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Re: Which URI should be persistent when redirects are used?

From: Alan Ruttenberg <alanruttenberg@gmail.com>
Date: Sun, 30 Sep 2007 00:06:21 -0400
Message-Id: <22563A44-8827-4CF0-A5A0-7AEFF59E949D@gmail.com>
Cc: Misha Wolf <Misha.Wolf@reuters.com>, W3C-TAG <www-tag@w3.org>, semantic-web-ig list <semantic-web-ig.list@reuters.com>
To: wangxiao@musc.edu

Richard, I am concerned, in this question, with content negotiation,  
although some of the same questions arise with redirection. Also, I  
am also not concerned with the http browser activities. My concern is  
the Semantic Web, and that the sort of answers and definitions which  
are offered for the traditional web do not seem to work, or at least  
are not  understandable to me in the context of the semantic web.

Xiaoshu,

You wrote:
> Content negotiation doesn't change the URI.  The server returns  
> different representations for a particular request depending on the  
> MIME type and q score, but all the representation is under the same  
> URI.

Let's examine the following situation:

3 URIs
   http://example.com/depict/alan
   http://example.com/depict/alan.jpg
   http://example.com/depict/alan.png

http://example.com/depict/alan does content negotiation, and  
depending on whether the agent wants jpeg or png, redirects to one of  
the two other URIs.  The bits for the jpg and the png are in a file  
on the server's file system.

The question is what http://example.com/depict/alan is.

By my understand of your instruction, a web server should, in some  
circumstances, when asked for the resource identified by http:// 
example.com/depict/alan sometimes return the bits from the jpg  
document, and sometimes return the bits from the png document. These  
two different sets of bits are both "identified" by the same URI,  
http://example.com/depict/alan, each of which should be considered a  
representation of http://example.com/depict/alan

Similarly, when the web server is asked for the resource identified  
by http://example.com/depict/alan.jpg it should  return the bits from  
the jpg document and we say that the resource returned is a  
representation of http://example.com/depict/alan.jpg

Now, I step back, and move into a language which I understand better.  
I consider the jpg and png files documents in the traditional and  
easier sense - they are a series of bits. They won't change. I  
consider a "copy" any other document that has exactly the same series  
of bits. I am thinking that I would like URIs to to identify this  
document. Naively, perhaps, I choose http://example.com/depict/ 
alan.jpg, and http://example.com/depict/alan.png. If anyone asks me  
what I mean by resource in this case I will say: "By resource, I mean  
document, in the sense described". If they ask me what I mean by  
representation in this case, I will say: "I don't know. Ask Xiaoshu".

 From this mindset, I will at some time later encounter http:// 
example.com/depict/alan. Upon accessing it, I get a series of bits.  
Upon examining the bits, I find that they are the same set of bits as  
the document. I say, oh, http://example.com/depict/alan identifies  
the same document as http://example.com/depict/alan. Conclusion  
http://example.com/depict/alan is an alias for http://example.com/ 
depict/alan

Some time later I access http://example.com/depict/alan with a  
different agent. Upon accessing it, I get a series of bits. Upon  
examining the bits, I find that they are the same set of bits as the  
document. I say, oh, http://example.com/depict/alan identifies the  
same document as http://example.com/depict/alan.png. Conclusion  
http://example.com/depict/alan is an alias for http://example.com/ 
depict/alan

But wait, it is worse, I now have a URI that seems to break the  
rules, and identifies two documents

OK.

Let me now examine your response


On Sep 29, 2007, at 9:59 AM, Xiaoshu Wang wrote:

> The relationship between a "resource/thing" and a URI is not  
> symmetric.  One thing can have many URIs but a URI can only  
> identify one "resource/thing".

So far, so good.

> In the web, we only work with URI -> representations, but not the  
> other way around, right?

This I don't understand. I still don't understand what a  
representation is.

> So, in your question, what is the exactly the "document" you have  
> in mind?  A file on a remote server? Or a cgi script? Or something  
> else?

I've explained what I mean, in detail above. Please let me know if  
there is any ambiguity in what I am trying to convey.

> My point is: the question that you posted, i.e, if a "document" can  
> have multiple URIs, is an implementation issue.

This is not the question I asked. I asked whether content  
negotiation, as basic consequence of its architecture, introduces URI  
aliasing.

> It is irrelevant to the Web architecture so you can freely infer  
> anything.

The issue of URI aliasing, and its undesirability, is of consequence  
to the web, and to the semantic web.

> What is important is what is identified by a URI

Indeed

> , i.e., given a URI, what you can get back.

I have been given to understand that there is more to what a URI  
identifies then what you can get back, as, for instance URIs can  
identify things (resources) that are not information resources.

---

On the semantic web, we want our agents to be able to know what  
things mean. At the moment, the meaning of http://example.com/depict/ 
alan, "representation", "resources", and "identifies" are unclear to me.

-Alan
Received on Sunday, 30 September 2007 04:06:27 UTC

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