How do you do capacity planning for a growing service?

What the interviewer wants to hear

• Forecast demand from historical growth and business events
• Measure capacity per instance with load testing
• Plan for N+1 or N+2 redundancy and peak traffic
• Account for lead time and cost
• Use autoscaling but know its limits, review regularly

Model answer

I start with demand: historical traffic trends plus known events such as sales, launches or festival seasons, and I plan for peak, not average. Then I measure supply by load testing to find how much traffic one instance can handle while still meeting the latency SLO, and which resource becomes the bottleneck first, often a database or a downstream service. From that I calculate instances needed at peak and add headroom, plus N+1 or N+2 so we survive losing a zone. I consider lead times for things like quotas, reserved capacity or database scaling, and I balance cost. Autoscaling helps with daily patterns but has limits like warm-up time, so I review the plan every quarter.

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