Design a Hotel Booking System (Airbnb/Booking.com)

To design a scalable booking system similar to Airbnb's, I would start by defining the core entities: Hosts, Properties, Rooms, and Reservations. Given the need for sub-second search responses across millions of listings, I'd propose an architecture where the Search Service uses Elasticsearch to handle complex filters like dates, price ranges, and geo-location, while the Inventory Service manages real-time availability in a low-latency store like Redis. For the reservation flow, the critical challenge is concurrency. When a user selects dates, the system must check availability and reserve the room atomically. I would implement a distributed lock mechanism or optimistic concurrency control with versioning in the database to ensure two users cannot book the same room simultaneously. If a conflict occurs, we return a 'busy' status immediately rather than failing after payment processing. For persistence, I'd use a sharded PostgreSQL cluster to handle write-heavy reservation updates, ensuring ACID compliance. To guarantee data consistency across services, I would employ the Saga pattern for long-running transactions, allowing us to compensate and release holds if a downstream step fails. Finally, caching strategies are vital; we should cache popular property details but invalidate them instantly upon a new booking to prevent stale data from showing up in search results.