Network Working Group L. Dusseault
Internet-Draft Xythos
Expires: May 12, 2004 November 12, 2003
Partial Document Changes (PATCH Method) for HTTP
draft-dusseault-http-patch-00
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Copyright Notice
Copyright (C) The Internet Society (2003). 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.
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1. Introduction
Three use cases initially motivated this proposal
1. WebDAV 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
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 [HTTP].
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 could be made to work in this
specification but they're not the only way to do partial
modifications. Since reliable and tested patch algorithms already
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exist, those are preferred.
Other delta encodings are defined for HTTP in [RFC3229]. 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. That
standard does not use MIME types to identify the delta algorithm, but
the mapping is trivial.
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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. See error handling section for details on status codes and
possible error conditions.
PATCH bodies are not cachable. 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 value indicating what the body type is. It MUST be a
format that has the semantics of defining a change to an existing
document (such as gdiff or vcdiff). The PATCH request MUST also use
one of the standard HTTP/1.1 mechanisms that let the server know when
the request body is done. The PATCH request body length MUST NOT be
indicated only by closing the connection when the body is complete,
because an incomplete PATCH body could conceivably corrupt the target
resource.
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 [WEBDAV] 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.
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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 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. 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"
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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. Every change document format must be a registered
MIME type. Servers advertise supported delta mechanisms by
advertising these MIME types, and clients specify which one they're
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using by including the MIME type in the Content-Type header.
This table outlines the delta format support requirements for a
server supporting this proposal.
Set of defined delta formats
Format Specification MIME type/ Requirements
GDIFF [W3C-GDIFF] application/gdiff
MUST support for all document types except XML.
MAY support for XML documents.
Servers supporting DeltaV MUST support for ALL
document types.
XCAP [XCAP] text/xcap+xml
MUST support for XML documents.
VCDIFF [] application/vcdiff???
MAY support for all document types.
DIFFE [RFC3229] ???
MAY support for all document types.
ISSUES: We only have a real mime type for GDIFF so far. It would be
a public service to register vcdiff and diffe as well if those are
used at all.
2.4 Advertising Support in OPTIONS: Patch header
The server advertises its support for the features described here
with an OPTIONS response header, sent on OPTIONS requests for any
resource. The Patch header on any OPTIONS response indicates that
the server supports the PATCH method and at least one delta format.
When the OPTIONS request addresses a specific modifiable resource,
the Patch header in the response indicates which delta formats may be
used for this specific resource. When an OPTIONS request addresses
the server as a whole (Request-URI = "*") the Delta header in the
response indicates the union of all delta formats supported by the
server.
OPTIONS request and response indicating Patch support
[request]
OPTIONS * HTTP/1.1
Host: www.example.com
[response]
HTTP/1.1 200 OK
Patch: application/gdiff, text/xcap+xml
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References
[1] van Hoff, A. and J. Payne, "Generic Diff Format Specification",
August 1997.
[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.
[3] Goland, Y., Whitehead, E., Faizi, A., Carter, S. and D. Jensen,
"HTTP Extensions for Distributed Authoring - WebDAV", RFC 2518.
[4] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", June 1999.
[5] Rosenberg, J., "The Extensible Markup Language (XML)
Configuration Access Protocol (XCAP)", October 2003.
[6] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046, November
1996.
Author's Address
Lisa Dusseault
Xythos Software, Inc.
2064 Edgewood Dr.
Palo Alto, CA 94303
US
EMail: lisa@xythos.com
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Appendix A. Changes
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