Curveballs
Halfway through, the interviewer changes a number, pushes on a choice, or goes quiet. None of it is a new question. It asks whether your decisions were tied to requirements, and the answer has the same shape every time.
What is a curveball in a system design interview?
It is the interviewer changing something after you have committed: a number goes up a hundred times, a constraint is added, a decision is challenged, or the room goes quiet. Most are planned. The interviewer wants to see which decision moves when a requirement moves, and whether you can say why the rest of the design stays. A design where every box has a reason makes that easy to answer.
How should I answer "your traffic just went up 100 times"?
Redo the envelope at the new rate out loud and rank the boxes by headroom, meaning how many times the old load each can take before its queue grows. The stateless app tier usually breaks first and is fixed by adding boxes. The first stateful box, usually the database primary, is the one that matters: shard it, cache in front of it, or relax a requirement. Then name the next box that will break.
Should I change my design when the interviewer pushes back?
Only if a requirement moved. Grant the alternative its real strength, name the requirement that decided your choice, and say the condition that would flip it. If the interviewer supplies that condition, switch and say why. Switching the moment you hear doubt reads as having no reasons; refusing to move when a requirement changed reads as not listening.
What do I do when the interviewer goes silent?
Keep driving. Say where you are on the clock, offer two directions, pick the one tied to the riskiest requirement, and start. Silence is usually deliberate: the interviewer is watching whether you can steer without prompts. Waiting for permission spends minutes that the failure phase needs.
What if the interviewer asks about a technology I do not know well?
Say so in one sentence, then reason from the mechanism it has to use: a replicated log, a partitioned store, a consensus group. Ask which property the design needs from it, such as ordering or durability, and answer in terms of that property. Bluffing internals is the one move that loses more than admitting the gap.