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Network Working Group                                       L. Dusseault
Internet-Draft                                                    Xythos
Expires: May 12, 2004                                  November 12, 2003


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

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on May 12, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

   Several applications extending HTTP require a feature to do partial
   resource modification.  Existing HTTP functionality only allows a
   complete replacement of a document. This proposal adds a new HTTP
   method, PATCH, to modify an existing HTTP resource.












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1. Introduction

   Three use cases initially motivated this proposal

   1.  WebDAV is used by authoring applications to store and share files
       on the internet.  For example, Adobe Photoshop has a Workgroup
       feature allowing the user to browse a repository and save the
       file.  Currently, Photoshop only publishes the file to the
       repository rarely, because Photoshop files are typically large
       and upload is slow.  Worse, large uploads are more likely to be
       interrupted.  Although HTTP provides byte range downloads, it
       cannot provide this simple a mechanism for uploads.

   2.  DeltaV extends WebDAV to do versioning.  In versioning
       environments, a large number of files may be updated with very
       small changes.  For example, a programmer may change the name of
       a function used in a hundred source files.  Versioning
       applications typically send deltas or 'diffs' to the server to
       modify these files, however DetaV does not yet have this
       functionality.

   3.  The SIMPLE WG is devising a way to store and modify configuration
       information. The biggest feature missing from HTTP is the ability
       to modify information in a very lightweight manner, so that the
       client that decides to change its presence state from "free" to
       "busy" doesn't have to upload a large document. This can be
       accomplished through changes to a HTTP resource as well.

   Other working groups (like netconf) are also considering manipulating
   large files using HTTP GET and PUT. Sometimes the files aren't that
   large but the device is small or bandwidth is limited, as when phones
   need to add a new contact to an address book file. This feature would
   allow much more efficient changes to files.

   This specification defines a new HTTP 1.1 method for patches [HTTP].
   A new method is necessary to improve interoperability and prevent
   errors. The PUT method is already defined to overwrite a resource
   with a complete new body, and MUST NOT be reused to do partial
   changes. Otherwise, proxies and caches and even clients and servers
   may get confused as to the result of the operation.

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



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   exist, those are preferred.

   Other delta encodings are defined for HTTP in [RFC3229]. That
   standard defines delta encodings for cache updates, not for user
   write operations.  It does mean that servers can reuse delta format
   algorithms to support both that standard and this proposal.  That
   standard does not use MIME types to identify the delta algorithm, but
   the mapping is trivial.











































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2. Mechanisms

2.1 PATCH Method

   The PATCH method requests that the request body (a patch document) be
   applied to the resource named in the Request-URI.  The resource named
   in the Request-URI MUST already exist (the server MUST NOT create a
   new resource with the body of the PATCH method).  The target
   resource's content type MUST be one to which the patch format
   applies.  See error handling section for details on status codes and
   possible error conditions.

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

   The PATCH request MUST have a body.  It MUST include the Content-Type
   header with a value indicating what the body type is.  It MUST be a
   format that has the semantics of defining a change to an existing
   document (such as gdiff or vcdiff).   The PATCH request MUST also use
   one of the standard HTTP/1.1 mechanisms that let the server know when
   the request body is done. The PATCH request body length MUST NOT be
   indicated only by closing the connection when the body is complete,
   because an incomplete PATCH body could conceivably corrupt the target
   resource.

   The PATCH request MUST only be used in a context which ensures that
   only one user may apply a patch at a time.  There are two reliable
   ways to do this. The first way is to find out the resource ETag at
   the time the body is downloaded, and use that Etag in the PATCH
   request to make sure the resource is still unchanged.  The second way
   to use WebDAV LOCK/UNLOCK [WEBDAV] to reserve the file (first LOCK,
   then GET, then PATCH, then UNLOCK).  PATCH collisions from multiple
   users are more dangerous than PUT collisions, because a PATCH that is
   not operating from a known base point may corrupt the resource.
   Therefore, if neither strong ETags nor LOCKS are available from the
   server, the client MUST use If-Last-Modified as a less-reliable
   safeguard.













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   Simple PATCH example


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

       0xd1, 0xff, 0xd1, 0xff
       4
       249,0,0,2
       2,'X','Y
       249,0,2,2
       249,0,1,4
       0


                                Figure 1

   This example illustrates use of the platform-portable 'gdiff'
   algorithm as one possible patch format.  In this case the resource is
   a text file.

2.2 PATCH Response

2.2.1 Success Response

   The basic success response code for PATCH is 204 No Content. For this
   new method, 200 OK is not used because 200 OK implies a body in the
   response, and 201 Created is not used because the resource must
   already exist.

   The server SHOULD provide a MD5 hash of the content after the delta
   was applied.  This allows the client to verify the success of the
   operation.  If the server supports ETags, the server MUST return a
   strong ETag for use in future client operations.  If the server does
   not support strong ETags, then the server MUST return the
   Last-Modified header instead.

   Successful PATCH response

       HTTP/1.1 204 No Content
       Content-MD5: Q2hlY2sgSW50ZWdyaXR5IQ==
       ETag: "e0023aa4e"






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2.2.2 Error handling

   This proposal uses the same mechanism as DeltaV to add much-needed
   info to base HTTP error responses.  Existing HTTP status codes are
   not infinitely extensible but XML elements and namespaces are more
   so, and it's simple to treat the HTTP error code as a rough category
   and put detailed error codes in the body.

   The PATCH method can return the following errors. Please note that
   the notation "DAV:foobar" is merely short form for expressing "the
   'foobar' element in the 'DAV:' namespace".  It has meaning only in
   English, not on the wire.  Also note that the string error codes are
   not meant to be displayed but instead as machine parsable known error
   codes (thus there is no language code).

   DAV:delta-format-unsupported: Used with 501 Not Supported status
      code.  Returned by the server when it doesn't support the delta
      format chosen by the client.

   DAV:delta-format-forbidden-on-resource: Used with 403 Forbidden when
      the delta format chosen by the client is supported by the server
      but not allowed on this kind of resource.

   DAV:delta-format-badly-formatted: Used with 400 Bad Request when the
      server finds that the delta document provided by the client was
      badly formatted and non-compliant.

   DAV:delta-empty-resource: Used with 409 Conflict when the resource
      addressed in the Request-URI exists but is empty, and the delta
      format cannot be applied to an empty document.  Note that some
      delta formats may be applied to an empty document, in which case
      this error wouldn't be used.

   DAV:patch-result-invalid: Used with 409 Conflict when the resource
      could be patched but the result of the patch would be a resource
      which is invalid.  This could mean, for example, that a XML
      resource would become an invalid XML file if the patch specified
      that a close element text line should be deleted.

   "404 Not Found" is used with no body/error element when the URL in by
   the Request-URI does not map to a resource.

2.3 Delta Formats

   A set of changes for a resource is itself a document, called a change
   document or delta.  Every change document format must be a registered
   MIME type.  Servers advertise supported delta mechanisms by
   advertising these MIME types, and clients specify which one they're



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   using by including the MIME type in the Content-Type header.

   This table outlines the delta format support requirements for a
   server supporting this proposal.

   Set of defined delta formats

       Format   Specification   MIME type/ Requirements
       GDIFF    [W3C-GDIFF]     application/gdiff
                                   MUST support for all document types except XML.
                                MAY support for XML documents.
                                   Servers supporting DeltaV MUST support for ALL
                                                document types.
       XCAP        [XCAP]          text/xcap+xml
                                   MUST support for XML documents.
       VCDIFF      []              application/vcdiff???
                                   MAY support for all document types.
       DIFFE       [RFC3229]       ???
                                   MAY support for all document types.

   ISSUES: We only have a real mime type for GDIFF so far.  It would be
   a public service to register vcdiff and diffe as well if those are
   used at all.

2.4 Advertising Support in OPTIONS: Patch header

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

   OPTIONS request and response indicating Patch support

       [request]

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

       [response]

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



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References

   [1]  van Hoff, A. and J. Payne, "Generic Diff Format Specification",
        August 1997.

   [2]  Mogul, J., Krishnamurthy, B., Douglis, F., Feldmann, A., Goland,
        Y., van Hoff, A. and D. Hellerstein, "Delta encoding in HTTP",
        RFC 3229, January 2002.

   [3]  Goland, Y., Whitehead, E., Faizi, A., Carter, S. and D. Jensen,
        "HTTP Extensions for Distributed Authoring - WebDAV", RFC 2518.

   [4]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
        Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
        HTTP/1.1", June 1999.

   [5]  Rosenberg, J., "The Extensible Markup Language (XML)
        Configuration Access Protocol (XCAP)", October 2003.

   [6]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
        Extensions (MIME) Part Two: Media Types", RFC 2046, November
        1996.


Author's Address

   Lisa Dusseault
   Xythos Software, Inc.
   2064 Edgewood Dr.
   Palo Alto, CA  94303
   US

   EMail: lisa@xythos.com


















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Appendix A. Changes


















































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   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION



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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.











































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