Designing Graceful Network Degradation for a Video Calling App How production video platforms keep a participant “in the room” when their network gets bad — instead of dropping them — through adaptive bitrate, layered video, jitter buffering, forward error correction, and state-machine-driven reconnection. 01 · Intro & History 02 ·…

Designing Group Video Calls That Gracefully Handle Mid-Call Join & Leave A production-grade system-design walkthrough for a messaging platform’s group video calling feature — media routing architecture, signaling, renegotiation on participant churn, scaling, and reliability — with interview-style deep dives. 01 · Intro & History02 · Requirements03 · Architecture04 ·…

Designing a Friendly Fraud Detection and Handling System A production-grade, interview-focused deep dive into building a system that recognises when a cardholder disputes a charge for something they genuinely ordered and received, builds a compelling evidence case automatically, and protects merchant revenue without wrongly treating honest customers as suspects. 01…

Detecting Coordinated Fraud Rings Testing Stolen Payment Credentials A production-grade system design deep dive on catching card-testing attacks that scatter tiny, forgettable transactions across many seemingly unrelated merchant accounts. A streaming entity graph, layered detection, community clustering, and adaptive scoring designed for the moment a single merchant sees a raindrop…

Designing a Real-Time Currency Conversion Fee Transparency System How payment platforms compute, cache, and display the exact markup applied over the market mid-rate on every cross-currency transaction — at a scale of millions of quotes per minute, with sub-second latency and audit-grade accuracy. A deep dive into rate ingestion, aggregation,…

Real-Time Detection of Unusual Foreign Transaction Patterns How card issuers catch a compromised card the moment its spending pattern stops looking like the actual cardholder travelling abroad — scored in real time, on every authorization, without freezing a genuine traveler’s card at passport control. 01 · Intro 02 · History…

Designing a Real-Time First-Party Fraud Detection System A complete, interview-depth system-design walkthrough for building a platform that spots customers who open accounts with real intent to use them — and then quietly plan to default, abuse promotions, or exploit credit — before that intent turns into loss. 01 · Intro…

Designing an End-to-End Encrypted Messaging System A complete, interview-grade walkthrough of building a messaging platform where the server can route, store, and deliver every message — while remaining mathematically incapable of reading a single one of them, by design rather than by policy. 01 · Introduction02 · Requirements03 · Architecture04…

Designing Disappearing Messages at Global Scale Building a messaging platform feature that guarantees time-bound message deletion consistently across every device, cache, and replica — the way Signal, WhatsApp, and Telegram do it. A deep dive into TTL indexes, sync fan-out, reconciliation sweeps, and the cross-device consistency guarantees that turn a…

Designing a Real-Time Chat Routing System for Customer Support A production-grade walkthrough of matching incoming support conversations to the best available agent, in real time, at scale — the way it is built at companies like Zendesk, Intercom, Salesforce Service Cloud, and Amazon Connect. Queueing theory, fairness, exactly-once assignment, and…