GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc5789

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.

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

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.

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], 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

Dusseault & 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 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.

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 2010

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.

Dusseault & Snell Standards Track [Page 7] RFC 5789 HTTP PATCH March 2010

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

Dusseault & 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. 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.

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

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

Dusseault & Snell Standards Track [Page 10]

/data/webs/external/dokuwiki/data/pages/rfc/rfc5789.txt · Last modified: 2010/03/19 01:27 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki