System Design Interview Questionsas rounds you can run
Reading a system design answer is not the same as giving one. Here every question is a round: draw your design on a board, send the problem’s real traffic through it, and watch which box breaks. Then an AI interviewer reviews your board and asks the follow-ups, from fresher to senior.
- rounds
- 8rounds
- follow-up questions
- 32follow-up questions
- load test, no sign-in
- Freeload test, no sign-in
- Users74k/s at 10:00
- Load balancer37% · 74k/s of 200k
- Booking serviceOverloaded · 79k/s of 5k
- PostgresOverloaded · 67k reads/s of 10k
What fixes it on the board: cache the availability, keep seat counts in Redis, and put a waiting room in front of booking.
How a round works
- 1Do the numbersRead the brief and estimate the load, the way interviews start.
- 2Draw the designBoxes for CDNs, services, caches, queues and databases, joined by arrows.
- 3Run the load testSend the real traffic through it, then fail a machine and see what holds.
- 4Defend itAn AI review of your board, then the follow-ups, Fresher to Senior.
Apps you use every day
Classic problems in familiar clothes: a ticket rush, payments, live video, live location, a flash sale. Every number is an assumption for the exercise, not the company’s.
- IRCTC Tatkal bookingStart here2 million people in five minutes, mostly for the same 10 trains.Waiting roomHot rowsPayment timeouts4 follow-ups, Fresher to Senior
- UPI payments10,000 payments a second, and a bank that may never reply.IdempotencyPending statesSharded ledger4 follow-ups, Fresher to Senior
- JioHotstar live IPL50 million viewers, and a wicket that brings them all back at once.Multi-CDNSpikesLive score4 follow-ups, Fresher to Senior
- Swiggy live tracking125,000 location updates a second, each for one customer.Write-heavyWebSocketsGeo search4 follow-ups, Fresher to Senior
- Flipkart flash sale1,000 phones, a million buyers, one stock count.Hot keysAtomic stockReservations4 follow-ups, Fresher to Senior
The classics
The three that come up most, and the usual place to start.
- URL shortener116,000 redirects a second, and every click counted.CachingCDNAsync events4 follow-ups, Fresher to Senior
- Rate limiter200,000 requests a second across 20 servers.Shared countersBurstsFail open or closed4 follow-ups, Fresher to Senior
- Notification system20,000 events a second, and an SMS provider that takes 2,000.Queue per channelProvider limitsExpiry4 follow-ups, Fresher to Senior
