How long should I actually spend on this?

About ten minutes of a 45 minute interview. Long enough to fix the feature set, the scale, the latency target and the read-write ratio, short enough to leave the bulk of the hour for design. If you are at fifteen minutes with no boxes drawn, state your assumptions and move.

The interviewer keeps saying "you decide". What do I do?

Decide, out loud, with a number. "I will assume 10 million daily actives, read-heavy at roughly 100 to 1" is the expected answer. The deflection is the test: they want to see whether you can commit to an assumption and design against it, not whether you can keep asking questions.

Is it a problem if my estimate is wrong?

Only if the order of magnitude is wrong. Being out by 40 percent changes nothing, because the decision the number drives is whether something fits on one machine or needs a fleet. State your assumptions as you go so the interviewer can correct an input rather than doubting the whole approach.

Fundamentalschapter 1 of 6 · 4 lessons

Scoping the Problem

The first ten minutes decide the interview, and they happen before you draw anything.

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Forty-five minutes on the clock. The interviewer says four words: design Twitter for us.

One candidate uncaps the marker. Load balancer, three app servers, a database, all inside ninety seconds. It looks decisive.

The other asks how many people use it, whether they can post video, and whether somebody seeing a post ten seconds late is a problem. Four minutes in, they have written on the board: 200 million daily users, text and images only, ten seconds of delay is fine.

The second candidate has drawn nothing and is already ahead. The first one is designing a system nobody described, and will spend forty minutes defending choices that were never asked for.

Lessons

4 in this chapter

The First Ten MinutesThe vagueness is the test, not an oversight.3 min
Functional RequirementsName two or three journeys, and say out loud what you are cutting.3 min
Turning Adjectives Into NumbersFast, big and reliable are not requirements. p99, DAU and nines are.3 min
Back of the EnvelopeThree multiplications that rule out most of the designs before you draw one.3 min