Design Uber
A worked system design answer for Uber: geohashing nearby drivers fast, and an atomic conditional update that prevents double-booking the same driver.
Design WhatsApp
A worked system design answer for WhatsApp: persistent connections, exactly-once delivery to an offline recipient, and ordering across multiple devices.
Design Twitter
A worked system design answer for a Twitter-like feed: fan-out on write vs fan-out on read, and the hybrid that solves the celebrity-follower problem.
Design Instagram
A worked system design answer for Instagram: async media processing, a CDN-first read path, and keeping feed fan-out cheap despite megabyte-sized posts.
Design a Web Crawler
A worked system design answer for a web crawler: a Bloom filter for 'already seen' at billion-URL scale, and per-domain queues that enforce politeness.
Design an E-commerce Checkout System
A worked system design answer for e-commerce checkout: reserving inventory before charging, idempotent payments, and recovering with the Saga pattern.