The TAG resolved a [semantic] web architecture issue (HTTPRange-14)
[1] about whether HTTP URIs without hashes could be used to refer to
things which were not documents (information resources), as follows:
"- If an "http" resource responds to a GET request with a 2xx
response, then the resource identified by that URI is an information
resource;
- If an "http" resource responds to a GET request with a 303 (See
Other) response, then the resource identified by that URI could be
any resource;
- If an "http" resource responds to a GET request with a 4xx (error)
response, then the nature of the resource is unknown."
The 303 response is used more or less to say, "The information
resource at the Location URI is not the thing you were asking for,
but is information about it".
It is really important that, as the things identified by the two URIs
are different, that the client of an XMLHTTP request be able to
distinguish what is returned from what was asked for.
However, the XMLHTTPRequest document [2] says:
"If the response is an HTTP redirect (status code 301, 302, 303 or
307), then it must be transparently followed (unless it violates
security, infinite loop precautions or the scheme isn't supported)."
This is a bug. The 303 response should be returned to the software,
so that it can interrogate the Location header, and pick up
information from the associated document should it wish to. An
alternative would be for the history of what happened to provided in
linked status blocks after a single call.
This is a problem currently in for example the Tabulator RDF browser.
Example: http://dig.csail.mit.edu/People/RRS is an identifier for
Ralph Swick.
$ curl -I http://dig.csail.mit.edu/People/RRS
HTTP/1.1 303 See Other
Date: Wed, 06 Sep 2006 18:28:37 GMT
Server: Apache/1.3.33 (Debian GNU/Linux) PHP/4.3.10-16 mod_ssl/2.8.22
OpenSSL/0.9.7e
Location: http://dig.csail.mit.edu/People/RRS/about
Content-Type: text/html; charset=iso-8859-1
http://dig.csail.mit.edu/People/RRS/about is an identifier for a
document about Ralph Swick.
For example, The tabulator [3], when given the URI for Ralph himself,
fetches it.
Because the redirection is followed transparently, it cannot
distinguish between the person and the document. It picks up the
title of the document, and uses that as the name for Ralph. It lists
my acquaintances as:
acquaintance Henry Story
Dan Brickley
About Ralph Swick
Joseph Scharf
etc...
It is also, by the way, very useful to be able to get at the 301 and
302 status details, particularly when many URIs redirect to the same
thing, which might already have been fetched.
There is also a problem with relative URIs in the destination
document. They should be parsed relative to the redirected URI (for a
302 Moved at , not the original URI, but the software knows nothing
about the redirection. This must apply also to other redirections.
Tim Berners-Lee
unofficially
[1] http://www.w3.org/2001/tag/issues.html#httpRange-14
[2] http://www.w3.org/TR/XMLHttpRequest/#xmlhttprequest
[3] http://dig.csail.mit.edu/2005/ajar/release/tabulator/0.7/tab.html
