Just over a month ago I
about my first experience with
those that don’t remember (shame on you!), zc.buildout is a Python tool
developed by Jim Fulton of Zope fame for constructing software
installations. “Buildouts”, if you will. The attraction to this over,
is that you can go from a bare-bones Python
installation and source code checkout (or tar-ball) to a fully
functional application, with dependencies and the trimmings, in a
predictable, sane way. The fact that dependencies are installed in a
local folder makes this especially appealing for deploying code on hosts
where you don’t have write access to site-packages. But enough
re-hashing my last zc.buildout post.
As I mentioned in my previous post, I’ve been using zc.buildout mostly
to deploy the web applications that power Creative
services These are written using
CherryPy, and were previously invoked as simple CGI scripts. However, as
usage grew, we needed to move them to independent processes, which
Apache proxied to using mod_rewrite. zc.buildout provided a great way
to generate the wrapper script for running that process, but early this
week we ran into a problem: we didn’t have a good way to detect if the
process died for some reason and needed to be restarted.
I decided that a simple approach was best for the initial go-round, and
wrote a small shell script that looks at the pid file, checks if
it’s still running and if not starts the process. Put this in a cron job
that runs every 10 minutes or so, and you have a poor man’s
watchdog:sup:1 <#fn8f2bac94-0bf0-492d-94de-4243a89c40c9>`_`. After
writing two of these, I realized that this is something that should
really be automated as part of my software installation: I always want
to have a “am I up?” check script available, and so my first custom
zc.buildout recipe, ``build_script, is born.
a recipe that takes a template file, fills in values using Python
marks it as executable. It has one template included, paster-check
that we’re using to check on a paster
process, and you can use templates that are part of the egg itself or
are part of your codebase (via the template_dir setting).
You can find the code for build_script itself in Creative Commons’
and an example of how we’re using it for our web services is
Suggestions, comments, questions all welcome, as are additional
templates that might be useful to include in the egg itself.
1I’m sure there’s a better, more robust way to do this, but
this is where we’re comfortable on the down time v. implementation time
continuum. That said, if someone knows of an easy, more robust way, of
course I’d like to hear about it.