7 Internet Engineering Task Force (IETF) L. Dusseault
8 Request for Comments: 5789 Linden Lab
9 Category: Standards Track J. Snell
10 ISSN: 2070-1721 March 2010
17 Several applications extending the Hypertext Transfer Protocol (HTTP)
18 require a feature to do partial resource modification. The existing
19 HTTP PUT method only allows a complete replacement of a document.
20 This proposal adds a new HTTP method, PATCH, to modify an existing
25 This is an Internet Standards Track document.
27 This document is a product of the Internet Engineering Task Force
28 (IETF). It represents the consensus of the IETF community. It has
29 received public review and has been approved for publication by the
30 Internet Engineering Steering Group (IESG). Further information on
31 Internet Standards is available in Section 2 of RFC 5741.
33 Information about the current status of this document, any errata,
34 and how to provide feedback on it may be obtained at
35 http://www.rfc-editor.org/info/rfc5789.
39 Copyright (c) 2010 IETF Trust and the persons identified as the
40 document authors. All rights reserved.
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47 to this document. Code Components extracted from this document must
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50 described in the Simplified BSD License.
58 Dusseault & Snell Standards Track [Page 1]
60 RFC 5789 HTTP PATCH March 2010
65 1. Introduction ....................................................2
66 2. The PATCH Method ................................................2
67 2.1. A Simple PATCH Example .....................................4
68 2.2. Error Handling .............................................5
69 3. Advertising Support in OPTIONS ..................................7
70 3.1. The Accept-Patch Header ....................................7
71 3.2. Example OPTIONS Request and Response .......................7
72 4. IANA Considerations .............................................8
73 4.1. The Accept-Patch Response Header ...........................8
74 5. Security Considerations .........................................8
75 6. References ......................................................9
76 6.1. Normative References .......................................9
77 6.2. Informative References .....................................9
78 Appendix A. Acknowledgements .....................................10
82 This specification defines the new HTTP/1.1 [RFC2616] method, PATCH,
83 which is used to apply partial modifications to a resource.
85 A new method is necessary to improve interoperability and prevent
86 errors. The PUT method is already defined to overwrite a resource
87 with a complete new body, and cannot be reused to do partial changes.
88 Otherwise, proxies and caches, and even clients and servers, may get
89 confused as to the result of the operation. POST is already used but
90 without broad interoperability (for one, there is no standard way to
91 discover patch format support). PATCH was mentioned in earlier HTTP
92 specifications, but not completely defined.
94 In this document, the key words "MUST", "MUST NOT", "REQUIRED",
95 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
96 and "OPTIONAL" are to be interpreted as described in [RFC2119].
98 Furthermore, this document uses the ABNF syntax defined in Section
103 The PATCH method requests that a set of changes described in the
104 request entity be applied to the resource identified by the Request-
105 URI. The set of changes is represented in a format called a "patch
106 document" identified by a media type. If the Request-URI does not
107 point to an existing resource, the server MAY create a new resource,
108 depending on the patch document type (whether it can logically modify
109 a null resource) and permissions, etc.
114 Dusseault & Snell Standards Track [Page 2]
116 RFC 5789 HTTP PATCH March 2010
119 The difference between the PUT and PATCH requests is reflected in the
120 way the server processes the enclosed entity to modify the resource
121 identified by the Request-URI. In a PUT request, the enclosed entity
122 is considered to be a modified version of the resource stored on the
123 origin server, and the client is requesting that the stored version
124 be replaced. With PATCH, however, the enclosed entity contains a set
125 of instructions describing how a resource currently residing on the
126 origin server should be modified to produce a new version. The PATCH
127 method affects the resource identified by the Request-URI, and it
128 also MAY have side effects on other resources; i.e., new resources
129 may be created, or existing ones modified, by the application of a
132 PATCH is neither safe nor idempotent as defined by [RFC2616], Section
135 A PATCH request can be issued in such a way as to be idempotent,
136 which also helps prevent bad outcomes from collisions between two
137 PATCH requests on the same resource in a similar time frame.
138 Collisions from multiple PATCH requests may be more dangerous than
139 PUT collisions because some patch formats need to operate from a
140 known base-point or else they will corrupt the resource. Clients
141 using this kind of patch application SHOULD use a conditional request
142 such that the request will fail if the resource has been updated
143 since the client last accessed the resource. For example, the client
144 can use a strong ETag [RFC2616] in an If-Match header on the PATCH
147 There are also cases where patch formats do not need to operate from
148 a known base-point (e.g., appending text lines to log files, or non-
149 colliding rows to database tables), in which case the same care in
150 client requests is not needed.
152 The server MUST apply the entire set of changes atomically and never
153 provide (e.g., in response to a GET during this operation) a
154 partially modified representation. If the entire patch document
155 cannot be successfully applied, then the server MUST NOT apply any of
156 the changes. The determination of what constitutes a successful
157 PATCH can vary depending on the patch document and the type of
158 resource(s) being modified. For example, the common 'diff' utility
159 can generate a patch document that applies to multiple files in a
160 directory hierarchy. The atomicity requirement holds for all
161 directly affected files. See "Error Handling", Section 2.2, for
162 details on status codes and possible error conditions.
164 If the request passes through a cache and the Request-URI identifies
165 one or more currently cached entities, those entries SHOULD be
166 treated as stale. A response to this method is only cacheable if it
170 Dusseault & Snell Standards Track [Page 3]
172 RFC 5789 HTTP PATCH March 2010
175 contains explicit freshness information (such as an Expires header or
176 "Cache-Control: max-age" directive) as well as the Content-Location
177 header matching the Request-URI, indicating that the PATCH response
178 body is a resource representation. A cached PATCH response can only
179 be used to respond to subsequent GET and HEAD requests; it MUST NOT
180 be used to respond to other methods (in particular, PATCH).
182 Note that entity-headers contained in the request apply only to the
183 contained patch document and MUST NOT be applied to the resource
184 being modified. Thus, a Content-Language header could be present on
185 the request, but it would only mean (for whatever that's worth) that
186 the patch document had a language. Servers SHOULD NOT store such
187 headers except as trace information, and SHOULD NOT use such header
188 values the same way they might be used on PUT requests. Therefore,
189 this document does not specify a way to modify a document's Content-
190 Type or Content-Language value through headers, though a mechanism
191 could well be designed to achieve this goal through a patch document.
193 There is no guarantee that a resource can be modified with PATCH.
194 Further, it is expected that different patch document formats will be
195 appropriate for different types of resources and that no single
196 format will be appropriate for all types of resources. Therefore,
197 there is no single default patch document format that implementations
198 are required to support. Servers MUST ensure that a received patch
199 document is appropriate for the type of resource identified by the
202 Clients need to choose when to use PATCH rather than PUT. For
203 example, if the patch document size is larger than the size of the
204 new resource data that would be used in a PUT, then it might make
205 sense to use PUT instead of PATCH. A comparison to POST is even more
206 difficult, because POST is used in widely varying ways and can
207 encompass PUT and PATCH-like operations if the server chooses. If
208 the operation does not modify the resource identified by the Request-
209 URI in a predictable way, POST should be considered instead of PATCH
212 2.1. A Simple PATCH Example
214 PATCH /file.txt HTTP/1.1
215 Host: www.example.com
216 Content-Type: application/example
217 If-Match: "e0023aa4e"
220 [description of changes]
226 Dusseault & Snell Standards Track [Page 4]
228 RFC 5789 HTTP PATCH March 2010
231 This example illustrates use of a hypothetical patch document on an
234 Successful PATCH response to existing text file:
236 HTTP/1.1 204 No Content
237 Content-Location: /file.txt
240 The 204 response code is used because the response does not carry a
241 message body (which a response with the 200 code would have). Note
242 that other success codes could be used as well.
244 Furthermore, the ETag response header field contains the ETag for the
245 entity created by applying the PATCH, available at
246 http://www.example.com/file.txt, as indicated by the Content-Location
247 response header field.
251 There are several known conditions under which a PATCH request can
254 Malformed patch document: When the server determines that the patch
255 document provided by the client is not properly formatted, it
256 SHOULD return a 400 (Bad Request) response. The definition of
257 badly formatted depends on the patch document chosen.
259 Unsupported patch document: Can be specified using a 415
260 (Unsupported Media Type) response when the client sends a patch
261 document format that the server does not support for the resource
262 identified by the Request-URI. Such a response SHOULD include an
263 Accept-Patch response header as described in Section 3.1 to notify
264 the client what patch document media types are supported.
266 Unprocessable request: Can be specified with a 422 (Unprocessable
267 Entity) response ([RFC4918], Section 11.2) when the server
268 understands the patch document and the syntax of the patch
269 document appears to be valid, but the server is incapable of
270 processing the request. This might include attempts to modify a
271 resource in a way that would cause the resource to become invalid;
272 for instance, a modification to a well-formed XML document that
273 would cause it to no longer be well-formed. There may also be
274 more specific errors like "Conflicting State" that could be
275 signaled with this status code, but the more specific error would
276 generally be more helpful.
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284 RFC 5789 HTTP PATCH March 2010
287 Resource not found: Can be specified with a 404 (Not Found) status
288 code when the client attempted to apply a patch document to a non-
289 existent resource, but the patch document chosen cannot be applied
290 to a non-existent resource.
292 Conflicting state: Can be specified with a 409 (Conflict) status
293 code when the request cannot be applied given the state of the
294 resource. For example, if the client attempted to apply a
295 structural modification and the structures assumed to exist did
296 not exist (with XML, a patch might specify changing element 'foo'
297 to element 'bar' but element 'foo' might not exist).
299 Conflicting modification: When a client uses either the If-Match or
300 If-Unmodified-Since header to define a precondition, and that
301 precondition failed, then the 412 (Precondition Failed) error is
302 most helpful to the client. However, that response makes no sense
303 if there was no precondition on the request. In cases when the
304 server detects a possible conflicting modification and no
305 precondition was defined in the request, the server can return a
306 409 (Conflict) response.
308 Concurrent modification: Some applications of PATCH might require
309 the server to process requests in the order in which they are
310 received. If a server is operating under those restrictions, and
311 it receives concurrent requests to modify the same resource, but
312 is unable to queue those requests, the server can usefully
313 indicate this error by using a 409 (Conflict) response.
315 Note that the 409 Conflict response gives reasonably consistent
316 information to clients. Depending on the application and the nature
317 of the patch format, the client might be able to reissue the request
318 as is (e.g., an instruction to append a line to a log file), have to
319 retrieve the resource content to recalculate a patch, or have to fail
322 Other HTTP status codes can also be used under the appropriate
325 The entity body of error responses SHOULD contain enough information
326 to communicate the nature of the error to the client. The content-
327 type of the response entity can vary across implementations.
338 Dusseault & Snell Standards Track [Page 6]
340 RFC 5789 HTTP PATCH March 2010
343 3. Advertising Support in OPTIONS
345 A server can advertise its support for the PATCH method by adding it
346 to the listing of allowed methods in the "Allow" OPTIONS response
347 header defined in HTTP/1.1. The PATCH method MAY appear in the
348 "Allow" header even if the Accept-Patch header is absent, in which
349 case the list of allowed patch documents is not advertised.
351 3.1. The Accept-Patch Header
353 This specification introduces a new response header Accept-Patch used
354 to specify the patch document formats accepted by the server.
355 Accept-Patch SHOULD appear in the OPTIONS response for any resource
356 that supports the use of the PATCH method. The presence of the
357 Accept-Patch header in response to any method is an implicit
358 indication that PATCH is allowed on the resource identified by the
359 Request-URI. The presence of a specific patch document format in
360 this header indicates that that specific format is allowed on the
361 resource identified by the Request-URI.
363 Accept-Patch = "Accept-Patch" ":" 1#media-type
365 The Accept-Patch header specifies a comma-separated listing of media-
366 types (with optional parameters) as defined by [RFC2616], Section
371 Accept-Patch: text/example;charset=utf-8
373 3.2. Example OPTIONS Request and Response
377 OPTIONS /example/buddies.xml HTTP/1.1
378 Host: www.example.com
383 Allow: GET, PUT, POST, OPTIONS, HEAD, DELETE, PATCH
384 Accept-Patch: application/example, text/example
386 The examples show a server that supports PATCH generally using two
387 hypothetical patch document formats.
394 Dusseault & Snell Standards Track [Page 7]
396 RFC 5789 HTTP PATCH March 2010
399 4. IANA Considerations
401 4.1. The Accept-Patch Response Header
403 The Accept-Patch response header has been added to the permanent
404 registry (see [RFC3864]).
406 Header field name: Accept-Patch
408 Applicable Protocol: HTTP
410 Author/Change controller: IETF
412 Specification document: this specification
414 5. Security Considerations
416 The security considerations for PATCH are nearly identical to the
417 security considerations for PUT ([RFC2616], Section 9.6). These
418 include authorizing requests (possibly through access control and/or
419 authentication) and ensuring that data is not corrupted through
420 transport errors or through accidental overwrites. Whatever
421 mechanisms are used for PUT can be used for PATCH as well. The
422 following considerations apply especially to PATCH.
424 A document that is patched might be more likely to be corrupted than
425 a document that is overridden in entirety, but that concern can be
426 addressed through the use of mechanisms such as conditional requests
427 using ETags and the If-Match request header as described in
428 Section 2. If a PATCH request fails, the client can issue a GET
429 request to the resource to see what state it is in. In some cases,
430 the client might be able to check the contents of the resource to see
431 if the PATCH request can be resent, but in other cases, the attempt
432 will just fail and/or a user will have to verify intent. In the case
433 of a failure of the underlying transport channel, where a PATCH
434 response is not received before the channel fails or some other
435 timeout happens, the client might have to issue a GET request to see
436 whether the request was applied. The client might want to ensure
437 that the GET request bypasses caches using mechanisms described in
438 HTTP specifications (see, for example, Section 13.1.6 of [RFC2616]).
440 Sometimes an HTTP intermediary might try to detect viruses being sent
441 via HTTP by checking the body of the PUT/POST request or GET
442 response. The PATCH method complicates such watch-keeping because
443 neither the source document nor the patch document might be a virus,
444 yet the result could be. This security consideration is not
450 Dusseault & Snell Standards Track [Page 8]
452 RFC 5789 HTTP PATCH March 2010
455 materially different from those already introduced by byte-range
456 downloads, downloading patch documents, uploading zipped (compressed)
459 Individual patch documents will have their own specific security
460 considerations that will likely vary depending on the types of
461 resources being patched. The considerations for patched binary
462 resources, for instance, will be different than those for patched XML
463 documents. Servers MUST take adequate precautions to ensure that
464 malicious clients cannot consume excessive server resources (e.g.,
465 CPU, disk I/O) through the client's use of PATCH.
469 6.1. Normative References
471 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
472 Requirement Levels", BCP 14, RFC 2119, March 1997.
474 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
475 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
476 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
478 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
479 Procedures for Message Header Fields", BCP 90, RFC 3864,
482 6.2. Informative References
484 [RFC4918] Dusseault, L., "HTTP Extensions for Web Distributed
485 Authoring and Versioning (WebDAV)", RFC 4918, June 2007.
506 Dusseault & Snell Standards Track [Page 9]
508 RFC 5789 HTTP PATCH March 2010
511 Appendix A. Acknowledgements
513 PATCH is not a new concept, it first appeared in HTTP in drafts of
514 version 1.1 written by Roy Fielding and Henrik Frystyk and also
515 appears in Section 19.6.1.1 of RFC 2068.
517 Thanks to Adam Roach, Chris Sharp, Julian Reschke, Geoff Clemm, Scott
518 Lawrence, Jeffrey Mogul, Roy Fielding, Greg Stein, Jim Luther, Alex
519 Rousskov, Jamie Lokier, Joe Hildebrand, Mark Nottingham, Michael
520 Balloni, Cyrus Daboo, Brian Carpenter, John Klensin, Eliot Lear, SM,
521 and Bernie Hoeneisen for review and advice on this document. In
522 particular, Julian Reschke did repeated reviews, made many useful
523 suggestions, and was critical to the publication of this document.
530 San Francisco, CA 94111
533 EMail: lisa.dusseault@gmail.com
538 EMail: jasnell@gmail.com
539 URI: http://www.snellspace.com
562 Dusseault & Snell Standards Track [Page 10]