Python Adventures

Today I was trying to put together these: ubuntu + nginx + gunicorn + start gunicorn from upstart + flask + virtualenv. After a whole afternoon, I managed to do it. I will write a detailed post about it, but now I just want to concentrate on one part, which was the most problematic.

Problem
I wanted to start gunicorn from upstart. So I issued the following command:

$ sudo service gunicorn start 
gunicorn start/running, process 21415
$ sudo service gunicorn status
gunicorn stop/waiting

Each time I tried to launch it, it failed.

Solution
First of all, I didn’t know where the upstart log files were located. I know, it’s lame, but after a few hours I relized that it’d be a good idea to google it… So, the log files are in this folder:

/var/log/upstart

The log of gunicorn is here:

/var/log/upstart/gunicorn.log

Now, I started to monitor the end of this file in another terminal with “tail -f /var/log/upstart/gunicorn.log“. With this information I could easily find the error. I put here my solution and then I add some explanation:

# /etc/init/gunicorn.conf
#
description "Gunicorn daemon for a Flask project"

start on (local-filesystems and net-device-up IFACE=eth0)
stop on runlevel [!12345]

# If the process quits unexpectadly trigger a respawn
respawn

setuid demo
setgid demo

script
. "/home/demo/.virtualenvs/myproj/bin/activate"
cd /home/demo/projects/myproj/api
exec gunicorn --name="my project" --bind=127.0.0.1:9000 --config /etc/gunicorn.d/gunicorn.py api:app --pid=gunicorn_from_nginx.pid
end script

And you know what caused the problem? Instead of “.” I tried to use “source”. This is what I found in every tutorial. But as it turned out, this script is executed with “/bin/sh“, not with “/bin/bash“, and sh doesn’t know source. Geez…

The normal user on this machine is called “demo“. My Flask project is within a virtual environment. You can create a script block and the steps within are executed with “sh”. So first we “source” the virtual environment, then enter the directory where the Python script to be executed is located (here “api” is the name of the folder, and in there I have a file called “api.py“, which contains the “app” variable). gunicorn is also installed in the virtual environment! You don’t need to install it globally with “sudo“!

Nginx is listening on port 80 and every connection that arrives there are forwarded to this gunicorn instance on port 9000. But I’ll write about it a bit later.

Appendix
The content of “/etc/gunicorn.d/gunicorn.py“:

"""gunicorn WSGI server configuration."""
from multiprocessing import cpu_count
from os import environ


def max_workers():
    return cpu_count() * 2 + 1

max_requests = 1000
worker_class = 'gevent'
workers = max_workers()