System design / 14 simulations
System Design Interview Questions
Classic "design X" interview questions, each one a simulation you can load, break and tune: WhatsApp, YouTube, Uber, Twitter and more.
Open the lab → Change the load, break a part, watch what happens. Free, no sign-up.
All topics: design interviews
- Design a URL shortenerBeginner · 7 min
Counter or random codes, collisions, a cache for hot links, and why 301 vs 302 changes your traffic and your analytics.
- Design a job schedulerIntermediate · 6 min
Workers claim jobs with leases. Crash one and the job is retried; make the lease too short and jobs run twice.
- Design a notification systemIntermediate · 6 min
Fan events out to push, SMS and email queues. Take the push provider down, fall back to SMS, and cap messages per user.
- Design WhatsApp: ticks, offline inboxes and retriesIntermediate · 6 min
Sent, delivered and read ticks from acks. Take Bob offline to fill his server inbox; drop acks and watch message ids stop duplicates.
- Design Instagram: photos through a CDNIntermediate · 5 min
Popular photos served from edge caches in each city. Turn the CDN off or send 4 MB originals and watch load time and origin traffic.
- Design YouTube: upload, transcode, publishIntermediate · 6 min
A worker pool turns uploads into 240p to 1080p. Split videos into segments, publish low quality first, and match workers to the upload rate.
- Design Netflix: adaptive bitrate streamingIntermediate · 5 min
The player picks 240p to 4K for every 4-second segment. Ride a train through tunnels and compare fixed, speed-based and buffer-based choices.
- Design Uber: finding the nearest driverIntermediate · 6 min
Drivers report locations; riders need the closest one fast. Compare scanning every driver with searching geo cells, and see stale locations.
- Design Google Maps: routes and live trafficIntermediate · 6 min
Roads as a weighted graph. Watch Dijkstra and A* explore the city, then ignore live traffic and see routes drive into a jam.
- Design Twitter/X: fan-out on write or on readAdvanced · 6 min
Copy each post into followers’ timelines, or merge at read time. One celebrity post floods the queue; the hybrid fixes it.
- Design Dropbox: chunks, dedupe and syncIntermediate · 5 min
Files as 4 MB chunks named by hash. Edit one slide and only one chunk moves; files the server already has upload instantly.
- Design a shopping cart: where it lives, what checkout checksBeginner · 5 min
Browser, cache with a TTL, or database: which carts survive a device switch or a long weekend? Then see why checkout must re-check price and stock.
- Design ticket booking: two people, one seatIntermediate · 6 min
Fans race for the centre seats. No lock sells seats twice, checking at payment means refunds, and seat holds with a timer fix both.
- Design a logging system: ship, buffer, index, searchIntermediate · 5 min
Agents ship log lines to an indexer. Start an incident and see errors lost without a Kafka buffer, and DEBUG logs flood storage and search.