Individual Submission L. Dusseault Internet-Draft OSAF Expires: September 30, 2004 April 2004 Partial Document Changes (PATCH Method) for HTTP draft-dusseault-http-patch-02 Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on September 30, 2004. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. 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. Dusseault Expires September 30, 2004 [Page 1] Internet-Draft HTTP PATCH April 2004 1. Introduction Three use cases initially motivated this proposal 1. 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 [3] provides byte range downloads, it cannot provide this simple a mechanism for uploads. 2. DeltaV 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. 3. 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 as defined MIME [1] Dusseault Expires September 30, 2004 [Page 2] Internet-Draft HTTP PATCH April 2004 document types, 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 rename or copy a different resource before applying PATCH. Dusseault Expires September 30, 2004 [Page 3] Internet-Draft HTTP PATCH April 2004 2. Mechanisms 2.1 PATCH Method The PATCH method requests that the request body (a patch document) be applied to the resource named in the Request-URI. The resource named in the Request-URI MUST already exist (the server MUST NOT create a new resource with the body of the PATCH method). 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) 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 either the Content-Type header with a MIME type value indicating what the body type is, or an IM header as defined in RFC 3229 [4], to identify the delta format. The IM header is only for gdiff and vcdiff, which are instance manipulations, and the Content-Type header is for delta formats which have registered MIME types. Either way, the identified format MUST have the semantics of defining a change to an existing document (such as gdiff). The PATCH request is subject to access control, which in turn may require authentication. The PATCH request SHOULD be subject to the same access control permissions as the PUT request. The PATCH request MUST only be used in a context which ensures that only one user may apply a patch at a time. 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). PATCH collisions from multiple users are more dangerous than PUT collisions, because a PATCH 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. Dusseault Expires September 30, 2004 [Page 4] Internet-Draft HTTP PATCH April 2004 Simple PATCH example PATCH /file.txt HTTP/1.1 Host: www.example.com IM: 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 platform-portable 'gdiff' algorithm as one possible patch format. In this case the resource is a text file. 2.2 PATCH Response 2.2.1 Success Response The basic success response code for PATCH is 204 No Content. For this new method, 200 OK is not used because 200 OK implies a body in the response, and 201 Created is not used because the resource must already exist. The server SHOULD provide a MD5 hash of the content after the delta was applied. This allows the client to verify the success of the operation. The PATCH method obviously MUST cause the ETag to change. So, if the server supports ETags, the server MUST return a strong ETag for use in future client operations. If the server does not support strong ETags, then the server MUST return the Last-Modified header instead. Successful PATCH response HTTP/1.1 204 No Content Content-MD5: Q2hlY2sgSW50ZWdyaXR5IQ== ETag: "e0023aa4e" Dusseault Expires September 30, 2004 [Page 5] Internet-Draft HTTP PATCH April 2004 2.2.2 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. 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 English, 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 501 Not Supported 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 and non-compliant. 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" is used with no body/error element when the URL in by the Request-URI does not map to a resource. 2.3 Delta Formats A set of changes for a resource is itself a document, called a change document or delta. The delta format is uniquely identified through a MIME type. Servers advertise supported delta mechanisms by Dusseault Expires September 30, 2004 [Page 6] Internet-Draft HTTP PATCH April 2004 advertising these MIME types, and clients specify which one they're using by including the MIME type in the Content-Type header of the PATCH request. The VCDIFF [5] format identifier is "vcdiff". The GDIFF [6] format identifier is "gdiff". Servers SHOULD support VCDIFF for all static resources. 2.4 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 * presents a bit of a special case, as it does not address a resource, and does not always show all the server's features. In responses to OPTIONS *, a server supporting this specification SHOULD include the PATCH method in the "Allow" header and SHOULD show the union of all supported diff methods in the "Accept-Patch" header. Accept-Patch = "Accept-Patch" ":" #Delta-identifer Delta-identifier = media-type | "gdiff" | "vcdiff" Example: OPTIONS * request and response indicating Patch support [request] OPTIONS * HTTP/1.1 Host: www.example.com [response] HTTP/1.1 200 OK Allow: GET, PUT, POST, OPTIONS, HEAD, TRACE, DELETE, PATCH Accept-Patch: gdiff, vcdiff, example/xcap+xml Dusseault Expires September 30, 2004 [Page 7] Internet-Draft HTTP PATCH April 2004 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 3 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] Korn, D., MacDonald, J., Mogul, J. and K. Vo, "The VCDIFF Generic Differencing and Compression Data Format", RFC 3284, June 2002. [6] van Hoff, A. and J. Payne, "Generic Diff Format Specification", August 1997. Dusseault Expires September 30, 2004 [Page 8] Internet-Draft HTTP PATCH April 2004 Author's Address Lisa Dusseault Open Source Application Foundation 2064 Edgewood Dr. Palo Alto, CA 94303 US EMail: lisa@osafoundation.org Dusseault Expires September 30, 2004 [Page 9] Internet-Draft HTTP PATCH April 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 and Alex Rousskov for review and advice on this document. Dusseault Expires September 30, 2004 [Page 10] Internet-Draft HTTP PATCH April 2004 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 since vcdiff and gdiff don't have registered MIME types (similar usage to RFC3229), and added use of IM header for same reason. 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. 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