Designing a Product Search & Filtering System for a Marketplace with Hundreds of Millions of Listings A complete, ground-up system design walkthrough: architecture, inverted-index internals, multi-attribute filtering and faceting, relevance ranking, sharding and replication, caching and load balancing, reliability, security, and the trade-offs real marketplaces make every day — from…

System Design · E-Commerce & Pricing Designing a Price-Match Guarantee Detection System How to build a system that watches competitor prices around the clock, matches products correctly, and automatically triggers a price match — at the scale of millions of SKUs and thousands of competitor sites. 01 · Intro &…

Designing a Photo-Sharing System That Serves Billions of Images in Under 100 ms, Globally A complete walkthrough of the upload pipeline, image processing, storage tiering, and global CDN architecture behind photo platforms that feel instant no matter where in the world you open them. 01 · Intro & History02 · Problem03…

Designing a System to Handle Lost Payment Confirmation Callbacks What happens when a payment processor successfully charges a customer’s card, but the network drops the confirmation message on its way back to your servers? How do you build a system that guarantees the customer is never charged without getting their…

Designing a Multi-Vendor Marketplace Cart & Checkout System How platforms like Amazon Marketplace, Etsy, Flipkart, and Alibaba let a single shopping cart hold items from dozens of independent sellers — and still check out as one smooth transaction, with split payments, partial-failure handling, and per-vendor fulfilment behind the scenes. 01…

System Design · Social Graphs Designing a Mutual Friends System at Billion-Connection Scale How do you tell two strangers, in under 100 milliseconds, who they both know — when the underlying graph has billions of people and hundreds of billions of edges? This is the full architecture, from adjacency lists…

Designing a Race-Condition-Safe Peer-to-Peer Money Transfer System How to guarantee that a sender with just enough balance for one transfer can never accidentally send money twice, even when two transfers race in at the exact same instant — at a scale of millions of requests per minute. 01 · Intro…

Designing a Multi-Warehouse Order Fulfillment Routing System How large-scale e-commerce platforms decide, in a few hundred milliseconds, which warehouse should ship each order — balancing inventory availability, distance, shipping cost, warehouse load, and business rules — without ever selling something that isn’t actually on a shelf. A full architectural walkthrough…

Detecting Stolen or Copied Listing Images at Marketplace Scale An interview-focused walkthrough of how to catch sellers who copy another seller’s product photos without permission — processing on the order of a million image events per minute, with sub-second per-image scoring and near-zero false accusations. 01 · Intro & History02…

Designing a Marketplace Dispute Resolution System A complete, beginner-to-advanced walkthrough of the least glamorous but most trust-critical system in any marketplace — evidence submission, case tracking, automated resolution, immutable audit trails, and safe money movement — built to gracefully survive a scenario of a million requests per minute during flash…