The probe

The inventory reservation problem. A listing is available on specific dates, and two users trying to book the same listing for overlapping dates cannot both succeed. The hard problem: prevent double-booking without making the availability check and reservation a single-threaded bottleneck.

Step 1 — Clarify

- Single room per listing (Airbnb) or multiple rooms (hotel)?

- Search: by location + date range + filters?

- Booking flow: instant book (reserve immediately) or request-to-book (host approves)? - How to handle the “someone else just booked it” race condition?

Step 2 — Data Model

listings: listing_id, host_id, title, location, price_per_night, instant_book (bool) availability: listing_id, date, status (available/blocked/reserved), booking_id (nullable) PRIMARY KEY: (listing_id, date) — one row per listing per night

bookings: booking_id, listing_id, guest_id, check_in, check_out, status

(pending/confirmed/cancelled),

total_price, idempotency_key, created_at

The availability table with one row per listing per night is the key data model decision. Date range availability check = SELECT COUNT(*) FROM availability WHERE listing_id=X AND date BETWEEN check_in AND check_out AND status=’available’. If count = (check_out - check_in) in days, all nights are available.

Get Access to GitHub Repo

Step 3 — Reservation flow (preventing double booking) BEGIN;

-- Check all nights available and lock them

SELECT COUNT(*) FROM availability

WHERE listing_id=:lid AND date BETWEEN :check_in AND :check_out

AND status=’available’

FOR UPDATE; -- locks these rows

-- If count != nights_requested: ROLLBACK (someone else has them)

-- Reserve all nights atomically

UPDATE availability SET status=’reserved’, booking_id=:booking_id

WHERE listing_id=:lid AND date BETWEEN :check_in AND :check_out; -- Create booking record

INSERT INTO bookings VALUES (:booking_id, ...);

COMMIT;

The FOR UPDATE lock on the availability rows means only one transaction can proceed for any given listing+date combination. The second concurrent booking attempt blocks, then fails the count check when it finally runs.

System Design Interview Roadmap

Design Airbnb Booking System — Walkthrough

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