Django in Production: Part 1 - The Stack

Everyone has their preferred way of doing things, and this is more and more true when there are many options available. In the Django world, this translates to everyone having their favourite web server, database, proxy, and so on.

In spite of this, I'm going to spend some time over the next few posts describing how I deploy Django applications in production from a high-level perspective. In the first part of this series, I'll talk about the core stack which serves as the basis of the application.

Gunicorn

Python-based web applications have traditionally been run under Apache, using a module such as mod_python or mod_wsgi . Apache, however, can be somewhat of a resource-hog, particularly on virtual servers which often have limited memory. Also, as I'll discuss in detail later in this series, deploying code frequently to an Apache-hosted application can be troublesome. A new solution is in order.

There is quite a choice of Python-based web servers for us "cool kids" to use, many of which are tested in this detailed (if a little old) benchmark by Nicholas Piël.

Gunicorn offer super simple configuration, an extremely small footprint, and it's pure Python - so you can install it with a quick pip install gunicorn .

Serving Your App

Running the Gunicorn server couldn't be simpler. Similar in nature to the Django development server, you simply execute python manage.py run_gunicorn from within your project's directory. In production, however, there's an even better method.

Gunicorn installs an executable script to your environment, called gunicorn_django . To use it, you specify which settings module to use on the command line: gunicorn_django path/to/project/settings . We can use this to serve an application from a process control system such as Supervisor.

Supervisor

On a production server, the application needs to start and stop with the operating system. To achieve this, a process control system is usually required (particularly for daemons that don't do this job themselves, like Gunicorn's). Supervisor is a particularly good example, and is pretty simple to setup. On Ubuntu, install it with sudo apt-get install supervisor . Then place a configuration file in /etc/supervisor/conf.d/app_name.conf with the required options. An example configuration, shown below, shows how a Django application might be served by Gunicorn under the www-data user from its own virtualenv, and save all its output to a log file. The server also starts automatically, and is restarted if it crashes.

[program:app_name] command = /path/to/env/bin/gunicorn_django app_name/settings directory = /path/to/app/ user = www-data autostart = true autorestart = true stdout_logfile = /path/to/app/django_logs/supervisord.log redirect_stderr = true

nginx

Due to the simplicity of Gunicorn, it isn't designed to be exposed to the world as a "bare" server. Instead, it's recommend that a reverse proxy is placed in front - which does the job of communicating with the client (browser) directly.

It's down to the way that worker processes are used, so a (perhaps deliberately) slow client can easily perform a Denial of Service (DoS) attack, which ends up taking down the entire application. For a more detailed explanation, see the deploy page in Gunicorn's documentation.

As it happens, nginx does a fine job of acting as a reverse proxy. It's also extremely fast, stable, and easy to configure. The following configuration sets up nginx to serve an application running on port 8000 (the default for Gunicorn):

upstream app_server { server localhost : 8000 ; } server { listen 80 ; client_max_body_size 4G ; keepalive_timeout 5 ; access_log /var/log/nginx/app_name.access.log ; location /static/ { alias /path/to/app/staticfiles/ ; } location / { proxy_pass http://app_server ; proxy_redirect off ; proxy_set_header Host $host ; proxy_set_header X-Real-IP $remote_addr ; proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for ; } }

You might have noticed, too, that this configuration serves static media from a directory called staticfiles , within the application's folder. I would recommend that the Django contrib app (by the same name) is used to manage the static media for the project - and this is easily done by executing python manage.py collectstatic on the server.

MySQL

The last thing I'd like to mention in this first part of the series is the database server. The Django community in general, I've found, tends to prefer PostgreSQL - though my leaning is towards MySQL. The decision here isn't really too important, so I'm not going to go into the pros and cons of each. In the end, as is often the case, it simply comes down to personal preference.

Database Migrations

When deploying code to an application server, it will often be necessary to perform database migrations. In this case, South is the perfect tool for the job. Also, the relevant libraries must be present on the server for your particular database. For MySQL, this is installable from pip as a module named mysql-python .

To begin with, it's difficult to get used to the workflow of creating schema migrations, applying them to the database, and then committing the tested files to the database. Once the initial hurdles are cleared, though, it makes an awful lot of sense. In a future post I'll talk about how to automate the process of migrating a production database using this very technique.

In the next post in this series, I'll be talking about executing long-running background tasks with Celery.

Part 2 of this series is now available: Background Tasks.