20130413 (Saturday, 13 April 2013)

Hosting multiple Lino applications

I read (again) Daniel’s and Audrey’s chapter about settings.py files. They recommend to use only “canned” (version-controlled) settings. And to use environment variables for private settings like SECRET_KEY.

One objection against this approach is that even the Django documentation itself says:

Since environment variables are process-wide, this doesn’t work when you run multiple Django sites in the same process. This happens with mod_wsgi.

To avoid this problem, use mod_wsgi’s daemon mode with each site in its own daemon process, or override the value from the environment by enforcing os.environ[“DJANGO_SETTINGS_MODULE”] = “mysite.settings” in your wsgi.py.

Q: Does this mean that I can avoid using mod_wsgi’s daemon mode if I do not use system variables in the Apache configuaration?

A: Maybe. But the question isn’t important since even the mod_wsgi docs recommend to use daemon mode:

(…) unless you are adept at configuring Apache, always use daemon mode when available. Overall, for large Python web applications you wouldn’t normally expect to see any significant difference between daemon mode and embedded mode, as the bottlenecks are going to be in the Python web application or any database access.

I still tend to prefer a “one directory per project” approach. This implies that each project directory contains a local settings.py file which is usually very small and not version controlled. The main reason for this is that when doing manual maintenance work in a shell via management commands I prefer to have the current working directory and not an environment variable determine which project I’m working on.

The trick is to have a local configure function called by both the manage.py and wsgi.py files. These files can then always be wsgi.py the same, just copies of a site-wide template.