Zope Paste Wsgi Applications In Zope 3 Using Paste Deploy

This is an introduction to zope.paste, a new package to allow configuring WSGI apps in Zope 3 using paste.deploy.

You can find the source code to zope.paste in the zope.org subversion repository.

what is it?

A simple package that enables one to configure WSGI applications to run inside Zope 3 using paste.deploy.

why?

Because Zope 3 already supported WSGI applications, but didn’t provide a simple way to create and configure them.

how to use it?

short version

Configuration is very simple. There are three steps that need to be performed:

  1. Configure a named IServerType utility (by default, we already define a utility named Paste.Main).
  2. Change the <server> directive on <INSTANCE_HOME>/etc/zope.conf to use the newly-created IServerType utility (out of the box, you can just swap out HTTP or WSGI-HTTP by Paste.Main).
  3. Configure a WSGI application using paste.deploy syntax in <INSTANCE_HOME>/etc/paste.ini

Here’s an example of how to configure the Paste.Main application using paste.deploy to use the Zope 3 publisher as a WSGI app:

[app:Paste.Main] paste.app_factory = zope.paste.application:zope_publisher_app_factory

long version

The narrative below applies to Zope 3.2.

When you create a Zope 3 instance, you have a choice of creating a zope.app.server (a.k.a. zserver) instance or a zope.app.twisted instance.

This package works with both, but applications that you develop using WSGI might choose to use only one of them.

After creating an instance, you should have a directory with some subdirectories like etclibvarlog, etc. Inside the etc directory you can find a file named zope.conf inside it.

This is the file that contains the bootstrap configuration for starting up a Zope 3 server.

Regardless of the flavor you choose (zserver or twisted) you will end up with a file that contains something just like this:

<server>
  type HTTP
  address 8080
</server>

The part that’s most interesting to us here is the type HTTP line. This is what controls what kind of server will be created.

When starting up, upon finding this directive, Zope 3 will lookup a named utility providing IServerType. If you don’t know what a named utility is, you should go read some documentation before proceeding.

Zope 3.2 has some support to WSGI applications. In fact, it does define a IServerType utility named WSGI-HTTP, which is also aliased to HTTP. So when you start up Zope 3 for the first time, you’re actually using WSGI already!

This package, zope.paste does define a new IServerType utility named Paste.Main. So, effectively, once you have this package installed you can change your zope.conf file to read:

<server>
  type Paste.Main
  address 8080
</server>

The Paste.Main utility defined in this package though doesn’t know how to create a WSGI application by itself. Instead, it relies on paste.deploy to create a WSGI application. By default, it will load a file named paste.ini in the etc directory of your Zope 3 instance.

When loading this file it will look for an application with the same name as the utility defined. So the simplest thing you can do is to create the paste.ini file with some content as follows:

[app:Paste.Main]
paste.app_factory = zope.paste.application:zope_publisher_app_factory

What this does is to create a WSGIPublisherApplication, which happens to be the same WSGI application that is created when you use the HTTP or WSGI-HTTP server type utilities.

Now, this is a lot of contortion just to do something that Zope 3 does out of the box no? Yes, I agree. But this is where stuff starts getting fun.

paste.deploy allows you to chain various WSGI entities together. There seems to be a distinction between ‘apps’ and ‘filters’ (also referred to as ‘middleware’). An example that might interest is applying a XSLT transformation to the output of the Zope 3 WSGI application.

Happily enough, someone seems to have already created a WSGI filter for applying a XSLT stylesheet. You can find it at:

http://www.decafbad.com/2005/07/xmlwiki/lib/xmlwiki/xslfilter.py

If you wanted to apply this WSGI filter to Zope 3, you would need three things:

  • Put the xslfilter.py file somewhere in PYTHONPATH. <INSTANCE>/lib/python is a good place.
  • Add this snippet to the bottom of xslfilter.py
def filter_factory(global_conf, **local_conf):
    def filter(app):
        return XSLFilter(app)
    return filter
  • Change the paste.ini file as follows
[pipeline:Paste.Main]
pipeline = xslt main

[app:main]
paste.app_factory = zope.paste.application:zope_publisher_app_factory

[filter:xslt]
paste.filter_factory = xslfilter:filter_factory

What this does is to define a pipeline. Learn more about this on the paste.deploy website. Refer to the source of xslfilter.py for information about how to pass a stylesheet to the filter.

Now, this far we only worked with a single WSGI application. If you wanted to host more than one WSGI application there are a couple ways of doing it:

  1. Using a composite application as described in paste.deploy.
  2. Setting up extra IServerType utilities.

I’m going to show you how to do the latter now.

The trick here is that as mentioned earlier here, you have the option to use both the zserver and the twisted WSGI servers. zope.paste is just glue code, so we defined a IServerType utility for each, and the only thing special is that the utility name is passed on to the WSGI application factory.

Here’s an excerpt from the configure.zcml as found on this package:

<configure zcml:condition="have zserver">
  <utility
      name="Paste.Main"
      component="._server.http"
      provides="zope.app.server.servertype.IServerType"
      />
</configure>

<configure zcml:condition="have twisted">
  <utility
      name="Paste.Main"
      component="._twisted.http"
      provides="zope.app.twisted.interfaces.IServerType"
      />
</configure>

Depending on which server is available, the right IServerType utility is registered. You are encouraged to use the same pattern when defining yours.

So suppose you want to have a second WSGI application. Here’s how you could do it.

  • Create a new IServerType utility. This excerpt could be added to a configure.zcml in your own package, or to a standalone file in etc/package_includes
<configure zcml:condition="have zserver">
  <utility
      name="Paste.Another"
      component="zope.paste._server.http"
      provides="zope.app.server.servertype.IServerType"
      />
</configure>

<configure zcml:condition="have twisted">
  <utility
      name="Paste.Another"
      component="zope.paste._twisted.http"
      provides="zope.app.twisted.interfaces.IServerType"
      />
</configure>
  • Change your zope.conf file to define a new server, using the newly-created Paste.Another utility
<server>
  type Paste.Main
  address 8080
</server>

<server>
  type Paste.Another
  address 8180
</server>
  • Define a WSGI application Paste.Another in paste.ini
[pipeline:Paste.Main]
pipeline = xslt main

[app:main]
paste.app_factory = zope.paste.application:zope_publisher_app_factory

[filter:xslt]
paste.filter_factory = xslfilter:filter_factory

[app:Paste.Another]
paste.app_factory = zope.paste.application:zope_publisher_app_factory

That’s it! For more information, learn about the different ways of configuring applications with paste.deploy.

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