Method challenge: estimate, then design
No requests-per-second figure this time: work it out. A social app has 2.16 million daily users. Each loads 60 pages and posts 3 updates a day, and the evening peak is five times the day's average rate. Pages are personalized, so a CDN cannot cache them, and posts stay in a relational database. Serve the peak with p99 under 200 ms, survive the loss of any instance, and spend at most 2,050 USD a month.
Traffic demand7,875 RPS95% reads, 5% writes
p99 latency target≤ 200 msSlowest 1% of requests
Availability≥ 99.9%N-1 failure survival (0 drops)
Monthly cloud budget≤ 2,050 USDA month, everything included