sudo apt install python3-pip python3-dev python3-venv nginx

python3 -m venv env
source env/bin/activate
pip3 install flask gunicorn

#~/project/app.py
from flask import Flask
app = Flask(__name__)
@app.route("/")
def hello():
    return "Welcome to Flask Application!"
if __name__ == "__main__":
    app.run(host='0.0.0.0')

#~/project/wsgi.py
from app import app
if __name__ == "__main__":
  app.run()

And deactivate the environment.

#vi /etc/systemd/system/flask.service
[Unit]
Description=Gunicorn instance to serve Flask
After=network.target
[Service]
User=root
Group=www-data
WorkingDirectory=/home/<user>/project
Environment="PATH=/home/<user>/project/env/bin"
ExecStart=/home/<user>/project/env/bin/gunicorn --bind unix:/home/<user>/project/request.sock wsgi:app
[Install]
WantedBy=multi-user.target

chown -R root:www-data /home/<user>/project
chmod -R 775 /home/<user>/project
systemctl daemon-reload
systemctl start flask
systemctl enable flask
systemctl status flask

> vim /etc/nginx/sites-available/flask.conf
server {
  listen <server_ip>:5002 ssl http2;
  listen <server_ip>:5001;

  ssl_certificate /home/<user>/project/cert.pem;
  ssl_certificate_key /home/<user>/project/key.pem;

  access_log /home/<user>/project/rest_api.access.log;
  error_log /home/<user>/project/rest_api.error.log;



  location / {
    include proxy_params;
    proxy_pass http://unix:/home/<user>/project/request.sock;
   }

}
> vim /etc/nginx/nginx.conf
stream {
    upstream http {
        server <server_ip>:5001;
    }

    upstream https {
        server <server_ip>:5002;
    }

    map $ssl_preread_protocol $upstream {
        default https;
        "" http;
    }

    # SSH and SSL on the same port
    server {
        listen 5000;

        proxy_pass $upstream;
        ssl_preread on;
    }
}

sudo ln -s /etc/nginx/sites-available/flask.conf /etc/nginx/sites-enabled
nginx -t
systemctl restart nginx