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Re: resources and URIs

From: Tim Berners-Lee <timbl@w3.org>
Date: Thu, 31 Jul 2003 15:06:26 -0400
Cc: Norman Walsh <Norman.Walsh@Sun.COM>, www-tag@w3.org
To: Sandro Hawke <sandro@w3.org>
Message-Id: <15668926-C38A-11D7-AB0E-000393914268@w3.org>


On Monday, Jul 28, 2003, at 15:43 US/Eastern, Sandro Hawke wrote:

>> representation of = something which portrays a resource; most often
>> in the arch doc a bag of bits with a MIME type and other metadata,
>
> That is: an expression in some identified language, a message with
> meaning.

Below you never mention the model which I assume.
A representation is not something which "portrays".
"portray" is to give information *about*.

A representation conveys information. Period.
"About" is an unnecessary and vague term we do not need in the 
architecture.
A representation is the expression of a message, thenn the thing 
identified by the URI is the message,
for that is what we expect to get an expression of when we dereference 
the URI.

We require much more than that the representation be *about* the same 
thing.

>  relationship between the message and its expression in Bits + MIME 
> Type == bits (raw information) + language
> identifier (indicating, by reference, the syntax and semantics of the
> language).  Sounds good.   In practice there's a kind of language
> layering and combining going on, with markup languages and header
> fields, but that can probably be ignored at this distance.
>
> But the important question is this: what is this "portrays"
> relationship between the resource and the representation.

If there is a "portrays" relationship, then it is between the car and 
the (bits+meta).
This is not very interesting for the architecture.
What does "Khubla Khan" portray? It is not easy to say, but it is not 
relevant either
so it doesn't matter.

As a web user. when I quote you
http://www.galegroup.com/free_resources/poets/poems/kublakhan.htm
What do I expect of this URI when I dereference it?

1. - That it is always something *about* stately pleasure domes?
2. - That it is always the poem "Khubla Khan"?
3. - That it is always a poem "Khubla Khan", as published in the 
Thompson/Gale collection?
4. - That it is always a particular set of bits?

The answer is  2 or 3.  That is what we expect to be invariant for the 
URI.
That is the basis on which I make hypertext links.
That is the basis upon which URIs are used.

That is why we say the URI identifies or denotes the poem.


What something "portrays" is *not* the important question for the 
architecture document.

I think that the word "representation"might have been historically 
unfortunate as it suggests "portrayal" (or a subject) rather than 
expression (of a message).

Tim
Received on Thursday, 31 July 2003 15:06:33 UTC

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