System Design Card 450 — Cost Efficiency / Check
Concern
Reasonable designs are not only fast and reliable; they are proportionate to the business value and expected usage. Precomputing everything, storing everything forever, or over-replicating can turn a technically plausible design into an economically poor one.
What Check means for this concern
In BASIC, the Check step is where you review the design for bottlenecks, failure modes, security gaps, observability, and cost. For Cost Efficiency, 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 review and stress-test before you hand the answer over. 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 optimizing for elegance or scale mythology instead of realistic cost trade-offs. 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 Check stage, how does Cost Efficiency change the architecture, the trade-offs, or the review checklist?”