Hotel Reservation System

Phase 1 — Clarify & Scope

Interviewer: Design a hotel reservation system — think Booking.com or Expedia. What do you want to clarify?

Candidate: A few questions to scope this:

1. Are we building the full platform (search + booking + inventory management) or a specific component?
2. What's the scale — how many hotels, rooms, and bookings per day?
3. Do we need real-time room availability, or can we tolerate slight staleness?
4. What's the consistency requirement — is double-booking catastrophic, or can we overbooking-compensate like airlines?
5. Multi-currency and multi-language?

Interviewer: Full platform — search, availability check, booking creation and cancellation. 500,000 hotels globally, 10 rooms average = 5 million rooms. 10 million bookings per day. Real-time availability required — double-booking is unacceptable (unlike airlines). Single currency/language for now.

Candidate: Let me size this:

$\text{Booking rate} = \frac{10\text{M bookings/day}}{86{,}400} \approx 116 \text{ bookings/sec average}$

$\text{Peak booking rate (10× average)} \approx 1{,}160 \text{ bookings/sec}$

Search queries vastly outnumber bookings (100:1 ratio):

$\text{Search queries} = 116 \times 100 = 11{,}600 \text{ searches/sec average}$

Room-nights stored (assuming average 3-night stay, 90-day search window):

$\text{Availability records} = 5\text{M rooms} \times 90 \text{ days} = 450\text{M records}$

Interviewer: What are the core functional requirements?

Candidate:

• Search hotels by location, dates, and filters (price, rating, amenities)
• Check real-time room availability for a date range
• Create a reservation (hold → confirm → paid)
• Cancel a reservation with refund processing
• Hotel management API: add/update room inventory, pricing

Non-functional:

• Search: <…