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Re: A question - use 301 instead of 406?

From: Pierre-Antoine Champin <swlists-040405@champin.net>
Date: Wed, 24 Mar 2010 09:06:46 +0100
Message-ID: <4BA9C816.1030701@champin.net>
To: Hugh Glaser <hg@ecs.soton.ac.uk>
CC: Linking Open Data <public-lod@w3.org>
Hi Hugh,

interesting suggestion; I have also been frustrated with dbpedia URIs in
the adress bar, which you have to (or forget to) change when you want to
feed them to an RDF agent...

However, 406 seems to be the most appropriate answer to give. Falling
back to 301 in too many different situations for the sake of forgiveness
is muddling the waters, IMHO...

I propose a middle way: the HTTP RFC [1] reads, regarding 406 responses:

> Unless it was a HEAD request, the response SHOULD include an entity
> containing a list of available entity characteristics and location(s)
> from which the user or user agent can choose the one most
> appropriate.

As a matter of fact, http://www.w3.org/ complies with this
recommandation (try to request it for image/svg, for example). It gives
you a page with alternative representations.

I think this would be a good principle for linked data as well, as
linked data relies heavily on Conneg. Note that if you request RDF for
http://foo/bar.html, the server could reply with 406, but include an
entity in RDF explaining where you can find other representations...
Your agent could then interpret that and fetch the new URI.


[1] http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.7

On 23/03/2010 23:50, Hugh Glaser wrote:
> I am not sure I even dare ask this, but here goes. (This is prompted
> by a real application implementation - it is not just a 
> hypothetical.)
> 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.
> But is this compatible with the principle of being as forgiving as
> possible as a server?
> I think it is clear what the agent wanted: Accept:
> application/rdf+xml http://foo/bar it is just that somehow the wrong
> URI got into the system.
> I know I have made the mistake of for example copying a dbpedia URI
> from the address bar when I was looking for the LD URI, and ended up
> wondering for a moment why Accept: application/rdf+xml
> http://dbpedia.org/page/Tim_Berners-Lee gives me a 406 before I
> remember I need to right click on the About and copy the link.
> 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.
> 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 08:07:21 UTC

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