Hi,
In my quest to make re-usable buildout components, I've started using a
pattern like this:
In a server-specific buildout file (e.g. prod-web-master.cfg) I have e.g.:
[buildout]
extends =
buildout.d/base.cfg
buildout.d/zope.cfg
buildout.d/relstorage.cfg
buildout.d/haproxy.cfg
[ports]
haproxy = 8100
[eggs]
main = my.package
...
Each file in buildout.cfg adds additional functionality, usually by
defining new parts and doing e.g.
parts += haproxy-build haproxy-config
[haproxy-build]
recipe = zc.recipe.cmmi
...
Sometimes, they also extend some shared variables, e.g.
[eggs]
main += RelStorage
With this type of setup, the order in which configuration is loaded and
processed becomes important, in particular for += and -= type operations
and straightforward overrides. For example, I may have a default
${ports:haproxy} defined in haproxy.cfg, but overridden in
prod-web-slave.cfg because the slave node wants it to run on a different
port, say.
I can't find much documentation about how this is supposed to work. I've
had situations where I've been surprised by the order in which things
are processed, in particular if I extend one file that in turn extends
another. So, a few questions
- Is the order well-defined? e.g. is it always doing a depth-first or
breath-first inclusion? Is the load order the same as the execution order?
- Do settings in the top-level config (the one run with bin/buildout
-c <file>) always override extended ones?
- More generally, if A extends B and B extends C, does configuration
in B always take precedence over configuration in C with the same
part/option? Does a += in B always extend the options set in C?
- If I have multiple files all extending a common base, e.g. A extends
B and C, and both B and C extend a common D, when does the common base
(D) get processed?
Cheers,
Martin
--
Author of `Professional Plone Development`, a book for developers who
want to work with Plone. See http://martinaspeli.net/plone-book
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