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Re: Valid representations, canonical representations, and what the SW needs from the Web...

From: Roy T. Fielding <fielding@apache.org>
Date: Sun, 2 Feb 2003 02:47:46 -0800
Cc: www-tag@w3.org
To: Sandro Hawke <sandro@w3.org>
Message-Id: <C40FD01E-369B-11D7-AD42-000393753936@apache.org>

> Roy, you seem to be saying that in the REST model, one cannot use a
> URI to identify the Sun.

I don't know how you came to that conclusion.  URIs are just names.
As such, a URI can identify anything.

> In REST, as I read your explanation, the Sun
> is not a resource.

REST is an architectural style for network-based applications, and
thus whether or not the Sun is a resource depends entirely upon whether
it can be both identified and faithfully represented within some
system based on REST, not on the model itself.

REST-based systems never transfer resources.  If the identifier is
applicable outside the information system, then how it is represented
within the information system does not constrain what is identified.

> To remove an issue about intent and errors, let
> revise my two-pages-about-the-Sun example to say sun-c is always a
> picture of the sun and sun-d is always text about it.  You say the
> resource identified by each URI is the intended persistent sameness
> associated with each URI.  If the URI authority says the resource is
> the Sun itself, that authority is speaking nonsense (or metaphor).
> The authority could perhaps rephrase: the resource is a web page about
> the Sun (and perhaps be more specific).
>
> Is that right?   (It seems like an excellent model to me.  I like the
> term "persistent sameness".)

If the URI authority makes contradictory assertions, then at least
one is false, yes.

If the URI identifies the Sun, then the resource has no constraints
other than being the Sun.  What I believe is that you cannot start an
example by saying both "X is the Sun" and "X is always represented as
a picture", since we both know that the Sun isn't always represented
as a picture.  However, you can say that "X is represented by this
picture".

If I provide a link to my dissertation and it always responds to GET
with an HTML format, does that mean my dissertation is always HTML?  No.
Unless I tell you that a URI is constrained, you don't know.  Once I do
tell you the URI points to the top of the HTML edition of my 
dissertation,
then I have constrained the resource so that it no longer means just my
dissertation.  It is still a very useful link, and most people still
use it to indirectly identify the information within my dissertation,
but I would break somebody's link semantics were I to replace its
representation with the PDF edition.

BTW, my dissertation doesn't exist within the Web any more than the
Sun exists within the Web.  Nevertheless, I could mint an http URI
to identify my dissertation if I wanted to and the system wouldn't
know the difference.

....Roy
Received on Sunday, 2 February 2003 06:11:02 GMT

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