[Distutils] RFC: Egg cache fro self-contained buildout

[Jim Fulton]

Problem
=======

For (stage and production) deployment purposed, we, ZC, use RPMs.
It's considered good hygene to produce source RPMs as well as binary
RPMS.  This led me to create zc.sourcerelease, which automates
creation of self-contained source tar balls, that, among other
benefits, provide input for making source RPMs, which feed into a
process for creating binary RPMs.

We're moving toward a continuous deployment pipeline, where binaries
are produced early in the development cycle and tested in a controlled
environment that matches production. It no-longer (never did actually)
makes sense to produce source RPMs that could be deployed in alternate
(untested) environments.

In general, our existing build process is grotesquely slow:

- we run a buildout to produce a source release.

- we run it again to build a source (and binary) rpm from the source
  release.

Both of these run in such a way that all of the eggs have to be
rebuilt. (But sources don't have to be downloaded.)

We'd like to move toward a model where we construct a build
environment for each controlled deployment environment.  In this build
environment, we never want to build a given distribution more than
once.  We need to produce application binaries that are self
contained.

Buildout allows you to use a shared eggs directory.  This can greatly
speed buildouts, because already-built distributions can be found and
used locally.  However, buildouts that use shared eggs directories
aren't self contained. They depend on the shared eggs directory.
I'd like to be able to reuse previously-built eggs, but have eggs
installed in my local buildout, so it's self contained.

Proposal: egg-cache
===================

If egg-cache is set to a directory, then when buildout builds an egg,
it will copy it to the egg cache. When looking for distributions, it
will look in the egg cache and, if it finds a matching egg there, it
will copy the egg to the buildout eggs directory.

The end result will be that an egg cache will have the same economy as
the current shared eggs directory, as far as building is concerned,
but it won't have the disk-space saving of a shared eggs directiory.
It will lead to buildouts that are self contained (at least wrt eggs)
and that can be copied to a deployment environment directly.

Thoughts?

Jim

-- 
Jim Fulton
http://www.linkedin.com/in/jimfulton