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Designing a Real-Time Chat Application for 500 Million Concurrent Users A complete, interview-grade walkthrough of building a globally distributed messaging system that delivers messages in under a second, at internet scale, without losing a single word your users type — the event-driven gateway fleet, consistent-hash presence routing, durable log-based fan-out,…
Designing a Real-Time Multi-Party Payment Splitting System A complete, interview-grade walkthrough of building a system like Splitwise-meets-Venmo: one expense, divided among several people, where each person pays their own share independently, at their own time, while the group sees balances update in real time — the independent-Payable state machines, sharded…
Designing Multi-Device Message State Synchronization for a Messaging Platform A complete, interview-ready system design walkthrough for keeping read, unread, and delivered status consistent when the same account is logged into a phone, a laptop, a tablet, and a web browser at the same time — at a scale of millions…
Detecting Payment Race Condition Fraud A complete, interview-ready system design walkthrough for engineers building fraud detection systems that catch attackers exploiting timing gaps in payment processing pipelines — combining structural prevention, real-time detection, and post-hoc forensics. 01 · Intro & History 02 · Attack Patterns 03 · Architecture 04 ·…
Designing Message Search at Scale — Years of Chat History, Millions of Messages, Sub-Second Queries A production-grade system design deep dive into building full-text, low-latency message search for a messaging platform — covering indexing pipelines, storage tiering, relevance ranking, sharding, security, and the trade-offs real companies make. 01 · Introduction02…
Designing a Micro-Lending Platform A complete, production-grade system design walkthrough for issuing very small, short-term loans to a very large population of borrowers, with fully automated, real-time underwriting decisions — the alternative-data pipelines, tiered risk models, event-driven lifecycle, idempotent disbursement, and cost-per-transaction discipline that make micro-lending economically possible where traditional…
Designing a Real-Time Loan Application Status Tracking System A deep, production-grade system-design walkthrough covering how a lending platform aggregates credit checks, document verification, and underwriting into one consistent, real-time, customer-facing status view — without ever showing two different truths on two different screens. 01 · Introduction02 · Architecture03 · Internal…
Designing a Live Polling & Q&A System for Large-Scale Virtual Events A production-grade walkthrough of real-time polling and Q&A for events with tens of thousands of concurrent attendees — ingest, sharded aggregation, push-based fan-out, reliability, and the interview-style deep dives that separate a good answer from a great one. 01…
Designing a Real-Time Live Chat Translation System for Global Broadcasts A production-grade deep dive into translating live chat messages into every viewer’s preferred language, in real time, at global broadcast scale — without either bankrupting the machine-translation budget or breaking the feeling of a shared live moment. 01 · Introduction02…
Designing a Scalable Customer Service Voice IVR System How to architect an Interactive Voice Response platform that handles millions of concurrent calls, routes each caller dynamically based on identity, history, and intent, and stays reliable when call volume spikes 10x without warning — where every architectural choice traces back to…