Practice series
Payments, ledgers & idempotency
Money movement, double-entry ledgers, retries, reconciliation, and billing-safe writes.
- 01Free
Ad Click Aggregator
Design an ad click aggregation system that ingests millions of click events per second and serves near-real-time plus historical aggregate metrics to advertiser...
Meta - 02Preview
Coupons and Deals Platform
Design a coupons and deals platform where users browse and search a large, frequently changing catalog of deals and redeem coupon codes exactly once per limit,...
Practice bank - 03Preview
Donation Platform
Design a donation platform like GoFundMe — a campaign can go viral overnight and receive a massive spike of small donations, and every dollar must be captured,...
Practice bank - 04Free
Facebook Messenger
Design Facebook Messenger — a real-time chat system delivering messages with low latency, ordering guarantees, and multi-device sync at massive scale.
Meta - 05Preview
Payment System
Design a payment system for an e-commerce product — money must move correctly between buyer, platform, and seller even when networks fail mid-transaction, and e...
Stripe - 06Preview
Robinhood (Stock Trading)
Design a retail stock trading platform like Robinhood that lets millions of users place buy/sell orders, tracks cash and positions with regulatory-grade accurac...
Robinhood - 07Preview
Spend Management System
Design a corporate spend management platform like Brex or Ramp — every card swipe must be authorized or declined against a live policy engine in under two secon...
Brex - 08Preview
Stripe
Design Stripe itself — a payments platform that other companies build on top of, exposing charge/webhook APIs to thousands of merchants while routing transactio...
Stripe - 09Free
Ticketmaster
Design Ticketmaster — an event ticketing system that sells reserved seats under extreme flash-sale contention without ever double-selling a seat.
Ticketmaster - 10Preview
Webhook
Design a webhook delivery platform (Stripe-webhooks-style) that reliably delivers event notifications to thousands of third-party consumer endpoints under retry...
Stripe - 11Preview
YouTube
Design YouTube’s upload and processing pipeline — accept massive volumes of user-generated video uploads and transform them into a playable, multi-resolution ca...
YouTube - 12Preview
Notification System
Design a multi-channel notification system that sends email, push, SMS, and in-app notifications on behalf of many internal services, respecting user preference...
Uber - 13Preview
Fraud Detection
Design a real-time ML fraud/risk scoring system that scores payment transactions in under 100 ms.
Stripe - 14Preview
IoT Device Platform
Design a platform managing ~10 million connected devices over MQTT: connection lifecycle and disconnect detection, telemetry ingest at fleet scale, and — the si...
Practice bank - 15Preview
Stock Exchange
Design the core trading system of a stock exchange: accept orders into a central limit order book, match at price-time priority, and when your own book cannot c...
Practice bank