PATCH Method for HTTP
RFC 5789

Internet Engineering Task Force (IETF)                      L. Dusseault
Request for Comments: 5789                                    Linden Lab
Category: Standards Track                                       J. Snell
ISSN: 2070-1721                                               March 2010

                         PATCH Method for HTTP

Abstract

   Several applications extending the Hypertext Transfer Protocol (HTTP)
   require a feature to do partial resource modification.  The existing
   HTTP PUT method only allows a complete replacement of a document.
   This proposal adds a new HTTP method, PATCH, to modify an existing
   HTTP resource.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5789.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1. Introduction ....................................................2
   2. The PATCH Method ................................................2
      2.1. A Simple PATCH Example .....................................4
      2.2. Error Handling .............................................5
   3. Advertising Support in OPTIONS ..................................7
      3.1. The Accept-Patch Header ....................................7
      3.2. Example OPTIONS Request and Response .......................7
   4. IANA Considerations .............................................8
      4.1. The Accept-Patch Response Header ...........................8
   5. Security Considerations .........................................8
   6. References ......................................................9
      6.1. Normative References .......................................9
      6.2. Informative References .....................................9
   Appendix A.  Acknowledgements .....................................10

1.  Introduction

   This specification defines the new HTTP/1.1 [RFC2616] method, PATCH,
   which is used to apply partial modifications to a 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 cannot 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.  POST is already used but
   without broad interoperability (for one, there is no standard way to
   discover patch format support).  PATCH was mentioned in earlier HTTP
   specifications, but not completely defined.

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

   Furthermore, this document uses the ABNF syntax defined in Section
   2.1 of [RFC2616].

2.  The 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.  If the Request-URI does not
   point to an existing resource, the server MAY create a new resource,
   depending on the patch document type (whether it can logically modify
   a null resource) and permissions, etc.

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   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 the 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 PATCH
   method affects the resource identified by the Request-URI, and it
   also MAY have side effects on other resources; i.e., new resources
   may be created, or existing ones modified, by the application of a
   PATCH.

   PATCH is neither safe nor idempotent as defined by [RFC2616], Section
   9.1.

   A PATCH request can be issued in such a way as to be idempotent,
   which also helps prevent bad outcomes from collisions between two
   PATCH requests on the same resource in a similar time frame.
   Collisions from multiple PATCH requests may be more dangerous than
   PUT collisions because some patch formats need to operate from a
   known base-point or else they will corrupt the resource.  Clients
   using this kind of patch application SHOULD use a conditional request
   such that the request will fail if the resource has been updated
   since the client last accessed the resource.  For example, the client
   can use a strong ETag [RFC2616] in an If-Match header on the PATCH
   request.

   There are also cases where patch formats do not need to operate from
   a known base-point (e.g., appending text lines to log files, or non-
   colliding rows to database tables), in which case the same care in
   client requests is not needed.

   The server MUST 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 NOT apply any of
   the changes.  The determination of what constitutes a successful
   PATCH can vary depending on the patch document and the type of
   resource(s) being modified.  For example, the common 'diff' utility
   can generate a patch document that applies to multiple files in a
   directory hierarchy.  The atomicity requirement holds for all
   directly affected files.  See "Error Handling", Section 2.2, for
   details on status codes and possible error conditions.

   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.  A response to this method is only cacheable if it

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   contains explicit freshness information (such as an Expires header or
   "Cache-Control: max-age" directive) as well as the Content-Location
   header matching the Request-URI, indicating that the PATCH response
   body is a resource representation.  A cached PATCH response can only
   be used to respond to subsequent GET and HEAD requests; it MUST NOT
   be used to respond to other methods (in particular, PATCH).

   Note that entity-headers contained in the request apply only to the
   contained patch document and MUST NOT be applied to the resource
   being modified.  Thus, a Content-Language header could be present on
   the request, but it would only mean (for whatever that's worth) that
   the patch document had a language.  Servers SHOULD NOT store such
   headers except as trace information, and SHOULD NOT use such header
   values the same way they might be used on PUT requests.  Therefore,
   this document does not specify a way to modify a document's Content-
   Type or Content-Language value through headers, though a mechanism
   could well be designed to achieve this goal through a patch document.

   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
   Request-URI.

   Clients need to choose when to use PATCH rather than PUT.  For
   example, if the patch document size is larger than the size of the
   new resource data that would be used in a PUT, then it might make
   sense to use PUT instead of PATCH.  A comparison to POST is even more
   difficult, because POST is used in widely varying ways and can
   encompass PUT and PATCH-like operations if the server chooses.  If
   the operation does not modify the resource identified by the Request-
   URI in a predictable way, POST should be considered instead of PATCH
   or PUT.

2.1.  A Simple PATCH Example

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

   [description of changes]

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

   Successful PATCH response to existing text file:

   HTTP/1.1 204 No Content
   Content-Location: /file.txt
   ETag: "e0023aa4f"

   The 204 response code is used because the response does not carry a
   message body (which a response with the 200 code would have).  Note
   that other success codes could be used as well.

   Furthermore, the ETag response header field contains the ETag for the
   entity created by applying the PATCH, available at
   http://www.example.com/file.txt, as indicated by the Content-Location
   response header field.

2.2.  Error Handling

   There are several known conditions under which a PATCH request can
   fail.

   Malformed patch document:  When the server determines that the patch
      document provided by the client is not properly formatted, it
      SHOULD return a 400 (Bad Request) response.  The definition of
      badly formatted depends on the patch document chosen.

   Unsupported patch document:  Can be specified using a 415
      (Unsupported Media Type) response when the client sends a patch
      document format that the server does not support for the resource
      identified by the Request-URI.  Such a response SHOULD include an
      Accept-Patch response header as described in Section 3.1 to notify
      the client what patch document media types are supported.

   Unprocessable request:  Can be specified with a 422 (Unprocessable
      Entity) response ([RFC4918], Section 11.2) when the server
      understands the patch document and the syntax of the patch
      document appears to be valid, but the server is incapable of
      processing the request.  This might include attempts 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.  There may also be
      more specific errors like "Conflicting State" that could be
      signaled with this status code, but the more specific error would
      generally be more helpful.

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   Resource not found:  Can be specified with a 404 (Not Found) status
      code when the client attempted to apply a patch document to a non-
      existent resource, but the patch document chosen cannot be applied
      to a non-existent resource.

   Conflicting state:  Can be specified with a 409 (Conflict) status
      code when the request cannot be applied given the state of the
      resource.  For example, if the client attempted to apply a
      structural modification and the structures assumed to exist did
      not exist (with XML, a patch might specify changing element 'foo'
      to element 'bar' but element 'foo' might not exist).

   Conflicting modification:  When a client uses either the If-Match or
      If-Unmodified-Since header to define a precondition, and that
      precondition failed, then the 412 (Precondition Failed) error is
      most helpful to the client.  However, that response makes no sense
      if there was no precondition on the request.  In cases when the
      server detects a possible conflicting modification and no
      precondition was defined in the request, the server can return a
      409 (Conflict) response.

   Concurrent modification:  Some applications of PATCH might require
      the server to process requests in the order in which they are
      received.  If a server is operating under those restrictions, and
      it receives concurrent requests to modify the same resource, but
      is unable to queue those requests, the server can usefully
      indicate this error by using a 409 (Conflict) response.

   Note that the 409 Conflict response gives reasonably consistent
   information to clients.  Depending on the application and the nature
   of the patch format, the client might be able to reissue the request
   as is (e.g., an instruction to append a line to a log file), have to
   retrieve the resource content to recalculate a patch, or have to fail
   the operation.

   Other HTTP status codes can also be used under the appropriate
   circumstances.

   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.

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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.  The PATCH method MAY appear in the
   "Allow" header even if the Accept-Patch header is absent, in which
   case the list of allowed patch documents is not advertised.

3.1.  The Accept-Patch Header

   This specification introduces a new response header Accept-Patch used
   to specify the patch document formats accepted by the server.
   Accept-Patch SHOULD 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.  The presence of a specific patch document format in
   this header indicates that that specific format is allowed on the
   resource identified by the Request-URI.

   Accept-Patch = "Accept-Patch" ":" 1#media-type

   The Accept-Patch header specifies a comma-separated listing of media-
   types (with optional parameters) as defined by [RFC2616], Section
   3.7.

   Example:

   Accept-Patch: text/example;charset=utf-8

3.2.  Example OPTIONS Request and Response

   [request]

   OPTIONS /example/buddies.xml HTTP/1.1
   Host: www.example.com

   [response]

   HTTP/1.1 200 OK
   Allow: GET, PUT, POST, OPTIONS, HEAD, DELETE, PATCH
   Accept-Patch: application/example, text/example

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

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4.  IANA Considerations

4.1.  The Accept-Patch Response Header

   The Accept-Patch response header has been added to the permanent
   registry (see [RFC3864]).

   Header field name:  Accept-Patch

   Applicable Protocol:  HTTP

   Author/Change controller:  IETF

   Specification document:  this specification

5.  Security Considerations

   The security considerations for PATCH are nearly identical to the
   security considerations for PUT ([RFC2616], Section 9.6).  These
   include authorizing requests (possibly through access control and/or
   authentication) and ensuring that data is not corrupted through
   transport errors or through accidental overwrites.  Whatever
   mechanisms are used for PUT can be used for PATCH as well.  The
   following considerations apply especially to PATCH.

   A document that is patched might be more likely to be corrupted than
   a document that is overridden in entirety, but that concern can be
   addressed through the use of mechanisms such as conditional requests
   using ETags and the If-Match request header as described in
   Section 2.  If a PATCH request fails, the client can issue a GET
   request to the resource to see what state it is in.  In some cases,
   the client might be able to check the contents of the resource to see
   if the PATCH request can be resent, but in other cases, the attempt
   will just fail and/or a user will have to verify intent.  In the case
   of a failure of the underlying transport channel, where a PATCH
   response is not received before the channel fails or some other
   timeout happens, the client might have to issue a GET request to see
   whether the request was applied.  The client might want to ensure
   that the GET request bypasses caches using mechanisms described in
   HTTP specifications (see, for example, Section 13.1.6 of [RFC2616]).

   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

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   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
   documents.  Servers MUST take adequate precautions to ensure that
   malicious clients cannot consume excessive server resources (e.g.,
   CPU, disk I/O) through the client's use of PATCH.

6.  References

6.1.  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.

6.2.  Informative References

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

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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 and also
   appears in Section 19.6.1.1 of RFC 2068.

   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, Michael
   Balloni, Cyrus Daboo, Brian Carpenter, John Klensin, Eliot Lear, SM,
   and Bernie Hoeneisen for review and advice on this document.  In
   particular, Julian Reschke did repeated reviews, made many useful
   suggestions, and was critical to the publication of this document.

Authors' Addresses

   Lisa Dusseault
   Linden Lab
   945 Battery Street
   San Francisco, CA  94111
   USA

   EMail: lisa.dusseault@gmail.com

   James M. Snell

   EMail: jasnell@gmail.com
   URI:   http://www.snellspace.com

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