PATCH Method for HTTP

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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.¶

This specification defines the new HTTP 1.1 [RFC2616] method PATCH that is used to apply partial modifications 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 can 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.¶

In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in [RFC2119].¶

The PATCH method requests that a set of changes described in the request entity be applied to the resource identified by the Request-URI. The set of changes is represented in a format called a "patch document" identified by a media type. PATCH is neither safe or idempotent as defined by [RFC2616] Section 9.1.¶

The difference between the PUT and PATCH requests is reflected in the way the server processes the enclosed entity to modify the resource identified by the Request-URI. In a PUT request, the enclosed entity is considered to be a modified version of the resource stored on the origin server and the client is requesting that stored version be replaced. With PATCH, however, the enclosed entity contains a set of instructions describing how a resource currently residing on the origin server should be modified to produce a new version. The changes described by the entity MAY result in the creation of one or more new resources on the server, however it is not intended that the body of the PATCH request be used as the content of such resources.¶

The server MUST always apply the entire set of changes atomically and never provide (e.g. in response to a GET during this operation) a partially-modified representation. If the entire patch document cannot be successfully applied then the server MUST fail the entire request, applying none of the changes. The determination of what constitutes a successful PATCH can vary depending on the patch document and the type of resource being modified. The actual method for determining how to apply the patch document to the resource is defined entirely by the origin server. See Error Handling in section 2.2 for details on status codes and possible error conditions.¶

If the request passes through a cache and the Request-URI identifies one or more currently cached entities, those entries SHOULD be treated as stale. Responses to this method are not cacheable, unless the response includes appropriate Cache-Control or Expires header fields or the response uses the 209 Content Returned status code as defined in Section 4. The 303 (See Other) response can be used to direct the user agent to retrieve a cacheable resource.¶

Collisions from multiple requests are more dangerous than PUT collisions, because a patch document that is not operating from a known base point may corrupt the resource. Clients wishing to apply a patch document to a known entity can first acquire the strong ETag of the resource to be modified, and use that Etag in the If-Match header on the PATCH request to verify that the resource is still unchanged. If a strong ETag is not available for a given resource, the client can use If-Unmodified-Since as a less-reliable safeguard.¶

It is RECOMMENDED that Servers provide strong ETags for all resources for which PATCH is supported.¶

If a PATCH request contains any entity-headers the server does not understand, the server MUST return a 501 (Not Implemented) response. A server that understands a particular entity-header can choose to ignore it; however, doing so can produce results that are unexpected or unintended by the client. All entity-headers contained in the request apply only to the contained patch document and MUST NOT be applied to the resource being modified.¶

If the Request-URI identifies a resource with multiple alternate representations, the server can choose to respond in a variety of ways. For instance, the server can decide which representation to alter and might even be able to change them all consistently depending on the patch format. A particular patch document might be able to identify specific representations to modify or might be capable of describing changes to multiple representations. If the server cannot choose a representation, it can reject the request with an error or the server can choose to redirect the request (e.g. using 301 Moved Permanently or 302 Found), in which case the user agent makes its own decision regarding whether or not to proceed with the request.¶

Clients are advised to take caution when sending multiple PATCH requests, or sequences of requests that include PATCH, over a pipelined connection as there are no guarantees that pipelined requests will be processed by the server in the same order in which the client sends them. Such sequences of requests can be made safer by using conditional request mechanisms such as If-Match. See [RFC2616] Section 8.1.2.2 for additional details regarding pipelining and non-idempotent requests.¶

There is no guarantee that a resource can be modified with PATCH. Further, it is expected that different patch document formats will be appropriate for different types of resources and that no single format will be appropriate for all types of resources. Therefore, there is no single default patch document format that implementations are required to support. Servers MUST ensure that a received patch document is appropriate for the type of resource identified by the Request-URI.¶

A server can advertise its support for the PATCH method by adding it to the listing of allowed methods in the "Allow" OPTIONS response header defined in HTTP/1.1.¶

The 209 "Content Returned" status code can be used to indicate that a response is equivalent to what would have been returned with a 200 status code response to a GET sent to the URI immediately following the successful completion of the request.¶

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 can be addressed through the use of mechanisms such as conditional requests using ETags and the If-Match request header.¶

Sometimes an HTTP intermediary might try to detect viruses being sent via HTTP by checking the body of the PUT/POST request or GET response. The PATCH method complicates such watch-keeping because neither the source document nor the patch document might be a virus, yet the result could be. This security consideration is not materially different from those already introduced by byte-range downloads, downloading patch documents, uploading zipped (compressed) files and so on.¶

Individual patch documents will have their own specific security considerations that will likely vary depending on the types of resources being patched. The considerations for patched binary resources, for instance, will be different than those for patched XML documents.¶

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.¶

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

• Should the Prefer section be separated out into a separate I-D?
• We need to determine how new preference codes are created/registered
• Are warn-codes ok or do we need a new response header?

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