How to create Systemd Unit File for a Flask App using SocketIO?

I want to use Flask and Flask SocketIO for realtime Data Visualization. I have set up the Flask App with this Digital Oceans Tutorial. Everything works. I want to create a systemd unit file for running flask with flask socketio in the background. I have tried to change the old systemd unit file (digital oceans tutorial) to run Flask with SocketIO but it doesn’t work.

Systemd Unit File

  [Unit]
    Description=Gunicorn instance to serve my_app
    After=network.target

    [Service]
    User=app_user
    Group=www-data
    WorkingDirectory=/home/user/my_app
    Environment="PATH=/home/user/my_app/venv/bin"
    ExecStart=/home/user/my_app/venv/bin/gunicorn --workers 3 --bind unix:my_app.sock -m 007 wsgi:app

    [Install]
    WantedBy=multi-user.target

My new NGINX Config File:

server {
    listen 80;
    server_name my_server;

    location / {
        include proxy_params;
        proxy_pass http://127.0.0.1:5000;
    }

    location /socket.io {
        include proxy_params;
        proxy_http_version 1.1;
        proxy_buffering off;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "Upgrade";
        proxy_pass http://127.0.0.1:5000/socket.io;
    }
}

I stopped the the systemd my_app service and tested the Flask App by typing:

(venv) $ gunicorn --worker-class eventlet -w 1 --bind 127.0.0.1:5000 wsgi:app

It works.

Unfortunately the changes in the systemd Unit File doesn’t work. I have tried:

ExecStart=/home/user/my_app/venv/bin/gunicorn --worker-class eventlet -w1 --workers 3 --bind 127.0.0.1:5000 wsgi:app

ExecStart=/home/user/my_app/venv/bin/gunicorn --worker-class eventlet -w1 --workers 3 --bind unix:my_app.sock -m 007 wsgi:app

Error Message (NGINX Error log): “failed (111: Connection refused) while connecting to upstream”

Changing User and Group doesn’t work either.

I have no idea how to solve this problem. Can anyone help? Thanks.