Challenge 6: Flash-sale catalog with a cold cache

A flash sale sends 20,000 requests a second to the product catalog, 98% reads, most of them for a few hundred products. Every response carries live stock and the shopper's cart, so a CDN cannot cache it, and orders stay in the relational database. After a deploy the caches start cold, and only half of reads hit. During a cold start, 95% of the extra misses are for the same hot products at the same moment. Survive the cold start and the loss of any instance without dropping a request, with p99 under 200 ms, for at most 3,500 USD a month.