Problems / Resilience, hard
Asked at Airbnb and Booking.com. One API serves search, listing pages and bookings from one Postgres database, and bookings call the card provider in the same thread pool. At 6 s holiday traffic rises by half; at 10 s the card provider gets eight times slower for 12 seconds, and searches start failing too.
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Success ≥ 99%, p99 ≤ 2.5 s and Apdex ≥ 0.90, for at most $700,000 a month.
Like a coding problem, a solution has to pass every case, not just the one it was built for. Submit in the lab runs all 3; 1 of them are hidden until then.
10,000 requests a second for 20 seconds: 70% search (reads), 25% listing (reads), 5% book (writes).
What each component does in this design, and what calls it. The simulator mimics each one from these settings.
| Component | Its job here | Settings |
|---|---|---|
| Load balancerLoad balancer | Traffic enters here.Calls Stays API.Layer 4 balancers route by IP and port; layer 7 balancers read HTTP and can route by path or header. | – |
| Stays APIApp service | Load balancer forwards to it.Calls Listings, calendars and bookings (Postgres), Card payments.Little's law: throughput ≈ concurrency ÷ latency. 200 slots at 50 ms serve about 4,000 req/s. | 16 replicas × 100 slots · timeout 10 s |
| Listings, calendars and bookings (Postgres)SQL database | Stays API calls it.Postgres runs one process per connection, which is why poolers like PgBouncer sit in front of busy databases. | 32 vCPU · 800 connections · 2 read replicas |
| Card paymentsPayment gateway | Stays API calls it for /book.Payment providers accept an idempotency key so a retried charge is not taken twice. | 500 slots |