When APM fails: A 502 troubleshooting tale

In an intriguing case study, the author explores the challenges of troubleshooting a persistent 502 error encountered in a multi-tier AWS environment using various monitoring tools such as AppDynamics, DataDog, and Sysdig. Despite having these commercial tools, the root cause of the issue remained elusive until the open-source tool Sysdig was employed. Sysdig's capture feature, which records every system call on a host, was pivotal in identifying the problem: an inverted HTTP header in the Java application's response that was being rejected by HAproxy. This case highlights the limitations of traditional monitoring tools in providing deep insights into specific issues and underscores the value of incorporating open-source tools like Sysdig into a developer's troubleshooting toolkit for more comprehensive diagnostics.