System Design Card 374 — Traffic Patterns / Implement
Concern
The read/write shape, burstiness, and fanout pattern determine the hot path and bottlenecks. A feed-read service with heavy fanout differs from a mostly write-once audit log.
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 Traffic Patterns, 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 designing a generic system that ignores where the load actually concentrates. 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 Traffic Patterns change the architecture, the trade-offs, or the review checklist?”