The Retry Storm – senior-level system design challenge

Your database is about to slow down by 400 ms. Your clients retry failures up to three times. Keep the API standing.

Requirements

• Traffic: 3,000 req/s
• Clients: retry up to 3 times, 1 s timeout
• p99 latency: ≤ 1 s
• Errors: ≤ 0.5%
• Must survive: the database slowing down by 400 ms

Goal

Hold p99 latency under 1,000ms, error rate under 0.5% through every stage of the test plan, then compare your design with a reference design.

The test plan

1. Normal: 3,000 requests per second.
2. Database slows down: 3,000 requests per second.

Hint

A slow database doesn't use API CPU - it holds API threads while requests wait. Once threads run out, requests fail, clients retry, and the retries add even more load. Stop requests waiting on the database: a cache answers most of them without touching it.

Solve it on the live simulator, free and in your browser.

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