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Grok, CherryPy, WSGI and Zope 3

Yesterday night I experimented with getting Zope 3/Grok to work CherryPy, through WSGI. This led to all kinds of interesting adventures and opportunities. Follow the path I took:

This started as I had been doing some work with CherryPy recently. I like what I see of CherryPy so far; a nice, Pythonic and powerful web server.

Currently Zope 3 uses Twisted as the default web server implementation. Using Twisted has lots of advantages. It's a high-quality framework with a lot of features. It's also maintained by people other than the Zope developers, which is good. Since Zope was one of the first in the Python web space, the Zope developers still maintain a version of Medusa for Zope 2 called ZServer. It made sense to do that at the time, but it doesn't now.

So if Twisted is so nice, why look at CherryPy at all? Because it's nice and because it's fun to try would be reasons that are good enough, but there are some other reasons that make CherryPy interesting for Zope too:

  • CherryPy supports threaded webservers in a non-begrudging manner. Since Zope uses a threaded model for web serving, that might be a nice match. Twisted supports threads just fine too, but when you do, one always gets this idea that thousands of Twisted developers screaming at you just outside the range of your hearing.
  • CherryPy is committed to supporting WSGI, the framework for Python web interoperability. Zope has committed to using WSGI as well.The Twisted community sometimes seems to be a bit more half-hearted about supporting this.
  • CherryPy installs cleanly as a Python egg. This means I can use setuptools and buildout easily. When I asked a few months ago I got the impression the Twisted developers don't like eggs very much and certainly have no plans to support them any time soon. Since Zope is moving towards using eggs, CherryPy is interesting.

Then I got slightly side-tracked. I figured that the easiest way to integrate Zope 3 with CherryPy was to use CherryPy's WSGI server and use Zope 3 as a WSGI app, which Zope 3 has support for. This gave me a nice opportunity to play with WSGI for a bit. Now CherryPy's WSGI server is not actually CherryPy's server, so there goes my goal of integrating the whole of CherryPy... I heard a few good things about CherryPy's WSGI server too though, and it was easier to do, so I tried that.

This became a nice opportunity to play with some other things. One of the reasons I wanted to try running Zope 3 and Grok with CherryPy is that it would allow me to experiment with what the minimal Zope 3 configuration of Zope 3 would need to be. Grok comes in nicely here: aiming for Grok support made that minimal configuration quite well-defined. I didn't need to aim to get Zope 3's own management UI up, just to get the basic Grok functionality to work.

One of my aims is to make Zope 3's startup procedure a lot faster when you're doing Grok development. Another aim is to be prepared for the time when Zope 3 comes as a whole bunch of eggs: we can just use the eggs that Grok needs and not anything else. Combined with buildout this makes for an easy and powerful development and deployment system.

So those are my aims and motivations - how far did I get? Quite far: I got a basic Grok view working with CherryPy's WSGI server when I stopped. Not needing the whole of Zope 3 to load up, the server also starts a bit faster, though not as fast as I'd like yet. Here it is:

There are some interesting parts. This is the site.zcml that includes the smallest minimum of ZCML I've managed to get to yet:

This is the WSGI-based server that hooks Zope 3 as a WSGI app into CherryPy's WSGI server:

Not a lot of code, thanks to WSGI!

This shows that Grok works at least minimally; we're registering a view for Zope 3 containers (such as the root folder):

A lot more work needs to be done if this wants to become anywhere near production ready. This code right now is just an experiment. I just thought I'd get my thoughts out there, and see who's interested.

Since it's all a buildout, you can install it safely and if you're on a sufficiently Unixy machine, make it work:

$ svn co svn:// megrok.cherry
$ cd megrok.cherry
$ python2.4
$ bin/buildout
$ bin/startserver

This starts a server running on port 8080. It doesn't do much yet, but what it does is allowing you to access:


This is rendered by the referenced before.

Let me know what you think!


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