Re: Question about the On Linking Alternative Representations TAG Finding

Thanks Raman.

Many people have added their bits to the thread. I will try to  
summarize the discussion and add a few thoughts of my own; and finally  
ask yet another question about a case that wasn't discussed yet.

First, it seems clear that this is not a question of Web architecture,  
but rather a question of good practices. Neither setup violates Web  
architecture, but different setups might be more or less useful to  

Second, it seems worth to mention terminology. The original resource  
(at /resource in my example) is the “real” resource we are interested  
in. It's a “generic resource” because it is available in different  
formats. The format specific URIs identifies specific *variants*,  
which are *representations* of this original resource. However, by  
assigning URIs to those representations, we are turning the  
representations into resources of their own right. (Because, by  
definition, everything that is identified by a URI is a resource.)

Third, many people have voiced this opinion: If the client indicates  
that it does not support the format available at this URI, then HTTP  
406 should be sent. (This is clearly a SHOULD, not a MUST, according  
to the language in RFC 2616.)

Fourth, I would like to point out that the case discussed under  
“third” does not arise if the client is a normal *Web browser*,  
because they all have */* in their accept headers and thus indicate at  
least *some* support for *every* format. But the case does arise with  
other HTTP clients that do not accept */*.

I'm still unclear about one particular case: What should be done if a  
client indicates support for several formats, but prefers one over the  
others; and the URI is specific to one of the non-preferred formats?  

     Accept: text/html;q=1.0, application/rdf+xml;q=0.5

This client supports HTML and RDF, but it prefers HTML over RDF.

If /resource.rdf is accessed, what should happen? The URI is  
specifically for the RDF variant. So we have a conflict between the  
resource (which is available in RDF) and the preference of the browser  
(which would rather have HTML than RDF). Should the server ignore the  
preference and send RDF anyway?


On 31 Jul 2008, at 18:03, T.V Raman wrote:

> Thanks for the clarification -- I understand your specific
> question much  better now.
> I dont think you should respond with a 406 -- since that is the
> least helpful of all to the client.
> If the client requests resquest.html, and says it *only* accepts
> mime-type json,
> and you actually have a json representation available, it is
> really a toss-up as to what you would send. I myself tend to lean
> toward sending the JSON representation in this case, mostly
> because I beleive that the content-type should be trusted more
> than the infered mime-type available through the URL extension
> --- but that is a contentious topic in its own right.
> But this is a good issue that you have raised --- and is an
> example of two pieces of the HTTP Request being in conflict, with
> the specifications giving no clear   guidance as to how to break
> the conflict --- especially when taken in context of how
> user-agents behave at present.
> Richard Cyganiak writes:
>> Raman,
>> My question was not at all about HTML vs. XHTML, Sebastien injected
>> that angle into the thread in a somewhat unhelpful way. I'm not
>> interested in hearing yet more people's opinions on the issue. I ask
>> for clarification of a detail in the TAG finding because I'm not sure
>> how to interpret the finding's intention.
>> Let's say I have
>> /resource      (generic information resource with HTML and JSON
>> variants)
>> /resource.html (a HTML specific URI)
>> /resource.json (a JSON specific URI)
>> Now let's say I request /resource.json with an Accept header of
>> "Accept: text/html". What should happen?
>> One opinion is that the JSON should be served anyway, because the URI
>> identifies a specific variant.
>> Another opinion is that the HTML should be served, or redirected to,
>> because that's what the client asked for and the server has it
>> available.
>> (A third opinion is that 406 should be answered, as suggested by
>> Sebastien.)
>> What I'm asking for is simply a clarification of the advice in the
>> spec. Did you intend that there be content negotiation on the
>> representation_i URIs?
>> Cheers,
>> Richard
>> On 31 Jul 2008, at 17:21, T.V Raman wrote:
>>> I'm a bit confused at this point by the question.
>>> Could you flesh out your example?
>>> HTML  is a particularly good example, depending on how much you
>>> know about your HTML  resource.
>>> Though there is much debate on this, in my experience,
>>> wel--formed  XHTML survives well when served either as text/html
>>> or application/xml+xhtml.
>>> So if you have a resource under your control whose  content you
>>> know is well-formed XHTML that you would rather serve as
>>> application/xml+xhtml, but you also know that many legacy agents
>>> only accept text/html
>>> In that case, you might definitely want to  check the accept
>>> header and serve the appropriate response.
>>> Sebastien Lambla writes:
>>>>> Is it ever appropriate to configure content negotiation on the
>>>>> *representation-specific URIs*? So, if someone requests the
>>>>> specific  URI
>>>>> for representation_1, but the Accept header indicates a
>>>>> preference  for
>>>>> representation_3, should content negotiation kick in and
>>>>> representation_3
>>>>> be served instead?
>>>> If your url is the representation-specific one, then the conneg
>>>> would fail
>>>> if the content-type of /resource.html is text/html and the Accept:
>>>> only
>>>> contains application/xhtml+xml, as the representation is not the
>>>> resource
>>>> and the url you requested is the one of the representation, not the
>>>> resource. I would return a 406.
>>>> I'd understand the reasoning as being that if you dereference /
>>>> resource.html
>>>> and get a 200 you can assert it is a document, if you were to
>>>> conneg to
>>>> another url from the specific url you loose that assertion as
>>>> defined in
>>>> httpRange-14
>>>> Sebastien Lambla
>>> -- 
>>> Best Regards,
>>> --raman
>>> Title:  Research Scientist
>>> Email:
>>> WWW:
>>> Google: tv+raman
>>> GTalk:,
>>> PGP:
> -- 
> Best Regards,
> --raman
> Title:  Research Scientist
> Email:
> WWW:
> Google: tv+raman
> GTalk:,
> PGP:

Received on Tuesday, 5 August 2008 14:19:41 UTC