RDFa Primer

1.1 HTML vs. XHTML

To date, because XHTML is extensible while HTML is not, RDFa has only been specified for XHTML 1.1. Web publishers are welcome to use RDFa markup inside HTML4: the design of RDFa anticipates this use case, and most RDFa parsers will recognize RDFa attributes in any version of HTML. The authors know of no deployed Web browser that will fail to present an HTML document as intended after adding RDFa markup to the document. However, publishers should be aware that RDFa will not validate in HTML4 at this time. RDFa attributes validate in XHTML, using the XHTML1.1+RDFa DTD.

2.1 Licensing your Work

In her blog's footer, Alice declares her content to be freely reusable, as long as she receives due credit when her articles are cited. The XHTML includes a link to a Creative Commons [CC] license:

...
All content on this site is licensed under
<a href="http://creativecommons.org/licenses/by/3.0/">
    a Creative Commons License
</a>.

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 resume. 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 the rel attribute (which we'll write as @rel so as not to repeat the word "attribute" too often), which defines the relationship between the current page and the linked page. The value of the attribute is license, an XHTML keyword reserved for just this purpose:

...
All content on this site is licensed under
<a rel="license" href="http://creativecommons.org/licenses/by/3.0/">
    a Creative Commons License
</a>.

With this small update, Bob's browser will now understand that this link has a flavor: it indicates the blog's license.

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.

2.2 Labeling the Title and Author

Alice is happy that adding XHTML flavor lets Bob find the copyright license on her work quite easily. But what about the article title and author name? Here, instead of marking up a link, Alice wants to augment existing text within the page. The title is a headline, and her name a sub-headline:

<div>
   <h2>The trouble with Bob</h2>
   <h3>Alice</h3>
   ...
</div>

To indicate that h2 represents the title of the page, and h3 the author, Alice uses @property, an attribute introduced by RDFa for the specific purpose of marking up existing text in an XHTML page.

<div xmlns:dc="http://purl.org/dc/elements/1.1/">
   <h2 property="dc:title">The trouble with Bob</h2>
   <h3 property="dc:creator">Alice</h3>
   ...
</div>

Why use dc:creator and dc:title, instead of simply creator and title? As it turns out, XHTML does not have reserved keywords for those two concepts. Alice could boldly choose to write property="title", but how does a program reading this know whether "title" here refers to the title of a work, a job title, or the deed for some real-estate property? And, if every web publisher laid claim to their own short keywords, the world of available properties would become quite messy, a bit like saving every file on a computer's desktop without any directory structure to organize them.

To enforce a modicum of organization, RDFa does not recognize property="title". Instead, Alice must indicate a directory somewhere on the web, using simply a URL, from where to import the specific creator and title concepts she means to express. Fortunately, the Dublin Core [DC] community has already defined a vocabulary of useful concepts for describing documents, including both creator and title, where title indeed means the title of a work. So, Alice:

1. imports the Dublin Core vocabulary using xmlns:dc="http://purl.org/dc/elements/1.1/", which associates the prefix dc with the URL http://purl.org/dc/elements/1.1/, and
2. uses dc:creator and dc:title. These are short-hands for the full URLs http://purl.org/dc/elements/1.1/creator, and http://purl.org/dc/elements/1.1/title.

In RDFa, all property names are, in fact, URLs.

Literal Properties: RDFa lets Alice connect not just one URL to another—for example to connect her blog entry URL to the Creative Commons license URL— but also to connect one URL to a string such as "The Trouble with Bob". All arrows are labeled with the corresponding property name, which is also a URL.

2.3 Multiple Items per Page

Alice's blog contains, 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 @about, an attribute for specifying the exact URL to which the contained RDFa markup applies:

<div xmlns:dc="http://purl.org/dc/elements/1.1/">
			
   <div about="/alice/posts/trouble_with_bob">
      <h2 property="dc:title">The trouble with Bob</h2>
      <h3 property="dc:creator">Alice</h3>
      ...
   </div>

   <div about="/alice/posts/jos_barbecue">
      <h2 property="dc:title">Jo's Barbecue</h2>
      <h3 property="dc:creator">Eve</h3>
      ...
   </div>

   ...

</div>

We can represent this, once again, as a diagram connecting URLs to properties:

Multiple Items per Page: each blog entry is represented by its own node, with properties attached to each. Here we've used the short-hands to label the arrows, in order to save space and clarify the diagram. The actual labels are always the full URLs.

Alice can use the same technique to give her friend Bob proper credit when she posts one of his photos:

   <div about="/alice/posts/trouble_with_bob">
      <h2 property="dc:title">The trouble with Bob</h2>
      
      The trouble with Bob is that he takes much better photos than I do:
	
      <div about="http://example.com/bob/photos/sunset.jpg">
        <img src="http://example.com/bob/photos/sunset.jpg" />
        <span property="dc:title">Beautiful Sunset</span>
        by <span property="dc:creator">Bob</span>.
      </div>
   </div>

Notice how the innermost @about value, http://example.com/bob/photos/sunset.jpg, "overrides" the outer value /alice/posts/trouble_with_bob for all markup inside the innermost div. And, once again, as a diagram that abstractly represents the underlying data of this new portion of markup:

Describing a Photo

3.1 Contact Information

Alice already has contact information displayed on her blog.

<div>
   <p>
     Alice Birpemswick
   </p>

   <p>
     Email: <a href="mailto:[email protected]">[email protected]</a>
   </p>
   
   <p>
     Phone: <a href="tel:+1-617-555-7332">+1 617.555.7332</a>
   </p>

</div>

The Dublin Core vocabulary does not provide property names for describing contact information, but the Friend-of-a-Friend [FOAF] vocabulary does. In RDFa, it is common and easy to combine different vocabularies in a single page. Alice imports the FOAF vocabulary and 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:

<div typeof="foaf:Person" xmlns:foaf="http://xmlns.com/foaf/0.1/">
...

Then, Alice can indicate which content on the page represents her full name, email address, and phone number:

<div typeof="foaf:Person" xmlns:foaf="http://xmlns.com/foaf/0.1/">
   <p property="foaf:name">
     Alice Birpemswick
   </p>

   <p>
     Email: <a rel="foaf:mbox" href="mailto:[email protected]">[email protected]</a>
   </p>
   
   <p>
     Phone: <a rel="foaf:phone" href="tel:+1-617-555-7332">+1 617.555.7332</a>
   </p>
</div>

Note how Alice didn't specify @about like she did when adding blog entry metadata. What is she associating these properties with, then? In fact, the @typeof on the enclosing div implicitly sets the subject of the properties marked up within that div. The name, email address, and phone number are associated with a new node of type foaf:Person. This node has no URL to identify it, so it is called a blank node.

A Blank Node: blank nodes are not identified by URL. Instead, many of them have a @typeof attribute that identifies the type of data they represent. This approach—providing no name but adding a type— is particularly useful when listing a number of items on a page, e.g. calendar events, authors on an article, friends on a social network, etc.

3.2 Social Network

Next, Alice wants to add information about her friends, including at least their names and homepages. Her plain XHTML is:

<div>
   <ul>
      <li>
        <a href="http://example.com/bob/">Bob</a>
      </li>
      <li>
        <a href="http://example.com/eve/">Eve</a>
      </li>
      <li>
        <a href="http://example.com/manu/">Manu</a>
      </li>
   </ul>
</div>

First, Alice indicates that all of these friends are of type foaf:Person.

<div xmlns:foaf="http://xmlns.com/foaf/0.1/">
   <ul>
      <li typeof="foaf:Person">
        <a href="http://example.com/bob/">Bob</a>
      </li>
      <li typeof="foaf:Person">
        <a href="http://example.com/eve/">Eve</a>
      </li>
      <li typeof="foaf:Person">
        <a href="http://example.com/manu/">Manu</a>
      </li>
   </ul>
</div>

Beyond declaring the type of data we're dealing with, each @typeof creates a new blank node with its own distinct properties, all without having to provide URL identifiers. Thus, Alice can easily indicate each friend's homepage:

<div xmlns:foaf="http://xmlns.com/foaf/0.1/">
   <ul>
      <li typeof="foaf:Person">
        <a rel="foaf:homepage" href="http://example.com/bob/">Bob</a>
      </li>
      <li typeof="foaf:Person">
        <a rel="foaf:homepage" href="http://example.com/eve/">Eve</a>
      </li>
      <li typeof="foaf:Person">
        <a rel="foaf:homepage" href="http://example.com/manu/">Manu</a>
      </li>
   </ul>
</div>

And, of course, each friend's name:

<div xmlns:foaf="http://xmlns.com/foaf/0.1/">
   <ul>
      <li typeof="foaf:Person">
        <a property="foaf:name" rel="foaf:homepage" href="http://example.com/bob/">Bob</a>
      </li>
      <li typeof="foaf:Person">
        <a property="foaf:name" rel="foaf:homepage" href="http://example.com/eve/">Eve</a>
      </li>
      <li typeof="foaf:Person">
        <a property="foaf:name" rel="foaf:homepage" href="http://example.com/manu/">Manu</a>
      </li>
   </ul>
</div>

Using @property, Alice specifies that the linked text ("Bob", "Eve", and "Manu") are, in fact, her friends' names. With @rel, she indicates that the clickable links are her friends' homepages. Alice is ecstatic 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 tired of repeatedly entering information about her friends in each new social networking sites. With RDFa, she can indicate her friendships on her own web page, and let social networking applications 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, in fact, knows them, she uses the FOAF property foaf:knows:

<div xmlns:foaf="http://xmlns.com/foaf/0.1/" about="#me" rel="foaf:knows">
   <ul>
      <li typeof="foaf:Person">
        <a property="foaf:name" rel="foaf:homepage" href="http://example.com/bob">Bob</a>
      </li>
      <li typeof="foaf:Person">
        <a property="foaf:name" rel="foaf:homepage" href="http://example.com/eve">Eve</a>
      </li>
      <li typeof="foaf:Person">
        <a property="foaf:name" rel="foaf:homepage" href="http://example.com/manu">Manu</a>
      </li>
   </ul>
</div>

Using rel="foaf:knows" once is enough to connect Bob, Eve, and Manu to Alice. This is achieved thanks to the RDFa concept of chaining: because the top-level @rel is without a corresponding @href, it connects to any contained node, in this case the three nodes defined by @typeof. (The @about="#me" is a FOAF/RDF 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. You are what you eat, but you are far more than just your homepage.)

Alice's Social Network

4.1 Custom Vocabularies

As Alice marks up her page with RDFa, she may discover the need to express data, e.g. her favorite photos, that is not covered by existing vocabularies like Dublin Core or FOAF. Since RDFa is simply a representation of RDF, the RDF schema mechanism that enables RDF extensibility is the same that enables RDFa extensibility. Once an RDF vocabulary created, it can be used in RDFa markup just like existing vocabularies.

The instructions on how to create an RDF schema are available in Section 5 of the RDF Primer [RDF-SCHEMA-PRIMER]. At a high level, the creation of an RDF schema for RDFa involves:

1. Selecting a URL where the vocabulary will reside, e.g. http://example.com/photos/vocab#.
2. Distributing and RDF document, at that URL, which defines the classes and properties that make up the vocabulary. For example, Alice may want to define classes Photo and Camera, as well as the property takenWith that relates a photo to the camera with which it was taken.
3. Using the vocabulary in XHTML+RDFa with the usual prefix declaration mechanism, e.g. xmlns: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 an RDF vocabulary and thus define new data fields they may wish to express. RDF and RDFa allow fully distributed extensibility of vocabularies.