Airbnb: a marketplace of nine million one-of-a-kind homes

How a stays marketplace with 9 million+ listings and 533 million nights a year works: search leaves that hold every listing's geo cell, a 365-night availability bit array and prices in memory, filtering by place then dates before a retrieve, rank and re-rank funnel and map pins chosen for attention; a unique key per listing-night at Reserve; iCal calendars from other sites polled by risk, diffed by source and refreshed when a guest presses Reserve; Smart Pricing inside the host's range; and guests' money held in a double-entry ledger and paid out the business day after check-in with an idempotency key per payout.