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Internet Engineering Task Force (IETF)                      L. DusseaultRequest for Comments: 5789                                    Linden LabCategory: Standards Track                                       J. SnellISSN: 2070-1721                                               March 2010PATCH Method for HTTPAbstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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.Dusseault & Snell            Standards Track                    [Page 1]

RFC 5789                       HTTP PATCH                     March 2010Table of Contents1. Introduction ....................................................22. The PATCH Method ................................................22.1. A Simple PATCH Example .....................................42.2. Error Handling .............................................53. Advertising Support in OPTIONS ..................................73.1. The Accept-Patch Header ....................................73.2. Example OPTIONS Request and Response .......................74. IANA Considerations .............................................84.1. The Accept-Patch Response Header ...........................85. Security Considerations .........................................86. References ......................................................96.1. Normative References .......................................96.2. Informative References .....................................9Appendix A.  Acknowledgements .....................................101.  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 inSection2.1 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.Dusseault & Snell            Standards Track                    [Page 2]

RFC 5789                       HTTP PATCH                     March 2010   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],Section9.1.   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 itDusseault & Snell            Standards Track                    [Page 3]

RFC 5789                       HTTP PATCH                     March 2010   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]Dusseault & Snell            Standards Track                    [Page 4]

RFC 5789                       HTTP PATCH                     March 2010   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 inSection 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.Dusseault & Snell            Standards Track                    [Page 5]

RFC 5789                       HTTP PATCH                     March 2010   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.Dusseault & Snell            Standards Track                    [Page 6]

RFC 5789                       HTTP PATCH                     March 20103.  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],Section3.7.   Example:   Accept-Patch: text/example;charset=utf-83.2.  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.Dusseault & Snell            Standards Track                    [Page 7]

RFC 5789                       HTTP PATCH                     March 20104.  IANA Considerations4.1.  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 specification5.  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 inSection 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 notDusseault & Snell            Standards Track                    [Page 8]

RFC 5789                       HTTP PATCH                     March 2010   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.  References6.1.  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.Dusseault & Snell            Standards Track                    [Page 9]

RFC 5789                       HTTP PATCH                     March 2010Appendix 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 inSection 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.comDusseault & Snell            Standards Track                   [Page 10]