Network Working Group J. Reschke
Internet-Draft greenbytes
Intended status: Standards Track September 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
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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>.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Protocol Extension . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Definition of 'Add-Member' URI . . . . . . . . . . . . . . 6
3.2. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2.1. p:add-member Property . . . . . . . . . . . . . . . . 6
3.2.2. Example . . . . . . . . . . . . . . . . . . . . . . . 6
3.2.3. add-member Link Relation . . . . . . . . . . . . . . . 7
3.3. Relation to AtomPub's 'Slug' Header . . . . . . . . . . . 8
3.4. Example Operation . . . . . . . . . . . . . . . . . . . . 8
4. Internationalization Considerations . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . . 9
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . . . 11
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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.
(...)
Note: the vCard Extensions to WebDAV (CardDAV)
([draft-ietf-vcarddav-carddav]) suffer from the same issue, and
may be able to take advantage of this specification.
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This specification defines a discovery mechanism through which
servers can advertise support for POST requests with the
aforementioned "add collection member" semantics.
Note: the author previously proposed a new HTTP method for exactly
this purpose ([draft-reschke-http-addmember]), but quite a few
reviewers pointed out that this would duplicate the original
semantics of POST. Thus this proposal that avoids adding a new
HTTP method.
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:
o Element names use the namespace
"http://purl.org/NET/webdav/post#". When an XML element type in
this namespace is referenced in this document outside of the
context of an XML fragment, the string "p:" will be prefixed to
the element name. [[ns: With sufficient support from the WebDAV
WG's former mailing list and/or the IESG Applications Area the
property defined later on could potentially be moved into the DAV:
namespace.]]
o Element ordering is irrelevant.
o Extension elements/attributes (elements/attributes not already
defined as valid child elements) may be added anywhere, except
when explicitly stated otherwise.
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
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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).
>>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>
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>>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.
3.2.3. add-member Link Relation
There may be cases in which it is more efficient to expose the Add-
Member URI in HTTP headers or in (X)HTML content. For this use case,
this specification defines a new link relation with the name:
http://purl.org/NET/webdav/post#add-member
It can be uses both in HTTP Link headers (see
[draft-nottingham-http-link-header]):
HEAD /collection/ HTTP/1.1
Host: example.com
HTTP/1.1 200 OK
Link: </collection/;add-member>;
rel="http://purl.org/NET/webdav/post#add-member"
...and in (X)HTML content:
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<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
<head>
<title>Contents of /collection/</title>
<link rel="http://purl.org/NET/webdav/post#add-member"
href="/collection/;add-member" />
</head
<body>...</body>
</html>
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.
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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
[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>.
[draft-nottingham-http-link-header]
Nottingham, M., "HTTP Header Linking",
draft-nottingham-http-link-header-02 (work in progress),
July 2008.
7.2. Informative References
[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.
[draft-ietf-vcarddav-carddav]
Daboo, C., "vCard Extensions to WebDAV (CardDAV)",
draft-ietf-vcarddav-carddav-01 (work in progress),
July 2008.
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[draft-reschke-http-addmember]
Reschke, J., "The HTTP ADDMEMBER Method",
draft-reschke-http-addmember-00 (work in progress),
February 2005.
Index
A
Add-Member URI 6
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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