W3C home > Mailing lists > Public > www-tag@w3.org > March 2008

Re: Uniform access to descriptions

From: Phil Archer <parcher@icra.org>
Date: Thu, 20 Mar 2008 13:02:42 +0000
Message-ID: <47E26072.4040205@icra.org>
To: Harry Halpin <hhalpin@ibiblio.org>, "www-tag@w3.org WG" <www-tag@w3.org>
CC: Jonathan Rees <jar@creativecommons.org>

Ah Harry... you've been reading our docs again. POWDER includes the very 
predicate you suggest (describedBy), see [1]. And I think the semantics 
are aligned here.

For usage in RDFa we say:

1  <html
2    xmlns="http://www.w3.org/1999/xhtml"
3    xmlns:wdr="http://www.w3.org/2007/05/powder#"
4  >
5    <head>
6      <title>The English Civil War</title>
7      <link rel="wdr:describedby"
           href="http://ecw.example.org/powder1.xml" />
8    </head>

(And the doc goes on to give an example of using describedby in a 
hyperlink so you can retrieve the description before deciding whether to 
fetch the linked resource or not).

Our thinking on the Link Header is that a simple

Link: <powder.xml> rel="powder"; /="/"; type="application/xml"

would suffice. It has the advantage of using a header that has other 
uses that are well understood - a browser may be pleased to be able to 
grab a stylesheet before it has begun to parse the HTML for instance.

A new header such as

describedBy: <powder.xml> /="/"; type="application/xml";

is, I would suggest, only better in that it avoids the discussion of 
relationship type disambiguation - the relationship is hard-wired into 
the header itself.

Now... maybe these could be combined in some way so we'd end up with the 
relationship type being added to the header field:

Link-rel: <...

where rel could be stylesheet, transform, powder etc. But I'm not sure 
this is truly advantageous over just using Link as Mark N has described 
in his updated draft [2].

A relationship type of describedBy would be a generalisation of our 
likely proposal of rel="powder" - that would be fine by the POWDER WG I 
think.

Incidentally, you may notice I'm talking about XML, not RDF. The doc 
referred to at [1] is new and is the first one to set out the developing 
POWDER model which is an XML format with an associated GRDDL Transform 
that can render the data in RDF/OWL. Another POWDER doc will be 
published within the next few days that updates the resource grouping 
work and those with W3C Member access can see it at [3].

Phil.


[1] http://www.w3.org/TR/2008/WD-powder-dr-20080317/#semlink
[2] 
http://www.ietf.org/internet-drafts/draft-nottingham-http-link-header-01.txt
[3] http://www.w3.org/2007/powder/Group/powder-grouping/20080317.html

Harry Halpin wrote:
> Thanks Jonathan,
> 
>     I do think it is *important* for the the TAG to ratify a way for
> some sort of normative description to be attached to a resource.  The
> current suggestion, to revive the "Link" header, is a good one. However,
> it would be a mistake to "just" let a revived Link header do all the
> work. Just like in the case of 303 redirection, many average people do
> not understand HTTP headers in general. What would be best would be a
> general purpose solution that deploys a new RDF predicate (something
> stronger than rdfs:seeAlso and resembling foaf:primaryTopic) with
> "normative" import, so that an application can expect to find
> "authoritative" metadata there. This would be like the distinction
> between informative references in W3C specs (much like rdfs:seeAlso) and
> normative ones (what in RDF already covers this?).
> 
> This new predicate, let's call it ex:describedBy, should be able to
> instantiate itself in 3 ways:
> 
> 1) As a typed link header, as in "Link:
> http://www.example.org/mydescription
> rel="http://www.w3c.org/example/describedBy"
> 
> This should be equivalent to some RDF.
> 
> 2) As a normal RDF statement, as "http://www.example.com/resource
> http://www.w3c.org/example/describedBy http://www.example.org/mydescription
> 
> This RDF should be able to be embedded in any HTML page, in case user
> does not know about HTTP headers or RDF.
> 
> 3) In HTML (and arbitrary XML):
> <HTML><HEAD>
> <LINK rel="http://www.w3c.org/example/describedB"
> href="http://www.example.org/mydescription">
> </HEAD>
> ....
> More detail to follow, but I think we're generally on the right track
> with reviving the Link header. But we need to be careful about
> inscribing distinctions like "information resource" and so on into
> Webarch forever, and make sure any solution can operate on the most
> common levels of the Web equally. We also should make sure any solution
> is *easy* to deploy over various levels and makes it perfectly clear
> what's going on (somewhat unlike 303, which is rather hard to deploy and
> minimalist). Hope this helps!
> 
>        thanks,
>                 hary
> 
> Jonathan Rees wrote:
>> Dear all,
>>
>> The issue of uniform access to metadata (or "descriptions", see wiki
>> page below) has resurfaced recently on the www-tag mailing list and
>> elsewhere.  In case this characterization doesn't ring a bell, this is
>> the problem that the revival of the Link: header is supposed to solve
>> - given a URI, obtain information about, or associated with, the
>> document / resource / thing.
>>
>> At the risk of delaying progress I think it's worthwhile to verify
>> that we have the right solution, and to consider benefits (and costs)
>> of possible improvements.  We may never get another chance to get this
>> right.
>>
>> I have done some research to locate as many interested parties as
>> possible, and now invite you all to discuss this issue on the
>> www-tag@w3.org mailing list.  Information about subscribing is at [2].
>> Subscribing is probably a more reliable way to participate than
>> inclusion in a cc: , but if you prefer to participate without
>> subscribing we'll do our best to include you.
>>
>> At Dan Connolly's suggestion I would like to start by collecting use
>> cases, which I volunteer to collate, and use them to drive discussion
>> of exactly what form the solution should take and what process should
>> be used to standardize it.  So please send your uses cases to www-tag.
>>
>> I have started gathering materials on this topic on a wiki page [3],
>> which you are invited to browse.  This is a followon to a similar page
>> started by TimBL [4].
>>
>> I'm happy to do what I can to coordinate the effort and drive it to a
>> speedy conclusion.
>>
>> Best
>> Jonathan Rees
>> Science Commons and TAG
>>
>> [1] http://lists.w3.org/Archives/Public/www-tag/2008Feb/0013.html
>> [2] http://www.w3.org/Mail/Request
>> [3] http://esw.w3.org/topic/FindingResourceDescriptions
>> [4] http://esw.w3.org/topic/LinkHeader
>>
>> Bcc:
>>   Stuart Williams, TAG
>>   Jonathan Rees, TAG
>>   Tim Berners-Lee, TAG
>>   Dan Connolly, TAG, HTML5
>>   Ian Hickson, HTML5
>>   Harry Haplin, GRDDL
>>   Phil Archer, POWDER
>>   Sean Palmer
>>   Jonathan Borden
>>   Mark Nottingham, HTTP WG
>>   Ed Davies
>>   Mikael Nilson
>>   Ian Davis
>>   Patrick Stickler
>>   Graham Klyne
>>   Alan Ruttenberg
>>
>>
> 
> 

-- 
Phil Archer
Chief Technical Officer,
Family Online Safety Institute
w. http://www.fosi.org/people/philarcher/
Received on Thursday, 20 March 2008 13:03:24 GMT

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