Black Friday – senior-level system design challenge

Traffic is 12,000 RPS and the CFO capped infra at $3,000/month. Keep p99 under 300ms and errors under 1%.

Requirements

• Peak traffic: 12,000 req/s
• Reads : writes: 49 : 1
• p99 latency: ≤ 300 ms
• Errors: ≤ 1%
• Budget: ≤ $3,000/month at peak

Goal

Hold p99 latency under 300ms, error rate under 1%, cost under $3,000/month through every stage of the test plan, then compare your design with a reference design.

The test plan

1. Normal day: 4,000 requests per second.
2. Black Friday: 12,000 requests per second.

Hint

Brute-force scaling busts the budget: the database is the most expensive tier to grow. A cache absorbs reads before they reach it. Price a CDN before adding one - at this request rate it costs more than everything else combined.

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

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