Airbnb

A two-sided marketplace with ~7M listings and ~100M nights booked per year. The hard parts: date-range availability search — "find homes in Lisbon for 4 guests, June 3–10, under $200/night" is a surprisingly gnarly query; no-double-booking with strong consistency on the calendar; and dynamic pricing + complex fees (cleaning fee, service fee, taxes that vary by jurisdiction) without ever showing the guest one number and charging another. Airbnb solved the "Amazon but for someone's spare bedroom" problem at planet scale.

Time budget: 45 min whiteboard. Draw architecture, estimate numbers, discuss tradeoffs.

• +2 Lead with the consistency split. "Search is eventually consistent; booking is strongly consistent; the booking saga re-verifies authoritatively." This sentence shows you've thought about the hard part.
• +2 Date-range availability bitmaps. Mention them by name. Most candidates don't know this representation; it's specific and correct.
• +2 Price quote locking. 10-minute quote window. Prevents "I see $200, I go enter my card, I see $230." A small detail that shows care for UX.
• +2 Saga, not 2PC. Airbnb is a canonical saga example. Don't propose distributed transactions across payment + calendar.
• +2 Reviews and trust. Often skipped. The blind-review system is a product-design decision with a data-model implication — worth mentioning.

Self-score: tally the points you would have mentioned unprompted. 7+ is interview-ready on this problem.

• No back-of-envelope estimation — jumps straight into components without quantifying scale for Airbnb
• Single point of failure — no replication, failover, or redundancy discussed
• Ignores data model and storage choices — hand-waves the database layer