Notes on switching my Djangos to mod_wsgi

I'm slowly trying to make my Django web servers conform to current best practices. I've set up an Nginx reverse proxy for serving static files, started using virtualenv to isolate my Python environments, and migrated my database to PostgreSQL. I ultimately want to implement memcached+Nginx caching in my reverse proxy, but the next task on my to-do list is switching from mod_python to mod_wsgi.

Within the past year (or maybe before), mod_wsgi has become the preferredmethod for serving Django applications. I also originally thought switching from mod_python to mod_wsgi would save me some much needed memory on my 256MB VPS. But after trying it out, running with a single Apache process in each case, the memory footprint was about the same. Even switching from mod_wsgi's embedded mode to daemon mode didn't make a significant difference. Likely the performance is better with mod_wsgi, though.

Here are my notes on installing mod_wsgi.

Configuration References

• Django docs: How to use Django with Apache and mod_wsgi
• mod_wsgi docs: Quick Configuration Guide
• A good blog article: Configs for nginx and Apache with mod_wsgi

Advice from mod_wsgi author Graham Dumpleton

• Load spikes and excessive memory usage in mod_python
• Using mod_wsgi when developing Django sites
• http://groups.google.com/group/django-users/browse_thread/thread/6d670b0fa7c0d733/4ff111a1f00f7629?q=worker+daemon#4ff111a1f00f7629
• http://groups.google.com/group/modwsgi/browse_thread/thread/cb21864a97d44ee9/38716433921a48cb?q=worker+daemon#38716433921a48cb

Install mod_wsgi and apache mpm-worker

I'm not 100% sure about prefork vs. worker mpm, but Graham Dumpleton favors worker mpm.

sudo apt-get install libapache2-mod-wsgi
sudo apt-get install apache2-mpm-worker

Create .wsgi application file

My virtualenv is located at /srv/python-environments/saltycrane. My Django settings files is at /srv/SaltyCrane/iwiwdsmi/settings.py.

/srv/SaltyCrane/saltycrane.wsgi:

import os
import sys
import site

site.addsitedir('/srv/python-environments/saltycrane/lib/python2.5/site-packages')

os.environ['DJANGO_SETTINGS_MODULE'] = 'iwiwdsmi.settings'

sys.path.append('/srv/SaltyCrane')

import django.core.handlers.wsgi
application = django.core.handlers.wsgi.WSGIHandler()

Edit Apache's httpd.conf file

I went back and forth between using embedded mode or daemon mode. I've ended up with embedded mode for now since it seems to use a tad less memory and is supposed to be a little bit faster. However, Graham Dumpleton seems to recommend daemon mode for people on VPSs. I may change my mind again later. To use daemon mode, I just need to uncomment the WSGIDaemonProcess and WSGIProcessGroup lines. I have StartServers set to 1 because I can only afford to have one Apache process running. This is assuming nginx is proxying requests to apache. For more on my nginx setup, see here.

Edit /etc/apache2/httpd.conf:

<IfModule mpm_worker_module>
    StartServers 1
    ServerLimit 1
    ThreadsPerChild 5
    ThreadLimit 5
    MinSpareThreads 5
    MaxSpareThreads 5
    MaxClients 5
    MaxRequestsPerChild 500</IfModule>

KeepAlive Off
NameVirtualHost 127.0.0.1:8080
Listen 8080

<VirtualHost 127.0.0.1:8080>
    ServerName www.saltycrane.com
    # WSGIDaemonProcess saltycrane.com processes=1 threads=5 display-name=%{GROUP}
    # WSGIProcessGroup saltycrane.com
    WSGIScriptAlias / /srv/SaltyCrane/saltycrane.wsgi</VirtualHost><VirtualHost 127.0.0.1:8080>
    ServerName supafu.com
    # WSGIDaemonProcess supafu.com processes=1 threads=5 display-name=%{GROUP}
    # WSGIProcessGroup supafu.com
    WSGIScriptAlias / /srv/Supafu/supafu.wsgi</VirtualHost><VirtualHost 127.0.0.1:8080>
    ServerName handsoncards.com
    # WSGIDaemonProcess handsoncards.com processes=1 threads=5 display-name=%{GROUP}
    # WSGIProcessGroup handsoncards.com
    WSGIScriptAlias / /srv/HandsOnCards/handsoncards.wsgi</VirtualHost>

Restart Apache

sudo /etc/init.d/apache2 restart