How to debug Istio Upstream Reset 502 UPE (old 503 UC)

Istio is a complex system. For the applications, the main component is the sidecar container Istio-Proxy, which proxies all traffic from all containers in Pod. And this can lead to some issues. This post describes one of the most complicated problems I have encountered in my career. The problem - Connection Reset 🐛 During Istio rollout on a huge system, with more than 40 different microservices, on a single endpoint, QA engineers found a bug. It was a POST endpoint, which was returning chunked data.