System Design Card 380 — API Design / Check
Concern
APIs reveal system boundaries, product semantics, idempotency, and the shape of client interaction. A create-notification endpoint should expose enough information about delivery state and retries to support real clients.
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 API Design, 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 staying at box level and never specifying how callers actually use the system. 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 API Design change the architecture, the trade-offs, or the review checklist?”