Airbnb

Home rental marketplace

Design a marketplace where hosts list homes and guests search by place and dates, book without ever double-booking a night, pay, and review each other.

Overview

A home rental marketplace connects hosts, who list homes, with guests, who search for a place to stay on particular dates. Unlike an online store, the inventory is not a count of identical items: each home on each night is unique, and it can be sold exactly once. Two guests must never both be confirmed for the same night.

That makes the design split cleanly. Search must answer 'what is free near here on these dates for four people?' over millions of listings, quickly, while hosts keep changing prices and calendars. It can be slightly stale, because nothing is sold from a search result. Booking must be exactly right, but it is a small, per-listing workload. The interviewer will test whether you keep the two apart and still make them agree.

Questions to ask first

How many listings, searches and bookings? — 7 million listings, 200 million searches and 400,000 bookings a day
How do guests search? — A map area or place, check-in and check-out dates, number of guests, filters
Can a guest book at once? — Yes for Instant Book listings; otherwise the host approves a request
When is money moved? — The guest pays when the booking is confirmed; the host is paid after check-in
Reviews? — Both sides review each other after the stay
How fresh must search be? — Seconds behind is fine; booking rechecks everything

What you will be able to do

Answer a map-plus-dates search with a geo index, precomputed availability and a final exact check.
Keep the search index fresh from a change stream, and say why booking never trusts it.
Make double-booking impossible with a database constraint on listing-nights.
Design Instant Book and booking requests, including holds that expire.
Take payment exactly once and pay hosts later; run double-blind reviews.

Key concepts

Geo search
Availability calendar
Two-stage retrieval
Search index freshness
Double-booking prevention
Booking requests with expiry
Idempotent payments
Delayed payout
Double-blind reviews

Functional requirements

Hosts create listings with photos, capacity, price and a calendar
Guests search by map area or place, dates and number of guests, with filters
Guests see a listing page with its calendar and the total price for their dates
Guests book instantly or send a request the host accepts or declines
Guests pay at confirmation; hosts are paid after check-in
After the stay, guest and host review each other

Non-functional requirements

Search returns in under 500 ms at p99 (a target we chose)
A night is never confirmed to two guests
Search reflects calendar and price changes within seconds
A guest is never charged twice for one booking
Booking stays available during the busiest travel seasons, at a few times average load

Interview questions

How do you prevent double-booking?
How does search handle dates as well as location?
How do you keep the search index fresh with thousands of updates a second?
How does a booking request that needs host approval work?
How do you avoid charging a guest twice?
When is the guest charged and when is the host paid?
How do reviews stay honest?
How do you shard the data?