Headroom, Growth, and Capacity Safety Margins
Capacity planning should account for expected growth, peak traffic, failover, and uneven distribution. If a cluster needs 8 nodes at normal peak, ask whether it can tolerate one node failing and still meet its latency target. Capacity that exists only when every component is healthy is fragile.
Use a target utilization below 100% so the system has room for bursts, garbage collection, maintenance, and rebalancing. The appropriate threshold depends on workload: CPU-bound stateless APIs and latency-sensitive databases have different safe operating ranges.
Plan with scenarios: current load, expected growth, a promotional spike, loss of one availability zone, and recovery after an outage. Capacity planning is iterative; monitor real demand and update estimates rather than treating an early spreadsheet as truth.
Peak factors, replication factors, and failover headroom represent different effects. State what each factor means to avoid double-counting or understating capacity.
- Plan for component loss, not just healthy-state traffic.
- Keep measurable headroom for spikes and maintenance.
- Revisit estimates when real telemetry becomes available.