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Copyright © The Internet Society (2004). All Rights Reserved.
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.
Three use cases initially motivated this proposal¶
Other working groups (like netconf) are also considering manipulating large files using HTTP GET and PUT. Sometimes the files aren't that large but the device is small or bandwidth is limited, as when phones need to add a new contact to an address book file. This feature would allow much more efficient changes to files.¶
This specification defines a new HTTP 1.1 method for patches. 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 MUST 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.¶
Note that byte ranges are already used in HTTP to do partial downloads (GET method). However, they are not defined for uploads, and there are some missing pieces for uploads. For example, the HTTP specification does not define a particularly informative error to send if the byte range in a PUT is invalid. Byte ranges (or some other kind of range) could be made to work in this specification but a more flexible mechanism (one that could also encompass XML patch formats) was desired, as well as a method that would not confuse caching proxies. Reliable and tested patch algorithms already exist, and this specification takes advantage of that existing work.¶
Other patch formats ("delta encodings") are defined for HTTP in RFC 3229 . That specification defines delta encodings for cache updates, not for user write operations. It does mean that servers can reuse delta encoding algorithms to support both that specification and this proposal.¶
This specification defines the new method PATCH to alter a single existing resource, in place, by applying a patch. The operation is atomic. Note that WebDAV MOVE and COPY requests, if supported by the HTTP server, can be useful to independently rename or copy a whole resource before applying PATCH to either the source or destination URL to modify the contents.¶
A set of changes for a resource is itself a document, called a patch document. The patch format is uniquely identified through a MIME type. Servers advertise supported patch formats by advertising these MIME types, and clients specify which one they're using by including the MIME type in the request. MIME types were specifically chosen so that there would be a well-defined way for other PATCH extensions to define their own patch formats and how to use them.¶
This specification only defines usage of the platform-portable gdiff  format identified as 'application/gdiff'. Servers SHOULD support gdiff for all authorable resources, that is all resources that support PUT. Some requirements apply only to specific patch formats, and in this specification those requirements are spelled out only for gdiff.¶
The PATCH method requests that the request body (a patch document) be applied to the resource identified by the Request-URI. The server MUST NOT create a new resource with the contents of the request body, although it MAY (depending on the patch document format) apply the request body to an empty entity to result in the content for the new resource. The target resource's content type MUST be one to which the patch format applies. The server MUST apply the entire patch atomically and never provide (e.g. in response to a GET during this operation) a partially-patched body. If the entire patch file cannot be successfully applied then the server MUST fail the entire request, applying none of the changes. See error handling section for details on status codes and possible error conditions.¶
In the model defined in RFC3230 , the patch document is modelled as an instance being sent to the server. Thus, if the server supports instance manipulations, the client MAY apply these manipulations to the patch document after it is generated (for example, a compression algorithm). On the receiving end, the server MUST undo the instance manipulation then apply the resulting document as a patch.¶
PATCH request bodies MUST NOT be cached. A cache MAY mark the resource identified in the Request-URI as stale if it sees a successful response to the PATCH request.¶
The PATCH request MUST have a body. It MUST include the Content-Type header with a MIME  type value identifying the patch format used in the request body. The request body MUST be in some format which has the semantics of defining a change to an existing document.¶
The PATCH request is subject to access control, which in turn may require authentication. If the server supports WebDAV Access Control , then the PATCH request SHOULD be subject to the same access control permissions as the PUT request.¶
If the gdiff format is used:¶
Simple PATCH example
PATCH /file.txt HTTP/1.1 Host: www.example.com Content-type: application/gdiff If-Match: "e0023aa4e" Content-Length: 100 [gdiff-binary-body]
This example illustrates use of the gdiff algorithm on an existing text file.¶
A successful response with the 204 No Content status code implies that no new resource was created. A successful response with the 201 Created status code informs the client that a new resource was created.¶
The server SHOULD provide a MD5 hash of the resource entity after the patch was applied. This allows the client to verify the success of the operation. As with PUT, the PATCH method MUST cause the ETag to change if the resulting entity is not identical to the original. If the server supports strong ETags, the server MUST return a strong ETag for use in future client operations. The server SHOULD return the Last-Modified header in any case, but the server MUST return the Last-Modified header if ETags aren't supported.¶
Successful PATCH response to existing text file
HTTP/1.1 204 No Content Content-MD5: Q2hlY2sgSW50ZWdyaXR5IQ== ETag: "e0023aa4e"
This proposal uses the same mechanism as DeltaV (defined in section 1.6 of RFC3253) to add much-needed info to base HTTP error responses. Existing HTTP status codes are not infinitely extensible but XML elements and namespaces are more so, and it's simple to treat the HTTP error code as a rough category and put detailed error codes in the body. Clients that do not use the extra information ignore the bodies of error responses.¶
The PATCH method can return the following errors. Please note that the notation "DAV:foobar" is merely short form for expressing "the 'foobar' element in the 'DAV:' namespace". It has meaning only in this specification, not on the wire. Also note that the string error codes are not meant to be displayed but instead as machine parsable known error codes (thus there is no language code).¶
"404 Not Found" can be used (with no body/error element) when the URL in by the Request-URI does not map to a resource and the server cannot apply the patch document to a new empty resource (thus this error wouldn't be used with gdiff patch documents).¶
The server advertises its support for the features described here with OPTIONS response headers. The "Allow" OPTIONS header is already defined in HTTP 1.1 to contain all the allowed methods on the addressed resource, so the server MUST add PATCH if it is allowed.¶
Clients also need to know whether the server supports special patch formats, so this document introduces a new OPTIONS response header "Accept-Patch". "Accept-Patch" MUST appear in the OPTIONS response for any resource where the PATCH method is shown as an allowed method.¶
OPTIONS * is not used to advertise support for PATCH because the patch formats supported are likely to change from one resource to another. A server MAY include the Accept-Patch header in response to OPTIONS *, and its value MAY be the union of known supported patch formats.¶
Accept-Patch = "Accept-Patch" ":" #media-type¶
Example: OPTIONS request and response for specific resource
[request] OPTIONS /example/buddies.xml HTTP/1.1 Host: www.example.com [response] HTTP/1.1 200 OK Allow: GET, PUT, POST, OPTIONS, HEAD, TRACE, DELETE, PATCH Accept-Patch: example/xcap+xml, application/gdiff
The examples show a server that supports PATCH generally, with two formats supported (one of them is fictional). On some resources, for example on XML files, different kinds of patch formats more appropriate to the resource may be supported.¶
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 and Jamie Lokier for review and advice on this document.¶
OPTIONS support: removed "Patch" header definition and used Allow and new "Accept-Patch" headers instead.¶
Supported patch formats: 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.¶
Removed references to XCAP - not yet a RFC.¶
Fixed use of MIME types (this "fix" now obsolete)¶
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.¶
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 patch formats.¶
Clarified what a static resource is.¶
Refixed use of MIME types for patch formats.¶
Limited the scope of some restrictions to apply only to 'gdiff' usage.¶
Various typographical, terminology consistency, and other minor clarifications or fixes.¶
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