Individual Submission L. Dusseault
Internet-Draft OSAF
Expires: January 13, 2005 July 15, 2004
Partial Document Changes (PATCH Method) for HTTP
draft-dusseault-http-patch-03
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Copyright (C) The Internet Society (2004). All Rights Reserved.
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.
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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 provides byte range downloads,
it does not provide a mechanism for partial uploads.
2. 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.
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, and this specification
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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.
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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.
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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:
o 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
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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.
o 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
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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.
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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
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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.
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[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.
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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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.
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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.
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