Lessons in reliability, load testing, observability and engineering leadership from supporting three Black Friday and Cyber Monday events. Some of my clearest memories from Groupon are not of writing code or reviewing an architecture document. They are of late nights in the office during Black Friday and Cyber Monday—sitting with other engineers, watching traffic climb on our dashboards and waiting for the peak. Weeks of preparation had led to those hours. Runbooks had been reviewed. Load tests had been executed and questioned. Capacity assumptions had been challenged. Dashboards and alerts had been improved. Plan B and Plan C had been discussed. Then came the waiting. There was always a degree of tension because production systems do not necessarily behave according to a plan. Something could slow down or fail abruptly. At the same time, there was confidence—not because we expected everything to behave perfectly, but because considerable thought had gone into preparing for the u...