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Introducing Embedded RDF

From: Ian Davis <iand@internetalchemy.org>
Date: Wed, 19 Oct 2005 09:29:48 +0100
Message-ID: <435603FC.70406@internetalchemy.org>
To: semantic-web@w3c.org

I've been working on an enhanced way to embed RDF into XHTML that 
doesn't require any new markup. There are existing methods such as the 
recommendations for expressing Dublin Core in HTML[1] but these are 
limited in scope and expressivity.

I've also spent quite a lot of time studying the underlying principles 
espoused by the Microformats community[2] who are reusing the semantics 
of XHTML to express commonly used data structures such as contact 
details or event descriptions. Some of the most important principles are:

* Visible Metadata - by making metadata visible consumers can easily 
form an opinion on whether to trust the author. Metadata hidden away in 
meta tags is easily abused for search engine placement or other gain 
since most visitors don't inspect the source code of the page. This 
principle also helps keep the metadata relevant. Hidden metadata is 
easily forgotten and can easily go stale whereas if it were visible to 
humans incaccuracies would soon be discovered and fixed.

* DRY (don't repeat yourself) - very often we maintain separate RDF 
documents with HTML equivilents. Unless these are automatically 
generated it's very easy for them to get out of synch. This principle 
suggests that the metadata should be expressed only once whether it's 
for humans or machines

* Reuse Not Reinvention - if we reuse existing formats then we 
immediately gain the benefit of being able to use existing tools to 
generate and consume the metadata. We also hook into the experience and 
knowledge of the thousands of people who have invested time and money 
getting to grips with existing technologies.

I've taken these principles on board in my design. Embedded RDF uses 
XHTML attributes such as 'rel', 'rev', 'class','href','src' and 'id' to 
embed RDF triples into an XHTML document. Triples can have a subject of 
the embedding document or of a fragment within that document. It's 
possible to use the 'rev' attribute to embed triples about other 
resources but the object of the triple must be the embedding document or 
               a fragment within it.

For example, the following XHTML:

<div><address class="dc-creator">Ian Davis</address> wrote this</div>

embeds the triple:

<> dc:creator "Ian Davis" .

Schema prefixes are declared using link elements in the head of the 
document, exactly as defined in the Dublin Core specification:

<link rel="schema.dc" href="http://purl.org/dc/elements/1.1/" />

The 'id' attribute is used to denote a seperate resource:

<p id="ian"><span class="foaf-name">Ian Davis</span> wrote this</p>

This generates a triple like this:

<#ian> foaf:name "Ian Davis" .

Anchor elements are used to refer to other resources:

<p id="ian">
   <a rel="foaf.homepage" href="http://example.org/home">my home page</a>

embeds the following triple:

<#ian> foaf:homepage <http://example.org/home> .

I hope this has given you a flavour of how Embedded RDF works. There is 
comprehensive documentation on our wiki here:


I'm also working on some examples. An example FOAF in XHTML document is 


And a sample DOAP in XHTML document:


There is also an RDF extraction service that scans an XHTML document for 
Embedded RDF and generates RDF/XML from it


This work is by no means complete, but I'm soliciting early feedback on 
the approach and the utility of embedding RDF into XHTML. Please feel 
free to share your thoughts on the wiki or email me if you have specific 
questions you'd like to have answered.


[1] http://www.dublincore.org/documents/dcq-html/
[2] http://microformats.org/
http://internetalchemy.org | http://purl.org/NET/iand
Working on... Silkworm <http://silkworm.talis.com/>
Received on Wednesday, 19 October 2005 10:09:47 UTC

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