Network Working GroupJ. Reschke
Internet-Draftgreenbytes
Intended status: Standards TrackSeptember 22, 2008
Expires: March 26, 2009

Using POST to add to Web Distributed Authoring and Versioning (WebDAV) Collections

draft-reschke-webdav-post-00

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Abstract

The Hypertext Transfer Protocol (HTTP) Extensions for the Web Distributed Authoring and Versioning (WebDAV) do not define the behavior for the "POST" method when applied to collections, as the base specification (HTTP) leaves implementers lots of freedom for the semantics of "POST".

This has lead to a situation where many WebDAV servers do not implement POST for collections at all, although it is well suited to be used for the purpose of adding new members to a collection, where the server remains in control of the newly assigned URL. As a matter of fact, the Atom Publishing Protocol (AtomPub) uses POST exactly for that purpose. On the other hand, WebDAV-based protocols such as the Calendar Extensions to WebDAV (CalDAV) frequently require clients to pick a unique URL, although the server could easily perform that task.

This specification defines a discovery mechanism through which servers can advertise support for POST requests with the aforementioned "add collection member" semantics.

Editorial Note (To be removed by RFC Editor before publication)

Please send comments to the Distributed Authoring and Versioning (WebDAV) working group at <mailto:w3c-dist-auth@w3.org>, which may be joined by sending a message with subject "subscribe" to <mailto:w3c-dist-auth-request@w3.org>. Discussions of the WEBDAV working group are archived at <http://lists.w3.org/Archives/Public/w3c-dist-auth/>.

Note that although discussion takes place on the WebDAV working group's mailing list, this is not a working group document.

XML versions, latest edits and the issues list for this document are available from <http://greenbytes.de/tech/webdav/#draft-reschke-webdav-post>.


1. Introduction

The Hypertext Transfer Protocol (HTTP) Extensions for the Web Distributed Authoring and Versioning (WebDAV) ([RFC4918], Section 9.5) do not define the behavior for the "POST" method when applied to collections, as the base specification (HTTP) leaves implementers lots of freedom for the semantics of "POST":

9.5 POST for Collections

Since by definition the actual function performed by POST is determined by the server and often depends on the particular resource, the behavior of POST when applied to collections cannot be meaningfully modified because it is largely undefined. Thus, the semantics of POST are unmodified when applied to a collection.

This has lead to a situation where many WebDAV servers do not implement POST for collections at all, although it is well suited to be used for the purpose of adding new members to a collection, where the server remains in control of the newly assigned URL. As a matter of fact, the Atom Publishing Protocol (AtomPub) uses POST exactly for that purpose ([RFC5023], Section 9.2):

9.2 Creating Resources with POST

To add members to a Collection, clients send POST requests to the URI of the Collection.

On the other hand, WebDAV-based protocols such as Calendaring Extensions to WebDAV (CalDAV) frequently require clients to pick a unique URL, although the server could easily perform that task ([RFC4791], Section 5.3.2):

5.3.2 Creating Calendar Object Resources

...

When servers create new resources, it's not hard for the server to choose an unmapped URI. It's slightly tougher for clients, because a client might not want to examine all resources in the collection and might not want to lock the entire collection to ensure that a new resource isn't created with a name collision. (...)

This specification defines a discovery mechanism through which servers can advertise support for POST requests with the aforementioned "add collection member" semantics.

2. Terminology

The terminology used here follows that in the WebDAV specification ([RFC4918]).

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

This document uses XML DTD fragments ([XML]) as a purely notational convention. In particular:

3. Protocol Extension

Due to the reasons stated in Section 1, clients can not rely on a specific server behavior when POST is applied to a collection. This problem is addressed by this specification by allowing servers to advertise a URI that has the desired "add member" semantics.

Note that ervers that already use POST for a different purpose can just expose a different URI for that purpose. Other servers can just advertise the collection's own URI, thus avoiding to mint another URI for a limited purpose.

3.1. Definition of 'Add-Member' URI

The "Add-Member" URI of a WebDAV collection is a URI that will accept HTTP POST requests, and will interpret these as requests to store the enclosed entity as a new internal member of the collection. The URI of the newly created resource is returned in the HTTP Location response header ([RFC2616], Section 14.30).

3.2. Discovery

3.2.1. p:add-member Property

The p:add-member property is defined on WebDAV collections, and contains the "Add-Member" URI for that collection (embedded inside a DAV:href element).

  <!ELEMENT p:add-member (href)>
  <!-- href: defined in [RFC4918], Section 14.7 -->

A PROPFIND/allprop request SHOULD NOT return this property (see [RFC4918], Section 9.1).

3.2.2. Example

>>Request

PROPFIND /collection/ HTTP/1.1
Host: example.com
Content-Type: application/xml; charset="utf-8"
Content-Lenght: 163

<?xml version="1.0" encoding="utf-8" ?> 
<propfind xmlns="DAV:" xmlns:p="http://purl.org/NET/webdav/post#">
  <prop>
    <p:add-member/>
  </prop>
</propfind>

>>Response

HTTP/1.1 207 Multi-Status 
Content-Type: application/xml; charset="utf-8" 
Content-Length: 385
  
<?xml version="1.0" encoding="utf-8" ?>
<multistatus xmlns="DAV:" xmlns:p="http://purl.org/NET/webdav/post#">
  <response>
    <href>/collection/</href>
    <propstat>
      <prop>
        <p:add-member>
          <href>/collection/;add-member</href>
        </p:add-member>
      </prop>
      <status>HTTP/1.1 200 OK</status>
    </propstat>
  </response>
</multistatus>

Note that in this case, the server has minted a separate URI for the purpose of adding new content.

3.3. Relation to AtomPub's 'Slug' Header

In the AtomPub protocol, clients can use the entity header "Slug" to suggest parts of the URI to be created (see [RFC5023], Section 9.7). Note that servers are free to ignore this suggestion, or to use whatever algorithm that makes sense to generate the new URI.

The same applies to the extension defined here: clients can use the "Slug" header as by its definition of a generic HTTP header. Servers should process it exactly the way defined by AtomPub.

3.4. Example Operation

>>Request

POST /collection/;add-member HTTP/1.1
Host: example.com
Content-Type: text/plain
Slug: Sample Text
Content-Lenght: 12

Sample text.

>>Response

HTTP/1.1 201 Created
Location: http://example.com/collection/sample%20text

4. Internationalization Considerations

This document does not introduce any new internationalization considerations beyond those discussed in Section 20 of [RFC4918].

5. IANA Considerations

This specification does not require any actions from IANA.

6. Security Considerations

All security considerations connected to HTTP/WebDAV and XML apply for this specification as well, namely, [RFC4918] (Section 20) and [RFC3470] (Section 7).

7. References

7.1. Normative References

Nottingham, M., “HTTP Header Linking”, Internet-Draft draft-nottingham-http-link-header-02 (work in progress), July 2008.
[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.
[RFC4918]
Dusseault, L., Ed., “HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)”, RFC 4918, June 2007.
[XML]
Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and F. Yergeau, “Extensible Markup Language (XML) 1.0 (Fourth Edition)”, W3C REC-xml-20060816, August 2006, <http://www.w3.org/TR/2006/REC-xml-20060816>.

7.2. Informative References

[draft-ietf-vcarddav-carddav]
Daboo, C., “vCard Extensions to WebDAV (CardDAV)”, Internet-Draft draft-ietf-vcarddav-carddav-01 (work in progress), July 2008.
[draft-reschke-http-addmember]
Reschke, J., “The HTTP ADDMEMBER Method”, Internet-Draft draft-reschke-http-addmember-00 (work in progress), February 2005.
[RFC3470]
Hollenbeck, S., Rose, M., and L. Masinter, “Guidelines for the Use of Extensible Markup Language (XML) within IETF Protocols”, RFC 3470, BCP 70, January 2003.
[RFC4791]
Daboo, C., Desruisseaux, B., and L. Dusseault, “Calendaring Extensions to WebDAV (CalDAV)”, RFC 4791, March 2007.
[RFC5023]
Gregorio, J. and B. de hOra, “The Atom Publishing Protocol”, RFC 5023, October 2007.

Index

A

Author's Address

Julian F. Reschke
greenbytes GmbH
Hafenweg 16
Muenster, NW 48155
Germany
Phone: +49 251 2807760
EMail: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/

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