davuseragent
============

The Problem
-----------

Zope 3 currently distinguishes different HTTP requests the following
way:

  * If it's not GET, POST or HEAD, it's a regular HTTP/WebDAV request

  * If it's POST and the content type starts with 'text/xml', it must
    be XML-RPC

  * Otherwise, it's a browser request

The problem with this approach is that WebDAV clients seeking to edit
a particular object are treated like a browser client when they do a
GET.  That way, they get to see the fully rendered default view of an
object instead of the filerepresentation source that they expect.  Of
course, clients are theoretically supposed to do a PROPFIND request
for the source, but no known client does so.


The Solution
------------

To solve this problem, Zope 2 invented a WebDAV source port, a
different port to which one could connect WebDAV clients and where a
GET request always led to the source of objects.  This approach is
impractical for a number of reasons:

  o Users invariably can't remember the port (and shouldn't have to)

  o Firewalls invariably block high ports (and firewall admins
    shouldn't have to know about DAV source port hacks)

  o When used behind Apache, security means SSL, and SSL+weird port
    numbers is yet-another-thing to manage.

With `davuseragent`, we follow a different approach: We decide based
on the User-Agent header of the incoming request whether we have a
WebDAV client or not.  That allows us to support a definite set of
clients that are known to work together with Zope 3's DAV support
(which is, admittedly, far from perfect).

`davuseragent` is not trying to please everyone.  It tries to make DAV
workable for a limited set of well-known and widely-used clients.  Of
course, given the variance amongst DAV clients, trying to claim
support for an arbitrary list has proven more than Zope can sign up
for.


How it works
------------

Upon an incoming HTTP request, Zope 3's HTTPServer delegates to a
request/publication factory.  The default one is located in
`zope.app.publication` and currently decides which request type to
instantiate based on the rule described in the first section of this
document.

`davuseragent` simply provides an alternate implementation of a
request/publication factory.  In consequence, it provides a new HTTP
server type which works like the regular `HTTP` server type, but uses
the alternate factory.


Installation
------------

In order to enable `davuseragent`, you'll have to install it as a
regular Zope 3 package in `package-includes` and change your `server`
section in `zope.conf` to use the new server type, e.g.::

  <server>
    type DAVUserAgent
    address 8080
  </server>

That's it!

Philipp von Weitershausen, philikon@philikon.de
Paul Everitt, paul@zope-europe.org