Why Most System Design Interviews Are Actually Testing Trade-off Reasoning

Most candidates walk into a system design interview trying to recall a diagram they memorized. That's the wrong instinct. What interviewers are actually evaluating is whether you can reason about trade-offs in real time - consistency versus availability, latency versus cost, simplicity versus scale.

Start every answer by clarifying requirements: read-heavy or write-heavy? What's the acceptable staleness window? Then build up the design incrementally, narrating the trade-off at each decision point.