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Deploying Flask apps on Linode.com

By Angela C

June 21, 2021 in hosting Flask

Reading time: 4 minutes.

In this post I am following the tutorials and guides on hosting Flask apps on Linode.com

A link to an example Flask app on https://github.com/abalarin is provided or you can use your own when following the guide. I used the example Flask app by Austin Balarin who also has a very helpful guide for deploying a Flask app on Linode.

Create an app

  • Create or clone the flask app then move into this folder.
git clone https://github.com/abalarin/Flask-on-Linode.git flask_app_project

  • Create a linode

  • SSH into the linode using root

  • apt-get update && apt-get upgrade to install software updates on Ubuntu distribution

  • Set the hostname: to identify the linode with an easy to remember name

hostnamectl set-hostname example_flask

  • list timezones: timedatectl list-timezones timedatectl set-timezone 'Europe/Dublin'

() dpkg-reconfigure tzdata is the newer and recommended way to reset timezone for Ubuntu)

  • date to check the date is correct

Secure your server

adduser user1

  • add a password for this user adduser user1 sudo to add the limited user to the sudo group

  • exit to exit from root then login again using the new user `` ssh user1@123.234.345.56

To run a command as administrator (user “root”), use “sudo ”. See “man sudo_root” for details.

  • Navigate to home cd /home

  • Pull application code from source control sudo git clone https://github.com/abalarin/Flask-on-Linode.git

  • Alternatively retrieving an application from your Local Machine:

    • SCP your Applications root directory into your Linode’s Home directory

    • scp -r flask_app/ user@<Linode-IP>:/home

    • sudo scp -r flask_app/ user1@123.234.345.56:/home

Configure Nginx

NGINX is a free, open-source, high-performance HTTP server and reverse proxy, as well as an IMAP/POP3 proxy server.

  1. Install Nginx

sudo apt install nginx 2. Create an NGINX Configuration file

sudo nano /etc/nginx/sites-enabled/flaskapp

Copy and paste the code here into the configuration file using the nano code editor.

replace the IP address with your Linodes own IP address.

server {
	listen 80;
	server_name <Your Linodes IP>;

	location / {
		proxy_pass http://127.0.0.1:8000;
		proxy_set_header Host $host;
		proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
	}
}

ctrl+x to exit the nano editor, then y to save.

  1. Unlink the NGINX default config

sudo unlink /etc/nginx/sites-enabled/default

  1. Reload your NGINX server

sudo nginx -s reload

Install Python and packages

  1. sudo apt install python3
  2. sudo apt install python3-pip
  3. Install Flask Packages/libraries:

cd Flask-on-Linode

pip3 install -r requirements.txt

sudo pip3 install -r flask_app/requirements.txt

Deploy your application

Gunicorn ‘Green Unicorn’ is a Python WSGI HTTP Server for UNIX. It’s a pre-fork worker model. The Gunicorn server is broadly compatible with various web frameworks, simply implemented, light on server resources, and fairly speedy FlaskDeployment tutorial

  1. Install Gunicorn

  2. Run Gunicorn from your Application’s root directory or a directory up from your Application’s entry point

The command gunicorn -w 3 flask_app:app tells Gunicorn to look for the WSGI instance named app in the flask_app directory. In this example project this WSGI instance named app is located in __init__.py.

The amount of workers to use with Gunicorn can be specified with the -w flag. As per the tutorial, a good rule of thumb is to double your CPU core’s and add 1 for a Nanode with 1 CPU core thats 3 workers.

gunicorn -w 3 flask_app:app

The application should now be live and you should be able to navigate to it by entering your Linodes IP into a web browser.

This worked and the example Flask app is here.

Configure Supervisor

Supervisor is a client/server system that allows its users to monitor and control a number of processes on UNIX-like operating systems. Supervisor is great for auto-reloading gunicorn if it crashes or if your Linode is rebooted unexpectedly. FlaskDeployment tutorial

  1. Install Supervisor

sudo apt install supervisor

  1. Create a supervisor program sudo nano /etc/supervisor/conf.d/flaskapp.conf
[program:flaskapp]
directory=/home/Flask-on-Linode
command=gunicorn -w 3 flask_app:app
autostart=true
autorestart=true
stopasgroup=true
killasgroup=true
stderr_logfile=/var/log/flaskapp/flaskapp.err.log
stdout_logfile=/var/log/flaskapp/flaskapp.out.log
  1. Create your log files
touch /var/log/flaskapp/flaskapp.out.log
touch /var/log/flaskapp/flaskapp.err.log
  1. Reload Supervisor to apply changes

sudo supervisorctl reload

How to keep the process running when I log out of ssh

The above was a Flask app following the tutorials referenced. Using Supervisor keeps the app running when I exit from the terminal.

For a dash app I am working on I want to be able to keep the app running when I logout or exit SSH. At the moment, running the app using gunicorn will make the app available on the internet at the public ip address. However then to exit or shut down my laptop I do ctrl + c. This stops the process and the app is no longer available.

gunicorn -w 3 app:server

I have to figure out how to configure supervisor to work with my dash app. In the meantime to followed the process below as outlined in the Linode Community on how to keep a process running after I disconnect from mu Linode using the bg command.

  1. run the command normally gunicorn -w 3 app:server This gets the dash app up and running

  2. ctrl + Z

  3. run bg

  4. Type disown

  5. Exit the terminal

My dash app is now running and available on the public internet.

References