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Designing a Same-Day Delivery Routing System How to dynamically route orders to the nearest fulfillment center that has both the inventory and the delivery capacity to get a package to a customer’s door before the sun goes down. 01 · Introduction02 · Problem03 · Concepts04 · Architecture05 · Routing Algorithm06…

Designing a Real-Time Return-Rate Anomaly Detection System How marketplaces detect, within one to two minutes, that returns for a category are behaving statistically abnormally — at a scale of millions of order and return events per minute, with z-score, EWMA, and CUSUM working together. 01 · Intro & History 02…

Designing a Recurring Subscription Billing System How to charge customers on a schedule, retry failed payments safely, and never lose track of a billing cycle — even when a million requests hit your system in a single minute. 01 · Intro & History02 · Problem03 · Core Concepts04 · Architecture05…

Designing a Real-Time Stock Alert System for an E-Commerce Platform A ground-up, interview-ready deep-dive into building a notification system that tells millions of waiting customers, within seconds, the moment an out-of-stock product comes back — without melting your database or spamming anyone twice. 01 · Introduction02 · Problem03 · Concepts04…

Designing a Real-Time Shipping Cost Recalculation System How marketplaces instantly re-price shipping as a customer adds or removes items from a cart — factoring in weight, dimensions, and destination — at a scale of millions of requests per minute. 01 · Intro & History 02 · Problem & Motivation 03…

Designing a Real-Time Multi-Seller Price Comparison System How do sites like PriceGrabber, Google Shopping, Amazon’s “Other Sellers” panel, or Skyscanner show you the cheapest price across dozens of sellers — updated within seconds of a seller changing it? This is the complete architecture, from seller feed ingestion to the moment…

Real-Time Price Propagation: Updating Millions of Rendered Product Pages Within Seconds A complete, beginner-to-interview-ready walkthrough of how large e-commerce platforms change a product’s price once and have that new price appear correctly everywhere — on cached pages, CDN edges, mobile apps, search results, and open browser tabs — in near…

Real-Time Personalized Product Recommendations How to architect the “Recommended for you” rail on an e-commerce homepage — a system that must blend a shopper’s lifetime purchase history with what they clicked thirty seconds ago, rank thousands of candidate products in under 100 milliseconds, and do this for tens of millions of…

System Design · Real-Time · E-Commerce Designing a Real-Time Fraud Detection System for E-Commerce A complete, ground-up engineering guide to building a system that catches fraudulent transactions in milliseconds — without turning away the honest customers who keep the business alive. 19 chapters Kafka · Flink · Redis · ML…