HTTP 502 status code means a proxy server (also known as gateway) didn’t receive a valid response from the upstream server (also known as 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 in 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 is Apache, Nginx, HAProxy, Varnish, and Caddy.
Make sure that upstream server:
- can respond to HTTP requests
- isn’t blocked by a firewall
- isn’t overloaded from too many requests
- and its hostname can be resolved
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