How do you do capacity planning for a service?

Summary

Forecast demand (organic growth, launches, seasonal peaks), measure how much load each instance or resource can handle through load testing, and provision for peak plus a safety margin and the loss of a failure domain (N+1 or N+2, zone loss). Revisit regularly and automate scaling where possible.

Longer Answer

Steps: know the service's bottleneck resource and its saturation point (load tests, production measurements); model demand per business driver (requests per active user); plan for redundancy (survive losing a zone at peak); include lead times for things that cannot autoscale (database instances, quotas, reserved capacity, hardware). Watch cost: overprovisioning is also a failure. Track forecast accuracy and adjust.

Key Points

• Forecast demand including peaks and launches.
• Measure per-unit capacity with load tests.
• Plan N+1 / zone loss.
• Account for lead times and quotas.