Designing a News Feed System for 500 Million Daily Active Users A complete, from-first-principles walkthrough of how to design a news feed — like the ones behind Facebook, Instagram, X (Twitter), and LinkedIn — that stays fast and fair even when some accounts have over 100 million followers. 01 ·…

System Design · Distributed Counters & Real-Time Fan-Out Designing a Real-Time Like & Share Counter for Millions of Concurrent Viewers How do apps like Instagram, YouTube, and X show a “like” count that updates in front of millions of eyeballs at once, without melting a database or lying to anyone…

Real-Time Inventory Synchronization Between E-Commerce and Physical Store POS Systems How Target, Walmart, and Zara make sure a shirt sold in a physical store at 2:14 PM disappears from the website by 2:14 PM and eleven seconds — without ever selling the same shirt twice. The full architecture, from event capture at…

Designing a Real-Time Credit Card Fraud Detection System (Sub-100ms Decisions) How to build a system that looks at a credit card swipe, scores it for fraud, and says APPROVE or DECLINE — all in less time than it takes you to blink — while processing millions of transactions per minute…

System Design · Messaging & Real-Time Designing Read Receipts Without Breaking Privacy or Performance Two grey ticks turn blue. That’s the entire user-visible feature. But behind those blue ticks sits a system that must track, for every message, exactly who has seen it, in real time, across one-on-one chats and…

Designing “People You May Know” at Social-Network Scale A complete, beginner-to-advanced walkthrough of building a personalized friend/connection recommendation system that works for hundreds of millions of users — covering the graph theory, the machine learning, the infrastructure, and the interview questions that come with it. 01 · Intro & History02…

System Design · Real-Time · E-Commerce Designing a Real-Time Personalized Email Trigger System for E-Commerce How to detect a user browsing a product category without buying — and automatically send them the right email, within seconds, at a scale of millions of behavioral events per minute. 20 chapters Kafka ·…

Designing a Real-Time Collaborative Story Writing Platform A complete, interview-focused architecture walkthrough for building a system where thousands of writers can co-author the same evolving story or fan-fiction universe — together, live, without stepping on each other’s sentences. 01 · Introduction02 · Problem03 · Concepts04 · Architecture05 · CRDT Engine06…

Designing a Real-Time Bidding System for a B2B Marketplace How do you let hundreds of buyers compete simultaneously for the same limited batch of supplier inventory, guarantee every bid is processed in a fair, strict order, and broadcast live updates to every participant within milliseconds — all without ever letting…

Designing a Privacy-Preserving Ad Targeting System How do you let an advertiser reach “people likely to be interested in hiking gear” without ever handing anyone a list of individuals, a raw browsing history, or a device identifier that can be traced back to a person? This tutorial builds that system…