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Re: httpRange-14 Change Proposal

From: Nathan <nathan@webr3.org>
Date: Wed, 28 Mar 2012 17:40:12 +0100
Message-ID: <4F733EEC.7020702@webr3.org>
To: Jeni Tennison <jeni@jenitennison.com>
CC: public-lod@w3.org, "www-tag@w3.org List" <www-tag@w3.org>
Jeni Tennison wrote:
> Nathan,
> 
> On 28 Mar 2012, at 16:07, Nathan wrote:
>> Jeni Tennison wrote:
>>> Yes, that's correct. With no constraining Accept headers, it could alternatively return HTML with embedded RDFa with a <link rel="describedby"> element, for example.
>> Is that universally true?
>>
>> Suppose /uri identified a PDF formatted ebook, or a digital image of a monkey in JPEG format, or even an RDF document.
> 
> Then it would return those things. I think that you may have leapt to the conclusion that /uri *always* returns the same as /uri-documentation. There's nothing to my knowledge that says that, indeed given that you can have several :describedby links it would be impossible.
> 
>> Question A:
>>
>> Currently we have:
>> <http://example.org/uri> - a JPEG image of a monkey.
>>
>> When you issue a GET on that URI the server currently responds
>>  200 OK
>>  Content-Type: image/jpeg
>>  Link: <http://example.org/uri-documentation>; rel="describedby"
>>
>> So under this new proposal, the server can return the contents of /uri-documentation with a status of 200 OK for a GET on /uri?
> 
> Under the proposal, the server would return the JPEG with a 200 OK for a GET on /uri. http://example.org/uri-documentation would return a description of the JPEG in some machine-readable format. 

Previously I asked:

"With this proposal though we'd be able to say issue a GET to /uri with 
an Accept header value of text/turtle, and the server could return back 
the contents of /uri-documentation, with a status of 200 OK"

And you replied "Yes, that's correct."

The above is exactly the same, so I'll ask again:

With this proposal can a server can return the contents of 
/uri-documentation with a status of 200 OK for a GET on /uri?

If the answers yes, then it must be yes for my previous "JPEG of a 
monkey" example (universality), if the answer is no then how does this 
proposal work? Apologies for my confusion.

Best,

Nathan

>> If yes, this seems like massively unexpected functionality, like a proposal to treat "Accept: some/meta-data" like a DESCRIBE verb, and seems to exaggerate the URI substitution problem (as in /uri would be taking as naming the representation of /uri-documentation).
>>
>> If no, where's the language which precludes this? (and how would that language go, given that it's exactly the same protocol flow and nothing has changed - other than the reader presuming that /uri now identifies something that does have a representation that can be transferred over HTTP vs identifying something that doesn't have a representation that can be transferred over HTTP).
> 
> I don't really understand what you think it needs to say I'm afraid.
> 
>> Question B:
>>
>> How would conneg work, and what would the presence of a Content-Location response header mean? Would HTTPBis need to be updated?
> 
> I can't see any way in which any of that would work differently from currently.
> 
>> Question C:
>>
>> Currently 303 "indicates that the requested resource does not have a representation of its own that can be transferred by the server over HTTP", and the Link header makes it clear that you are dealing with two different things (/uri and /uri-documentation), but where does this proposal make it clear at transfer protocol level that the representation included in the http response is a representation of another resource which describes the requested resource (rather than it being as the spec defines "a representation of the target resource")?
> 
> The proposal says that applications can draw no conclusions from information at the transfer protocol level about /uri. In particular, it can't tell whether the representation that is returned with /uri is *the content* of /uri or *the description* of /uri. Further information about /uri (eg that it is a foaf:Person) may help the application work out that the representation was *a description*.
> 
> However, an application can draw conclusions about /uri-documentation, assuming it gives a 2XX response, because it has been retrieved as the result of following a :describedby link (or if it were the target of a 303 redirection). The application can tell that the representation from /uri-documentation is *the content* of /uri-documentation and *the description* of /uri.
> 
>>> Either way, there is no implication that what you've got from http://example.org/uri is the content of http://example.org/uri (or that http://example.org/uri identifies an information resource), but there is an implication that what you get from http://example.org/uri-documentation is the content of http://example.org/uri-documentation (and that http://example.org/uri-documentation is an information resource).
>> Sorry I don't follow, how is there an implication from a 200 OK for <uri-a> that it's not an IR and for <uri-b> that it is an IR?
> 
> Because /uri-documentation was reached through a :describedby link. This extra information allows the application to draw the conclusion that the representation from /uri-documentation is *the content* of /uri-documentation.
> 
>> If there was a Set of all Things (Set-A), then that set would have two sets, "the set of all things which can be transferred via a transfer protocol like HTTP" (Set-B), and then everything else (Set-C) which comprises Set-A minus Set-B. As far as I can tell, the one thing that determines whether something is a member of the Set-B or Set-C, for HTTP, is that 200 OK in response to a GET, hence why we need the 303.
>>
>> This proposal appears to try and override that "rule" (fact) by saying let the content of a representation define what is a member of Set-B or Set-C, however the act of dereferencing itself is what determines whether an identified thing is a member of Set-B, as Set-B is the set of all things that can be dereferenced. Hence my confusion at this proposal.
> 
> 
> The "fact" that a 200 OK determines whether something is a member of Set-A or Set-B is a design choice made by httpRange-14, not a fundamental truth of the universe. The proposal makes a different design choice, in saying that you need more than just a 200 OK response to say, beyond all doubt, that a URI refers to something that is member of Set-B.
> 
> Cheers,
> 
> Jeni
Received on Wednesday, 28 March 2012 16:41:32 UTC

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