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Designing an Idempotent Payment Processing System How to guarantee a transaction is charged exactly once, even when a client retries after a network timeout — at a scale of millions of requests per minute. A ground-up walkthrough of the client-generated idempotency key, the layered defences behind it, and every trade-off,…
Designing a Hashtag Trending System How do platforms like X, Instagram, and Threads figure out — in near real-time, across hundreds of millions of posts a day — which topics are suddenly “hot”? This is a complete, ground-up walkthrough of building a trending-topics engine: the math, the data structures, the…
Designing a Multi-Channel Gift Card Marketplace System A complete, ground-up walkthrough of how to design a gift card platform that can issue cards, track balances accurately, and support redemption across web, mobile, retail point-of-sale, call centers, and third-party marketplace partners — without ever losing a cent, and without ever letting…
Designing a Scalable Communities & Groups Platform How to architect a “Groups” or “Communities” feature — like Facebook Groups, Reddit subreddits, or Slack/Discord servers — that can host millions of members per community, serve a fast personalized internal feed, enforce configurable rules, and support moderation at scale, all without falling…
System Design · Trust & Safety Designing a Fake Account & Bot-Driven Engagement Detection System Behind every platform’s follower count, comment section, and trending list there is a quieter number nobody sees: how many of those accounts actually belong to a real person. This tutorial builds the system that answers…
Designing a “For You” Algorithmic Feed for a Billion Users A ground-up, production-grade walkthrough of how platforms like TikTok, Instagram, and YouTube build ranking systems that juggle three competing forces at once — freshness, relevance, and diversity — while serving billions of feed requests a day within milliseconds. 01 ·…
Designing a Flash Sale Checkout System A complete architecture for a checkout system that can survive 50,000 concurrent buyers hammering “Buy Now” on a limited-stock item at the exact same second — and guarantee that not one single unit is ever oversold, without the system grinding to a halt under…
Designing an Ephemeral Stories System That Expires in 24 Hours A full walkthrough of how to build ephemeral, self-expiring content at massive scale — where the hardest problem isn’t showing content, it’s guaranteeing it reliably disappears on schedule. 01 · Introduction02 · Problem03 · Requirements04 · Architecture05 · TTL Storage06…
Designing a System to Detect & Prevent Engagement Farming at Platform Scale A production-grade blueprint for identifying bait tactics, coordinated engagement pods, and bot-driven metric inflation at platform scale — architecture, algorithms, trade-offs, and the questions interviewers actually ask. 01 · Intro & History02 · Problem03 · Core Concepts04 ·…
Designing a Duplicate & Copyrighted Content Detection System A complete, interview-ready walkthrough of how large user-generated content (UGC) platforms fingerprint, match, and act on duplicate or copyrighted uploads at the scale of hundreds of millions of files a day — inspired by systems like YouTube Content ID, Facebook Rights Manager,…