Partial Document Changes (PATCH Method) for HTTP

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Copyright Notice

Abstract

Several applications extending HTTP [3] 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.¶

1. Introduction

Three use cases initially motivated this proposal¶

WebDAV [2] is used by authoring applications to store and share files on the internet. For example, Adobe Photoshop has a Workgroup feature allowing the user to browse a repository and save the file. Currently, Photoshop only publishes the file to the repository rarely, because Photoshop files are typically large and upload is slow. Worse, large uploads are more likely to be interrupted. Although HTTP provides byte range downloads, it does not provide a mechanism for partial uploads.
DeltaV [6] extends WebDAV to do versioning. In versioning environments, a large number of files may be updated with very small changes. For example, a programmer may change the name of a function used in a hundred source files. Versioning applications typically send deltas or 'diffs' to the server to modify these files, however DetaV does not yet have this functionality.
The SIMPLE WG is devising a way to store and modify configuration information. The biggest feature missing from HTTP is the ability to modify information in a very lightweight manner, so that the client that decides to change its presence state from "free" to "busy" doesn't have to upload a large document. This can be accomplished through changes to a HTTP resource as well.

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 has no way for the server to send errors 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 diffs) 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 delta encodings are defined for HTTP in RFC 3229 [4]. That standard defines delta encodings for cache updates, not for user write operations. It does mean that servers can reuse delta format algorithms to support both that standard and this proposal.¶

This standard defines the new method PATCH to alter a single existing resource, in place, by applying a delta or diff file. 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.¶

2. Delta Formats

A set of changes for a resource is itself a document, called a patch document or delta. The delta format is uniquely identified through a MIME type. Servers advertise supported delta 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 those formats.¶

This specification only defines usage of the platform-portable gdiff [9] 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.¶

3. Mechanisms

3.1. PATCH Method

The PATCH method requests that the request body (a patch document) be applied to the resource named in 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 [5], 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 delta.¶

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 [1] type value identifying the delta 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 the WebDAV Access Control standard [8], then the PATCH request SHOULD be subject to the same access control permissions as the PUT request.¶

If the gdiff format is used:¶

The client MUST verify that it is applying the patch document to a known entity. There are two reliable ways to do this. The first way is to find out the resource ETag at the time the body is downloaded, and use that Etag in the PATCH request to make sure the resource is still unchanged. The second way to use WebDAV LOCK/UNLOCK to reserve the file (first LOCK, then GET, then PATCH, then UNLOCK). Gdiff collisions from multiple users are more dangerous than PUT collisions, because a gdiff that is not operating from a known base point may corrupt the resource. Therefore, if neither strong ETags nor LOCKS are available from the server, the client MUST use If-Last-Modified as a less-reliable safeguard.
If the Request-URI identifies an unmapped URL, the server SHOULD (subject of course to access control and other restrictions) create a resource with an empty body and apply the gdiff changes to that empty entity. A client SHOULD verify that the URL is unmapped, as expected, with use of the "If-None-Match: *" header.

Simple PATCH example


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

    0xd1, 0xff, 0xd1, 0xff
    4
    249,0,0,2
    2,'X','Y 
    249,0,2,2
    249,0,1,4
    0

Figure 1

This example illustrates use of the gdiff algorithm on an existing text file.¶

3.2. PATCH Response

3.2.1. Success Response

A successful response SHOULD have the 204 No Content status code or the 201 Created status code if a new resource was created. Either response indicates that the server successfully applied the delta and that the response contains no body.¶

The server SHOULD provide a MD5 hash of the resource entity after the delta was applied. This allows the client to verify the success of the operation. 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"

3.2.2. Error handling

This proposal uses the same mechanism as DeltaV 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).¶

DAV:delta-format-unsupported:: Used with 403 Forbidden status code. Returned by the server when it doesn't support the delta format chosen by the client.
DAV:delta-format-forbidden-on-resource:: Used with 403 Forbidden when the delta format chosen by the client is supported by the server but not allowed on this kind of resource.
DAV:delta-format-badly-formatted:: Used with 400 Bad Request when the server finds that the delta document provided by the client was badly formatted or non-compliant. The definition of badly formatted or non-compliant depends on the delta format chosen, but generally if the server finds it can't handle the diff entity even though it supports the format used, this error ought to be appropriate.
DAV:delta-empty-resource:: Used with 409 Conflict when the resource addressed in the Request-URI exists but is empty, and the delta format cannot be applied to an empty document. Note that some delta formats may be applied to an empty document, in which case this error wouldn't be used.
DAV:patch-result-invalid:: Used with 409 Conflict when the resource could be patched but the result of the patch would be a resource which is invalid. This could mean, for example, that a XML resource would become an invalid XML file if the patch specified that a close element text line should be deleted.

"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).¶

3.3. Advertising Support in OPTIONS

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 allowable methods on the addressed resource, so it's natural to add PATCH.¶

Clients also need to know whether the server supports special diff 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 delta 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 delta 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 diff formats more appropriate to the resource may be supported.¶

4. References

[1]: Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types”, RFC 2046, November 1996.
[2]: Goland, Y., Whitehead, E., Faizi, A., Carter, S., and D. Jensen, “HTTP Extensions for Distributed Authoring -- WEBDAV”, RFC 2518, February 1999.
[3]: 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.
[4]: Mogul, J., Krishnamurthy, B., Douglis, F., Feldmann, A., Goland, Y., van Hoff, A., and D. Hellerstein, “Delta encoding in HTTP”, RFC 3229, January 2002.
[5]: Mogul, J. and A. Van Hoff, “Instance Digests in HTTP”, RFC 3230, January 2002.
[6]: Clemm, G., Amsden, J., Ellison, T., Kaler, C., and J. Whitehead, “Versioning Extensions to WebDAV (Web Distributed Authoring and Versioning)”, RFC 3253, March 2002.
[7]: Korn, D., MacDonald, J., Mogul, J., and K. Vo, “The VCDIFF Generic Differencing and Compression Data Format”, RFC 3284, June 2002.
[8]: Clemm, G., Reschke, J., Sedlar, E., and J. Whitehead, “Web Distributed Authoring and Versioning (WebDAV) Access Control Protocol”, RFC 3744, May 2004.
[9]: van Hoff, A. and J. Payne, “Generic Diff Format Specification”, W3C NOTE-gdiff-19970901, August 1997.

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 and Jamie Lokier 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 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.¶

B.2. Changes from -01

Removed references to XCAP - not yet a standard.¶

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 a delta of 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 formats.¶

Clarified what a static resource is.¶

Refixed use of MIME types for delta formats.¶

Limited the scope of some restrictions to apply only to 'gdiff' usage.¶

Author's Address

Lisa Dusseault
Open Source Application Foundation
2064 Edgewood Dr.
Palo Alto, CA 94303
US
EMail: lisa@osafoundation.org

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