[Grok-dev] Re: Grok component reuse in the Zope 3 world (was Re: Re: ANN: megrok.quarry)

Martijn Faassen faassen at startifact.com
Sun May 6 13:09:10 EDT 2007

Hey there,

Gary Poster wrote:
> To be clear, the win of zcml, as I'm trying to present it, can be done 
> without zcml--I gave the example of a separate Python file with a 
> grok-like approach, for instance, as one of the possible 
> replacements--

As I tried to point out elsewhere, this is not a Grok-like approach. I 
think I need to go into this idea some more, as your idea sounds so 
simple and straightforward to apply, but it really isn't.

One important idea of Grok is that development agility is badly hurt by 
this separation. Instead of having to be in one place, you need to be in 
two places (implementation and configuration). That the second place is 
a separate Python file or a ZCML file doesn't really matter very much - 
I believe the fact that this second place exists in the first place 
that's hurting agility the most, not the particular spelling used.

In most cases Grok actually deduces the need for registration from the 
base class used (for a view, adapter, etc), together with some class 
annotations (which are optional as they have defaults). Most of the time 
in Grok-based code, configuration information cannot be separated from 
the code that is being configured. Code and configuration are actually 
the same thing, and that's an essential characteristic of Grok's approach.

(note that configuration does happen at a very different stage than code 
import or execution time - the code is introspected and registrations 
are made during this introspection phase. This actually happens during 
the same phase as ZCML registrations occur, as it's triggered from ZCML. 
I.e,. this is a good idea of ZCML we don't want to give up)

Now if the need to reuse implementation and not the accompanying 
configuration were very common, the effort to separate configuration 
from code makes sense. I do not consider this to be common, however: 
it's the exceptional case where you want to reuse a package and not 
reuse its configuration. ZCML's approach to enable such separate reuse, 
separation of configuration information from the code, I consider to 
have a high cost in agility. That is why I think we need to come up with 
alternative solutions, instead of separation to tackle the issue of 
overriding or skipping configuration.

ZCML has some other important beneficial properties besides the 
potential it has for reuse of code without its configuration. As I 
already mentioned above, a concept Grok carries over is making 
configuration happen in a separate phase. Moreover, ZCML is a limited, 
explicit language. It makes the act of configuration very clear. This 
really forces developers to think about the act of configuration, and 
about what's configuration and what's not (and frequently disagree, but 
at least think about it). This has been extremely helpful.

Doing configuration with Python code runs the risk a thousand 
inconsistent API will grow and configuration code will become an 
unpredictable mess. I think our approach with Grok has shown a way to 
avoid those risks. Grok, for the most part, isn't expressing ZCML 
configuration directives in the equivalent Python. It's about weaving 
the configuration information declaratively together with the actual 
code that it is about.

Choices to separate or integrate happen frequently in programming 
language design. Docstrings or external API documentation? Separate 
header declarations versus deducing this from the class declaration? 
Doctests in docstrings or in separate files? For RMI, interface 
description languages or introspection?

Related to this is explicit versus implicit - explicit static type or 
implicit dynamic types? Explicit interfaces or informal protocols?

There is no simple answer about what the right way to go is in any of 
these cases. It depends on the circumstances. Generally the choice to 
make something more explicit and more separate has a cost in agility, 
with hopefully a gain in explicitness and reconfigurability. The choice 
to integrate something tends to mean a gain in agility and to reduce 
conceptual load on the developer. There are a lot of trade-offs.

Grok is in the happy position of being able to explore integrated 
configuration information with an explicit configuration already in 
place to learn from. We also let go of some of the benefits of ZCML at 
the same time, hopefully to regain them, later, in a different way.

It also means that the ZCML way of thinking about configuration doesn't 
really work for Grok - applying the idea of separating the code that is 
being configured from the configuration information sounds like a 
relatively straightforward simple approach from the perspective of 
someone used to ZCML. It would also, in my opinion, remake Grok into 
something completely different.



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