To deploy Flask web applications with uWSGI

The Flask web applications communicates data by the Web Server Gatway Interface (WSGI) specification. It cannot be readily understood by the current web servers such as nginx at the front end, hence the need for a middle layer that does the translation of communciation protocols.

If we use uWSGI for this purpose, to install it,

sudo yum install gcc python3-devel
pip3 install --user uwsgi

uWSGI needs be configured for the web application. Below is an example configuration file uwsgi.ini.

[uwsgi]
socket = :7777
master = true
processes = <number_of_processes>
module = <name_of_webapp_module>:app
master-fifo = /home/ec2-user/www/uwsgi.fifo
logto = /home/ec2-user/www/uwsgi.log
stats = /home/ec2-user/www/stats.socket

<number_of_processes> is the number of concurrent uWSGI server processes to handle requests. <name_of_webapp_module> is the name of the Python module you have developed the Flask web application under. For example, for this Flask official tutorial, the name of webapp module should be flaskr. Lastly, for the above configuration, all the runtime files will be placed under the directory /home/ec2-user/www and one has to make sure it exists.

To start the web application and the uWSGI server in the background, in the shell, do

uwsgi --ini uwsgi.ini &

The front end web server nginx shall now be configured to delegate requests for the web application to uWSGI. Go to /etc/nginx/nginx.conf, and modify the relevant server section,

to mount the web app on the root path ‘/’, so that the URL looks http://<host>,

  server {
    ...
    location / {
		uwsgi_pass 127.0.0.1:7777;
		include uwsgi_params;
	}
    ...
  }

or to mount the web app on path ‘/bbb’, so that the URL looks http://<host>/bbb,

  server {
    ...
    location = /bbb {
        return 301 $scheme://$host$uri/$is_args$args;
    }
    location /bbb/ {
        try_files $uri @bbb;
    }
    location @bbb {
		uwsgi_pass 127.0.0.1:7777;
		include uwsgi_params;
    }
    ...
  }

Eventually, reload or restart the web server in the shell,

sudo service nginx force-reload

When uWSGI is running, in the shell, we could print text command to the fifo file /home/ec2-user/www/uwsgi.fifo for control, such as,

# to quit the web app and uwsgi process
echo q >/home/ec2-user/www/uwsgi.fifo

# to restart the web app and uwsgi process
echo r >/home/ec2-user/www/uwsgi.fifo