Using POST to add Members to Web Distributed Authoring and Versioning (WebDAV) Collectionsgreenbytes GmbHHafenweg 16MuensterNW48155Germany+49 251 2807760julian.reschke@greenbytes.dehttp://greenbytes.de/tech/webdav/
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.
Please send comments to the Distributed Authoring and Versioning (WebDAV)
working group at , which may
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WEBDAV working group are archived at
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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 .
The Hypertext Transfer Protocol (HTTP) Extensions for the Web Distributed
Authoring and Versioning (WebDAV) (, 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
(, 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 (, 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. (...)
As a matter of fact, letting the server choose the member URI not only is a
simplification for certain types of clients, but can also reduce the complexity
of the server (in that it doesn't need to persist an additional
client-supplied identifier where it already has an internal one like
a UUID or a primary key).
Note: the vCard Extensions to WebDAV (CardDAV) ()
suffer from the same issue, and may be able to take advantage of this
specification.
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 (),
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.
The terminology used here follows that in the WebDAV specification
().
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 .
This document uses XML DTD fragments
() as a purely notational convention. In particular:
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.
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.Element ordering is irrelevant.Extension elements/attributes (elements/attributes not already defined
as valid child elements) may be added anywhere, except when explicitly
stated otherwise.
Due to the reasons stated in , 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 servers 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 minting another URI for a limited
purpose.
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.
It MUST identify a resource on the same server as the WebDAV collection
(the host and port components (, Section 3.2.2) of the URIs must match).
If there are pre-conditions related to creating a resource in the collection
using a PUT request, then those same pre-conditions apply to the new POST
request behavior, and the same HTTP response body will returned on failure.
The URI of the newly created resource is returned in the HTTP Location
response header (, Section 14.30).
Note: the fact that a server advertises an "Add-Member" URI
does not imply any special semantics of the collection itself. For instance,
member URIs assigned by the server are not necessarily unique over time (a member
URI may be assigned again to a new resource when it previously was removed).
Note: the "Add-Member" URI can be the identical to the collection's
URI (in which case the server just advertises the fact that POST to the
WebDAV collection's URI is supported as defined within this specification).
But it can also be different from it, in which case it doesn't need to
have any relation to the collection's URI.
Given a collection URI of all of the URIs below might occur as "Add-Member" URIs:
The remainder of the document uses the same format just for reasons
of consistency; any other HTTP URI would do as well.
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).
]]>
A PROPFIND/allprop request SHOULD NOT return this property (see
, Section 9.1). Servers
MUST implement the DAV:supported-live-property-set property defined
in Section 3.1.4 of , and report the property
p:add-member as a supported live property.
>>Request
]]>>>Response/collection//collection;add-member/HTTP/1.1 200 OK]]>
Note that in this case, the server has minted a separate URI for the purpose
of adding new content.
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:
It can be used both in HTTP Link headers (see ):
>>Request>>Response;
rel="http://purl.org/NET/webdav/post#add-member"
]]>
...and in (X)HTML content:
Contents of /collection/
...