Designing a Digital Wallet System with Strict Balance Consistency A ground-up, interview-ready deep-dive into building a digital wallet that keeps every customer’s balance perfectly accurate, even with millions of concurrent transfers, deposits, and withdrawals hitting the system every minute — with zero tolerance for double-spends or lost money. 01 ·…

System Design · Digital Bank Real-Time Account Balance Checks, Safe Through a Database Failover A deep, from-first-principles walkthrough of how a digital bank answers millions of “what’s my balance?” requests every minute — in well under a second — while guaranteeing the number shown is never wrong, even in the…

Designing a Threaded Comment System for Millions of Nested Replies A complete, from-first-principles walkthrough of how to architect a comment platform that can host viral posts with millions of nested, threaded replies — covering data modeling, storage, caching, ranking, moderation, real-time delivery, and the trade-offs real companies make in production.…

Designing a Personal Data Export System A complete, ground-up walkthrough of building a platform that lets any user request and download their entire personal data archive, in line with data portability laws such as GDPR Article 20, the CCPA, and India’s DPDP Act. 01 · Intro02 · Problem03 · Concepts04…

Designing a Direct Messaging System: One-on-One and Massive Group Chats How to build a chat platform that works just as well for two friends texting as it does for a 50,000-member community group — covering architecture, protocols, scaling, and the hard trade-offs behind products like WhatsApp, Slack, and Discord. 01…

Designing a Coupon & Promo Code Abuse Detection System How to build a real-time fraud detection platform that stops users from farming one-time discounts through multi-accounting, device spoofing, and referral abuse — without punishing your honest customers — using layered blocklists, deterministic rules, streaming velocity checks, graph-based entity resolution, and…

Designing a Cryptocurrency Exchange — Accurate, Auditable Balances at Scale A system design deep dive into building an exchange that maintains accurate, auditable balances while processing thousands of trades per second, scaled to millions of requests per minute — combining a stock-exchange-grade matching engine with a bank-grade custody and ledger…

Designing a Daily Interest Accrual Engine for Millions of Loans A production-grade, interview-ready deep dive into building a system that calculates and applies daily interest across millions of active loans with heterogeneous terms, rates, and compounding rules — while surviving traffic spikes of a million requests per minute, without ever…

Designing a Creator Monetization Platform for Subscriber-Only Content at Scale A complete, ground-up walkthrough of how to design a system like Patreon, Substack, or subscriber-only video platforms — one that lets millions of creators sell subscriber-only content to millions of paying subscribers, reliably, securely, and at global scale. 01 ·…

Designing a Real-Time, Cross-Device Shopping Cart Sync System A shopper adds a jacket to their cart on the subway using their phone. Twenty minutes later they open their laptop at home — and the jacket is already sitting in the cart, waiting for checkout. This guide designs that system end…