Beautiful Sunset</span>
by <span property="creator">Bob</span>.
</div>
</div>
Notice how the innermost resource value,
http://example.com/bob/photos/sunset.jpg, "overrides" the outer
value /alice/posts/trouble_with_bob for all markup inside the
containing div. Once again, here is a diagram that represents the
underlying data of this new portion of markup:
Figure 5: Describing a Photo
Fig. 5two separate nodes, each with two properties
2.1.2
Exploring Further: Social networks
2.1.2.1
Contact Information
Alice would also like to make information about herself, such as her email
address, phone number, and other details, easily available to her friends'
contact management software. This time, instead of describing the properties of a
web page, she's going to describe the properties of a person: herself.
Alice already has contact information displayed on her blog.
The Dublin Core vocabulary does not provide property names for describing contact
information, but the Friend-of-a-Friend [foaf] vocabulary does. Alice therefore
decides to use the FOAF vocabulary. As a first step, she declares a FOAF
"Person". For this purpose, Alice uses typeof, an RDFa attribute
that is specifically meant to declare a new data item with a certain type:
Alice realizes that she only intends to use the FOAF vocabulary at this point, so
she uses the vocab attribute to simplify her markup further (and
overriding the effects of any vocab attributes that may have been
used in, for example, the body element at the top).
Note how Alice did not specify a resource like she did when adding
blog entry metadata. But, if she is not declaring what she is talking about, how
does the RDFa Processor know what she's identifying? In RDFa, in the absence of a
resource attribute, the typeof attribute on the
enclosing div implicitly sets the subject of the properties marked
up within that div. That is, the name, email address, and phone
number are associated with a new node of type Person. This node has
no URL to identify it, so it is called a blank node as shown on the
figure:
Figure 6: A Blank Node: blank nodes are not
identified by URL. Instead, many of them have an RDFa typeof
attribute that identifies the type of data they represent.
(We've used a short-hand to label the arrows, in order to save space and
clarify the diagram. The actual labels are always the full URLs.)
Fig. 6single 'blank' node with 4 properties
2.1.2.2
Describing Social Networks
Alice continues to mark up her page by adding information about her friends,
including at least their names and homepages. She starts with plain HTML:
First, Alice indicates that the friends she is describing are people, as opposed
to animals or imaginary friends, by using again the Person type in
typeof attributes.
Beyond declaring the type of data we are dealing with, each typeof
creates a new blank node with its own distinct properties. Thus, Alice can
indicate each friend's homepage:
Alice would also like to improve the markup by expressing each person's name
using RDFa, too. That can be done by adding a separate span element
and the relevant property:
Alice is happy that, with so little additional markup, she's able to fully
express both a pleasant human-readable page and a machine-readable dataset.
Alice is a member of 5 different social networking sites. She is tired of
repeatedly entering information about her friends in each new social networking
site, so she decides to list her friends in one place-on her website, combining
it with her own FOAF data. With RDFa, she can indicate her friendships on her own
web page and let social networking sites read it automatically. So far, Alice has
listed three individuals but has not specified her relationship with them; they
might be her friends, or they might be her favorite 17th century poets. To
indicate that she knows them, she uses the FOAF property foaf:knows:
With this, Alice could describe here social network:
Figure 7: Alice's social network. Note that, with
RDFa, Alice could express a fairly complex set of information that others can
use.
Fig. 78 node network with 12 relationships
2.1.3 Repeated Patterns
We have seen, in a previous section, how Alice can use RDFa to include Creative Commons statements on her blog. However, the solution in that section assigned these statements to the whole page, and not to individual blog items. This may be an issue if the page includes multiple items. Indeed, Alice may be forced to repeat the relevant statements like this:
HTML+RDFa introduces the notion of "Property copying" to alleviate this situation. Using this feature Alice can "collect" a number of statements as a pattern, and refer to that pattern from other parts of the page. This is done using the magic property rdfa:copy and the magic type rdfa:Pattern as follows:
(Alice may choose to use CSS to make the CC statements invisible on the screen if she wants.) The effect of this structure is to, conceptually, "copy" all the RDFa statements appearing in the pattern to replace the link element, yielding the following structure:
Figure 8: Creative Commons statements added to each blog item separately.
Fig. 88 node network with 12 relationships
2.1.4
Internal References
Alice may want to add her personal data to her individual blog items, too. She
decides to combine her FOAF data with the blog items, i.e.:
The structured data she generates looks like this:
Figure 9: Alice's blog item with data about herself.
Fig. 9The simple blog structure extended with Alice's foaf data as blank node
Unfortunately, this solution is not optimal in two respects. First of all, notice
that Alice had to use the full URI for the creator property: this is
because the vocab attribute is used to set the FOAF terms, i.e., the
simple creator value would have been misinterpreted. We will come back
to the issue of using several vocabularies in another
section below.
The other issue is that Alice would like to design her Web page so that her personal
data would not appear on the page in each individual blog item but, rather, in one
place like a footnote or a sidebar. I.e., what she would like to see is something
like:
Figure 10: Structure of Alice's Site: individual blog
items on the left, personal data, linked from the blog using RDFa terms, in a
sidebar.
Fig. 10Mock-up of Alice's blog page design, with blogs on the left and personal data on the right
If the FOAF data were included in each blog item, Alice would have to create a
complex set of CSS rules to achieve the visual effect she wants.
To solve this, Alice decides to make use of the structure she already used for her
FOAF data but, this time, assigning it a separate URI using the resource
attribute:
It is actually considered as a good practice to use real URIs whenever possible,
i.e., Alice's new alternative should be preferred in general. Indeed, if a real URI
is used, then it becomes possible to unambiguously refer to that particular piece of
information, whereas that becomes more complicated with blank nodes.
Note
The resource="#me" markup (which, by the way, also presupposes that the target is in the
same HTML scope) is a FOAF convention: the URL that represents
the person Alice is http://example.com/alice#me. It should not
be confused with Alice's homepage, http://example.com/alice. Of course,
Alice could have used a different URI if, for example, her blog and her personal
homepage were kept separate; e.g., she could have used
resource="http://alice.example.com/alice/home#myself" instead of
resource="#me".
Using the explicit URI for her FOAF data Alice can add a direct reference to the blog
item using again the resource attribute:
The resource attribute appears, in this case, together with
propertyon the same element: in this situation
resource indicates the "target" of the relation. Usage of this attribute
allows Alice to "distribute" the various parts of her structured data on her page.
What she gets is a slightly modified version of the previous structure, where the
only difference is the usage of an explicit URI instead of a blank node:
Figure 11: Alice's blog item with data about herself,
using an explicit URI for her FOAF data.
Fig. 11The simple blog structure extended with Alice's foaf data with an explicit URI
Using this approach, it becomes very easy to also add references to the same data from different blog posts:
Figure 12: Several of Alice's blog items with data
about herself, using an explicit URI for her FOAF data.
Fig. 12The simple blog structure with two blogs extended with Alice's foaf data with an explicit URI
Note
Combined with property, the resource attribute plays
exactly the same role as href, already used for "links with flavor",
except that it does not provide a clickable link to the browser like
href does. Also, the resource attribute can be used on
any HTML element, as opposed to href whose usage is restricted,
in HTML, to the a and link elements.
Note
There is a similarity between this issue and its solution and the issue and the approach taken in the section on property copying. There is, however, a subtle but important difference between the two. The solution using the resource attribute introduces a new node in the graph, as shown on Figure 12, whereas copying the properties does not. Which of the two approaches should be adopted is often based on the vocabulary that is used.
2.1.5
Using Multiple Vocabularies
The previous examples show that, for more complex cases, multiple vocabularies have
to be used to express the various aspects of structured data. We have seen Alice
using the Dublin Core, as well as the FOAF and the Creative Commons vocabularies, but
there may be more. For example. Alice may want to add vocabulary elements defined by
search engines on their schema.org site [schema].
Alice can use either full URLs for all the terms, or can use the vocab
attribute to abbreviate the terms for the predominant vocabulary. But, in some cases,
the vocabularies cannot be separated easily, which means that the usage of
vocab may become awkward. Here is, for example, the kind of HTML she
might end up with:
Example 27
<html>
<head>
...
</head>
<body vocab="http://schema.org/">
<div resource="/alice/posts/trouble_with_bob" typeof="BlogPosting">
<h2 property="http://purl.org/dc/terms/title">The trouble with Bob</h2>
...
<h3 property="http://purl.org/dc/terms/creator" resource="#me">Alice</h3>
<div property="articleBody">
<p>The trouble with Bob is that he takes much better photos than I do:</p>
</div>
...
</div>
...
</body>
</html>
Note that the schema.org and the Dublin Core terms are intertwined for a specific
blog, and it becomes an arbitrary choice whether to use the vocab
attribute for http://purl.org/dc/terms/ or for
http://schema.org/. We have seen the same problem in a previous section when FOAF and Dublin Core terms were
mixed.
To alleviate this problem, RDFa offers the possibility of using prefixed
terms: a special prefix attribute can assign prefixes to represent URLs
and, using those prefixes, the vocabulary elements themselves can be abbreviated. The
prefix:reference syntax is used: the URL associated with
prefix is simply concatenated to reference to create a full
URL. (Note that we have already used this convention to simplify our figures.) Here
is how the HTML of the previous example looks like when prefixes are used:
Example 28
<html>
<head>
...
</head>
<body prefix="dc: http://purl.org/dc/terms/ schema: http://schema.org/">
<div resource="/alice/posts/trouble_with_bob" typeof="schema:BlogPosting">
<h2 property="dc:title">The trouble with Bob</h2>
...
<h3 property="dc:creator" resource="#me">Alice</h3>
<div property="schema:articleBody">
<p>The trouble with Bob is that he takes much better photos than I do:</p>
</div>
...
</div>
</body>
</html>
The usage of prefixes can greatly reduce possible errors by concentrating the
vocabulary choices to one place in the file. Just like vocab, the
prefix attribute can appear anywhere in the HTML file, only affecting
the elements below. prefix and vocab can also be mixed, for
example:
Example 29
<html>
<head>
...
</head>
<body vocab="http://purl.org/dc/terms/"prefix="schema: http://schema.org/">
<div resource="/alice/posts/trouble_with_bob" typeof="schema:BlogPosting">
<h2 property="title">The trouble with Bob</h2>
...
<h3 property="creator" resource="#me">Alice</h3>
<div property="schema:articleBody">
<p>The trouble with Bob is that he takes much better photos than I do:</p>
</div>
...
</div>
</body>
</html>
Note
An important issue may arise if the html element contains a large number
of prefix declarations. The character encoding (i.e., UTF-8, UTF-16, ASCII, etc.)
used for an HTML5 file is declared using a meta element in the header.
In HTML5 this meta declaration must fall within the first 512 bytes of the page, or
the HTML5 processor (browser, parser, etc.) will try to detect the encoding using
some heuristics. A very "long" html tag may therefore lead to problems.
One way of avoiding the issue is to place most of the prefix declarations on the
body element.
2.1.5.1
Repeating properties
The previous example, whereby the Dublin Core and the schema.org vocabularies are
used within the same blog post, raises another issue. It so happens that not only
Dublin Core, but also schema.org has a property called creator.
Because RDFa uses URIs to denote properties that, by itself, is not a problem.
However, if Alice wants to use both these properties in the same blog
post (e.g., because she wants search engines to manage her blog post but, at the
same times, she wants Dublin Core aware applications, like catalogs, to handle
her blog post, too) this is what she may have to do:
Example 30
<html>
<head>
...
</head>
<body prefix="dc: http://purl.org/dc/terms/ schema: http://schema.org/">
<div resource="/alice/posts/trouble_with_bob" typeof="schema:BlogPosting">
<h2 property="dc:title">The trouble with Bob</h2>
...
<h3 property="dc:creator" resource="#me"><span property="schema:creator" resource="#me">Alice</span></h3>
<div property="schema:articleBody">
<p>The trouble with Bob is that he takes much better photos than I do:</p>
</div>
...
</div>
</body>
</html>
Which is a bit awkward. Fortunately, RDFa allows the value of a
property attribute to be a list of values, i.e., she can also write:
Example 31
<html>
<head>
...
</head>
<body prefix="dc: http://purl.org/dc/terms/ schema: http://schema.org/">
<div resource="/alice/posts/trouble_with_bob" typeof="schema:BlogPosting">
<h2 property="dc:title">The trouble with Bob</h2>
...
<h3 property="dc:creator schema:creator" resource="#me">Alice</h3>
<div property="schema:articleBody">
<p>The trouble with Bob is that he takes much better photos than I do:</p>
</div>
...
</div>
</body>
</html>
yielding the structure:
Figure 13: Alice's blog item using two different
vocabularies, including two properties with the same context and target.
Fig. 13The simple blog structure with two different creator properties
Similarly to property, typeof also accepts a list of values. For example,
schema.org also has a notion of a Person, similar to FOAF; Alice may choose to use both:
A number of vocabularies are very widely used by the Web community with
well-known prefixes—the Dublin Core vocabulary is a good example. These common
vocabularies tend to be defined over and over again, and sometimes Web page
authors forget to declare them altogether.
To alleviate this issue, RDFa introduces the concept of an initial
context that defines a set of default prefixes. These prefixes, whose list
is maintained and regularly updated by the W3C, provide a number of pre-defined
prefixes that are known to the RDFa processor. Prefix declarations in a document
always override declarations made through the defaults, but if a web page author
forgets to declare a common vocabulary such as Dublin Core or FOAF, the RDFa
Processor will fall back to those. The list of default prefixes are available on the Web for
everyone to read.
For example, the following example does not declare the dc:
prefix using a prefix attribute:
However, an RDFa processor still recognizes the dc:title and
dc:creator short-hands and expands the values to the corresponding
URLs. The RDFa processor is able to do this because the dc prefix is
part of the default prefixes in the initial context.
Note
Default prefixes are used as a mechanism to correct RDFa documents where authors
accidentally forgot to declare common prefixes. While authors may rely on these
to be available for RDFa documents, the prefixes may change over the course
of 5-10 years, although the policy of W3C is that once a prefix is defined as
part of a default profile, that particular prefix will not be changed or
removed. Nevertheless, the best way to ensure that the prefixes that document
authors use always map to the intent of the author is to use the
prefix attribute to declare these prefixes.
Since default prefixes are meant to be a last-resort mechanism to help novice
document authors, the markup above is not recommended. The rest of this document
will utilize authoring best practices by declaring all prefixes in order to make
the document author's intentions explicit.
2.2
Going Deeper: RDFa Core
As we have seen in the previous sections, RDFa Lite is fairly powerful. Alice could
indeed express complex sets of structured information. However, there are cases when the
set of attributes presented so far does not cover all the needs, or make the resulting HTML
structure a bit awkward and possibly error-prone. In those cases additional RDFa
possibilities, provided through additional RDFa attributes, may come to the rescue; some
of these will be presented in this section.
Note
RDFa Lite does not define a separate class of RDFa processors. In other words conforming
RDFa processors are supposed to handle all RDFa features, not only those listed used by
RDFa Lite.
2.2.1
Using the content attribute
When creating her blog, Alice decided to use this simple structure to add Dublin Core
information to her blog post (see also Figure 2):
However, to do that, Alice had to accept a small compromise. Indeed, although the
string "2011-09-10" unambiguously identifies a date for a machine, it does not looks
very natural for a human reader. Surely a native English reader would prefer
something like "10th of September, 2011". On the other hand, although it is of course
possible for a machine to parse and interpret that string as a date, too, it is
clearly more complicated to do so. The problem is that, as a default, RDFa uses the
textual content of the element for the property value. While this works well in most
of the cases, sometimes, like in this example, this has awkward consequences.
To alleviate this problem RDFa makes it possible to re-use the content
attribute of HTML. The blog entry could be written as follows:
The resulting structure is exactly the same as before (i.e., Figure
2). The difference is the presence of the content attribute: it
instructs the RDFa processor to overrule the default behavior of using the textual
content, and to use the value of the content attribute instead. Using
this attribute Alice could provide a more readable date, while maintaining an
unambiguous content for machines using the structured data.
The content attribute has another important usage. The "traditional"
approach to add simple metadata to a Web page has been to use the document header
through the link and the meta elements. While there is no
problem using link in RDFa Lite (which uses the href
attribute, i.e., can be used to define "flavored" links), the fact that, in a
conforming HTML file, the meta element may have no text content means
that the only way of using the header for such statements is to use the
content attribute. For example, using the meta element is
the approach suggested by Facebook for the Open Graph Protocol [ogp] vocabulary;
i.e., if Alice wants to make use of the "Like" button in her posts, this is what she
would add to her header:
In this example the prefix for the Open Graph Protocol vocabulary is defined via the
prefix attribute. Alas, many authors forget to do so. Fortunately, the
og prefix is part of the initial context for RDFa, i.e., the resulting
information will be valid even without the prefix declaration…
2.2.2
Datatypes
Alice has already put license information on her page:
but she would like to complete this by recording the date of her copyright statement
as a structured data, too. She can use the date term of Dublin Core:
However, the value used for the date may be ambiguous for machines. Of course, if a
program "knows" that that http://purl.org/dc/terms/date refers to a
date, then of course it can find out that the string "2011" stands for a year. But
there may be processors that, for example, provide a visual presentation of all the
structured data on a specific page, and would like to use a different "widget" to
represent a year and again another one to represent, say, an integer number. How
would such a processor know which one to choose?
Alice may decide to be helpful by adding an additional information to that item in
the form of a datatype. This additional information can be conveyed to the
RDFa processor using the datatype RDFa attribute as follows:
where xsd:gYear stands for
http://www.w3.org/2001/XMLSchema#gYear, and is one of the standard
datatypes defined by W3C's Datatype
specification [xmlschema11-2] which contains such types as booleans, integers, dates,
or doubles. (xsd is one of the default
prefixes for RDFa.)
2.2.3
Alternative for setting the context: about
Alice has used the following patterns to define structured data for the individual
blogs:
The role of the resource attribute in the div element is to
set the "context", i.e., the subject for all the subsequent statements. Also, when
combined with the property attribute, resource can be used
to set the "target", i.e., the object for the statement (much as href).
This pattern is perfectly fine, but it may become too verbose in some cases. Indeed,
let us suppose that Alice would like to set up a separate index page for all her
blog posts, and the only information she would like to put there, as structured data, is
references to the titles. Following the same pattern, she would have to do something
like:
This of course works, but it is a bit convoluted. Merging the information into one
element, i.e.:
Example 42
<ul resource="/alice/posts/trouble_with_bob">
<li resource="/alice/posts/trouble_with_bob" property="title">The trouble with Bob</li>
...
</ul>
would not be correct; the combination of property and
resource would generate a different statement than originally intended.
RDFa introduces a separate attribute, called about, that can be used as
an alternative to resource in setting the the context. Using that
attribute, Alice could write:
The fundamental difference between about and resource is
that the former is only used to set the context, whether combined with the
property attribute on the same element or not. This also means that, for
such usage, about and resource are interchangeable; i.e.,
in her original blog item, Alice could have chosen to write:
Each "branch" in the list sets a separate object (blank nodes in this example) and
the same property (foaf:knows) is used to bind them to the same context.
The property="knows" had to be repeated in each list element to define
the corresponding property. If this structure is generated by some CMS systems, this
is of course not a problem. However, if such structure is authored manually, it is
clearly error prone: the property name can be misspelled or forgotten.
Instead, Alice could use another RDFa attribute, namely rel. Using this
attribute the corresponding HTML would look as:
In contrast to property, relnever considers the
textual content of an element (or the value of the content attribute).
Instead, if no clear target has been specified for a link via, e.g., a
resource or an href attribute, the processor is supposed to
go “down” and find one or more targets in the hierarchy and use those. This is what
happens in this case: the knows attribute on the ul
element does not include any obvious target; however, the processor finds those in
the individual li elements and will use those. This
pattern is typical for the usage of rel.
Note
In many situations, property and rel are interchangeable
when the intended structured data involves (flavored) links. There are, however,
subtle differences involving, for example, “chaining” that must be used with care.
The interested reader should consult the relevant section of the RDFa 1.1
specification for further details.
In general, it is advised to use property, when possible.
3.
You Said Something about RDF?
RDFa benefits from the power of RDF [rdf11-primer], the W3C's standard for interoperable
machine-readable data. Although readers of this document are not expected to understand RDF,
some may be interested in how these two specifications interrelate.
RDF, the Resource Description Framework, is the abstract data representation we have drawn
out as graphs in the examples above. Each arrow in the graph is represented as a
subject-property-object triple: the subject is the node at the start of the arrow, the
property is the arrow itself, and the object is the node or literal at the end of the arrow.
A set of such RDF triples is often called an "RDF graph", and is typically stored in what is
often called a "Triple Store" or a "Graph Store".
Consider the first example graph:
Fig. 14relationship value is text
The two RDF triples for this graph are written, using the Turtle syntax [turtle] for RDF,
is as follows:
Example 47
<http://www.example.com/alice/posts/trouble_with_bob>
<http://purl.org/dc/terms/title> "The Trouble with Bob" ;
<http://purl.org/dc/terms/created> "2011-09-10" .
The TYPE arrows we drew are no different from other arrows. The
TYPE is just another property that happens to be a core RDF property, namely
rdf:type. The rdf vocabulary is located at
http://www.w3.org/1999/02/22-rdf-syntax-ns#. The contact information example
from above should thus be diagrammed as:
Fig. 15blank node with rdf:type foaf:Person
The point of RDF is to provide a universal language for expressing data and relationships. A
unit of data can have any number of properties that are expressed as URLs. These URLs can be
reused by any publisher, much like any web publisher can link to any web page, even ones they
did not create themselves. Using data in the form of RDF triples, collected from various
locations, and also using the RDF query language SPARQL [sparql11-query], one can search for
"friends of Alice's who created items whose title contains the word 'Bob'," whether those
items are blog posts, videos, calendar events, or other data types.
RDF is an abstract data model meant to maximize the reuse of vocabularies. RDFa is a way to
express RDF data within HTML, in a way that is machine-readable, and by reusing the existing
human-readable data in the document.
3.1
Custom Vocabularies
As Alice marks up her page with RDFa, she may discover the need to express data, such as
her favorite photos, that is not covered by existing vocabularies. If she needs to, Alice
can create a custom vocabulary suited for her needs. Once a vocabulary is created, it can
be used in RDFa markup like any other vocabulary.
The instructions on how to create a vocabulary, also known as an RDF Schema, are
available in the RDF Primer [rdf11-primer]. At a high level, the creation of
a vocabulary for RDFa involves:
Selecting a URL where the vocabulary will reside, for example:
http://example.com/photos/vocab#.
Publishing the vocabulary document at the specified vocabulary URL. The vocabulary
document defines the classes and properties that make up the vocabulary. For example,
Alice may want to define the classes Photo and Camera, as well
as the property takenWith that relates a photo to the camera with which it
was taken.
Using the vocabulary in an HTML document either with the vocab attribute
or with the prefix declaration mechanism. For example: prefix="photo:
http://example.com/photos/vocab#" and typeof="photo:Camera".
It is worth noting that anyone who can publish a document on the Web can publish a
vocabulary and thus define new data fields they may wish to express. RDF and RDFa allow
fully distributed extensibility of vocabularies.
4.
RDFa Tools
There is a wide variety of tools that can be used to generate or process RDFa data. Good
sources for these are the RDFa page of the W3C
Semantic Web Wiki, although care should be taken that some tools may be related to a previous
version of RDFa. Another source may be the RDFa community site’s
implementation page. Both these sources are constantly evolving. By the way, the latter is
part of a more general community page that
contains further examples for using RDFa, general information, as well as information on how to get involved.
In particular, RDFa fragments can be tested using the
real-time RDFa 1.1 editor that can also display a
visual representation of the underlying structural data.
5.
Acknowledgments
At the time of publication, the active members of the RDF Web Application Working Group were:
Stéphane Corlosquet, Massachusetts General Hospital
Ivan Herman, W3C
Gregg Kellogg (Invited Expert)
Niklas Lindström (Invited Expert)
Shane McCarron, Applied Testing and Technology, Inc. (Invited Expert)
Steven Pemberton, Centre Mathematics and Computer Science
Manu Sporny, Digital Bazaar (Chair, Invited Expert)
Ted Thibodeau, OpenLink Software
Thanks also to Grant Robertson and Guus Schreiber who, though not part of the Working Group,
have provided useful comments on earlier drafts of this note.
