Partial Document Changes (PATCH Method) for HTTP

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Copyright Notice

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.¶

1. Introduction

Three use cases initially motivated this proposal¶

WebDAV [3] 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 [1] provides byte range downloads, it does not provide a mechanism for partial uploads.
DeltaV [4] 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 patches 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 to apply a delta encoding, or a "patch", 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 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) as defined in RFC2616. 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 delta encodings) was desired, as well as a method that would not confuse caching proxies. Reliable and tested delta encodings already exist, and this specification takes advantage of that existing work.¶

Some delta encodings for use in HTTP GET responses are defined in RFC 3229 [2]. 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 delta encoding. A patch request body is modelled as a manipulation of an instance, where the instance would have been the entire document had it been PUT to the server, following the model of RFC3229 [2]. 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 Encodings

A set of changes for a resource is itself a document, called a delta encoding. The delta encoding is uniquely identified through a instance manipulation as defined in RFC3229. Servers advertise supported delta encodings for PATCH by advertising these algorithms, and clients specify which one they're using by including the name in the request. Not all instance-manipulations defined in the IANA registry are delta encodings; as of October 2004, the instance manipulations which are also delta encodings are vcdiff, diffe, and gdiff.¶

Servers SHOULD support PATCH and the vcdiff delta encoding 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 vcdiff.¶

3. Mechanisms

3.1. PATCH Method

The PATCH method requests that the request body (a delta encoding) 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 delta encoding) apply the request body to an empty entity to result in the content for the new resource.¶

The server MUST always 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.¶

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 IM header with a single valid delta encoding. The PATCH request MAY include a Content-Type header which is the content-type of the resource to which the delta encoding is to be applied. The request body MUST be in the delta encoding format specified in the IM header.¶

If the vcdiff format is used:¶

The client MUST verify that it is applying the delta encoding 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 If-Match header on 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). Vcdiff collisions from multiple users are more dangerous than PUT collisions, because a vcdiff 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-Unmodified-Since as a less-reliable safeguard.
If the Request-URI does not identify an existing resource, the server SHOULD (subject of course to access control and other restrictions) create a resource with an empty body and apply the vcdiff changes to that empty entity. A client SHOULD verify that the URL is unmapped, as expected, with use of the "If-None-Match: *" header.
The Content-Type header specifies the client's intended Content-Type for the resource being patched. Thus, if the server creates a new resource it MUST assign this Content-Type, or assign a generic one if the Content-Type header was not provided. If the server modifies an existing resource, the server MUST change the Content-Type if a new Content-Type was provided by the client; otherwise the resource's Content-Type should remain unchanged.

Simple PATCH example


    PATCH /file.txt HTTP/1.1
    Host: www.example.com
    Content-type: text/plain
    IM: vcdiff
    If-Match: "e0023aa4e"
    Content-Length: 100

    [vcdiff-bytes]

Figure 1

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

3.2. PATCH Response

3.2.1. Success Response

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 send the Content-MD5 header in responses to PATCH. This allows the client to verify the success of the operation.¶

As with PUT, the PATCH method MUST change the resource's ETag 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 MUST return the Last-Modified header if it does not support strong ETags.¶

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 (defined in section 1.6 of RFC3253) to add machine-parsable information to provide more detail than HTTP status codes can. 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. These error codes are not meant to be displayed directly to end-users, so there is no language code or other display information. Clients MUST ignore any unrecognized elements within the XML response body because extensions allow implementors to add custom debug information to the response.¶

The PATCH method can return the following errors. All these errors are represented as XML elements in an XML document, where the specific error element appears inside a root element called "error" in the "DAV:" namespace. The new elements defined in this specification are all in the "urn:ietf:params:xml:ns:patch" namespace.¶

delta-encoding-badly-formatted:: Used with 400 Bad Request when the server finds that the delta encoding provided by the client was badly formatted or non-compliant. The definition of badly formatted or non-compliant depends on the delta encoding chosen, but generally if the server finds it can't handle the current patch even though it supports the format used, this error ought to be appropriate.
delta-encoding-unsupported:: Used with 501 Unsupported when the client sends a delta encoding that the server doesn't support on this resource, or a delta encoding that the server never supports.
patch-empty-resource:: Used with 409 Conflict when the resource addressed in the Request-URI exists but is empty, and the delta encoding cannot be applied to an empty document. Note that some delta encodings may be applied to an empty document, in which case this error wouldn't be used.
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 delta encoding to a new empty resource.¶

Other status codes defined in RFC2616 may also be used under the appropriate circumstances, with no response body. For example, an unauthenticated user may be prompted to authenticate, in order to use PATCH, with "401 Unauthorized". An authenticated user who does not have sufficient privilege to use PATCH may receive a "403 Forbidden" response.¶

3.2.2.1. Example error response with body detail

    HTTP/1.1 409 Conflict
    Content-Type: text/xml; charset="utf-8"
    Content-Length: xxx
            
    <?xml version="1.0" encoding="utf-8" ?>  
    <D:error xmlns:D="DAV:">
      <P:patch-result-invalid 
            xmlns:P="urn:ietf:params:xml:ns:patch"/>
    </D:error>

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 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 delta encodings for all types of resources.¶

Accept-Patch = "Accept-Patch" ":" #instance-manipulation¶

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: vcdiff, gdiff, diffe, example-xcap-xml

The examples show a server that supports PATCH generally, with all the delta encodings defined in RFC3229 plus one fictional XML-oriented delta encoding. On some resources, for example on XML files, different kinds of delta encodings more appropriate to the resource may be supported.¶

4. Interdependencies with other Standards

4.1. PATCH and Access Control (RFC3744)

If the server supports WebDAV Access Control [5], then the PATCH request SHOULD be subject to the same access control permissions as the PUT request.¶

5. IANA Considerations

This document uses URNs to describe XML namespaces and XML schemas conforming to a registry mechanism described in [RFC3688].¶

Registration request for the patch namespace:¶

URI: urn:ietf:params:xml:ns:patch¶

Registrant Contact: See the "Author's Address" section of this document.¶

XML: None. Namespace URIs do not represent an XML specification.¶

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 is addressed through use of MD5 digests.¶

7. References

7.1. Normative References

[1]: 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.
[2]: Mogul, J., Krishnamurthy, B., Douglis, F., Feldmann, A., Goland, Y., van Hoff, A., and D. Hellerstein, “Delta encoding in HTTP”, RFC 3229, January 2002.

7.2. Non-Normative References

[3]: Goland, Y., Whitehead, E., Faizi, A., Carter, S., and D. Jensen, “HTTP Extensions for Distributed Authoring -- WEBDAV”, RFC 2518, February 1999.
[4]: Clemm, G., Amsden, J., Ellison, T., Kaler, C., and J. Whitehead, “Versioning Extensions to WebDAV (Web Distributed Authoring and Versioning)”, RFC 3253, March 2002.
[5]: Clemm, G., Reschke, J., Sedlar, E., and J. Whitehead, “Web Distributed Authoring and Versioning (WebDAV) Access Control Protocol”, RFC 3744, May 2004.

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)¶

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.¶

Clarified what header server MUST use to return MD5 hash.¶

Reverted to using 501 Unsupported for unsupported delta encodings.¶

Appendix C. Notes to RFC Editor

The RFC Editor should remove this section and the Changes section.¶

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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