Problems / Async and streaming, medium
Payment events go through a queue that delivers at least once. Some acknowledgements are lost, so some customers are charged twice.
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No duplicate side effects, 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.
500 requests a second for 30 seconds: 100% webhook (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 Webhook API.Layer 4 balancers route by IP and port; layer 7 balancers read HTTP and can route by path or header. | – |
| Webhook APIApp service | Load balancer forwards to it.Calls Payment queue (publishes).Little's law: throughput ≈ concurrency ÷ latency. 200 slots at 50 ms serve about 4,000 req/s. | 2 replicas × 100 slots · timeout 1.0 s |
| Payment queueMessage queue | Webhook API publishes to it.Calls Payment worker.If a consumer does not finish within the visibility timeout, the message is delivered again. | – |
| Payment workerWorker | Payment queue consumes from it.Queues deliver at least once, so a worker can see the same message twice. | 64 concurrent jobs |