System Design Card 394 — Caching / Implement
Concern
Caching can remove load from the hot path, but only if keys, invalidation, and staleness are acceptable. Caching short-link lookups or popular feed fragments can change capacity needs dramatically.
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 Caching, 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 adding a cache as decoration without clarifying what is cached and how it expires. 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 Caching change the architecture, the trade-offs, or the review checklist?”