System Design Card 369 — Capacity Estimation / Implement
Concern
Rough numbers create the pressure model that justifies caches, queues, partitioning, or simpler choices. Daily active users, QPS, object size, retention, and peak bursts all shape what 'reasonable' means.
What Implement means for this concern
In BASIC, the Implement step is where you walk the design into existence in a controlled order, deepening the risky parts first. For Capacity Estimation, that means the candidate should make this concern visible at the right moment instead of bolting it on at the end.
Design move
A good move is to transcribe the plan instead of improvising. Tie the concern back to the user flow, the workload, and the dominant trade-off. That keeps the design grounded and makes it easier for the interviewer to follow why a cache, queue, replica, partition, or rate limiter is actually necessary.
Common miss
The miss is naming databases and caches without a load model. BASIC helps because the staged flow keeps this concern proportional to the prompt and connected to the rest of the architecture.
BASIC prompt
“When I reach the Implement stage, how does Capacity Estimation change the architecture, the trade-offs, or the review checklist?”