Design the booking flow for a ticketing platform: browse a live seat map, hold seats for a few minutes, then confirm the booking.
Most of the week it idles. Then a headline act goes on sale and everyone shows up in the same minute: seat-map traffic jumps 12× and the hottest seats are contested by thousands of buyers at once. A seat must never be sold twice, a confirmed booking must never disappear, and the seat map people are frantically refreshing may be at most one second behind reality. Holds expire on their own; payment processing is out of scope (assume the charge succeeds).