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Re: Using OPTIONS for optional feature discovery -- advice

From: Scott Lawrence <scott@skrb.org>
Date: Tue, 20 Apr 2004 22:39:02 -0400
To: Lisa Dusseault <lisa@osafoundation.org>
Cc: ietf-http-wg@w3.org
Message-Id: <1082515141.16857.255.camel@localhost.localdomain>

On Tue, 2004-04-20 at 21:21, Lisa Dusseault wrote:
> I am working on the next version of the HTTP PATCH method proposal:
> http://www.ietf.org/internet-drafts/draft-dusseault-http-patch-00.txt
> We've had some discussions amongst WebDAV people of the best way for 
> clients to discover server feature support.  In this case, the client 
> wants to discover:
>   - if the server supports PATCH at all
>   - if so, what delta or diff formats can be used on this resource.
> For that purpose, is a new header on OPTIONS still considered to be the 
> way to go?  Can a server omit this header on responses to OPTIONS * if 
> it only supports the feature in part of its namespace?  (E.g. if a java 
> servlet supplies support for this feature only in the namespace hosted 
> by that servlet)

00>            Partial Document Changes (PATCH Method) for HTTP
00>                     draft-dusseault-http-patch-00

00> 1. Introduction
00>   Note that byte ranges are already used in HTTP to do partial
00>   downloads (GET method).   However, they are not defined for uploads,
00>   and there are some missing pieces for uploads. For example, the HTTP
00>   specification has no way for the server to send errors if the byte
00>   range in a PUT is invalid. Byte ranges could be made to work in this
00>   specification but they're not the only way to do partial
00>   modifications.  Since reliable and tested patch algorithms already
00>   exist, those are preferred.

Just a comment here... the Range mechanism can (theoretically at least)
operate on units other than bytes, and can do multiple ranges.  I
think that it could be used for the sort of thing you envision, but I
agree that a patch-like mechanism is probably more robust.

00> 2.1 PATCH Method
00>   PATCH bodies are not cachable.  A cache MAY mark the resource
00>   identified in the Request-URI as stale if it sees a successful
00>   response to the PATCH request.

I think that what you want to say here is that PATCH _request_ bodies
MUST NOT be cached.  Whether or not the response can be cached depends
on the nature of the response, and the server has available ways to
control whether or not it can be cached.

00>   The PATCH request MUST have a body.  [...]
00>                                        The PATCH request MUST also use
00>   one of the standard HTTP/1.1 mechanisms that let the server know when
00>   the request body is done. The PATCH request body length MUST NOT be
00>   indicated only by closing the connection when the body is complete,
00>   because an incomplete PATCH body could conceivably corrupt the target
00>   resource.

That is confusing, and needn't be said at all - it's never valid to
terminate a _request_ body by closing the connection - only a response

00> 2.4 Advertising Support in OPTIONS: Patch header

00>   The server advertises its support for the features described here
00>   with an OPTIONS response header, sent on OPTIONS requests for any
00>   resource.  The Patch header on any OPTIONS response indicates that
00>   the server supports the PATCH method and at least one delta format.
00>   When the OPTIONS request addresses a specific modifiable resource,
00>   the Patch header in the response indicates which delta formats may be
00>   used for this specific resource.  When an OPTIONS request addresses
00>   the server as a whole (Request-URI = "*") the Delta header in the
                                                    ^^^^^ - Patch?
00>   response indicates the union of all delta formats supported by the
00>   server.                             ^^^^^-patch?

There is already a way to advertise support for a method - the 'Allow'
header (rfc2616#14.7), which may be sent on any response.  For an
'OPTIONS' request whose request-uri was '*', the Allow header can
refer only to the server as a whole, which doesn't tell you whether or
not a method in the Allow header will be acceptable for any particular
URI (just as you describe above); for that you need to probe the URI
that you're interested in, either with OPTIONS or some other method.
In a WebDAV context, where multiple requests for the same resource are
common, I would think that there would often be an opportunity to
piggyback this on something else.

I'd also suggest that for clarity you use 'Accept-Patch' as the
response header to indicate what patch types are supported by the

So your first example:

00>   OPTIONS request and response indicating Patch support

00>       [request]

00>       OPTIONS * HTTP/1.1
00>       Host: www.example.com

00>       [response]

00>       HTTP/1.1 200 OK
00>       Patch: application/gdiff, text/xcap+xml

should, I think, be:


          OPTIONS * HTTP/1.1
          Host: www.example.com


          HTTP/1.1 200 OK
          Allow: PATCH, ...
          Accept-Patch: application/gdiff, text/xcap+xml

and a specific use case:


          OPTIONS /foo/bar.xml HTTP/1.1
          Host: www.example.com


          HTTP/1.1 200 OK
          Accept-Patch: text/xcap+xml

and a PUT example showing that the capability can be passed with
another response:


          PUT /foo/bar.xml HTTP/1.1
          Host: www.example.com


          HTTP/1.1 201 Created
          Accept-Patch: text/xcap+xml
          Etag: "xi30qkd0a"

Scott Lawrence        
  Pingtel Corp.   
  sip:slawrence {at} pingtel.com  
  +1.781.938.5306 x162
Received on Tuesday, 20 April 2004 22:39:19 UTC

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