Deploy_Flask_App.md

Deploy Flask App on VPS:

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Install required packages:

Update preinstalled packages

sudo apt update 

Now install python3, python3-pip & Nginx

sudo apt install python3-pip python3-dev nginx

Create a directory for flask app and virtual environment:

Lets install virtual environment

sudo pip3 install virtualenv

Lets create a directory to host our flask application

mkdir myFlaskApp && cd myFlaskApp

Run the following command to create a virtual environment named env

virtualenv env

Finally activate the virtual environment

source env/bin/activate

Now install flask and gunicorn using flask

pip3 install flask gunicorn

Create a sample project & wsgi entry point:

Let us now create a sample project by entering the command below:

vim app.py

Paste the below code to app.py file. Here debug = true is written. For the first time to deploy the app, then this is for testing purpose. Then make it false. This is actually for development purpose.

from flask import Flask
app = Flask(__name__)
@app.route('/')
def hellow_world():
    return "Hello WOrld!"
if __name__ == '__main__':
    app.run(debug = True, host = '0.0.0.0')

Now run the project. If everything is ok, then go further.
Next, we will create a file that will serve as the entry point for our application. This will tell our Gunicorn server how to interact with the application.

vim wsgi.py

Copy the contents below to wsgi.py

from app import app

if __name__ == "__main__":
    app.run()

We can test gunicorn's ability to serve our project by running the command below

gunicorn --bind 0.0.0.0:5000 wsgi:app

Folder structure so far:

myFlaskApp
  |____ app.py
  |____ wsgi.py
  |____ env

Lets deactivate the virtual environment now

deactivate

Creating a systemd service:

When your server will reboot for any reason then the flask app will rerun automatically. Lets now create a systemd service using:

vim /etc/systemd/system/app.service

Now paste the contents below to this file and save it:

[Unit]
#  specifies metadata and dependencies
Description=Gunicorn instance to serve myproject
After=network.target
# tells the init system to only start this after the networking target has been reached
# We will give our regular user account ownership of the process since it owns all of the relevant files
[Service]
# Service specify the user and group under which our process will run.
User=sayan
# give group ownership to the www-data group so that Nginx can communicate easily with the Gunicorn processes.
Group=www-data
# We'll then map out the working directory and set the PATH environmental variable so that the init system knows where our the executables for the process are located (within our virtual environment).
WorkingDirectory=/home/sayan/myFlaskApp/
Environment="PATH=/home/sayan/myFlaskApp/env/bin"
# We'll then specify the commanded to start the service
ExecStart=/home/sayan/myFlaskApp/env/bin/gunicorn --workers 3 --bind unix:app.sock -m 007 wsgi:app
# This will tell systemd what to link this service to if we enable it to start at boot. We want this service to start when the regular multi-user system is up and running:
[Install]
WantedBy=multi-user.target

[NOTE: Change the username according to you. Change the WorkingDirectory according to your project and also specify the Environment path.]
Activate this service by typing

sudo systemctl start app
sudo systemctl enable app

A file named app.sock will be automatically created. If there is no .sock file, you did a mistake. Folder structure so far

myFlaskApp
  |____ app.py
  |____ wsgi.py
  |____ env
  |____ app.sock

Configure Naginx

Create a file named app inside /etc/nginx/sites-available

sudo vim /etc/nginx/sites-available/app

Now copy the below contents to this file:

server {
    listen 80;
    server_name 165.232.177.116; # This is your local ip address

    location / {
        include proxy_params;
        # tell where the files of the site
        proxy_pass http://unix:/home/satan/myFlaskApp/app.sock;
    }
}                

Activate this configuration by executing this

sudo ln -s /etc/nginx/sites-available/app /etc/nginx/sites-enabled

Restart nginx and your website should work fine!

sudo systemctl restart nginx

Firewall Setup

Execute below commands

sudo ufw allow ssh
sudo ufw enable
sudo ufw allow 'Nginx Full'
sudo ufw allow http
sudo ufw allow https

What to do if you changes sometimg to your project?

After changing the project, next have to execute below commands

sudo service nginx restart
sudo service app restart

How to serve static files?

In you project, there will be static files also. To serve that files also, type these commands:

sudo vim /etc/nginx/sites-availabe/app

Next have to change the file. After changing the file, the file should look like this

server {
    listen 80;
    server_name 165.232.177.116;
    
    location /static {
        alias /home/sayan/myFlaskApp/static/;
    }
    
    location / {
        include proxy_params;
        proxy_pass http://unix:/home/sayan/myFlaskApp/app.sock;
    }
}