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z3c.baseregistry 2.0.0

Manage IComponents instances using Python code and ZCML.

The purpose of this package is to define, populate and use multiple IComponents instances using filesystem-based development in other words, Python code and ZCML.

Detailed Documentation

Base Components

The purpose of this package is to define, populate and use multiple IComponents instances using filesystem-based development – in other words, Python code and ZCML.

Motivation

The current state of the component architecture allows us to

1 – create a global components registry, populate it using ZCML, and use it
via the zope.component API functions.
2 – define local sites (local components registries), populate them with local
(persistent) components, and use them selectively based on location – commonly defined by the path of the URL.

Unfortunately, it is impossible to populate local sites with ZCML. The main reason is the lack of addressability of local sites during the initial startup process.

However, on the other hand we have a very advanced UI configuration system that involves views, resources, layers and skins. So let’s compare the two.

1 – Views/Resources in the UI are like registered components in the component
architecture.
2 – Skin Layers in the UI behave very much like registries. The default skin
is like the global base registry. Skins, like local sites, are activated during traversal, but can be populated using ZCML.
3 – Layers are really base layers to the skin layer. The equivalent in the
component architecture is to specify bases for a components registry, which is possible since the Great Component Architecture refactoring for Zope 3.3 in 2006.

But layers can be defined and configured via ZCML. The purpose of this package is to be able to create base components registries and then populate them using ZCML. (As a side note: As skin layers and layers are practically the same components, there is no difference between the concept of global, local and base components registries.)

The second feature is specific to the Zope application server. It provides an UI to set the bases on a local site manager. The user can select among all registries that have been registered as IComponents utilities.

There are also a few options that could be considered in the future. For example, it would be simple to integrate the zope:registerIn directive (see below for documentation) into the zope:configure directive.

If the above text is too dry and theoretical for you, here is the summary. This package

1 – implements Steve Alexander’s long dream (at least 3 years) of defining
local sites via ZCML.
2 – solves all of my (Stephan Richter) problems I am having with a complex
Application Service Provider (ASP) setup.
3 – implements a missing feature that you and everyone else really wanted,
even if you did not know it yet.

Thanks goes to Jim Fulton, whose outstanding design of the zope.configuration and zope.component packages made the implementation of the feature such a breeze. I also want to thank Fred Drake for helping with the initial design ideas.

“Base Components” Registries

Base registries are global component registries implementing the IComponents interface. In comparison to the base global registry (also known as globalSiteManager), these registries are not necessarily available via module globals and must be registered with a parent registry, most commonly the base global registry:

>>> from z3c.baseregistry import baseregistry
>>> import zope.component
>>> myRegistry = baseregistry.BaseComponents(
...     zope.component.globalSiteManager, 'myRegistry')
>>> myRegistry
<BaseComponents myRegistry>

Another VERY IMPORTANT requirement is that zope.component hooks are in place. Install the hooks now:

>>> import zope.component.hooks
>>> zope.component.hooks.setHooks()

Since this registry does not implement any of the IComponents API itself, it is not necessary to demonstrate those features here. Please see the corresponding documentation in the zope.component package.

One feature of global registries must be that they pickle efficiently, since they can be referenced in persisted objects. As you can see, the base registry pickles quite well:

>>> import cPickle
>>> jar = cPickle.dumps(myRegistry)
>>> len(jar)
100

However, when reading the jar, we get an error:

>>> cPickle.loads(jar)
Traceback (most recent call last):
...
ComponentLookupError: (ComponentLookupError(<InterfaceClass zope.interface.interfaces.IComponents>, 'myRegistry'),
                       <function BC at ...>,
                       (<BaseGlobalComponents base>, 'myRegistry'))

This is because we have not registered the registry in its parent as an IComponents utility, yet:

>>> from zope.component.interfaces import IComponents
>>> zope.component.provideUtility(myRegistry, IComponents, 'myRegistry')
>>> cPickle.loads(jar)
<BaseComponents myRegistry>

Thus it is very important that you always register your base registry with its parent!

Like any other components registry, a base registry can also have bases:

>>> myOtherRegistry = baseregistry.BaseComponents(
...     zope.component.globalSiteManager, 'myRegistry', (myRegistry,))
>>> myOtherRegistry.__bases__
(<BaseComponents myRegistry>,)

Let’s now have a look at how base registries can be defined and used via ZCML, which is the usual mode of operation.

Defining Base Registries

The above tasks are more commonly done in ZCML. Base components registries – or any IComponents implementation for that matter – can be seen as utilities providing the aforementioned interface and are distinguishable by name. So let’s define a “custom” registry:

>>> custom = baseregistry.BaseComponents(
...     zope.component.globalSiteManager, 'custom')

Let’s make sure that the parent of the custom registry is the base registry:

>>> custom.__parent__
<BaseGlobalComponents base>

The registry is then registered using the standard utility directive. After loading the meta directives for this package,

>>> from zope.configuration import xmlconfig
>>> context = xmlconfig.string('''
... <configure i18n_domain="zope">
...   <include package="z3c.baseregistry" file="meta.zcml" />
...   <include package="zope.component" file="meta.zcml" />
... </configure>
... ''')

we can register the registry:

>>> context = xmlconfig.string('''
... <configure xmlns="http://namespaces.zope.org/zope" i18n_domain="zope">
...
...   <utility
...       component="README.custom"
...       provides="zope.component.interfaces.IComponents"
...       name="custom" />
...
... </configure>
... ''', context=context)

The new registry can now be accessed as follows:

>>> custom = zope.component.getUtility(IComponents, name='custom')
>>> custom
<BaseComponents custom>

Populating Different Registries

Now to the interesting part. Let’s register components for both the global base and the “custom” registry. Let’s first create some utilities we can register:

>>> import zope.interface
>>> class IExample(zope.interface.Interface):
...     name = zope.interface.Attribute('Name of Example')
>>> class Example(object):
...     zope.interface.implements(IExample)
...     def __init__(self, name):
...         self.name = name
...     def __repr__(self):
...         return '<%s %r>' %(self.__class__.__name__, self.name)
>>> example1 = Example('example1')
>>> example2 = Example('example2')

Create some adapters we can register:

>>> class IToAdapt1(zope.interface.Interface):
...     pass
>>> class IToAdapt2(zope.interface.Interface):
...     pass
>>> class IAdapted(zope.interface.Interface):
...     pass
>>> @zope.component.adapter(IToAdapt1)
... @zope.interface.implementer(IAdapted)
... def adapter1(context):
...     return "adapted1"
>>> @zope.component.adapter(IToAdapt2)
... @zope.interface.implementer(IAdapted)
... def adapter2(context):
...     return "adapted2"
>>> class ToAdapt1(object):
...     zope.interface.implements(IToAdapt1)
...     def __init__(self, name):
...         self.name = name
...     def __repr__(self):
...         return '<%s %r>' %(self.__class__.__name__, self.name)
>>> toAdapt1 = ToAdapt1('toAdapt1')
>>> class ToAdapt2(object):
...     zope.interface.implements(IToAdapt2)
...     def __init__(self, name):
...         self.name = name
...     def __repr__(self):
...         return '<%s %r>' %(self.__class__.__name__, self.name)
>>> toAdapt2 = ToAdapt2('toAdapt2')

Let’ now register “example1”, adapter1 in the global registry and “example2”, “adapter2” in our custom registry:

>>> context = xmlconfig.string('''
... <configure xmlns="http://namespaces.zope.org/zope" i18n_domain="zope">
...
...   <utility component="README.example1"
...            name="example1" />
...   <adapter
...         factory="README.adapter1"
...         name="adapter1"/>
...
...   <registerIn registry="README.custom">
...     <utility component="README.example2"
...              name="example2" />
...     <adapter
...         factory="README.adapter2"
...         name="adapter2"/>
...   </registerIn>
...
... </configure>
... ''', context=context)

Let’s now make sure that the utilities have been registered in the right registry:

>>> zope.component.getUtility(IExample, name="example1")
<Example 'example1'>
>>> zope.component.getUtility(IExample, name="example2")
Traceback (most recent call last):
...
ComponentLookupError: (<InterfaceClass README.IExample>, 'example2')

Let’s now make sure that the adapters have been registered in the right registry:

>>> zope.component.getAdapter(toAdapt1, IAdapted, name="adapter1")
'adapted1'
>>> zope.component.getAdapter(toAdapt2, IAdapted, name="adapter2")
Traceback (most recent call last):
...
ComponentLookupError: (<ToAdapt2 'toAdapt2'>, <InterfaceClass README.IAdapted>, 'adapter2')
>>> custom = zope.component.getUtility(IComponents, name='custom')
>>> custom.getUtility(IExample, name="example1")
Traceback (most recent call last):
...
ComponentLookupError: (<InterfaceClass README.IExample>, 'example1')
>>> custom.getUtility(IExample, name="example2")
<Example 'example2'>
>>> custom.getAdapter(toAdapt1, IAdapted, name="adapter1")
Traceback (most recent call last):
...
ComponentLookupError: (<ToAdapt1 'toAdapt1'>, <InterfaceClass README.IAdapted>, 'adapter1')
>>> custom.getAdapter(toAdapt2, IAdapted, name="adapter2")
'adapted2'

Let’s now register other instances of the Example class without a name. This should not cause a conflict error:

>>> example3 = Example('example3')
>>> example4 = Example('example4')
>>> context = xmlconfig.string('''
... <configure xmlns="http://namespaces.zope.org/zope" i18n_domain="zope">
...
...   <utility component="README.example3" />
...
...   <registerIn registry="README.custom">
...     <utility component="README.example4" />
...   </registerIn>
...
... </configure>
... ''', context=context)
>>> zope.component.getUtility(IExample)
<Example 'example3'>
>>> custom.getUtility(IExample)
<Example 'example4'>

Using Base Registries

Most commonly base registries will be used in local site managers. So let’s create a local site:

>>> from zope.site.folder import Folder
>>> site = Folder()
>>> from zope.site.site import LocalSiteManager
>>> site.setSiteManager(LocalSiteManager(site))
>>> sm = site.getSiteManager()

Initially only the base global registry is a base of the local site manager:

>>> sm.__bases__
(<BaseGlobalComponents base>,)

Now only registrations from the base site are available:

>>> sm.getUtility(IExample)
<Example 'example3'>
>>> sm.getUtility(IExample, name="example1")
<Example 'example1'>
>>> sm.getUtility(IExample, name="example2")
Traceback (most recent call last):
...
ComponentLookupError: (<InterfaceClass README.IExample>, 'example2')
>>> sm.getAdapter(toAdapt1, IAdapted, name="adapter1")
'adapted1'
>>> sm.getAdapter(toAdapt2, IAdapted, name="adapter2")
Traceback (most recent call last):
...
ComponentLookupError: (<ToAdapt2 'toAdapt2'>, <InterfaceClass README.IAdapted>, 'adapter2')

But if we add the “custom” registry, then things look more interesting:

>>> sm.__bases__ += (custom,)
>>> sm.__bases__
(<BaseGlobalComponents base>, <BaseComponents custom>)
>>> sm.getUtility(IExample)
<Example 'example3'>
>>> sm.getUtility(IExample, name="example1")
<Example 'example1'>
>>> sm.getUtility(IExample, name="example2")
<Example 'example2'>
>>> sm.getAdapter(toAdapt1, IAdapted, name="adapter1")
'adapted1'
>>> sm.getAdapter(toAdapt2, IAdapted, name="adapter2")
'adapted2'

But where is the registration for example 4? Well, the order of the bases matters, like the order of base classes in Python matters. The bases run from must specific to most generic. Thus, if we reverse the order,

>>> bases = list(sm.__bases__)
>>> bases.reverse()
>>> sm.__bases__ = bases
>>> sm.__bases__
(<BaseComponents custom>, <BaseGlobalComponents base>)

then our “custom” registry effectively overrides the global one:

>>> sm.getUtility(IExample)
<Example 'example4'>
>>> sm.getUtility(IExample, name="example1")
<Example 'example1'>
>>> sm.getUtility(IExample, name="example2")
<Example 'example2'>

Edge Cases and Food for Thought

Duplicate Registrations

Like before, duplicate registrations are detected and reported:

>>> context = xmlconfig.string('''
... <configure xmlns="http://namespaces.zope.org/zope" i18n_domain="zope">
...
...   <registerIn registry="README.custom">
...     <utility component="README.example3" name="default" />
...     <utility component="README.example4" name="default" />
...   </registerIn>
...
... </configure>
... ''', context=context)
Traceback (most recent call last):
...
ConfigurationConflictError: Conflicting configuration actions
    For: (<BaseComponents custom>,
          ('utility', <InterfaceClass README.IExample>, u'default'))
...

But as we have seen before, no duplication error is raised, if the same registration is made for different sites:

>>> context = xmlconfig.string('''
... <configure xmlns="http://namespaces.zope.org/zope" i18n_domain="zope">
...
...   <utility component="README.example3" name="default" />
...
...   <registerIn registry="README.custom">
...     <utility component="README.example4" name="default" />
...   </registerIn>
...
... </configure>
... ''', context=context)
Overriding ZCML

Overriding should behave as usual. If I define something within a particular site, then it should be only overridable in that site.

In the following example, base-overrides.zcml overrides only the global registration of the following snippet to “example3”:

>>> context.includepath = ('base.zcml', 'original.zcml')
>>> context = xmlconfig.string('''
... <configure xmlns="http://namespaces.zope.org/zope" i18n_domain="zope">
...
...   <utility component="README.example1" />
...
...   <registerIn registry="README.custom">
...     <utility component="README.example2" />
...   </registerIn>
...
... </configure>
... ''', context=context, execute=False)
>>> context.includepath = ('base.zcml',)
>>> context = xmlconfig.string('''
...   <includeOverrides package="z3c.baseregistry.tests"
...                     file="base-overrides.zcml" />
... ''', context=context)
>>> zope.component.getUtility(IExample)
<Example 'example3'>
>>> custom.getUtility(IExample)
<Example 'example2'>

In the next example, custom-overrides.zcml overrides only the custom registration of the following snippet to “example3”:

>>> context.includepath = ('base.zcml', 'original.zcml')
>>> context = xmlconfig.string('''
... <configure xmlns="http://namespaces.zope.org/zope" i18n_domain="zope">
...
...   <utility component="README.example1" />
...
...   <registerIn registry="README.custom">
...     <utility component="README.example4" />
...   </registerIn>
...
... </configure>
... ''', context=context, execute=False)
>>> context.includepath = ('base.zcml',)
>>> context = xmlconfig.string('''
... <configure xmlns="http://namespaces.zope.org/zope" i18n_domain="zope">
...
...   <includeOverrides package="z3c.baseregistry.tests"
...                     file="custom-overrides.zcml" />
...
... </configure>
... ''', context=context)
>>> zope.component.getUtility(IExample)
<Example 'example1'>
>>> custom.getUtility(IExample)
<Example 'example3'>

Note: Sorry for the convoluted test sequence; this is just how it works. :-(

Nested Registry Usage

I thought about this one for a long time, but I think it is better not allowing to nest zope:registerIn directives, because the logic of manipulating the discriminator would be very complex for very little added benefit.

>>> context = xmlconfig.string('''
... <configure xmlns="http://namespaces.zope.org/zope" i18n_domain="zope">
...
...   <registerIn registry="README.custom">
...     <registerIn registry="zope.component.globalregistry.base">
...       <utility component="README.example4" />
...     </registerIn>
...   </registerIn>
...
... </configure>
... ''', context=context)
Traceback (most recent call last):
...
ZopeXMLConfigurationError: File "<string>", line 5...
  ConfigurationError: Nested ``registerIn`` directives are not permitted.
Cleanup

Just unregister the zope.component hooks:

>>> zope.component.hooks.resetHooks()
Global Non-Component-Registration Actions

ZCML is not only responsible for populating the components registries, but also to do other global configuration, such as defining security and assigning interfaces to classes. On the other hand, the registerIn directive works by manipulating the discriminator by prefixing it with the current registry. While I assert that this is the right approach for component registrations, it does not work for those other global configurations.

In order to address the issue, I need somehow more information. A balance must be struck between the need to change existing directives and making the solution non-monolithic. Here are some design ideas:

  1. A Special Discriminator Prefix

All directives that globally manipulate the state of the system and do not register a component have as their first discriminator entry a special string, like “StateChange”. The directive can then look for those entries and not change the discriminator at this point.

Advantages include the ability to use those directives inside the registerIn directive and allow gradual upgrading. In the other hand, util directives are adjusted, conflict resolution will not be available for those scenarios.

  1. A Registry of Global Action Callables

Here this package provides a registry of callables that change the state of the system. Directive authors can then subscribe their callables to this registry.

The big advantage of this approach is that you can make it work now for all built-in directives without changing any implementation. The disadvantage is that the solution hides the problem to directive authors, so that detailed documentation must be provided to ensure integrity and avoid surprises. Another disadvantage is the complexity of yet another registry.

  1. Autodetection with False-Positives

As far as I can tell, all actions that manipulate the components registries use the zope.component.zcml.handler function. Okay, so that allows me to detect those. Unfortunately, there might be directives that do not manipulate the state, for example ensuring the existence of something. There are a bunch of those directives in the core.

The advantage here is that for the core it should just work. However, 3rd party directive developers might be tripped by this feature. Also, we could only issue warnings with this solution and probably need to be able to turn them off.

I have not implemented any of those suggestions, waiting for input from the community.

CHANGES

2.0.0 (2012-11-17)

  • zope.configuration changed action tuples to action dicts. This version works with the new action dict given from zope.configuration since version 3.8.0. This version is not compatible with zope.configuration version less then 3.8.0

1.3.0 (2010-10-28)

  • Fundamental change in the way how baseregistry hooks into ZCA. Now it uses hooks.setSite, which requires that zope.component hooks are in place. Usually they are installed by zope.app.appsetup. Unless you use zope.app.appsetup, install the hooks with zope.component.hooks.setHooks(). This applies to zope.component versions >= 3.9.4.

1.2.0 (2009-12-27)

  • Moved browser dependencies to zmi extras

1.1.0 (2009-03-19)

  • Fix base registry management form failure in case, when a site has its parent’s local site manager (that isn’t registered as utility) in its __bases__.
  • Use zope.site instead of zope.app.component.
  • Drop unused dependencies on zope.app.i18n and zope.app.pagetemplate.

1.0.0 (2008-01-24)

  • Initial Release
 
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z3c.baseregistry-2.0.0.zip (md5) Source 2012-11-17 35KB