Re: Content Negotiated Media Resources -- Clarification Needed

[Joe Gregorio]

On 6/9/06, Andreas Sewe <[EMAIL PROTECTED]> wrote:

Here is an extension of an, IMHO, valid example of using APP. (It builts
on an example posted by me to <atom-syntax>, [1].) But since I am not
entirely sure whether it violates the spec (PaceMediaEntries5, to be
precise) or not, I would like to hear the WG's opinion on it.

So consider the following client/server exchange:

First, the clients posts an image to the collection (at
<http://example.org/entries>).

     POST /entries HTTP/1.1
     Host: example.org
     Content- Type: image/png
     Content- Length: nnn
     Title: A picture of the beach

     {binary data}

Now the server creates both a media resource of type "image/png" (at
<http://example.org/media/1.png>) and a media link entry (at
<http://exmaple.org/entries/1>). But furthermore the server
automatically converts the media resource to type "image/gif" (made
available at <http://example.org/media/1.png>).

Finally the media link entry refers to a content-negotiated resource (at
<http://example.org/media/1>) which makes available both the "image/png"
and "image/gif" representations. But since the original media resource
was of type "image/png", only the "image/png" representation is editable
by the client; the "image/gif" representation obtained by conversion is not.

     HTTP/1.1 201 Created
     Date: Fri, 7 Oct 2005 17:17:11 GMT
     Content- Length: nnn
     Content- Type: application/atom+xml; charset="utf-8"
     Content- Location: http://example.org/entries/1
     Location: http://example.org/entries/1

     <?xml version="1.0"?>
     <entry xmlns="http://www.w3.org/2005/Atom";>
       <title>A picture of the beach</title>
       <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
       <updated>2003-12-13T18:30:02Z</updated>
       <author><name>John Doe</name></author>
       <summary type="text" />
       <!-- editable media link entry -->
       <link rel="edit"
        href="http://example.org/entries/1"; />
       <!-- editable media resource -->
       <link rel="edit-media" type="image/png"
        href="http://example.org/media/1.png"; />
       <link rel="alternate" type="image/png"
        href="http://example.org/media/1.png"; />
       <!-- read-only media resource -->
       <link rel="alternate" type="image/gif"
        href="http://example.org/media/1.gif"; />
       <!-- content negotiated read-only media resource -->
       <content
        src="http://example.org/media/1"; />
     </entry>

Note that there is no content/@type attribute present since the media
resource (at <http://example.org/media/1>) is content negotiated. This
violates a SHOULD of RFC 4287, however, which is an issue already
discussed on <atom-syntax> [1].

But what is worse is the fact that the above at least seems to violate a
MUST of PaceMediaEntries5:

   The media link entry MUST have a "content" element with a "src"
   attribute which links to the media resource

And indeed that URI does point to the media "resource", there just happens to
be two different "representations" that can be returned from that URI.

 -joe

--
Joe Gregorio        http://bitworking.org