How a system design interview works
Forty-five minutes, a vague question, and someone scoring how you think.
Someone says "design a system for booking cinema seats". Then they go quiet. That is the whole question. It is vague on purpose. The first thing they test is whether you make the question clear before you start answering it.
In the room, you talk for most of the time. You draw boxes on a whiteboard or a shared doc. The interviewer stops you with questions. They are not waiting for the end to grade your drawing. They score the conversation while it happens.
A common way to fail has nothing to do with knowledge. It is drawing too early. Someone hears "cinema seats", feels a little panic, and starts drawing a load balancer and a database. They have not asked how many users there are, whether seats have numbers, or what happens when two people pick the same seat. Every box after that is a guess, and the interviewer can tell.
Another is going silent. If you think for ninety seconds without speaking, the interviewer learns nothing about you. Thinking out loud is not a bonus here. It is what is being scored. A slightly weaker design that you explain clearly beats a better design you built in silence.
A third is answering a different question. If they ask what happens when a whole region goes down, and you talk about your cache, you have not answered. They notice.
The first 17 minutes happen before you draw the real design. People who skip them spend the last 14 defending boxes they cannot explain.
- The question is vague on purpose. Making it clear is the first thing they score.
- You talk for most of the time. Silent thinking teaches the interviewer nothing.
- Drawing too early is a very common failure, and it is not about knowledge.
- They interrupt on purpose. Being interrupted is normal, not a bad sign.