Almost nobody fails a system design round for choosing the wrong database. They fail for designing in silence, assuming requirements, or presenting one architecture as though alternatives did not exist.
The interviewer is watching how you think when the problem is deliberately underspecified — because that is what the job is.
“Design Twitter” is not a question, it is an invitation to ask questions. How many users, read-heavy or write-heavy, what latency matters, what can be eventually consistent.
Five minutes of clarifying is not lost time — it is the first thing being scored. A candidate who starts drawing boxes immediately has told the interviewer they would do the same thing on a real project.
Write the constraints down where both of you can see them, then design against those numbers rather than against a general idea of scale.
Every meaningful decision in the round has a cost. Caching buys latency and pays in staleness. Sharding buys write throughput and pays in cross-shard queries and operational pain.
Say the cost out loud. “I would put a cache here, which means accepting up to sixty seconds of staleness — fine for a feed, not fine for a balance” is the sentence the round exists to hear.
When you do not know something, say so and reason from what you do know. Bluffing is the fastest way to lose a technical interviewer, and they will follow up on exactly the thing you were vaguest about.
Senior candidates are separated from mid-level ones here. What happens when this queue backs up, this region goes down, this dependency starts returning 500s at ten percent?
Nobody expects a complete answer. Raising the question unprompted is most of the mark.
Think out loud. An unspoken thought scores zero — the interviewer can only mark what they hear.
Justify every component from the constraints you established. Unjustified complexity reads as inexperience, not seniority.
Name the alternative you rejected and why. That comparison is the clearest evidence of judgment you can give.
System design, trade-offs, and war stories from production. You will be scored on technical communication, engineering depth, ownership & collaboration, with the transcript attached.
The first question in almost every interview, and the one most people lose the room on. Here is the structure that works and the timing that matters.
What behavioral rounds are actually testing, how to structure an answer with STAR, and the questions worth preparing stories for.