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Re: 200 OK with Content-Location might work

From: Phil Archer <phila@w3.org>
Date: Sun, 07 Nov 2010 19:35:51 +0000
Message-ID: <4CD6FF97.7060903@w3.org>
To: John Sheridan <johnlsheridan@yahoo.com>
CC: Niklas Lindström <lindstream@gmail.com>, Ian Davis <me@iandavis.com>, public-lod@w3.org
I share John's unease here. And I remain uneasy about the 200 C-L solution.

I know I sound like a fundamentalist in a discussion where we're trying 
to find a practical, workable solution, but is a description of a toucan 
a representation of a toucan? IMO, it's not. Sure, one can imagine an 
HTTP response returning a very rich data stream that conveys the entire 
experience of having a toucan on your desk - but the toucan ain't 
actually there.

I've been toying with the idea of including a substitution rule in a 200 

Following the practice of using /id/ for NIRs and /doc/ for their 
descriptions, suppose a GET on http://example.com/id/toucan returned:

200 OK
Apply-URI-substitution: s/id/doc/
Content-type: text/html
Blah blah...

In just one trip, user agents would then be able to interpret this as a 
document whose URI can be derived by performing the substitution, 
knowing that the returned data describes the thing identified by the 
original URI.

This approach, and C-L approach, both require client side implementation 
of course.

My worry is that any 200-based solution is going to be poorly 
implemented in the real world by both browsers and LOD publishers (Talis 
excepted of course!) so that IRs and NIRs will be indistinguishable 'in 
the wild'.

303 works already, and that is still the one that feels right to me. I'm 
happy that the discussion here is centred on adding a new method cf. 
replacing 303, especially as the HTTP-Bis group seems to have made its 
use for LOD and explicit part of the definition.


On 07/11/2010 15:07, John Sheridan wrote:
> Niklas,
> In general I am supportive of your and Ian's thinking. 200 OK with
> Content-Location might work.
> However, three points from my perspective:
> 1) debating fundamental issues like this is very destabilising for those
> of us looking to expand the LOD community and introduce new people and
> organisations to Linked Data. To outsiders, it makes LOD seem like its
> not ready for adoption and use - which is deadly. This is at best the
> 11th hour for making such a change in approach (perhaps even 5 minutes
> to midnight?).
> 2) the 303 pattern isn't *that* hard to understand for newbies and maybe
> even helps them grasp LOD. Making the difference between NIRs and IRs so
> apparent, I have found to be (counter-intuitively) a big selling point
> for LOD, when introducing new people to the paradigm. Let's not be too
> harsh on 303 - it does make an important distinction very clear for new
> adopters and, in my experience, it seems to be an approach new people
> grok quite quickly and easily.
> 3) I see much to commend in what Ian suggests, in practical terms. If
> the community is going to move in that direction what we need is a clear
> roadmap. An alternative pattern (say, 200 OK plus Content-Location)
> needs to be (*very* quickly) alighted upon and then used in practice. We
> would have to reconcile ourselves to the 303 pattern and the
> alternative, operating side-by-side, for some period of time (years?).
> Only once there is some breadth of usage, should the community seek to
> deprecate the use of 303s. If this is a pattern the community wishes to
> change, we have to gradually evolve our way to something different. We
> can't just leap.
> Hope these thoughts help,
> John.
> On Sun, 2010-11-07 at 14:42 +0000, John Sheridan wrote:
>> One use-case that we have with the Linked Data work for UK Government,
>> is where we have a URI for a NIR at one (notionally more stable) domain
>> which 303s to an IR at a different (less stable, organisationally
>> orientated) domain.
>> Often the NIR URI is something like
>> http://{something}.data.gov.uk/id/something whereas the IR is on an
>> organisation's own website.
>> We do this because organisations in the public sector are unstable and
>> subject to continual change (creation, merger, abolition) whereas the
>> government as a whole is very stable.
>> To give an example, the Open Government Licence (for the NIR of the
>> licence) is http://reference.data.gov.uk/id/open-government-licence
>> which 303s to
>> http://www.nationalarchives.gov.uk/doc/open-government-licence/ (the IR
>> of the current licence text, currently published by The National
>> Archives, with HTML and RDF representations selected through conneg)
>> We are looking at a similar pattern for local authorities. Each Council
>> would have a NIR URI at (something like)
>> local.data.gov.uk/id/{local-council-identifier} which would 303 to IR
>> about that Council on the Council's own website.
>> Our thinking is that the {something}.data.gov.uk URI is more likely to
>> survive machinery of government changes, but the organisation
>> responsible for (say) the Open Government Licence is always going to
>> want to publish the IR about that on its own website, and should be
>> encouraged to do so.
>> The 303 pattern helps enable this pattern, which fits well in general
>> with some of the challenges on Linked Data in the public sector.
>> I would like to understand a little better how Ian's proposal maps to
>> this use case.
>> Grateful for comments,
>> John.
>> On Sun, 2010-11-07 at 12:11 +0100, Niklas Lindström wrote:
>>> +1 indeed. Content-Location has definitely been overlooked. During
>>> conneg, it is used to differ between a resource and its
>>> representation(s), which are obviously different resources (well, not
>>> necessarily the same). This distinction could certainly be enough to
>>> remove the fundamental need for 303:ing on NIR:s (provided consensus
>>> and some formal resolution).
>>> (I pondered on a similar issue in
>>> <http://lists.w3.org/Archives/Public/public-rdf-in-xhtml-tf/2010Feb/0007.html>,
>>> regarding the identity of fragments. Perhaps that discussion would be
>>> worth revisiting again in light of this?)
>>> Best regards,
>>> Niklas
>>> On Fri, Nov 5, 2010 at 5:55 PM, Nathan<nathan@webr3.org>  wrote:
>>>> Mike Kelly wrote:
>>>>> http://tools.ietf.org/html/draft-ietf-httpbis-p2-semantics-12#page-14
>>>> snipped and fuller version inserted:
>>>>    4.  If the response has a Content-Location header field, and that URI
>>>>        is not the same as the effective request URI, then the response
>>>>        asserts that its payload is a representation of the resource
>>>>        identified by the Content-Location URI.  However, such an
>>>>        assertion cannot be trusted unless it can be verified by other
>>>>        means (not defined by HTTP).
>>>>> If a client wants to make a statement  about the specific document
>>>>> then a response that includes a content-location is giving you the
>>>>> information necessary to do that correctly. It's complemented and
>>>>> further clarified in the entity body itself through something like
>>>>> isDescribedBy.
>>>> I stand corrected, think there's something in this, and it could maybe
>>>> possibly provide the semantic indirection needed when Content-Location is
>>>> there, and different to the effective request uri, and complimented by some
>>>> statements (perhaps RDF in the body, or Link header, or html link element)
>>>> to assert the same.
>>>> Covers a few use-cases, might have legs (once HTTP-bis is a standard?).
>>>> Nicely caught Mike!
>>>> Best,
>>>> Nathan
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Phil Archer
W3C Mobile Web Initiative

Received on Sunday, 7 November 2010 19:36:26 UTC

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