Hi there,

We've recently had some discussions on where to place the implementation 
of various ZCML directives. This post is to try to summarize the issue 
and analyze the options we have.

Right now ZCML directives are implemented in packages that contain other 
implementation. For example, zope.component implements various ZCML 
directives, and zope.security implements some more.

In the case of zope.component, a special [zcml] extras dependency 
section is  declared. If the ZCML dependencies are asked for, using 
zope.component will suddenly pull in a much larger list of dependencies 
than the original zope.component dependency list. The ZCML directives 
are component-related, but do offer extra options that need bits from 
the wider Zope 3 framework, such as the security infrastructure.

In the case of zope.security, this isn't the case. As far as I can see, 
it doesn't declare any dependency beyond zope.configuration to allow it 
to implement its ZCML directives.

The dependency situation for the ZCML implementations in zope.component 
doesn't appear ideal. It was therefore proposed to move the ZCML 
implementations to another package. This could be a new package, or it 
could be created.

Following up on that, it was considered we should move *all* directives 
from the packages that implement them now into special packages. This 
would allow some packages to lose the dependency on zope.configuration, 
which is a relatively minor gain.

We have several ways to go:

a) continue with the current extra dependencies situation like in 
zope.component, and in fact clean up other packages that define ZCML to 
declare ZCML extra dependencies.

b) pull out all ZCML implementations from where they are now and put 
them in special ZCML implementation packages. We could for instance have 
zcml.component, or zope.component_zcml, or zope.configuration.component. 
We had a bit of a side-tracked discussion about naming and namespace 
packages here.

c) pull out only those ZCML implementations that cause extra 
dependencies beyond zope.configuration. So, we extract the bits of 
zope.component into a new package, but we don't extract bits from 
zope.security.

I personally don't like extras. I think the ideal situation would be if 
packages had *no* extras at all (even test extras), as it complicates 
reasoning about the dependency structure. I think with improved 
structuring of the dependency graph, a package should have enough in its 
implementation dependencies to implement its tests. We've already 
experienced some of that last week at the sprint. For ZCML, with 
improved structuring of the dependency graph a package should only need 
zope.configuration as an extra package to implement ZCML.

For that reason, a) is not really an option for me. That leaves b) and 
c). I think for now we should go with c), as it's the smallest step 
forward that will help clean up things. That is, we either find and 
appropriate package that makes sense for the ZCML implementations in 
zope.component, or we create a new package. Of course if we create a new 
package we still have a naming discussion ahead and I risk sparking 
another naming discussion (as it's easy to have an opinion about names).

So, what do people think about option c)?

Regards,

Martijn



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