Designing a Push-to-Talk Walkie-Talkie System for Team Collaboration A deep, interview-ready walkthrough of how to build an instant, one-touch voice communication feature where every millisecond of delay matters more than pristine audio — the exact opposite priority ordering of a normal phone call. 01 · Introduction02 · Architecture03 · Internal…

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…