W3C home > Mailing lists > Public > www-tag@w3.org > October 2007

Re: HTTP URIs and authority

From: Pat Hayes <phayes@ihmc.us>
Date: Wed, 24 Oct 2007 12:40:47 -0500
Message-Id: <p0623090bc3452f9c5332@[]>
To: wangxiao@musc.edu
Cc: noah_mendelsohn@us.ibm.com, Alan Ruttenberg <alanruttenberg@gmail.com>, Mikael Nilsson <mikael@nilsson.name>, Richard Cyganiak <richard@cyganiak.de>, W3C-TAG Group WG <www-tag@w3.org>

>Pat Hayes wrote:
>>>Pat Hayes writes:
>>>>  > [Noah Mendelsohn wrote]:
>>>>  >  If I say that it's for the poem, and in a year or so someone
>>>>  >comes up with a font I like better, I see no problem with my changing
>>>>  >page to use that.
>>>>  Neither do I: but that doesn't mean that the URI denotes anything
>>>>  font-less, like the 'real' poem. It just means that your resource
>>>>  here has a changing font.
>>>Let's make the example more complicated.  I mint the URI and claim, at
>>>least insofar as you're willing to allow me too :-), that it represents
>>>the poem itself, not a particular presentation of it.  Because I'm a cool
>>>Web guy, I support content negotiation.  On day one, in addition to that
>>>fancy 25 point italic version served as text/html, I also offer
>>>text/plain, with each line of the poem given on one line of the returned
>>Thats still a particular document inscription of the poem, not the 
>>poem itself.
>>>  Now, as in the first example, I decide I've found a better font for
>>>the HTML, and I leave the text version unchanged.  You seem to imply that
>>>the resource itself has surely changed?
>>Something has changed, yes. I'm presuming that was the resource in 
>>question, no? Its a small change, admittedly, but it is a change.
>You seems contradicting yourself here.  User your term, the 
>"document inscription" has changed, the peom has not.

No, I'm here assuming the overall webarch framework and terminology 
(which I think you do not), according to which it is (literally) 
impossible to put an actual poem at an http endpoint, so the 
'resource' being talked about here is an inscription in html, or a 
complex of such inscriptions in various fonts. It is an 'information 

>>>Your view seems to be that the resource needs to, at least in some sense,
>>>be isomorphic to the representation, so you infer that when the
>>>representation changes the resource must have changed.
>>Well, that last point doesn't need the isomorphism assumption 
>>exactly. But yes, I'm assuming that webarch:representation is 
>>something like taking an imprint from a platen. It has to in some 
>>sense be a 'faithful' representation of 'all' of the resource. I 
>>agree this needs to be said more carefully to allow for content 
>Then, in that case, the scope of URI must be scaled back to our file systems.

For information resources, yes. But those are all things that already 
fit into our file systems.

>   Who is there to judge "faithful"?

True, true. Which is one reason I don't like the webarch usage of 
"representation". But let us say that what counts as 'faithful' 
should be determined by the content type. For HTML it is clear: it 
would not be acceptable for example to return only the first half of 
the HTML on a web page, or every other line, on the grounds that this 
was just an alternative kind of 'representation' of the HTML source.

>>>  My preferred view
>>>is that there is allowance for changing policy as to how a particular
>>>resource is represented, and that such changes to not necessarily imply
>>>that the resource itself has changed.
>>Well, that is a coherent position, I admit, but I don't like it, as 
>>it seems to reduce the webarch notion of 'representation' to 
>>vacuity. If I can change my resource without its 
>>webarch:representations changing, what *is* the relationship 
>>between the representation that GET delivers and the resource 
>>itself? It isn't, apparently, determined by HTTP or by media type 
>>or content negotiation. In fact, its not determined by anything at 
>>all. So it can be anything at all. I could say that this resource:
>Resource owner and URI owner are two different entities.

Well, OK, but who owns Moby Dick? Or change my example so that the 
jpeg URI denotes, I will say, a paper I wrote (and own the copyright 
of), say the dialog to be found here:


Now, what's to stop me claiming that the image is just a rather 
unusual representation of the text? According to Noah, I can choose 
my own 'style' for my representations. But this seems silly to me.

>   The URI owner gives you his/her *representation* of the resource. 
>For a given resource, there can potentially have multiple URIs, when 
>grounded to the web, someone will provide a better representation of 
>the resource than the other.  Eventually we choose those better ones 
>and disregard the rest. Doesn't this sound right?

Well, but now you are raising a different point. That does sound 
nice, but the problem I have with it is how to know which of the 
representations is best. We have literally NO access to the actual 
resources, and if representations can have arbitrary 'stylistic' 
differences, then we can't rely on any fixed semantics. So we would 
seem to be lost in a cloud of unknowing.


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Received on Wednesday, 24 October 2007 17:41:01 UTC

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