Problems and cost lab
Practise on real problems
14 system design problems, a load test of your own design, a free cloud bill lab and a queue of your own mistakes.
The cost lab is free and needs no sign-up.
14 problems
From a URL shortener to ticket booking
Classic questions, and the ones Indian engineers meet at work: UPI payments, a 10 am train booking rush, a flash sale. Each one is a mock interview with an AI interviewer.
- 01Design a URL Shortener (like bit.ly)Start hereProve itMedium45 min
- 02Design an API Rate LimiterMedium45 min
- 03Design a Notification SystemProve itMedium45 min
- 04Design a Social Media News FeedProve itMedium45 min
- 05Design a Background Job QueueReal-worldMedium45 min
- 06Design a Webhook Delivery SystemReal-worldMedium45 min
- 07Design a Photo and Video Upload PipelineReal-worldMedium45 min
- 08Design Search Autocomplete (Typeahead)Real-worldProve itMedium45 min
- 09Design a Chat and Messaging AppProve itHard45 min
- 10Design a Distributed Key-Value StoreHard45 min
- 11Design a Payment System (UPI and Cards)Real-worldProve itHard45 min
- 12Design a Ticket Booking System (Movies and Trains)Real-worldProve itHard45 min
- 13Design a Flash Sale SystemReal-worldProve itHard45 min
- 14Design Nearby Drivers and ETA for a Ride-Hailing AppReal-worldHard45 min
Opening a problem needs a free account. 8 problems have a Prove it load test inside the mock.
Start with the first oneProve it
Say what your design can hold. Then run the test
In a mock, Prove it turns the design you drew into a load test with the problem’s own numbers. This is the real test, on the URL shortener. It starts with a design that breaks. Can you fix it?
About 40 new links and 390 redirects a second on average; peak about 1,200 redirects a second (100 million links and 1 billion redirects a month).
Today’s peak, rounded up: 1,250 redirects a second
Broke.
It broke at between 750 and 1,031 redirects a second. The database ran out first: responses got too slow (p95 over 500 ms).
- p95 time
- over 10 s
- Busiest part
- database 148%
- Cost a month
- $108 of $150
- Cache hit rate
- no cache
ThinkAt peak, reads are about 30 times the average writes, and the database answers every one. What could answer a repeat read without it?
The same load-test code that runs inside a mock. It is a model, so read the result as a check of your design’s shape, not a benchmark.
No sign-up needed to try it here. Inside a mock, the interviewer reads your result and asks about it.
Cost lab
What does your design cost a month?
Interviewers ask. Build a bill for 8 real-looking workloads, read it line by line, and bring it under the budget.
- Free, and it runs in your browser. No AI, no sign-up.
- Every line shows its sum, like “4 servers x price x hours”, so you can say it out loud.
- Defend your bill. When you solve one, the interviewer asks 3 questions about your own design. The right answers come from the bill, not from a guess.
List prices, checked 2026-10-19, may change. They are not a quote: taxes (GST), support plans, free tiers and discounts are left out. Region: AWS ap-south-1 (Mumbai).
- Compute$3,539
- Network$1,483
- Database$1,418
- Monitoring$848
- Storage$70
- Handles the peak, passes
- Availability: 99.95% (survives a zone loss), passes
- Latency: p99 under 300 ms, passes
- Logs kept at least 30 days, passes
- The choices fit the workload, passes
- Within budget ($2,575 a month), fails
It is $4,782 over budget. Find the cuts without breaking the checks.
- Food delivery in 3 Indian citiesCore
- UPI-style payments APIStretch
- IPL live score and match metadataSenior
- E-commerce sale monthStretch
- B2B SaaS analytics dashboardCore
- Photo-sharing appSenior
- Notification serviceCore
- AI chatbot feature (LLM token costs)Stretch
Review
Learn from your own mistakes
Each card is a point you missed in your own report. If you get it, it comes back later. If not, it comes back tomorrow.
- Short. At most 5 cards a day, so it stays a few minutes.
- Your words. The card quotes what you said, then shows what a stronger answer adds.
- It grows. After each right answer the wait is longer: 1, 3, 7, 16, 35 days.
More ways to practise
Two rounds that live elsewhere
What it costs
What is free, and what is paid
Free, no account: the cost lab, and the Load simulator (each level opens after the one before, or press Try anyway). Free with an account: Review, and your first AI system design session, with Prove it inside. Paid: more sessions, with 3 Prove it runs each. It is one payment and nothing renews. See pricing.
Free to read
Lessons behind this page
Worked case studies and number drills for the problems on this page.
Questions
Before you start
Do I need an account to open a problem?
Yes, a free one. Opening a problem starts a mock interview. The cost lab does not need an account.
Is Prove it the same as the Load simulator?
It uses the same engine, but it asks a different question. The simulator teaches you level by level. Prove it takes the design you drew in a mock and tests it against that problem’s own numbers. You get 3 runs in each session.
Are the cost lab prices real?
They are list prices for one region (AWS Mumbai), checked on a date the lab shows. They are not a quote: GST, support plans and discounts are left out. Use them to learn how a bill is built.
When does a review card come back?
If you get it, the wait grows: 1, 3, 7, 16, 35 days. If you do not, it comes back the next day. You see at most 5 cards a day.
Start with what is free
Open the cost lab now, no sign-up. Or start today’s Drill with a free account.