Individual Submission L. Dusseault
Internet-Draft OSAF
Expires: April 29, 2008 J. Snell
October 27, 2007
PATCH Method for HTTP
draft-dusseault-http-patch-10
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Copyright Notice
Copyright (C) The IETF Trust (2007).
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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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The PATCH Method . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. A simple PATCH example . . . . . . . . . . . . . . . . . . 5
2.2. Error handling . . . . . . . . . . . . . . . . . . . . . . 5
3. Advertising Support in OPTIONS . . . . . . . . . . . . . . . . 6
3.1. The Accept-Patch Header . . . . . . . . . . . . . . . . . 7
3.2. An example OPTIONS request and response . . . . . . . . . 7
4. 209 Content Returned . . . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5.1. The 'Accept-Patch' Response Header . . . . . . . . . . . . 8
5.2. HTTP Status codes . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . . 9
7.2. 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
B.9. Changes from -08 . . . . . . . . . . . . . . . . . . . . . 12
B.10. Changes from -09 . . . . . . . . . . . . . . . . . . . . . 12
Appendix C. Notes to RFC Editor . . . . . . . . . . . . . . . . . 12
Appendix D. Editorial Notes . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . . . 14
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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",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in [RFC2119].
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. 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.
If the request passes through a cache and the Request-URI identifies
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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 or the response uses the 209 Content Returned status code as
defined in Section 4. 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.
If a PATCH request contains any entity-headers the server does not
understand, the server MUST return a 501 (Not Implemented) response.
A server that understands a particular entity-header can choose to
ignore it; however, doing so can produce results that are unexpected
or unintended by the client. All entity-headers contained in the
request apply only to the contained patch document and MUST NOT be
applied to the resource being modified.
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
request.
Clients are advised to take caution when sending multiple PATCH
requests, or sequences of requests that include PATCH, over a
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 8.1.2.2 for additional details regarding pipelining
and non-idempotent requests.
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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.
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]
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
ETag: "e0023aa4f"
Content-MD5: Q2hlY2sgSW50ZWdyaXR5IQ==
2.2. Error handling
There are several known conditions under which a PATCH request can
fail.
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
appropriate.
Unsupported patch document: Specified using a 415 Unsupported Media
Type when the client uses 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
formats are supported.
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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 until the first modification request is
complete.
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.
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.
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3.1. The Accept-Patch Header
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
factors.
3.2. An 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 documents.
4. 209 Content Returned
The 209 "Content Returned" status code can be used to indicate that a
response is equivalent to what would have been returned with a 200
status code response to a GET sent to the URI immediately following
the successful completion of the request.
5. IANA Considerations
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5.1. 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
5.2. HTTP Status codes
This specification defines the 209 Content Returned status code
(Section 3) to be updated in the registry at
<http://www.iana.org/assignments/http-status-codes>.
6. 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
documents.
7. References
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7.1. 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.
7.2. 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
location.
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
used.
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-
Since.
Add language that clarifies the difference between PUT and PATCH.
Add language that clarifies the issues with PATCH and Content
Negotiation.
Use of Accept-Patch on any response implies that PATCH is supported.
Add language advising caution when pipelining PATCH requests.
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B.9. Changes from -08
Addition of the 209 Content Returned status code
Addition of the Prefer header field mechanism
Removed the paragraph discussing the use of 200+Content-Location.
This is replaced by the 209 Content Returned status code.
B.10. Changes from -09
Move the prefer header to a separate document
Restructure the document sections.
Appendix C. Notes to RFC Editor
The RFC Editor should remove this section and the Changes section.
Appendix D. Editorial Notes
There are several outstanding issues with the Prefer section:
o Should the Prefer section be separated out into a separate I-D?
o We need to determine how new preference codes are created/
registered
o Are warn-codes ok or do we need a new response header?
Authors' Addresses
Lisa Dusseault
Open Source Application Foundation
2064 Edgewood Dr.
Palo Alto, CA 94303
US
Email: lisa@osafoundation.org
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James M Snell
Phone:
Email: jasnell@gmail.com
URI: http://www.snellspace.com
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