Question

A cloud engineer deploys a new version of a web application behind a load balancer. Immediately after deployment, users report intermittent HTTP 502 errors. The load balancer health checks show all backend instances as healthy, and CPU, memory, and network metrics on the instances are well within normal ranges. Application logs on the instances show requests being served successfully with 200 responses, but the load balancer access logs record a mix of 200 and 502 responses. Following a structured troubleshooting methodology, what is the MOST likely cause of the 502 errors?

Accepted Answer

The backend application's idle keep-alive timeout is shorter than the load balancer's, causing the backend to close connections the load balancer still tries to reuse

Answer

DNS records for the application have not fully propagated, so some users reach an old endpoint

Answer

The backend instances are undersized and exhausting CPU during peak traffic

Answer

The load balancer's TLS certificate has expired, breaking the handshake for some clients