Finding a Needle in Haystack: Fixing Mysterious Bad Gateway

This article details a senior software engineer's root cause analysis of intermittent HTTP 502 Bad Gateway errors in a distributed system. The author describes building an alert system for server-side failures, then discovering 502 errors that never reached the application. By examining proxy server logs and throughput metrics, the engineer rules out application code issues and high traffic. The investigation leads to TCP connection handling and HTTP keep-alive mechanisms, explaining how connection reuse can cause proxy-server communication failures. The article provides practical debugging insights for backend engineers dealing with gateway errors.