W3C home > Mailing lists > Public > public-lod@w3.org > March 2010

Re: A question - use 301 instead of 406?

From: Richard Cyganiak <richard@cyganiak.de>
Date: Wed, 24 Mar 2010 13:13:56 +0000
Cc: Linking Open Data <public-lod@w3.org>
Message-Id: <76595DB5-A948-49F6-BDF9-DF29640938F2@cyganiak.de>
To: Hugh Glaser <hg@ecs.soton.ac.uk>

On 23 Mar 2010, at 22:50, Hugh Glaser wrote:
> Assuming that we are in the usual situation of http://foo/bar doing  
> a 303 to
> http://foo/bar.rdf when it gets a Accept: application/rdf+xml http://foo/bar
> what should a server do when it gets a request for
> Accept: application/rdf+xml http://foo/bar.html ?
> OK, the answer is 406.

No. The answer is 200, with the HTML representation. Content  
negotiation should happen on the “generic” URI, e.g., <http://foo/ 
bar>, but not on the representation format specific URIs.

The reason for having the representation format specific URIs </ 
bar.html> and </bar.rdf> in the first place is to allow users to  
override their user agent's Accept header.

For example, normal web browsers accept text/html but not application/ 
rdf+xml. There is no way how an average user can change the browser's  
behaviour in this regard. Thus, if I direct my browser to </bar> I  
would always get HTML. If, for whatever reason, I want to see the RDF/ 
XML, there's no way how I can do it. But if the </bar.rdf> URI is  
configured to always returns RDF/XML, no matter what the Accept header  
says, then the HTML can include a link to </bar.rdf> and say, “go here  
if you really want RDF/XML.” Problem solved.

Sending 406 (or 301) on the representation format specific URIs like </ 
bar.html> and </bar.rdf> negates the entire purpose of having those  
URIs in the first place.

A key bit of text from RFC 2616:

>> Note: HTTP/1.1 servers are allowed to return responses which are  
>> not acceptable according to the accept headers sent in the request.  
>> In some cases, this may even be preferable to sending a 406 response.

Amen. 406 is actually counterproductive IMO. It just forces user  
agents to include something like "*/*;q=0.01" in the Accept header to  
work around those overeager content negotiation implementations that  
are just looking for an excuse not to send a representation to the  

> That's OK if all that happens is I use the wrong URI straight away.
> But what happens if I then enter it into a form that requires a LD  
> URI, and
> then perhaps goes into a DB, and becomes a small part of a later  
> process?
> Simply put, the process will fail maybe years later, and the  
> possibility and
> knowledge to fix it will be long gone.
> Maybe the form validation is substandard, but I can see this as a  
> situation
> that will recur a lot, because the root cause is that the address  
> bar URI
> changes from the NIR URI. And most html pages do not have links to  
> the NIR
> of the page you are on - I am even told that it is bad practice to  
> make the
> main label of the page a link to itself - wikipedia certainly doesn't,
> although it is available as the "article" tab, which is not the  
> normal thing
> of a page. SO in a world where wikipedia itself became LD, it would  
> not be
> clear to someone who wanted the NIR URI where to find it.

This is a serious problem. It is a UI problem and should be solved on  
the UI level, not on the transfer protocol level. We have lots of  
protocol people here and few UI people, so everyone tries to fix  
everything in the protocols.

A similar problem has plagued RSS in its early years. The solution was  
the feed autodiscovery convention for the HTML header, and the  
universal feed icon. Linked data needs something similar.


> So that is some of the context and motivation.
> If we were to decide to be more forgiving, what might be done?
> How about using 301?
> <<Ducks>>
> To save you looking it up, I have appended the RFC2616 section to this
> email.
> That is
> Accept: application/rdf+xml http://foo/bar.html
> Should 301 to http://foo/bar
> It seems to me that it is basically doing what is required - it  
> gives the
> client the expected access, while telling it (if it wants to hear)  
> that it
> should correct the mistake.
> One worry (as Danius Michaelides pointed out to me) is that the  
> caching may
> need careful consideration - should the response indicate that it is  
> not
> cacheable, or is that not necessary?
> So that's about it.
> I am unhappy that users doing the obvious thing might get frustrated  
> trying
> to find the URIs for heir Things, so really want a solution that is  
> not just
> 406.
> Are there other ways of being nice to users, without putting a serious
> burden on the client software?
> I look forward to the usual helpful and thoughtful responses!
> By the way, I see no need to 301 to http:/foo/bar if you get a
> Accept: text/html http://foo/bar.rdf as the steps to that might lead  
> to this
> would require someone looking at an rdf document to decide to use it  
> as a
> NIR, which is much less likely. And the likelihood is that there is an
> eyeball there to see the problem.
> But maybe it should?
> Best
> Hugh
> 10.3.2 301 Moved Permanently
>   The requested resource has been assigned a new permanent URI and any
>   future references to this resource SHOULD use one of the returned
>   URIs.  Clients with link editing capabilities ought to automatically
>   re-link references to the Request-URI to one or more of the new
>   references returned by the server, where possible. This response is
>   cacheable unless indicated otherwise.
>   The new permanent URI SHOULD be given by the Location field in the
>   response. Unless the request method was HEAD, the entity of the
>   response SHOULD contain a short hypertext note with a hyperlink to
>   the new URI(s).
>   If the 301 status code is received in response to a request other
>   than GET or HEAD, the user agent MUST NOT automatically redirect the
>   request unless it can be confirmed by the user, since this might
>   change the conditions under which the request was issued.
>      Note: When automatically redirecting a POST request after
>      receiving a 301 status code, some existing HTTP/1.0 user agents
>      will erroneously change it into a GET request.
Received on Wednesday, 24 March 2010 13:14:31 UTC

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