Challenge 8: Checkout with a slow dependency

Shoppers browse (70% of 3,000 personalized requests a second) and check out (30%), and orders stay in the relational database. Checkout calls a separate inventory service, which sometimes slows to 2 seconds. When it does, browsing must keep working: no request that does not need inventory may fail. Any instance can fail too. p99 under 300 ms, at most 1,350 USD a month.