DSA TrainerSystem Design · Unit 1
How system design interviews work
The interviewer says "design Twitter" and slides you a blank whiteboard. You have 45 minutes. There is no single correct answer, no test cases, nothing telling you where to start. For most people who are new to this, that blank page is the scary part, not any particular piece of technology.
Here is the reassuring truth: strong candidates are not smarter in the moment, they are just following a plan. They clarify what they are actually building, get a rough sense of how big it is, and then design it in a fixed order, talking the whole time. The interview is testing judgment and communication, not whether you memorized a reference architecture.
This unit teaches that plan. Every other unit in this track, caching, load balancing, databases, plugs into one of these steps. Learn the flow first and the rest has somewhere to go.
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Why it feels impossible: there's no single right answer
In an algorithm problem there is a correct output. In system design there is not. Two good engineers can design the same product differently and both pass, because what is being scored is how you reason: do you ask the right questions, make sensible tradeoffs, and explain why.
That is actually good news. You are not hunting for one hidden answer. You are demonstrating a thought process. Once you stop looking for "the" answer and start showing your reasoning, the blank page gets a lot smaller.
In a system design interview, what is the interviewer primarily evaluating?