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Re: PATCH thoughts...

From: Lisa Dusseault <lisa@osafoundation.org>
Date: Fri, 30 Apr 2004 14:03:51 -0700
Message-Id: <E214FC72-9AE9-11D8-8BCF-000A95B2BB72@osafoundation.org>
Cc: HTTP working group <ietf-http-wg@w3.org>, Jamie Lokier <jamie@shareable.org>, Justin Chapweske <justin@chapweske.com>
To: Jeffrey Mogul <Jeff.Mogul@hp.com>

I originally thought all HTTP messages with bodies needed to indicate 
Content-Type.
I assumed that if the Content-Type header were used then that was the
obvious place to say what diff format is being provided.  Thus PATCH
has currently:

     PATCH /file.txt HTTP/1.1
     Host: www.example.com
     Content-Type: application/gdiff
     If-Match: "e0023aa4e"
     Content-Length: 100

    [body]

However, clearly RFC3229 doesn't take that approach -- some responses 
are
shown with bodies (implied) but with only a Content-encoding header to 
indicate
that there's even a body on the message.  Modelling on the RFC3229 
approach,
PATCH could look like this instead:

       PATCH /file.txt HTTP/1.1
       Host: foo
       If-Match: "def"
       Content-encoding: gzip
       IM: vcdiff

     [body]

Now that I've reviewed this part of 2616 more carefully, it seems 
RFC32229
takes a justified approach.  RFC2616, section 7.2.1 only requires 
Content-Type
for entity bodies, not for other non-entity request bodies:

    When an entity-body is included with a message, the data type of that
    body is determined via the header fields Content-Type and Content-
    Encoding.

WebDAV seems to have interpreted this requirement more broadly,
as if it were worded  "When a body is included with a message, ..."
For example, PROPFIND requests contain a body that is not an entity 
body,
but instead additional command detail.  Since the body must be XML, each
PROPFIND request has the "Content-Type: application/xml" header.
Implementors also seem to have interpreted this requirement broadly.
I know of server implementations that do not expect to see a message 
body
if the Content-Type header is missing and they interpret the body, if 
sent,
as the next request.

One compromise is to inlude Content-Type with a reasonably appropriate
MIME type, but one that doesn't need to specify the diff format used.

       PATCH /file.txt HTTP/1.1
       Host: foo
       If-Match: "def"
       Content-encoding: gzip
       IM: gdiff
       Content-Type: application/octet-stream

     ...

This certainly routes around the MIME registration issue and may be
preferable.  I have no strong opinions on that.

Lisa


On Apr 30, 2004, at 1:34 PM, Jeffrey Mogul wrote:

>     So please voice your opinions on this issue to help me off this 
> fence.
>     Should PATCH require support for one diff format, and if so, what?
>     Should it use MIME types or strings to identify diff formats?  
> Should
>     it register MIME types for VCDiff and/or GDiff (ouch)?
>
> Please bear with me if I'm just being ignorant ... I haven't read
> much of the WebDAV spec, especially not within the last couple
> of years (and I certainly don't have time to read it now).
>
> But why should this be a MIME type at all?  HTTP has a clear
> distinction between MIME _content_ types (I emphasize the
> word _content_ here) and several _separate_ name spaces for
> representations used for on-the-wire encodings.  These
> include content-codings and content-transfer-codings in
> RFC2616, and RFC3229 introduces "instance-manipulations"
> as a third (orthogonal) space.
>
> I can assure you that we did not do this just to make more
> work for IANA.
>
> I can anticipate an argument along the form of "why create
> a new namespace when an existing one is almost the right
> thing?".  This is very dangerous; "almost the right thing"
> can lead to extremely messy workarounds.
>
> For example, with delta encoding, our original thought
> was that this is "obviously just a new content-coding".
> We then spent months thrashing around how to deal with
> all of the resultant complexity that arises when you try
> to do this and compose delta encoding with things like
> compression and byte ranges.  It was only after the "aha"
> that delta encoding was NOT the same as a content-coding
> that we came up with a clear, clean design.
>
> There's some rationale for this in RFC3229, and some more
> in my WWW 2002 paper (http://www2002.org/CDROM/refereed/444.pdf)
>
> -Jeff
Received on Friday, 30 April 2004 17:18:27 GMT

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