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Re: Terminology Question concerning Web Architecture and Linked Data

From: Richard Cyganiak <richard@cyganiak.de>
Date: Sun, 22 Jul 2007 18:00:33 +0200
Message-Id: <C796D851-2FA0-4001-95FE-9201DF14659F@cyganiak.de>
Cc: Mark Baker <distobj@acm.org>, Chris Bizer <chris@bizer.de>, www-tag@w3.org, semantic-web@w3.org, Linking Open Data <linking-open-data@simile.mit.edu>
To: Dan Brickley <danbri@danbri.org>

On 22 Jul 2007, at 15:16, Dan Brickley wrote:
>> IMO, those URIs identify different resources.  I say this because  
>> they
>> all return different representations when I dereference them.  If  
>> they
>> identified the same resource then their representations would be
>> identical (see Roy's definition of resource in his REST  
>> dissertation).
> I am suprised to hear you say that. The Web allows that you might  
> receive different resource representations, or indeed nothing, on  
> each dereference. For example - a weather site, or a fortunate  
> cookie site. TimBL is not the weather, nor is he a fortune cookie,  
> admittedly. But it doesn't follow from the fact that you've  
> received different representations in different HTTP conversations,  
> that those conversations were about distinct things. Even if you  
> hold language negotiation, content negotiation, user agent etc  
> constant, even if you hold time constant, incoming packet size,  
> HTTP referer ... you've no right to assume you'll always get "the"  
> representation.

I think Roy's abstract model from REST goes something like this: A  
resource (these days we should say: information resource) is a  
function that maps a request Q received at time t to a representation  
R. If this function is the same for two information resources, then  
they are the same resource.

So the function can return different representations at different  
times, and it can return different representations at the same time  
for different requests (content negotiation).

And it's important to remember that this function is an  
implementation detail of the server. We cannot determine the function  
by inspecting the resource through its HTTP interface, because for  
that we'd have to be able to travel through time.

This means that only the resource owner can determine if two  
information resources are the same.

I think Roy's work is by far the best and definitive account of the  
things we now call information resources. But it does *not* account  
for any of the new stuff we're doing with URIs these days. Web  
architecture has evolved since Roy wrote his thesis. Unfortunately,  
many REST proponents are still stuck in the pre-httpRange-14 past,  
and are likely to remain there for a few more years.


> cheers,
> Dan
Received on Sunday, 22 July 2007 16:01:04 UTC

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