Design it like you'll be on call for it
Each challenge gives you a product, requirements and a scale. Build the architecture in the lab, simulate it at the target traffic and submit it for review. The review tells you what's missing and asks the questions a senior architect would ask. It doesn't hand you the answer: reference notes unlock only after your first review.
- System design
Design a URL shortener
Marketing teams paste a long URL and get back a 7-character link like sho.rt/aZ3kQ9x. Anyone who clicks it is redirected within milliseconds. Links are printed on posters and shared in emails, so they must keep working for years. A handful of viral links get the vast majority of clicks.
New links: 100M / monthRead:write ratio: ≈ 100 : 14k rps - System design
Design a hotel booking engine
Travellers search for hotels by city and dates, compare prices, and book a room with a card. Hotels update availability and prices throughout the day. The worst possible bug is selling the same room twice; the second worst is a confirmation email that never arrives.
Hotels: 2M propertiesSearches: 50M / day2k rps - System design
Design a tutoring marketplace (Preply-like)
Learners find a tutor, watch their intro video, buy a package of lessons and book slots in the tutor's calendar. Tutors get paid after each lesson. Lessons happen over a third-party video provider. Everyone gets reminders before a lesson. The team is 6 engineers.
Learners: 300k activeTutors: 20k300 rps - System design
Design a video platform
Creators upload videos; the platform converts them into several resolutions and makes them watchable worldwide with smooth playback. Viewers browse a home feed, search, and watch. Uploads can be several gigabytes; a popular video can be watched by millions of people in its first hour.
Daily viewers: 50MUploads: 500 hours of video / minute50k rps - System design
Design a chat app (WhatsApp-lite)
Build the backend for a mobile messaging app used by 1 billion people. Users send 1:1 and small-group messages, see whether contacts are online, share photos and videos, and get a push notification when a message arrives while the app is closed. Messages must never be lost, and must arrive in order within a conversation.
Users: 1B (500M daily active)Messages: ≈ 100B / day150k rps - AI architecture
Design an AI customer support system
An online retailer wants an AI assistant that answers customers 24/7. It must answer from the company's help centre and policies, look up the customer's own orders, issue refunds within policy, log every conversation in the CRM, and hand over to a human agent when it is unsure or the customer asks. A wrong refund costs money; leaking another customer's order is a disaster.
Customers: 2MConversations: 20k / day15 rps - AI architecture
Design an AI hotel concierge
A hotel group wants a concierge in each guest's phone. Guests ask about breakfast hours, the spa, local restaurants; they order room service, book a spa slot or ask for extra towels. Each hotel has its own information and services. Requests that need staff must reach the right team quickly, and the concierge must never act on another room's behalf.
Hotels: 200Guests: 40k / day8 rps - AI architecture
Build an AI hotel operating system
Architect mode. Design the platform that runs 5,000 hotels: reservations, check-in, payments, housekeeping, maintenance, dynamic pricing and guest messaging, with AI agents working alongside staff. Every operational event (a check-out, a broken AC, a booking spike) should be visible to every part of the system that cares. You decide the boundaries, the data flows and where AI is allowed to act on its own.
Hotels: 5,000Rooms: 1M3k rps