1. Create new directory project

   mkdir /var/www/flask-app

2. Change location to your new created directory project

   cd /var/www/flask-app

3. Create new Virtualenv for specific python version

   virtualenv venv --python=python3.9

4. Activate your virtual environment (venv)

   source venv/bin/activate

5. Install Flask using pip

   pip install flask

6. Create new file app.py in root project directory

   from flask import Flask, request, abort, jsonify
   
   app = Flask(__name__)
   
   @app.route('/getSquare', methods=['POST'])
   def get_square():
       if not request.json or 'number' not in request.json:
           abort(400)
       num = request.json['number']
       return jsonify({'answer': num ** 2})
   
   if __name__ == '__main__':
       app.run(host='127.0.0.1', port=8080, debug=True)

7. Finally, your project directory should be like this

   ├── app.py
   ├── venv
   │   ├── bin
   │   ├── lib
   │   ├── share

   ---

   Testing

   1. Make sure you've been activating your virtual environment

      source venv/bin/activate

   2. Run flask app

      python app.py

   3. Create new request

      curl -i -H "Content-Type: application/json" -X POST -d '{"number":5}' http://127.0.0.1:8080/get-square

      After running that command, you should get this message

      HTTP/1.0 200 OK
      Content-Type: application/json
      Content-Length: 19
      Server: Werkzeug/0.16.0 Python/3.9.0
      Date: Mon, 23 Nov 2020 15:18:14 GMT  
      
      { 
          "answer": 25
      }
      

1. Install Gunicorn package

   pip install gunicorn

2. Create new file on your root project directory named wsgi.py

   from app import app
   
   if __name__ == '__main__':
       app.run()

3. Finally, your project directory should look like this

   ├── app.py
   ├── venv
   │   ├── bin
   │   ├── lib
   │   ├── share
   ├── wsgi.py

   ---

   Testing

   1. Make sure you've been activating virtual environment

      source venv/bin/activate

   2. Run flask app using Gunicorn web service

      gunicorn --bind 127.0.0.1:8080 wsgi:app

      You should get this message after running that command

      [2020-11-23 06:49:16 +0000] [4166] [INFO] Starting gunicorn 20.0.4
      [2020-11-23 06:49:16 +0000] [4166] [INFO] Listening at: http://127.0.0.1:8080 (4166)
      [2020-11-23 06:49:16 +0000] [4166] [INFO] Using worker: sync
      [2020-11-23 06:49:16 +0000] [4181] [INFO] Booting worker with pid: 4181

      Alternative, you can use IP address "0.0.0.0:8080", instead of "127.0.0.1:8080". Especially in production server.

1. Create new file named gunicorn.py inside root project directory

   import multiprocessing
   
   workers = multiprocessing.cpu_count() * 2 + 1
   bind = 'unix:flask-app.sock'
   umask = 0o007
   reload = True
   
   #logging
   accesslog = '-'
   errorlog = '-'

2. Create systemd file to automatically start Gunicorn service

   sudo nano /etc/systemd/system/flask-app.service

3. Below is the content of flask-app.service file

   [Unit]
   Description=Gunicorn instance to serve flask application
   After=network.target
   
   [Service]
   User=root
   Group=www-data
   WorkingDirectory=/var/www/flask-app/
   ExecStart=/var/www/flask-app/venv/bin/gunicorn --config gunicorn.py wsgi:app
   Environment="PATH=/home/root/flask_rest/flaskvenv/bin"
   Environment="FOO=Bar"
   
   [Install]
   WantedBy=multi-user.target

   You could add some configuration (DB connection, App version, etc) by using Environment="FOO=Bar". Just add some couple config there.

4. Let's start and enable the service

   sudo systemctl start flask-app.service
   sudo systemctl enable flask-app.service

5. Finally, you can check your service whether its running or not

   flask-app.service - Gunicorn instance to serve flask application
      Loaded: loaded (/etc/systemd/system/flask-app.service; enabled; vendor preset: enabled)
      Active: active (running) since Mon 2020-11-23 04:55:01 UTC; 1h 4min ago
    Main PID: 19207 (gunicorn)
       Tasks: 11 (limit: 4647)
      CGroup: /system.slice/flask-app.service
      ...

1. Create Apache virtual host config file

   sudo nano /etc/apache2/sites-available/flask-app.conf

2. Below is the content of flask-app.conf

   <VirtualHost *:80>
   
     ServerAdmin [email protected]
     ServerName  flask-app.example.com
   
     ErrorLog ${APACHE_LOG_DIR}/flask-app-error.log
     CustomLog ${APACHE_LOG_DIR}/flask-app-access.log combined
   
     <Location />
       ProxyPass unix:/var/www/flask-app/flask-app.sock|http://127.0.0.1:8080/
       ProxyPassReverse unix:/var/www/flask-app/flask-app.sock|http://127.0.0.1:8080/
     </Location>
   
   </VirtualHost>

3. Enable new Apache config

   sudo a2ensite flask-app.conf

4. Reload Apache service

   sudo service apache2 reload

Congratulation, you have been created new project using Flask and Gunicorn. Now, it's time to test our API.

1. Create new request using cURL

   curl -i -H "Content-Type: application/json" -X POST -d '{"number":5}' http://flask-app.example.com

2. You should get this message

   HTTP/1.0 200 OK
   Content-Type: application/json
   Content-Length: 19
   Server: Werkzeug/0.16.0 Python/3.9.0
   Date: Mon, 23 Nov 2020 15:18:14 GMT  
   
   { 
       "answer": 25
   }
   

1. Save your pip package into text file and install them when deploy to your server
   1. Save pip packages into text file

      pip freeze > requirements.txt

   2. Installpip packages from text file

      pip install -r requirements.txt

Reference : https://medium.com/@thishantha17/build-a-simple-python-rest-api-with-apache2-gunicorn-and-flask-on-ubuntu-18-04-c9d47639139b