Problems / Resilience, medium

Checkout: retries into an outage

Checkout calls one payment provider. The API retries failed calls 3 times immediately. At 10 s the provider is down for 15 seconds, and the retries triple the pile-up.

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Your goals

Success ≥ 99% through the outage, p99 ≤ 3 s.

Test cases

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.

  1. As given. The problem’s own traffic, scheduled failures and seed.
  2. Peak hour. 25% more traffic than the problem states, with the same failures.
  3. A slow dependency. The main data store or model answers twice as slowly for part of the run.

The traffic

300 requests a second for 30 seconds: 90% reads, 10% writes.

The system

What each component does in this design, and what calls it. The simulator mimics each one from these settings.

ComponentIts job hereSettings
Load balancerLoad balancerTraffic enters here.Calls API.Layer 4 balancers route by IP and port; layer 7 balancers read HTTP and can route by path or header.–
APIApp serviceLoad balancer forwards to it.Calls Postgres, Payment API (provider A).Little's law: throughput ≈ concurrency ÷ latency. 200 slots at 50 ms serve about 4,000 req/s.3 replicas × 200 slots · timeout 3.0 s
PostgresSQL databaseAPI calls it.Postgres runs one process per connection, which is why poolers like PgBouncer sit in front of busy databases.8 vCPU · 60 connections
Payment API (provider A)External API (mock)API calls it.Third-party APIs have their own rate limits and outages that you cannot fix.400 slots · 60,000 calls/min

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