Doing my ZCML


09/05/2008 by Gilles Lenfant

Howdy folks,

Lots of us, poor Plone components developers, have been somehow stuck discovering the new Plone 3 architecture before having some enlightenments from documentation or various blogs or better, from Martin Aspeli essential book.

Across most new programming behaviours, you noticed the massive use of ZCML coming along with the various components, packages or products(*) that ship with Plone 3 bundle.

At first glance, I had some pain trying to understand what’s behind the scene and as Zope 3 dummy, didn’t understand why we should use such a cryptic damned notation. Okay, why should we learn another language when we can do all this in Python? I heard ZCML is for site integrators. Well, for those integrators who already know how to read Python source but certainly not the ones who expand products then play in ZMI to add features in a Plone site.

In addition, finding good tutorials on ZCML usage in Plone 3 was painful before fall 2007.

So far, so good, I started to upgrade some public Plone extension products using ZCML without really understanding fully the “how what why” of all this, and it worked. But why?

Among the products I’m prettily proud to maintain, there’s FileSystemStorage (FSS) that’s now “componentized” the Plone 3 way to ease its installation with buildout (but that’s another story). For those who didn’t click here, FSS let your Archetypes content types store fieds values directly in the file system rather than in plain ZODB object attributes or annotations. This saves memory in Plone sites that serve lots of big files.

There’s two ways to use FSS from content types.

(a) Using FSS in your own content types in the content type schema as below:

# Usual Zope/CMF/Plone/Archetypes imports
from Products.FileSystemStorage.FileSystemStorage import FileSystemStorage
my_schema = Schema((

(b) Using FSS for third party content types as done below for the standard ATFile:

from Products.FileSystemStorage.FileSystemStorage import FileSystemStorage
from Products.ATContentTypes import atct
field = atct.ATFile.schema['file'] = FileSystemStorage()

Okay, this works but the pythonic glue of FSS with content types has two caveats.

At first, we should let the site integrators decide wether he should use FSS for such or such content type attribute depending on the use, amount of content (…) of the site without adding lines of Python.

The second one: we depend on the public API of FSS and inners of Archetypes to do this. This sucks somehow because we plan to transform the FSS product into a Zope 3 component, this will change the imports. In addition we use Archetypes low level API (field.registerLayer) to re-initalize the field storage. That API is subject to changes in the future as it is not in the public API.

So what? We need to provide a higher level setup for FSS that may support the furure and we don’t want to freeze FSS API and we want to honour potential Archetypes inner changes.

We can’t use ZMI/PMI settings because the storage layer of Archetypes content must be set at an early stage of Zope startup, means before any content object is waken up from ZODB.

Using a ZConfig dedicated schema could be an option but ZConfig is not well suited to such situations.

Well, the best option is… Creating a ZCML directive for this. Let’s have a look on how to store ATFile content using FSS service from any “configure.zcml”:

     fields="file" />

Self explanative, doesn’t require any comment for Plone integrators, doesn’t expose or require any specific API.

Now I have defined this, we now need to add the machinery that makes the glue between that elegant ZCML bunch and the deep inners behind the scene.

Digging deep into the Zope startup process, notably through Five, and we can notice that:

  • Five processes a bunch all “meta.zcml”, that define the various namespaces and elements that may be used in the “configure.zcml”.
  • Then it processes all “configure.zcml”, those “configure.zcml” mainly make the high level glue between the various components and products of the instance.
  • Finally it processes “overrides.zcml”, if any that may change standard settings provided in the above “configure.zcml”.

This is a short version and curious people will have a look at $INSTANCE_HOME/etc/site.zcml to see the details on how to customize the ZCML setup. Note that with some magic of Five, you don’t need to change anything here if your main zcml files (“meta.zcml”, “configure.zcml” and “override.zcml”) are in a Zope 2 style product.

Okay, aren’t you asleep or lost reading this long and boring technical recipe? No? So let’s continue and grab into the depth of the code!

As stated above, I need to define the schema and handler of the element “<fss:typeWithFSS …” from a “meta.zcml” and a bunch of Python modules.

The meta directive
In my case, this is a simple element (**) and stuff will be somehow easy since I got the ZCML enlightenment. We can define the such primary definition in the “meta.zcml”:


Let’s dig into it:

Line 3: Don’t forget to say we’re defining a meta, defining its namespace.
Line 5: We define a simple directive, the simplest possible.
Line 6: The name of our diective.
Line 7: The namespace for this directive as seen above in the sample “configure.zcml”.
Line 8: The schema (read attributes in our case) of that directive is defined by a Zope3 interface.
Line 9: The handler function that will be invoked for each “<fss:typeWithFSS …” directive found at Zope startup.

To keep the things simple, the schema and the handler belong to the same module, “”

The schema

The people who already made components based on Zope 3 formlib will be familiar with such notation. A scema is just a Zope 3 interface that defines in its Python attributes the XML attributes expected from your directive.

from zope.interface import Interface
from zope.configuration.fields import GlobalObject, Tokens, PythonIdentifier
class ITypeWithFSSDirective(Interface):
    """Schema for fss:typeWithFSS directive"""

    class_ = GlobalObject(
        description=u'Dotted name of class of AT based content type using FSS',

    fields = Tokens(
        description=u'Field name or space(s) separated field names',

All these configuration fields are defined in the “zope.configuration.fields” package. Have a look to this package to see what other attribute types I could have used. Note that the attribute types inherit from zope.schema resources that are familiar to Zope 3 formlib developers.

Some comments:

  • If your attribute is not required, you may give a default value.
  • You may use a Tokens attribute type where you need multivalued attributes from the configuration, as I needed for the “fields” attribute. In that case, you define the type for each single value in the “value_type” keyword argument.

In our case, you can notice the “class_” attribute of the interface maps automatically to the “class” attribute expected from the configuration directive. This is a magic that maps all XML attributes named with a Python keyword as “class”, “for” and the rest.

The handler

The handler will be invoked with all directive objectized attributes as arguments.

def typeWithFSS(_context, class_, fields):
    """Register our monkey patch"""

        args=(class_, fields)

def patchATType(class_, fields):
    """Processing the type patch"""

    for fieldname in fields:
        field = class_.schema[fieldname] = FileSystemStorage()
        LOG("Field '%s' of %s is stored in file system.", fieldname, class_.meta_type)

The signature of the handler:

  • _context: a context object defined and documented in details in zope.configuration.interfaces.IConfigurationContext interface. We’ll talk about it later.
  • The various attributes in the order of the schema. Optional attributes should be passed as keywords args with a default value, but we have no optional argument in our example.

You can see that we could immediately patch the content type, using directly the “patchATType” function body inside the handler.

But it’s a much better practice to let the zcml machinery execute as late as possible that function, and warn on potential conflicts of ZCML configuration directives.

In that intent, I prefer to register the FSS in the content type invoking the “_context.action(…)” with a discriminator on the class name that will automatically warn on duplicate FSS patch on the same content type class.

Have a look at the interface of the “_context” object. There are in lots of other valuables services that you may use in other situations.


I hope this small recipe brought you the “eureka” of the ZCML. As said earlier in that recipe, you may make more complex ZCML directives, said “grouping directives”. As you got the starter kit in this article about making a simple directive, you’ll get much information on complex directives looking at the “meta.zcml” from Five or GenericSetup, and their associated modules.

(*) Should we clarify the words used here?

(**) Digging the code that handles “meta.zcml”, you’ll notice that we may define more complex configuration schemas.


2 thoughts on “Doing my ZCML

  1. […] by linuxos on Tue 02-12-2008 Zope3 on App Engine – Redux Saved by FilthyWhore on Tue 18-11-2008 Doing my ZCML Saved by fain on Tue 18-11-2008 Zope3 for Djangoers. Part 2: Writing your models Saved by ewuzrab […]

  2. […] Of course, you may grep all “meta.zcml” files in your instance, search for the appropriate namespace, then the appropriate directive. And finally open the Python file that defines the schema of the mysterious ZCML, just as I explained in another ZCML related blog post. […]

Leave a Reply

Please log in using one of these methods to post your comment: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s


%d bloggers like this: