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Learn · Practice · Coach, free to start
Learn the concepts, practise real problems step by step, and always know what to do next. From entry level to staff.
Free during the beta: all learning content, the coach and 4 guided sessions a month. Paid plans are coming soon.
Your answer · API design
A buyer can lose their seats while paying
Paying takes a minute or more, and here the seats are only claimed once the charge succeeds. Someone else can take them mid-checkout.
“Charge the card, then mark the seats sold.”Endpoint 2 · POST /bookings
Read:Learn: API contract design
Buyers can check where their order stands · credited as a valid alternative: polling endpoint 3 instead of a push confirmation
Next stop: revise endpoint 2then step 4
Designed by FAANG engineers
Learn the ideas, practise them on real problems, and let the coach plan the route to your interview date.
Every problem is a line of steps like this. A flaw that would sink you at your level is always a Blocking issue, however good the rest is.
What to build, how big it is and what’s out of scope, stated up front.
Endpoints, tables and request walkthroughs instead of an essay, plus a whiteboard.
A question on the weakest point of your answer, so you practise defending it.
A clear band, your own words as evidence, credit for valid alternatives. Then revise.
Would sink the answer at your level. Fix it first.
The core works. An important point is missing.
Stands up at your level.
Our limit, not yours. It doesn’t count against you. Check again for free.
One guided session, start to finish: a step answered and checked, a wrong answer caught, a design drawn, a deep dive, and the practice result. Narrated, with captions.
This is a guided session on SystemRound, designing a URL shortener. First, pick your level, from Entry to Staff: the route grows with it, and you’re graded against that level’s bar. At Mid-level, it’s seven steps: five must-pass, the parts an interviewer won’t let slide, and two optional. Each step has a task, your answer, and a Guide with what a strong answer covers.
Step three is the API. For each call: the request, the response, the errors. We’ve planted a common mistake: the short link returns a 301, a permanent redirect. Press Check. Before any feedback, you get one follow-up question, aimed at your weakest point: your chance to fix it yourself.
We’ll stand by our answer, and it’s flagged as wrong: a browser may keep a 301 for a long time, so later clicks never reach you, and never get counted. The feedback quotes your own words, names what’s missing, and says why interviewers care. Beside your answer is a strong one, with only the missing part marked. The full strong answer is one click away, but try fixing yours first.
Design steps are drawn. Add components, then draw each flow as numbered arrows, on the board or by picking From and To. The board is checked too: only drawn flows count. Here the redirect reads the cache, and the database only on a miss. Solid, first try, and the feedback points to the hop that earned it.
Steps five to seven are deep dives. After the high-level design, interviewers probe failure, scale and edge cases. That’s where they separate levels. Here, support deletes a scam link. Our answer deletes the row, but Redis keeps the redirect for a day, so the scam link keeps working. The follow-up goes straight to that gap. Then why interviewers care, and what a strong Mid-level answer does. At Senior and Staff, the route adds more deep dives: a viral link, abuse, and other regions.
We answer the other two deep dives the same way, and end. The practice result shows each step’s band, and a verdict on a four-point scale, from No hire to Hire. Here: Lean hire. What to fix first: step three’s 301 clashes with step six, so a deleted link could keep working. It’s practice under our rubric, never a prediction that you’ll pass.
Your dashboard picks your next step, and the Coach plans the rest.
Every lesson, the coach and 4 guided sessions a month, on any problem or variant. Paid plans are coming soon.
Still deciding? Read a lesson first. It’s free, and it’s the quickest way to see how we teach.
Yes, during the beta. You get all the learning content, the coach, and 4 guided sessions a month, on any problem or variant. Paid plans are coming soon; nothing is charged until they open.
Anyone preparing for a system design interview, from entry level to staff. You pick the level for each guided session: Entry level, Mid-level, Senior or Staff. The bar and the feedback change with it.
Against the problem’s rubric at the level you chose, and a rule decides each band. A flaw that would sink the answer at your level is a Blocking issue, whatever else was good. An important gap is Gaps to fix. Otherwise the step is Solid. A disqualifying finding is checked a second, independent time before it counts, and every must-fix quotes your own words.
Yes. Valid alternatives get credit. If your approach really meets the requirement, it counts, even when it isn’t the one in the sample answer.
Tell it your interview date and the level you’re aiming for, and it builds a plan to that date. After every guided session it gives you one clear next step, targeted exercises from what you missed, and spaced repeats, ideally on a different variant, so the fix sticks. Readiness is shown per skill and based on your practice. It is never a prediction that you will pass. The coach is on every plan, Free included. On Free, it plans around your 4 guided sessions a month and fills the weeks between them with lessons and exercises.
No. Grading every guided session has a real running cost, so the paid plans will cover the weeks you’re preparing: monthly, quarterly or yearly.
Read a lesson, try a guided step, see exactly what to fix. Free, with 4 guided sessions a month.
Lessons, patterns and technologies with diagrams, worked examples and the mistakes interviewers look for. Free on every plan.
Start from the full brief. Build each answer with endpoints, tables and a whiteboard. Get honest feedback on every step.
Classic problems: URL shortener, Rate limiter, Chat, News feed
A plan to your interview date, and one clear next step after every guided session. Missed skills come back later. On every plan, Free included.