Something-driven development

Software development thoughts around Ubuntu, Python, Golang and other tools

A generic juju charm for Django apps

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After experimenting with juju and puppet the other week, I wanted to see if it was possible to create a generic juju charm for deploying any Django apps using Apache+mod_wsgi together with puppet manifests wherever possible. The resulting apache-django-wsgi charm is ready to demo (thanks to lots of support from the #juju team), but still needs a few more configuration options. The charm currently:

  1. Enables the user to specify a branch of a Python package containing the Django app/project for deploy. This python package will be `python setup.py install`’d on the instance, but it also
  2. Enables you to configure extra debian packages to be installed first so that your requirements can be installed in a more reliable/trusted manner, along with the standard required packages (apache2, libapache2-mod-wsgi etc.). Here’s the example charm config used for apps.ubuntu.com,
  3. Creates a django.wsgi and httpd.conf ready to serve your app, automatically collecting all the static content of your installed Django apps to be served separately from the same Apache virtual host,
  4. When it receives a database relation change, it creates some local settings, overriding the database settings of your branch, sync’s and migrates the database (a noop if it’s the second unit) and restarts apache (See the database_settings.pp manifest for more details).

Here’s a quick demo which puts up a postgresql unit and two app servers with these commands:

$ juju deploy --repository ~/charms local:postgresql
$ juju deploy --config ubuntu-app-dir.yaml --repository ~/apache-django-wsgi/ local:apache-django-wsgi
$ juju add-relation postgresql:db apache-django-wsgi
$ juju add-unit apache-django-wsgi

Things that I think need to be improved or I’m uncertain about:

  1. `gem install puppet-module` is included in the install hook (a 3rd way of installing something on the system :/). I wanted to use the vcsrepo puppet module to define bzr resource types and puppet-module-tool seems to be the way to install 3rd-party puppet modules. Using this resource-type enables a simple initial_state.pp manifest. Of course, it’d be great to have ‘necessary’ tools like that in the archive instead.
  2. The initial_state.pp manifest pulls the django app package to /home/ubuntu/django-app-branch and then pip installs it on the system. Requiring the app to be a valid python package seemed sensible (in terms of ensuring it is correctly installed with its requirements satisfied) while still allowing the user to go one step further if they like and provide a debian package instead of a python package in a branch (which I assume we would do ultimately for production deploys?)
  3. Currently it’s just a very simple apache setup. I think ideally the static file serving should be done by a separate unit in the charm (ie. an instance running a stripped down apache2 or lighttpd). Also, I would have liked to have used an ‘official’ or ‘blessed’ puppet apache module to benefit from someone else’s experience, but I couldn’t see one that stood out as such.
  4. Currently the charm assumes that your project contains the configuration info (ie. a settings.py, urls.py etc.), of which the database settings can be simply overridden for deploy. There should be an additional option to specify a configuration branch (and it shouldn’t assume that you’re using django-configglue), as well as other options like django_debug, static_url etc.
  5. The charm should also export an interface (?) that can be used by a load balancer charm.
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Written by Michael

November 22, 2011 at 5:59 pm

Posted in django, juju

4 Responses

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  1. [...] A generic juju charm for Django apps (micknelson.wordpress.com) [...]

  2. [...] have been some attempts at making a generic, reusable Django charm.  Michael Nelson made one that uses Puppet and a custom config.yaml for each project.  While this works, it has two [...]

  3. […] writing charms for Juju, one of the things that I struggled with was testing the hook code – specifically the install […]

  4. […] writing charms for Juju, one of the things that I struggled with was testing the hook code – specifically the install […]


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