5xx Server ErrorCommonly usedRFC 9110
502Bad Gateway
Upstream server returned an invalid response.
What it means
The server, while acting as a gateway or proxy, received an invalid response from an upstream server. Usually means a downstream service (API, database, microservice) is down or returning errors.
When to use it
- ✓Upstream API or microservice is returning errors
- ✓Load balancer cannot reach the application server
- ✓Reverse proxy (nginx) cannot connect to Node.js app
Common causes
- →Application server crashed or is restarting
- →nginx cannot reach Node.js (wrong port)
- →Upstream microservice is down
- →Network issue between services
Code Examples
nginx configuration
nginx
# 502 often means nginx can't reach your app
# Check your proxy_pass matches your app port:
server {
location / {
proxy_pass http://localhost:3000; # Match your app port
proxy_read_timeout 60s;
}
}Quick Facts
Code502
CategoryServer Error
SpecRFC 9110
CommonYes