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A complete system design walkthrough for a verified badge identity-verification program at scale: applicant intake, encrypted evidence storage, asynchronous fan-out pipeline (document forensics, liveness/face-match, deduplication, eligibility, risk scoring), three-lane decision routing (auto-approve / human review / auto-reject), badge issuance, tamper-evident audit log, appeals, and data-minimising retention — with inline SVG…

Designing a Real-Time Sales Tax Calculation System Across Thousands of US Jurisdictions A marketplace tax engine that is instant, correct, and survives one million requests a minute — built from first principles through to production concerns. 01 · Intro02 · History03 · Problem04 · Core Concepts05 · Architecture06 · Internals07…

A complete system design walkthrough for a Try Before You Buy apparel marketplace: saga-coordinated trial orders, card authorisation holds, soft and hard inventory reservation, order and return state machines, automated pickup scheduling, courier anti-corruption adapters, warehouse QC, refund and restock flow, capacity estimation, security, and fraud prevention — illustrated with…

Designing a Triangulation Fraud Detection System A complete, from-first-principles walkthrough of how to design a real-time system that catches “triangulation fraud” — where a seller lists items they do not own and fulfils orders using another retailer’s stolen payment details — at a scale of millions of requests per minute.…

Designing a Platform-Wide “Trending Now” Page That Updates in Seconds A complete, ground-up walkthrough of how to build a real-time trending system — one that watches hundreds of millions of events a minute across an entire platform and reflects a genuine shift in global attention on the page within seconds,…

A complete, beginner-to-production system design walkthrough for handling supplier inventory feed delays and failures without ever overselling: circuit breakers, safety stock buffers, Kafka-backed ingestion pipelines, reservation with optimistic locking, reconciliation jobs, and end-to-end monitoring, illustrated with inline architecture and sequence diagrams and Java code.

Designing a Transaction Reconciliation System How do you prove, every single day, that millions of transactions your platform says happened actually match what the banks say happened — without losing a single rupee, dollar or cent and without a human reading every row? 01 · Intro & History02 · Problem03…

Designing a Spam Link Detection & Prevention System A production-grade blueprint for catching and stopping malicious and unwanted links across billions of daily direct messages and comments — without slowing down a single legitimate conversation. 01 · Intro & History02 · Problem03 · Core Concepts04 · Architecture05 · Internals06 ·…

Designing a Checkout Abandonment Recovery System A complete, from-scratch walkthrough of how large e-commerce platforms detect abandoned shopping carts and automatically re-engage customers to bring them back to complete their purchase — covering architecture, data flow, scaling, reliability, security and real production patterns used by companies like Amazon, Flipkart and…