|Network Working Group||J. Reschke|
|Updates: 2616 (if approved)||March 2009|
|Intended status: Experimental|
|Expires: September 2009|
The Hypertext Transfer Protocol (HTTP) Entity Tag ("ETag") Response Header in Write Operations
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The Hypertext Transfer Protocol (HTTP) specifies a state identifier, called "Entity Tag", to be returned in the "ETag" response header. However, the description of this header for write operations such as PUT is incomplete, and has caused confusion among developers and protocol designers, and potentially interoperability problems.
This document explains the problem in detail and suggests both a clarification for a revision to the HTTP/1.1 specification (RFC2616) and a new header for use in responses, making HTTP entity tags more useful for user agents that want to avoid round-trips to the server after modifying a resource.
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XML versions, latest edits and the issues list for this document are available from <http://greenbytes.de/tech/webdav/#draft-reschke-http-etag-on-write>.
|I edit (type: edit, status: open)|
|email@example.com||2006-08-10||Umbrella issue for editorial fixes/enhancements.|
The Hypertext Transfer Protocol (HTTP) specifies a state identifier, called "Entity Tag", to be returned in the "ETag" response header (see [RFC2616], Section 14.19). However, the description of this header for write operations such as PUT is incomplete, and has caused confusion among developers and protocol designers, and potentially interoperability problems.¶
This document explains the problem in detail and suggests both a clarification for a revision to [RFC2616] and a new header for use in responses, making HTTP entity tags more useful for user agents that want to avoid round-trips to the server after modifying a resource.¶
Note that there is a related problem: modifying content-negotiated resources. Here the consensus seems to be simply not to do it. Instead, the origin server should reveal specific URIs of content that is not content-negotiated in the Content-Location response header ([RFC2616], Section 14.14), and user agents should use this more specific URI for authoring. Thus, the remainder of this document will focus on resources for which no content negotiation takes place.¶
For a long time, nobody realized that there was a problem at all, or those who realized preferred to ignore it.¶
Server implementers added code that would return the new value of the "ETag" header in a response to a successful PUT request. After all, the client could be interested in it.¶
User agent developers in turn were happy to get a new "ETag" value, saving a subsequent HEAD request to retrieve the new entity tag.¶
However, at some point of time, potentially during a Web Distributed Authoring and Versioning (WebDAV, [RFC2518], obsoleted by [RFC4918]) interoperability event, client programmers asked server programmers to always return "ETag" headers upon PUT, never ever to change the entity tag without "good reason", and - by the way - always to guarantee that the server stores the new content octet-by-octet.¶
From the perspective of client software that wants to treat an HTTP server as a file system replacement, this makes a lot of sense. After all, when one writes to a file one usually expects the file system to store what was written, and not to unexpectedly change the contents.¶
However, in general, an HTTP server is not a file system replacement. There may be some that have been designed that way, and some that expose some parts of their namespace that have this quality. But in general, HTTP server implementers have a lot of freedom in how resources are implemented. Indeed, this flexibility is one of the reasons for HTTP's success, allowing it to be used for a wide range of tasks, of which replacing file systems is just one (and not necessarily the most interesting one).¶
In particular: ¶
As long as servers store the content octet-by-octet, and return exactly what the client wrote, there is no problem at all.¶
Things get more interesting when a server does change the content, such as in the "simple authoring" example given in Appendix A.1. Here, the server does change the content upon writing to the resource, yet no harm is done, because the final state of the resource on the server does not depend on the client being aware of that.¶
All of the content rewriting examples mentioned above have this quality: the client can safely continue to edit the entity it sent, because the result of the transformation done by the server will be the same in the end. Formally, if we call the server-side transformation "t", the initial content "c", and the client-side editing steps "e1" and "e2", then¶
t(e2(e1(c))) = t(e2(t(e1(c))))
e.g., it is irrelevant whether the client obtained the current entity body before doing its second edit.
Problems will only occur if the client uses the entity body it sent, and the entity tag it obtained in return, in subsequent requests that only transfer part of the entity body, such as GET or PUT requests using the "Range" request header (see [RFC2616], Section 14.35).¶
Furthermore, some clients need to expose the actual contents to the end user. These clients will have to ensure that they really have the current representation.¶
The ETag response-header field provides the current value of the entity tag for the requested variant. Sections 14.24, 14.26 and 14.44 describe the headers used with entity tags. The entity tag MAY be used for comparison with other entities from the same resource (see Section 13.3.3).
The response-header fields allow the server to pass additional information about the response which cannot be placed in the Status-Line. These header fields give information about the server and about further access to the resource identified by the Request-URI.
The "ETag" response header itself is mentioned mainly in the context of cache validation, such as in Section 13.3.2. What is missing is a coherent description on how the origin server can notify the user-agent when the entity tag changes as result of a write operation, such as PUT.¶
A 201 response MAY contain an ETag response header field indicating the current value of the entity tag for the requested variant just created, see Section 14.19.
The "ETag" response header is mentioned again in the definition of 206 Partial Content (Section 10.2.7) and 304 Not Modified (Section 10.3.5), but notably missing are statements about other 2xx series status codes that can occur upon a successful PUT operation, such as 200 OK (Section 10.2.1) and 204 No Content (Section 10.2.5).¶
While working on the revision of [RFC2518], the IETF WebDAV working group realized that this is a generic problem that needs attention independently of WebDAV. An initial attempt was made with [draft-whitehead-http-etag] in February 2006, but no progress was made since.¶
Since then, two specifications based on HTTP were just published by the RFC Editor, taking two opposite approaches: ¶
In essence, this makes it impossible to implement an HTTP resource that conforms to both specifications. Due to the differing use cases of XCAP and CalDAV, this may not be a problem in practice, but the disagreement in itself is scary. Publication of these specifications on the standards track will make it much harder for future protocols to deal with this topic in a meaningful way (comments were sent during IETF Last Call for CalDAV, see <http://www1.ietf.org/mail-archive/web/ietf/current/msg43021.html>).¶
This section describes a minimal change to [RFC2616], proposed in <http://lists.w3.org/Archives/Public/ietf-http-wg/2006JanMar/0003.html>.¶
The response MAY contain an ETag response header field indicating the current value of the entity tag (Section 14.19) for the requested variant. The value SHOULD be the same as the one returned upon a subsequent HEAD request addressing the same variant.
A 201 response MAY contain an ETag response header field indicating the current value of the entity tag for the requested variant just created, see Section 14.19.
In essence, this moves the statement about entity tags in write operations from the specific case of status 201 Created into the more generic description of the 2xx series status codes.¶
Note that the term "requested variant" is somewhat misleading, in particular in the context of write operations (see <http://www.tools.ietf.org/wg/httpbis/trac/ticket/69> on the RFC2616bis issues list).¶
The 'Entity-Transform' entity header provides information about whether a transformation has been applied to an entity body.¶
When used in an HTTP request, its meaning is undefined. In an HTTP response, it provides information whether the server has applied a transformation when the entity was stored last.¶
In general, entity headers may be stored in intermediates. The main use of this header however applies to the HTTP PUT method, of which by default the results are not cacheable (see [RFC2616], Section 9.6). In addition, the value format is defined so that a client can reliably detect whether the information is fresh.¶
Entity-Transform = "Entity-Transform" ":" entity-transform-spec entity-transform-spec = entity-transform-keyword SP entity-tag entity-transform-keyword = "identity" | "unspecified"| token entity-tag = <entity-tag: defined in [RFC2616], Section 3.11> token = <token: defined in [RFC2616], Section 2.2>
The entity-tag specifies the entity body to which this information applies.¶
An entity-transform-keyword of "identity" specifies that the origin server has stored the entity octet-by-octet, thus the user agent MAY use a local copy of the entity body with the given entity-tag for subsequent requests that rely on octet-by-octet identity (such as a PUT with "Range" request header).¶
Both the absence of this response header and any entity-transform-keyword value other than "identity" specify that the origin server may have transformed the entity before storage, thus a subsequent retrieval will not necessarily return an exact copy of the submitted entity.¶
Content was stored octet-by-octet:
HTTP/1.1 200 OK ETag: "1" Entity-Transform: identity "1"
Content was transformed:
HTTP/1.1 200 OK ETag: "2" Entity-Transform: unspecified "2"
Response containing a stale "Entity-Transform" header:
HTTP/1.1 200 OK ETag: "3" Entity-Transform: unspecified "2"
Note that in this case the newly assigned entity tag and the entity tag returned in "Entity-Transform" do not match, thus the client is aware that the header value is stale and can't be used.
The clarification of [RFC2616] (see Section 3) makes it clear that user agents can use "ETag" headers obtained in write operations, as long as they do not require octet-by-octet identity. In particular, a new entity tag can be returned for any method, such as a WebDAV PROPPATCH (see [RFC4918], Section 9.2). This helps dealing with the problem described in Appendix A.3. See Appendix A.4 for details.¶
The addition of the "Entity-Transform" header (see Section 4) enables origin servers to signal that they stored an exact copy of the content, thus allowing clients not to refetch the content. Note that by default (in absence of the response header), a client can not make any assumptions about the server's behavior in this regard. Thus clients will only benefit from new servers explicitly setting the new header.¶
This document specifies the new HTTP header listed below.¶
|[RFC2119]||Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, March 1997.|
|[RFC2616]||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.|
|[draft-whitehead-http-etag]||Whitehead, J., “Design Considerations for State Identifiers in HTTP and WebDAV”, Internet-Draft draft-whitehead-http-etag-00 (work in progress), February 2006.|
(Not updated since.)
|[HTML]||Raggett, D., Hors, A., and I. Jacobs, “HTML 4.01 Specification”, W3C REC-html401-19991224, December 1999, <http://www.w3.org/TR/html401/>.|
|[RFC2518]||Goland, Y., Whitehead, E., Faizi, A., Carter, S., and D. Jensen, “HTTP Extensions for Distributed Authoring -- WEBDAV”, RFC 2518, February 1999.|
|[RFC4791]||Daboo, C., Desruisseaux, B., and L. Dusseault, “Calendaring Extensions to WebDAV (CalDAV)”, RFC 4791, March 2007.|
|[RFC4825]||Rosenberg, J., “The Extensible Markup Language (XML) Configuration Access Protocol (XCAP)”, RFC 4825, May 2007.|
|[RFC4918]||Dusseault, L., Ed., “HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)”, RFC 4918, June 2007.|
|[RFC5023]||Gregorio, J., Ed. and B. de hOra, Ed., “The Atom Publishing Protocol”, RFC 5023, October 2007.|
A server implementing this specification would instead respond with:¶
>> Response (2c)
HTTP/1.1 200 OK Content-Type: text/plain ETag: "2" Entity-Transform: unspecified "2"
This indicates that the content that was sent is not octet-by-octet identical to what a subsequent GET request would retrieve. It is then up to the client to decide whether it wants to re-fetch the content before continuing edits.¶
In the first step, the client obtains the current representation for <http://example.com/test.html>:¶
>> Request (1)
GET /test.html HTTP/1.1 Host: example.com
>> Response (1)
HTTP/1.1 200 OK Content-Type: text/html ETag: "A" <html> <head> </head> <body> </body> </html>
Next, it adds one paragraph to the <body> element, and gets back a new entity tag:¶
>> Request (2)
PUT /test.html HTTP/1.1 Host: example.com If-Match: "A" <html> <head> </head> <body> <p>First paragraph.</p> </body> </html>
>> Response (2)
HTTP/1.1 200 OK ETag: "B"
>> Request (3)
PROPPATCH /test.html HTTP/1.1 Host: example.com Content-Type: application/xml <proppatch xmlns="DAV:"> <set> <prop> <title xmlns="http://ns.example.org/" >A sample document</title> </prop> </set> </proppatch>
>> Response (3)
HTTP/1.1 207 Multi-Status Content-Type: application/xml <multistatus xmlns="DAV:"> <response> <href>/test.html</href> <propstat> <prop> <title xmlns="http://ns.example.org/"/> </prop> <status>HTTP/1.1 2OO OK</status> </propstat> </response> </multistatus>
The server knows how to propagate property changes into the HTML content, so it updates the entity by adding an HTML title document accordingly. This causes the entity tag changing to "C".¶
The new entity body is shown below, but the client does not realize that it did change at all.
<html> <head> <title>A sample document</title> </head> <body> <p>First paragraph.</p> </body> </html>
A subsequent attempt by the client to update the entity body will fail, unless it realizes that changing WebDAV properties may affect the entity as well. In this case, it would have had to get the current entity tag before proceeding. Of course, this introduces an additional round-trip, and a timing window during which overlapping updates by other clients would go unnoticed.¶
Below we repeat the example from above (Appendix A.3), but here the origin server returns entity tags for all write operations, and the user agent knows how to deal with them. That is, both take advantage of what [RFC2616] already allows.¶
>> Request (3)
PROPPATCH /test.html HTTP/1.1 Host: example.com Content-Type: application/xml <proppatch xmlns="DAV:"> <set> <prop> <title xmlns="http://ns.example.org/"> A sample document </title> </prop> </set> </proppatch>
>> Response (3)
HTTP/1.1 207 Multi-Status Content-Type: application/xml ETag: "C" <multistatus xmlns="DAV:"> <response> <href>/test.html</href> <propstat> <prop> <title xmlns="http://ns.example.org/"/> </prop> <status>HTTP/1.1 2OO OK</status> </propstat> </response> </multistatus>
As before, this causes the entity to change, and a new entity tag to be assigned. But in this case, the origin server actually notifies the client of the changed state by including the "ETag" response header.¶
The client now will be aware that the requested entity change, and can use the new entity tag in subsequent requests (potentially after refreshing the local copy).¶
Add and resolves issues "entity-header" and "extensibility", by removing the extension hooks, and by redefining the header to it can be used as an Entity header.¶
Update APP and CALDAV references. Remove RFC3986 reference (not needed anymore after the simplication in draft 01). Fix typo in header description ("submitted entity", not "stored entity"). Remove comparison about how XCAP and CALDAV profile RFC2616: after all, both mandate a behaviour that was legal but optional before. Add "Updates: RFC2616".¶
In the references, note that there was no activitiy on draft-whitehead-http-etag-00 anymore. Change intended status to "Experimental". Update APP reference. Update statements about current status of XCAP and CALDAV. Add and resolve "clarify-extension".¶
Update XCAP reference. Update header definition to use prose rules rather than comments. Clarify "identity" keyword when appearing with a weak entity-tag. In PROPPATCH/HTML example, fix whitespace so that the HTML title element has exactly the same value as the one that was PROPPATCHed. Mention that ETags changing due to a PROPPATCH could even occur in presence of a WebDAV write lock. Expand keyword substitution example with variant where Entity-Transform is returned.¶
Update APP reference. Update CalDAV reference (published as RFC4791). Update "prior work" (RFC4791 being published, XCAP having been approved).¶
Update XCAP reference (published as RFC4825). Update "prior work" (RFC4825 being published).¶
Replace RFC2518 references with RFC4918 where appropriate. Update APP reference.¶
Update APP reference. Reference the "requested variant" issue on the RFC2616bis issues list (<http://www.w3.org/Protocols/HTTP/1.1/rfc2616bis/issues/#i69>).¶