Problems / Real world, easy
The monthly renewal run charges 300 subscriptions a second through a payment provider that allows 12,000 calls a minute.
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Success ≥ 99%.
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 5; 2 of them are hidden until then.
300 requests a second for 30 seconds: 100% 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 Billing service.Layer 4 balancers route by IP and port; layer 7 balancers read HTTP and can route by path or header. | – |
| Billing serviceApp service | Load balancer forwards to it.Calls Payment provider.Little's law: throughput ≈ concurrency ÷ latency. 200 slots at 50 ms serve about 4,000 req/s. | 3 replicas × 200 slots · timeout 4.0 s |
| Payment providerPayment gateway | Billing service calls it.Payment providers accept an idempotency key so a retried charge is not taken twice. | 500 slots · 12,000 calls/min |