Individual Submission                                       L. Dusseault
Internet-Draft                                                      OSAF
Expires: December 3, 2007                                       J. Snell
                                                               June 2007

                         PATCH Method for HTTP

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   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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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  PATCH Method . . . . . . . . . . . . . . . . . . . . . . .  3
       2.1.1.  Example  . . . . . . . . . . . . . . . . . . . . . . .  5
     2.2.  Error handling . . . . . . . . . . . . . . . . . . . . . .  5
     2.3.  Advertising Support in OPTIONS . . . . . . . . . . . . . .  7
       2.3.1.  Example  . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
     3.1.  The 'Accept-Patch' Response Header . . . . . . . . . . . .  8
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   5.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     5.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     5.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . .  9
   Appendix B.  Changes . . . . . . . . . . . . . . . . . . . . . . .  9
     B.1.  Changes from -00 . . . . . . . . . . . . . . . . . . . . .  9
     B.2.  Changes from -01 . . . . . . . . . . . . . . . . . . . . .  9
     B.3.  Changes from -02 . . . . . . . . . . . . . . . . . . . . . 10
     B.4.  Changes from -03 . . . . . . . . . . . . . . . . . . . . . 10
     B.5.  Changes from -04 . . . . . . . . . . . . . . . . . . . . . 10
     B.6.  Changes from -05 . . . . . . . . . . . . . . . . . . . . . 10
     B.7.  Changes from -06 . . . . . . . . . . . . . . . . . . . . . 11
     B.8.  Changes from -07 . . . . . . . . . . . . . . . . . . . . . 11
   Appendix C.  Notes to RFC Editor . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
   Intellectual Property and Copyright Statements . . . . . . . . . . 13

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1.  Introduction

   This specification defines the new HTTP 1.1 [RFC2616] method PATCH
   that is used to apply partial modifications to a HTTP resource.

   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 can 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.

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   and "OPTIONAL" are to be interpreted as described in [RFC2119].

2.  Mechanisms

2.1.  PATCH Method

   The PATCH method requests that a set of changes described in the
   request entity be applied to the resource identified by the Request-
   URI.  The set of changes is represented in a format called a "patch
   document" identified by a media type.  PATCH is neither safe or
   idempotent as defined by [RFC2616] Section 9.1.

   The difference between the PUT and PATCH requests is reflected in the
   way the server processes the enclosed entity to modify the resource
   identified by the Request-URI.  In a PUT request, the enclosed entity
   is considered to be a modified version of the resource stored on the
   origin server and the client is requesting that stored version be
   replaced.  With PATCH, however, the enclosed entity contains a set of
   instructions describing how a resource currently residing on the
   origin server should be modified to produce a new version.  The
   changes described by the entity MAY result in the creation of one or
   more new resources on the server, however it is not intended that the
   body of the PATCH request be used as the content of such resources.

   The server MUST always apply the entire set of changes atomically and
   never provide (e.g. in response to a GET during this operation) a
   partially-modified representation.  If the entire patch document
   cannot be successfully applied then the server MUST fail the entire
   request, applying none of the changes.  The determination of what
   constitutes a successful PATCH can vary depending on the patch
   document and the type of resource being modified.  The actual method
   for determining how to apply the patch document to the resource is
   defined entirely by the origin server.  See Error Handling in section
   2.2 for details on status codes and possible error conditions.

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   If the request passes through a cache and the Request-URI identifies
   one or more currently cached entities, those entries SHOULD be
   treated as stale.  Responses to this method are not cacheable, unless
   the response includes appropriate Cache-Control or Expires header
   fields.  However, the 303 (See Other) response can be used to direct
   the user agent to retrieve a cacheable resource.

   Collisions from multiple requests are more dangerous than PUT
   collisions, because a patch document that is not operating from a
   known base point may corrupt the resource.  Clients wishing to apply
   a patch document to a known entity can first acquire the strong ETag
   of the resource to be modified, and use that Etag in the If-Match
   header on the PATCH request to verify that the resource is still
   unchanged.  If a strong ETag is not available for a given resource,
   the client can use If-Unmodified-Since as a less-reliable safeguard.

   It is RECOMMENDED that Servers provide strong ETags for all resources
   for which PATCH is supported.

   A PATCH response with a 2xx status code indicates that the PATCH
   request was a success.  When the server responds with a status code
   of 200 OK, it MUST include a representation of the modified resource.
   A 200 response whose entity payload is empty indicates that the
   result of the PATCH request is an empty resource.  If the server
   chooses not to return a representation of the modified resource, it
   can use 204 No Content.  With either a 200 or 204 response, the
   server MAY include appropriate entity headers applied to the modified
   resource to allow the client to verify the success of the operation.

   The server MUST NOT ignore any Content-* (e.g.  Content-Range)
   headers that it does not understand or implement and MUST return a
   501 (Not Implemented) response in such cases.

   If the Request-URI identifies a resource with multiple alternate
   representations, the server can choose to respond in a variety of
   ways.  For instance, the server can decide which representation to
   alter and might even be able to change them all consistently
   depending on the patch format.  A particular patch document might be
   able to identify specific representations to modify or might be
   capable of describing changes to multiple representations.  If the
   server cannot choose a representation, it can reject the request with
   an error or the server can choose to redirect the request (e.g. using
   301 Moved Permanently or 302 Found), in which case the user agent
   makes its own decision regarding whether or not to proceed with the

   Clients are advised to take caution when sending multiple PATCH
   requests, or sequences of requests that include PATCH, over a

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   pipelined connection as there are no guarantees that pipelined
   requests will be processed by the server in the same order in which
   the client sends them.  Such sequences of requests can be made safer
   by using conditional request mechanisms such as If-Match.  See
   [RFC2616] Section for additional details regarding pipelining
   and non-idempotent requests.

   There is no guarantee that a resource can be modified with PATCH.
   Further, it is expected that different patch document formats will be
   appropriate for different types of resources and that no single
   format will be appropriate for all types of resources.  Therefore,
   there is no single default patch document format that implementations
   are required to support.  Servers MUST ensure that a received patch
   document is appropriate for the type of resource identified by the

2.1.1.  Example

   Simple PATCH example

       PATCH /file.txt HTTP/1.1
       Content-type: application/example
       If-Match: "e0023aa4e"
       Content-Length: 100

       [description of changes]

   This example illustrates use of a hypothetical patch document on an
   existing text file.

   Successful PATCH response to existing text file

       HTTP/1.1 200 OK
       ETag: "e0023aa4f"
       Content-MD5: Q2hlY2sgSW50ZWdyaXR5IQ==
       Content-Type: text/plain

       [modified resource]

2.2.  Error handling

   There are several known conditions under which a PATCH request can

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   Malformed patch document:  Can be specified using a 400 Bad Request
      when the server finds that the patch document provided by the
      client was improperly formatted.  The definition of badly
      formatted depends on the patch document chosen, but generally if
      the server finds it cannot handle the patch due to the
      serialization of the patch document, this response ought to be
   Unsupported patch document:  Specified using a 415 Unsupported Media
      Type when the client sends a patch document that the server
      doesn't support for the resource identified by the Request-URI.
      Such a response SHOULD include an Accept-Patch response header as
      described in Section 2.3 to notify the client what patch document
      formats are supported.
   Unprocessable request:  Can be specified with a 422 Unprocessable
      Entity [RFC4918] when the server understands the patch document
      and the syntax of the patch document appears valid, but the server
      is incapable of processing the request.  There are a number of
      situations that could lead to such a result, for example:
      *  The client attempted to apply a patch document to an empty
         resource, but the patch document chosen cannot be applied to an
         empty resource.
      *  The client attempted to apply a structural modification and the
         structures assumed to exist did not exist (e.g. a patch which
         specifies changing element 'foo' to element 'bar' but element
         'foo' doesn't exist).
      *  The client attempted to modify a resource in a way that would
         cause the resource to become invalid.  For instance, a
         modification to a well-formed XML document that would cause it
         to no longer be well-formed.
      *  The client attempted to modify a resource that has multiple
         representations but the server was unable to choose which
         representation to modify.
   Conflicting modification:  Specified with a 412 Precondition Failed
      when a client uses either the If-Match or If-Unmodified-Since
      request headers and attempts to apply a patch document to a
      resource whose state has changed since the patch was created.  If
      the server detects a possible conflicting modification and neither
      the If-Match or If-Unmodified-Since request headers are used, the
      server can return a 409 Conflict response.
   Concurrent modification:  When a server receives multiple concurrent
      requests to modify a resource, those requests SHOULD be queued and
      processed in the order in which they are received.  If a server is
      incapable of queuing concurrent requests, all subsequent requests
      SHOULD be rejected using a 409 Conflict response until the first
      modification request is complete.

   Other HTTP status codes can also be used under the appropriate

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   The entity body of error responses SHOULD contain enough information
   to communicate the nature of the error to the client.  The content-
   type of the response entity can vary across implementations.

2.3.  Advertising Support in OPTIONS

   A server can advertise its support for the PATCH method by adding it
   to the listing of allowed methods in the "Allow" OPTIONS response
   header defined in HTTP/1.1.

   Clients also need to know whether the server supports specific patch
   document formats, so this specification introduces a new response
   header "Accept-Patch" used to specify the patch document formats
   accepted by the server.  "Accept-Patch" MUST appear in the OPTIONS
   response for any resource that supports the use of the PATCH method.
   The presence of the "Accept-Patch" header in response to any method
   is an implicit indication that PATCH is allowed on the resource
   identified by the Request-URI.

   Accept-Patch = "Accept-Patch" ":" #( media-range )

   The Accept-Patch header specifies a listing of media ranges as
   defined by [RFC2616], Section 14.1.  Note that, unlike the HTTP
   Accept request header, the Accept-Patch header does not use quality

2.3.1.  Example

   Example: OPTIONS request and response for specific resource


       OPTIONS /example/buddies.xml HTTP/1.1


       HTTP/1.1 200 OK
       Accept-Patch: application/example, text/example

   The examples show a server that supports PATCH generally using two
   hypothetical patch documents.

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3.  IANA Considerations

3.1.  The 'Accept-Patch' Response Header

   The 'Accept-Patch' response header should be added to the permanent
   registry (see [RFC3864]).

       Header field name: Accept-Patch

       Applicable Protocol: HTTP

       Status: standard

       Author/Change controller: IETF

       Specification document: this specification

4.  Security Considerations

   The security considerations for PATCH are nearly identical to the
   security considerations for PUT.  In addition, one might be concerned
   that a document that is patched might be more likely to be corrupted,
   but that concern can be addressed through the use of mechanisms such
   as conditional requests using ETags and the If-Match request header.

   Sometimes an HTTP intermediary might try to detect viruses being sent
   via HTTP by checking the body of the PUT/POST request or GET
   response.  The PATCH method complicates such watch-keeping because
   neither the source document nor the patch document might be a virus,
   yet the result could be.  This security consideration is not
   materially different from those already introduced by byte-range
   downloads, downloading patch documents, uploading zipped (compressed)
   files and so on.

   Individual patch documents will have their own specific security
   considerations that will likely vary depending on the types of
   resources being patched.  The considerations for patched binary
   resources, for instance, will be different than those for patched XML

5.  References

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5.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              September 2004.

5.2.  Informative References

   [RFC4918]  Dusseault, L., "HTTP Extensions for Web Distributed
              Authoring and Versioning (WebDAV)", RFC 4918, June 2007.

Appendix A.  Acknowledgements

   PATCH is not a new concept, it first appeared in HTTP in drafts of
   version 1.1 written by Roy Fielding and Henrik Frystyk.

   Thanks to Adam Roach, Chris Sharp, Julian Reschke, Geoff Clemm, Scott
   Lawrence, Jeffrey Mogul, Roy Fielding, Greg Stein, Jim Luther, Alex
   Rousskov, Jamie Lokier, Joe Hildebrand, Mark Nottingham and Michael
   Balloni for review and advice on this document.

Appendix B.  Changes

B.1.  Changes from -00

   OPTIONS support: removed "Patch" header definition and used Allow and
   new "Accept-Patch" headers instead.

   Supported delta encodings: removed vcdiff and diffe as these do not
   have defined MIME types and did not seem to be strongly desired.

   PATCH method definition: Clarified cache behavior.

B.2.  Changes from -01

   Removed references to XCAP - not yet a RFC.

   Fixed use of MIME types (this "fix" now obsolete)

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   Explained how to use MOVE or COPY in conjunction with PATCH, to
   create a new resource based on an existing resource in a different

B.3.  Changes from -02

   Clarified that MOVE and COPY are really independent of PATCH.

   Clarified when an ETag must change, and when Last-Modified must be

   Clarified what server should do if both Content-Type and IM headers
   appear in PATCH request.

   Filled in missing reference to DeltaV and ACL RFCs.

   Stopped using 501 Unsupported for unsupported delta encodings.

   Clarified what a static resource is.

   Refixed use of MIME types for patch formats.

   Limited the scope of some restrictions to apply only to usage of
   required diff format.

B.4.  Changes from -03

   Various typographical, terminology consistency, and other minor
   clarifications or fixes.

B.5.  Changes from -04

   Moved paragraphs on ACL and RFC3229 interoperability to new section.

   Added security considerations.

   Added IANA considerations, registration of new namespace, and
   discontinued use of "DAV:" namespace for new elements.

   Added example of error response.

B.6.  Changes from -05

   Due to various concerns it didn't seem likely the application/gdiff
   registration could go through so switching to vcdiff as required diff
   format, and to RFC3229's approach to specifying diff formats,
   including use of the IM header.

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   Clarified what header server MUST use to return MD5 hash.

   Reverted to using 501 Unsupported for unsupported delta encodings.

B.7.  Changes from -06

   The reliance on RFC 3229 defined patch documents has been factored
   out in favor of delta encodings identified by MIME media type.

   The required use of DeltaV-based error reporting has been removed in
   favor of using basic HTTP status codes to report error conditions.

   The Accept-Patch response header has been redefined as a listing of
   media-ranges, similar to the Accept request header.

   Added James Snell as a co-author.

B.8.  Changes from -07

   Terminology change from "delta encoding" to "patch document"

   Added clarification on the safety and idempotency of PATCH

   Updated the caching rules of PATCH responses

   200 responses MUST include a representation of the modified resource.
   204 responses are used to indicate successful response without
   returning a representation.

   Suggest using 422 Unprocessable Entity to indicate that a properly
   formatted patch document cannot be processed

   Clarify the use of 412 and 409 to indicate concurrent and conflicting
   resource modifications.

   Added registration for the Accept-Patch header.

   Relaxed the requirements for the use of If-Match and If-Unmodified-

   Add language that clarifies the difference between PUT and PATCH.

   Add language that clarifies the issues with PATCH and Content

   Use of Accept-Patch on any response implies that PATCH is supported.

   Add language advising caution when pipelining PATCH requests.

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Appendix C.  Notes to RFC Editor

   The RFC Editor should remove this section and the Changes section.

Authors' Addresses

   Lisa Dusseault
   Open Source Application Foundation
   2064 Edgewood Dr.
   Palo Alto, CA  94303


   James M Snell


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