How to fix the bug 502 Bad Gateway: webpage error due to Gunicorn issue

Want to visit your website all the time, even if you close the terminal? Here is the way:

How to set the Python flask app on AWS EC2 is always available?

But sometimes, it doesn’t work. Here are some ways to check the reason.

Check the systemd service that manage Gunicorn.

sudo nano /etc/systemd/system/myapp.service

The code are similar to:

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

[Service]
User=ec2-user
Group=nginx
WorkingDirectory=/home/ec2-user/firstweb
Environment="PATH=/home/ec2-user/firstweb/venv_winjob/bin"
ExecStart=/home/ec2-user/firstweb/venv_winjob/bin/gunicorn --workers 3 --bind 0.0.0.0:5000 app:app

[Install]
WantedBy=multi-user.target

Note: the following part tells the program the location of gunicorn:

ExecStart=/home/ec2-user/firstweb/venv_winjob/bin/gunicorn --workers 3 --bind 0.0.0.0:5000 app:app

Then reload and restart the service:

sudo systemctl daemon-reload
sudo systemctl enable myapp.service
sudo systemctl start myapp.service

Then check the status

sudo systemctl start firstweb.service

If it is still not work, check the journal logs for detailed error messages:

sudo journalctl -u firstweb.service

Sometimes, it may due to Unicorn Executable Path:

source /home/ec2-user/firstweb/venv_winjob/bin/activate
which gunicorn

For example, if the result

~/.local/bin/gunicorn

and it is not as same as the code in the setting file:

ExecStart=/home/ec2-user/firstweb/venv_winjob/bin/gunicorn --workers 3 --bind 0.0.0.0:5000 app:app

we need to change it to keep the same.

ExecStart=/home/ec2-user/firstweb/.local/bin/gunicorn --workers 3 --bind 0.0.0.0:5000 app:app

Or reinstall gunicorn:

Reload and Restart the Service:

sudo systemctl daemon-reload
sudo systemctl restart firstweb.service