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

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

> > The value of httpRange-14, in my eyes, is simply this: It affirms
> > 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

> 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
> > 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
   [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

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 
> apache/lucene/java/ which has the 
> following instructions.
> It seems to me that it's pretty hard to argue that http:// 
> 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

Hewlett-Packard Limited registered Office: Cain Road, Bracknell, Berks
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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