[Zope3-dev] what is ZCML?

[Martijn Faassen]

Hi there,

In this mail I'd like to make explicit some competing design influences 
on ZCML.

The first interpretation of what ZCML is:

"""
ZCML is a configuration language that provides abstract directives for 
configuring Zope applications. If we're setting up a page, we use the 
page directive. If we're setting up a skin, we use a skin directive. If 
we're setting up an adapter, we use an adapter directive.

Even if two directives underneath only set up adapters (such as 
browser:page and zope:adapter), we still think it's valuable to have two 
directives, as we make explicit what we are doing.

ZCML should be readable without having to consult Python code a lot. 
I.e. if we set up an adapter, we know what interface it's working for 
and we know what interface the adapter is providing just by looking at 
the ZCML.
"""

I believe that the first interpretation is the traditional 
interpretation of ZCML.

A newer interpretation of ZCML is:

"""
ZCML is a configuration language that configures a number of basic 
directives for configuring the component architecture and security: 
adapters, utilities, security requirements, and little else. Everything 
else should be done in Python code, as configuration in Python code is 
more flexible and packages can form a more self-contained whole. ZCML 
should reflect the underlying universality of the component architecture.

If two directives work with, say, adapters underneath, they should 
really be one directive. ZCML should be simple and minimal so it is easy 
to grasp.

ZCML is not readable standalone. ZCML is simply used to turn on various 
adapters and such, hooking them into the system, but we do not get a 
clue what the adapters are doing by just looking at the ZCML - Python 
code needs to be consulted.
"""

I believe that this interpretation is the up-and-coming interpretation 
of ZCML.

(the third interpretation of ZCML is that it's evil and should be 
destroyed I'd like to leave out of this discussion - the outcome doesn't 
matter that much if you're of that persuasion)

Of course, these interpretations have never been made very explicit. We 
have discussions where they are implicitly present, though. Reducing the 
namespaces in ZCML drastically makes more sense from the second 
perspective than from the first. Adding a new ZCML directive to support 
annotations makes more sense from the first and doesn't make much sense 
from the second interpretation.

Which one of these interpretations is the right one for the future?

I realize that the interpretations I sketch out may be extreme ends of a 
spectrum. I haven't seen a lot of advocacy the removal of browser:page, 
for instance. It may be that the real ZCML should be in the middle of 
these two interpretations. If so we should make our criteria explicit 
somehow, too - when do we really want to add a directive, and when do we 
really not want to remove a directive?

Perhaps there is whole other perspective possible on ZCML that resolves 
this whole issue. Let us know if you have one!

Regards,

Martijn