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Resources and Checksums (was RE: Subgroup to handle semantics of HTTP etc?)

From: Williams, Stuart (HP Labs, Bristol) <skw@hp.com>
Date: Tue, 23 Oct 2007 10:53:43 +0100
Message-ID: <C4B3FB61F7970A4391A5C10BAA1C3F0DEE031C@sdcexc04.emea.cpqcorp.net>
To: "Alan Ruttenberg" <alanruttenberg@gmail.com>, "Richard Cyganiak" <richard@cyganiak.de>
Cc: <wangxiao@musc.edu>, <www-tag@w3.org>, "Dan Connolly" <connolly@w3.org>, "Booth, David (HP Software - Boston)" <dbooth@hp.com>, "Jonathan A Rees" <jar@mumble.net>, "Tim Berners-Lee" <timbl@w3.org>


> -----Original Message-----
> From: Alan Ruttenberg [mailto:alanruttenberg@gmail.com] 
> Sent: 22 October 2007 21:22
> To: Richard Cyganiak
> Subject: Re: Subgroup to handle semantics of HTTP etc?

<snip/>
> > The value of httpRange-14, in my eyes, is simply this: It affirms
that 
> > web page URIs still identify web pages, even in the Semantic Web.
> 
> Web pages "the generic". This says that the URI identifies 
> something that could have a representation which is html, or 
> jpeg, or svg. But there is still a desire to be able to 
> identify each of these individually. It matters, for 
> instance, who you hire to do an update to each of these - 
> they require different tools and skills to change.
> 
> It seems to me that if you want to be able to denote these 
> with a URI, you are forced to accept that the appropriate 
> response for a web server is to respond 303.

I don't think that is necessarily the case. If you have such a resource,
you can do conneg, and you can provide a distinguihsed URI to refer to
the a specific resource (more specific in the sense that it provides
only a specific, possibly singular, subset of the responses available
from the generic resource) using a Content-Location header along with a
200 - OK response.

> Remember the test I proposed, that you seems to agree to?
> If it's an information resource, you can't get a checksum of it. 

This looks interesting... failed to find the exchange... do you have a
reference?

> If you can checksum it, it can't be an information resource. 
> Each of the three items I have suggested I want to denote 
> have can be checksummed.

I'd say that each of the particular representations that you speak of
can be - though the checksum is not an invariant - though likely
different for each representation. ie. the checksum is a property of the
representation not, in general, the resource). 

> > I like the "Halpin Test" [1]:
> >
> > "I would say that if there is a URI that is used to identify a 
> > resource one would want to make logical statements about, and these 
> > statements do not apply to possible representations of that
resource, 
> > then one should use the "hash" or 303 redirection to separate these 
> > URIs."
> >
> > To me, that's good enough as an every-day sniff test.
> 
> A good test, indeed. But suppose I am looking from the 
> outside and want to make a statement about such resources. In 
> other discussions we've concluded that pretty anything goes 
> as far as what the possible representations can be. How am I, 
> not the owner, able to figure out what the possible resources 
> are?

1) Experience of the resources and intuition - what you perceive them to
be.
   [ie direct experience of representations.]

2) What someone else (that you trust?) tells you about the resource.
   [1st and 3rd party descriptions of the resource].

> And what happens when I want to say more creative things 
> than the owner thought of, things that do not apply equally 
> to all the representations that she serves?

Maybe you have to temporally scope your statements... (eg. see the once
proposed tdb/duri URI schemes once proposed by Larry Masinter
<http://larry.masinter.net/duri.html>)

Maybe there are other scoping things that you need to be able to say. In
general URIs alone don't have the precision to designate particular
representations - sometimes the 'desired' effect is accomplished by
creating a resources whose sole purpose is to serve an invariant
representation or from an invariant set of representations - eg W3C
practice for TR page URIs.

> An how can I, as resource owner, decide that I want to mint a 
> URI to denote things that some might call representations? 

Set up a resource that serves the singular representation that you the
resource owner want it to serve - and baptise it with a URI. While
there's a 1-1 correspondence between resource and (the type of[*])
representations in provides - they are still (IMO) distinct - eg. the
resource can respond to requests whereas its representations are pretty
inert (and the latter can be checksummed).

> How am I to do that. Take, as an example,  the zip files on 
> http://mirror.nyi.net/ apache/lucene/java/ which has the 
> following instructions.
> 
> It seems to me that it's pretty hard to argue that http:// 
> mirror.nyi.net/apache/lucene/java/lucene-2.2.0-src.zip isn't 
> intended to denote something that can be checksummed, hence 
> not a resource.

I suspect that you and Richard weren't careful with the quantifier - or
maybe you were in baiting the trap :-)

> -Alan

Stuart
--
Hewlett-Packard Limited registered Office: Cain Road, Bracknell, Berks
RG12 1HN
Registered No: 690597 England
--
[*] By "type of representation" I am referring to one lack of clarity
wrt to representations. Two messages (representations) that have
identical bit/byte sequences are distinct messages (they occupy
different temporal extents and potentially traverse different paths in
(network) space). There is an ambiguity about whether the term
representation refers to a message occupying some particlar space/time
or a somewhat timeless bit sequence. One can think of the bit/byte
sequence as a type for all messages that carry that bit sequence. (on
account of having been inspired by Pat to read some Quine).
Received on Tuesday, 23 October 2007 09:54:16 UTC

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