Day 52 drills

Paper first. Warm-up before the timed run.

D1 (why request-response does not work for chat, what a persistent connection gives you, and the new problem it creates):

D2 (routing: how A on gateway 7 reaches B on gateway 2000, and what store holds the user-to-gateway map):

D3 (offline: B is offline for three messages, where they go, how B syncs on reconnect, and what guarantees order and no duplicates):

D4 (group fan-out: cost of fan-out on write to a 500-person group, when you would not do it, and how it is the Day 18 celebrity problem):

D5 (presence: why “online / last seen / typing” is so expensive at scale, and the honest cheap answer):