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Re: TB16 Re: Comments on arch doc draft

From: Paul Prescod <paul@prescod.net>
Date: Wed, 03 Jul 2002 10:49:02 -0700
Message-ID: <3D23390E.AF568A85@prescod.net>
To: Patrick Stickler <patrick.stickler@nokia.com>
CC: WWW TAG <www-tag@w3.org>

Patrick Stickler wrote:
> A picture of a rock is not an HTTP representation of the rock.

Well, this is the core issue of the debate. Fielding claims that was his

"The key abstraction of information in REST is a resource. Any
information that can be
named can be a resource: a document or image, a temporal service (e.g.
“today’s weather
in Los Angeles”), a collection of other resources, a non-virtual object
(e.g. a person), and
so on. In other words, any concept that might be the target of an
author’s hypertext
reference must fit within the definition of a resource. A resource is a
conceptual mapping
to a set of entities, not the entity that corresponds to the mapping at
any particular point in

> Web applications do not have access to the actual resource, no, but
> a representation of the resource is analogous to getting the actual
> resource. For that to happen, the resource has to be digitally
> encoded in some fashion.

The word "analogous" does not give me a warm and fuzzy. Either you can
get the real resource or you cannot. HTTP says you cannot.

The resource could be something concrete like an XML file.
It could be something transient, like the result of a database query.
It could be something abstract like a person.

If Web clients *cannot* tell the difference because the resource is
hidden, then why is it useful to make the distinction architecturally?

> A rock (at least for the moment) cannot be digitally encoded, and therefore
> is not web-accessible.

No. A representation of it is.

> A picture of a rock is not the rock. A prose description of a rock
> is not the rock. Yet the picture, the description, and the rock all
> may be separately denoted by URI.
> If you use the same URI to denote the rock and the picture of the
> rock, how do you say anything about either without ambiguity? You
> can't.

Let's change the language above:

"An XHTML representation of an XML file is not the XML file. A
Formatting Objects representation of an XML file is not the XML file....

If you use the same URI to denote the XHTML representation and the
formatting objects representation, how do you say anything about either
without ambiguity? You can't."

This goes back to Noah's point. Semantic web languages should have ways
of saying whether you are asserting something about the resource or some
particular representation. If the language does this then the appearance
of overloading disappears.
> >> ...
> >> A schema is not a representation of a vocabulary. Sorry. Nope. It's
> >> something that uses the terms of a vocabulary, but the vocabulary itself
> >> is abstract, just as are the terms.
> >
> > Why not?
> A vocabulary is an abstract set of terms with associated semantics.
> A schema is a document that describes vocabulary terms.

Yes I see the difference. But it it is totally irrelevant. Things that
are representations of other things are *always different* otherwise why
bother with the representation?

You might as well say that "An HTML entity is not a representation of an
XML entity" because XML and HTML entities are different. 

> And, e.g. an XML Schema does not define a vocabulary. 

Nobody said that an XML Schema defines a vocabulary. I said that it
*represents* a vocabulary as an HTML rendition is a lossy representation
of an XML original.

Come discuss XML and REST web services at:
  Open Source Conference: July 22-26, 2002, conferences.oreillynet.com
  Extreme Markup: Aug 4-9, 2002,  www.extrememarkup.com/extreme/
Received on Wednesday, 3 July 2002 13:49:37 UTC

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