502 Bad Gateway

The HTTP 502 status code means a proxy server (also known as a gateway) didn’t receive a valid response from the upstream server (also known as the origin server).

Having a proxy server in front of the application (upstream) server is a common pattern in production web apps. Web servers (acting as reverse proxies) are more efficient at and capable of terminating TLS connections, dealing with slow clients, compressing requests, and serving static files quickly. Some popular open-source software used as proxies are Apache, Nginx, HAProxy, Varnish, and Caddy.

Make sure that the upstream server:

• can respond to HTTP requests
• isn’t blocked by a firewall
• isn’t overloaded with too many requests
• and its hostname can be resolved