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Re: [httpRange-14]: New Draft Finding "Dereferencing HTTP URIs"

From: Norman Walsh <ndw@nwalsh.com>
Date: Wed, 30 May 2007 11:04:03 -0700
To: www-tag@w3.org
Message-ID: <877iqq4298.fsf@nwalsh.com>
/ "Williams, Stuart (HP Labs, Bristol)" <skw@hp.com> was heard to say:
| A new draft TAG finding, "Dereferencing HTTP URIs" is available for
| review at:
|
| http://www.w3.org/2001/tag/doc/httpRange-14/2007-05-31/HttpRange-14

I inadvertantly sent these comments to the TAG member list. Here's a
public copy:

[...]
|2 Information Resources
|
|   Information resources are resources, identified by URIs and whose
|   essential characteristics can be conveyed in a message [AWWW].

Suggest: s/, identified by URIS and/identified by URIs/

|   It is not appropriate for any of the individual representations that
|   Angela is considering to be returned by dereferencing the URI that
|   identifies the concept of the meter. Not only do the representations she
|   is considering fail to represent the concept of the meter, they each have
|   a different essence and so they should each have their own URI.

I like the example, but because the resource identified is a concept,
it's a little subtle. On any given Thursday afternoon, I could easily
be persuaded that the following text/plain representation identifies
quite clearly the concept of a meter:

  A meter is a unit of measurement. It is the length of 1,650,763.73
  wavelengths of the orange-red emission line in the electromagnetic
  spectrum of the krypton-86 atom in a vacuum. It is a little longer
  than three feet in the antiquated American style of measurement.

I have sometimes found it easier to understand the distinction we are
drawing by discussing physical objects: the laptop I am composing this
message on, for example.

Whereas one might imagine that an information resource could convey
everything there is to be known about the "metre-ness" of a meter, no
one is likely to be convinced by any quantity of information that they
possess the physical artifact unter my fingers if they do not, in
fact, possess it.

|           +----------------------------------------------------------------+
|           | Editorial note                       |                         |
|           |----------------------------------------------------------------|
|           | There is a question in my mind about how much we can infer     |
|           | from receiving a 303, and in particular the fact that we       |
|           | didn't get a 200. We say that a response of 200 means that the |
|           | dereferenced resource is an information resource. Also, the    |
|           | HTTP spec says that the URI returned with a 303 is "not a      |
|           | substitute reference for the originally requested resource"    |
|           | (quote from RFC2616). Hence the URIs are distinct, and so are  |
|           | the resources. Does that mean we can never get a               |
|           | representation from the original URI? If so, we should be able |
|           | to infer that it can't refer to an information resource? The   |
|           | question is are there any reasons for getting a 303 from an    |
|           | information resource? Can the behaviour of a 303 ever be       |
|           | associated with an information resource? For example, could a  |
|           | 303 ever be a valid response from a failure of conneg? I don't |
|           | think so, because it would be being used to say "I don't have  |
|           | a representation of this but you might find one over there"    |
|           | This is at odds with the 303 statement about the redirected    |
|           | URI not being a substitute for the originally requested one.   |
|           +----------------------------------------------------------------+

I think it would be possible to concoct a scenario where 303 made
sense for an information resource. The things that occur to me
immediately, information resources previously available but now known
to be unavailable but about which other sources of information (but
not copies which would be 307) might be known, seem pretty far
fetched.

I could live with a statement that 303 should not be used with
information resources, but I wouldn't personally fight for it :-)

                                        Be seeing you,
                                          norm

-- 
Norman Walsh <ndw@nwalsh.com> | What is more wonderful than the delight
http://nwalsh.com/            | which the mind feels when it *knows*?
                              | This delight is not for anything beyond
                              | the knowing, but is in the act of
                              | knowing. It is the satisfaction of a
                              | primary instinct.--Mark Rutherford

Received on Wednesday, 30 May 2007 18:04:21 GMT

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