Practice for system design interviews

Think it through. Out loud.

A system design interview with a voice interviewer that follows your reasoning, not a script. Sketch on a shared canvas, pause for coaching when you're stuck, and read a Report against the bar for your level.

Private beta · 30 free minutes when you're in

Design a URL shortener

InterviewerSpeaking15:25

“Back to your 301. The same person clicks that link a second time: what does their browser do, and what does your service see?”

Fig. 1 An example session. Asked about the 301, the Candidate works out that the second click never reaches the service, and switches to a 302.

The code got automated. The thinking is yours.

What stays yours is judgment: deciding what to build and why, weighing the trade-offs, and explaining a design out loud when someone pushes back.

Think Aloud is where you practise it, as often as you need. It is practice, never help during a real interview.

It follows you

Start wherever you like. The next question comes from what you just said.

Nothing is read from a script. Two Candidates got the same brief and opened differently, and the interviewer followed each one.

The brief both Candidates heard“Design a URL shortener. Someone pastes a long link and gets a short one back; anyone who opens the short link lands on the original page.”

Opening A · starts from the click

“Okay, I'll start with the part that matters most, which is somebody clicking a short link. So there's a browser. It asks for our domain, slash, the code…”

The interviewer's next question“You expect the redirect side to get hammered. Before you size anything, what numbers would you want to know?”

Opening B · pins down scale first

“Before I draw anything, can I pin down what's in and out? … A hundred million over eighty-six thousand, about twelve hundred a second.”

1,200 links/s115k redirects/s230k at peak

The interviewer's next question“A hundred and eighty billion links alive, and the redirect carries the load. What does fast enough mean for someone clicking a short link?”

Thirty minutes, one real interview.

A desktop browser and a microphone. 30 or 45 minutes.

The brief
You talk and sketchafter the interviewer reads the brief
Coachingif you pause
Back to the interviewsame canvas, same clock
You wrap upthe last four minutes

Your Reporta few minutes later

Talk it through

Ask for numbers, weigh the trade-offs, change your mind out loud. The interviewer hears you and asks about the part you skipped.

Sketch as you go

A shared canvas the interviewer reads while you draw. It can point at a box you made and ask what happens there.

Read your Report

What you showed against the bar for your level, each point with the words you said.

Coaching

Stuck? Pause. Learn it. Pick up where you left off.

Pause mid-interview and a coach walks you through the part you're missing. Then you go back to the same interviewer, the same canvas, and the clock. Pauses can use up to a fifth of your time.

Interview pausedCoaching · 5:58 leftResume

Coach

“You already said every click goes to the database. Don't stop at slower: think about how much the database was sized for, and what it gets in that minute.”

Report · Attempt 1 · Mid barDesign a URL shortener

Framed before designingShown

Sized with numbersShown

Laid out both paths end to endShown

Chose the record, its key and the callsShown

Separated the redirect path from creationShown

Designed for failurePartly

Owned trade-offsShown

Explained how the mechanism worksShown

Canvas qualityShown

Sized with numbersShown

Does arithmetic out loud from a given or assumed number to a new figure the interviewer did not read out, and says its unit.

“Redirects, ten billion over eighty-six thousand, roughly a hundred and fifteen thousand a second.”In context

The Report

No score. No verdict. Your own words, against a written bar.

Every point quotes what you said, and one click shows the moment you said it. What the interview never reached is marked as not judged, not held against you.

The bars

What does good look like at your level?

Each part of the interview has a written bar, and it rises with the level. Your Report reads you against the bar you chose. Here is one of them: estimation.

See all twelve bars →

JuniorTurns one given number into a new figure, out loud, and uses a number to justify one choice.
Mid+ Says the unit, and uses a figure they worked out to size a named component.
Senior+ Sizes both sides: a figure for the writes and a figure for the reads.
Staff+ Names the figure that limits the design, and says why.

Interviews you can practise today

System design · 30 min · Junior, Mid

Pricing

Free while we're in private beta.

After that, you pay per minute. Prices are announced before payments open.

Minutes never expire.
A session that fails costs nothing.
Every account starts with 30 free minutes.

Questions

Will it understand my accent?: It transcribes what it hears, and your Report quotes you word for word so you can check it. The Report judges what you reason and draw, not how you sound.
Is my voice recorded?: No. Your voice reaches the interviewer while the session runs and isn't kept. The transcript and your canvas are kept to write your Report, and only you can open them. Privacy Policy
How is it different from a mock with a person?: A person brings judgment of their own. Think Aloud brings the repetition: the same written bar every time, any evening, as often as you need.
Can I use it during a real interview?: No. It's a place to practise, and nothing in it listens in on a real interview.
What do I need?: A desktop browser and a microphone. Sessions last 30 or 45 minutes, and your Reports read fine on a phone.
Can my company use it?: Not yet. An assessment mode is on the way: the same interview, with coaching off and the Report sent to the employer.

Practise the part that's yours.

Start with the URL shortener. Thirty minutes, out loud.