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System Design Interviews

This section does not teach answers. It teaches how to run the conversation.

The problem this section addresses​

The system design interview evaluates something specific: how you reason under ambiguity, with incomplete information and little time. The prompt is vague on purpose — "design Twitter" — because the first thing evaluated is whether you notice that it is vague.

The most common mistake is not technical. It is starting to draw. Whoever draws first is answering a problem they invented, and the interviewer sees that immediately.

The second mistake is the opposite of what traditional preparation produces: candidates who memorized a reference architecture and recite it regardless of the prompt. It works until the first follow-up question.

What you will find here​

Structure of the conversation. How to distribute time between clarification, estimation, design and deep dive. Having structure is half the evaluation — it shows you have done this before.

Clarification. What questions to ask and in what order. Separating functional from non-functional requirements out loud.

Estimation. Back-of-the-envelope calculations: volume, storage, bandwidth, connections. Not to get the number right, but so that the architecture has a declared scale — without it, every decision has no criterion.

Design. API design, data modeling and high-level architecture.

Deep dive. Bottleneck identification, scaling and failure handling. This is where the interview actually separates candidates.

Communication. How to state a trade-off out loud while drawing. This is the most valued competency and the least trained.

Common mistakes. The patterns that make interviews go wrong, with what to do instead.

The order this section trains​

Problem → Requirements → Questions to Ask → Capacity Estimates
→ Possible Architectures → Trade-offs → Recommended Approach

Note that Questions to Ask comes before any architecture. It is the order of the real interview, and it is the habit this section trains.

Each document carries an Interview Example — the exchange with the interviewer, with the follow-up questions they would ask. The long exercises of the path live in the other sections; here the training is the order, not the prompt.

A note on preparation​

Memorizing reference architectures is the most popular way to prepare and one of the least effective. It works while the prompt matches what was memorized, and collapses at the first variation — which the interviewer will introduce precisely to test that.

What transfers is the method: clarify, estimate, decompose, identify the bottleneck, state the trade-off. That method works on any prompt, including ones you have never seen.

By the end​

You run the conversation instead of reacting to it. You ask the right questions before drawing. You state assumptions out loud, which lets the interviewer correct course early.

And you can say "I would choose X, but if the consistency requirement were different, I would choose Y" — which is exactly what the interview is looking for.

Case Studies for the version with no time pressure, and Trade-offs for the argumentation material.