Designing a Global SMS Gateway: Multi-Carrier Routing Across the World A complete, interview-ready system design walkthrough for building an SMS gateway that reliably delivers text messages to every country, routing intelligently through different telecom carriers and aggregators, at a scale of millions of messages per minute. 01 · Introduction02 ·…

Screen Sharing for 100-Participant Video Calls A deep, interview-ready walkthrough of how to architect real-time screen sharing that stays sharp and smooth even when a hundred people are watching at once — covering encoding, media routing, scaling, and the trade-offs real platforms like Zoom, Meet, and Teams have made. 01…

Designing a Suspicious Transaction Regulatory Reporting System A production-grade, interview-focused deep dive into building a system that continuously watches financial transactions, flags suspicious activity, coordinates human investigation, and files legally binding reports to regulators within hard, unforgiving deadlines — where a missed clock is not a lost feature, it is…

Large-Scale Webinar Broadcast to 50,000 Attendees A deep, interview-ready walkthrough of how to build a webinar platform that streams a single presenter to tens of thousands of concurrent viewers with minimal end-to-end latency — without the fan-out cost exploding. WebRTC-grade ingest, CDN-scale fan-out, and everything in between. 01 · Intro…

Designing a Synthetic Identity Fraud Detection System A complete, production-grade system design walkthrough for detecting synthetic identities — fraud built from a mix of real and fake personal details — at the scale of millions of financial account applications, layering deterministic rules, calibrated ML scoring, and an identity graph into…

Designing a Spam & Phishing Link Detection System for Direct Messages at Scale A deep, interview-ready walkthrough of how large messaging platforms detect and stop spam and phishing links in direct messages — built to handle millions of messages per minute with low latency, low false positives, and a continuously-adapting…

Designing Real-Time Collaborative Annotation for Video Calls A complete system design walkthrough for building a shared drawing and highlighting layer on top of screen sharing in a live video call — covering conflict resolution, synchronization, rendering, scalability, and reliability, written for interview preparation and real production understanding. 01 · Introduction02…

Designing Delivery Confirmation: Read Receipts and ‘Delivered’ Status at Scale A production-grade system design deep dive into building sent / delivered / read status tracking for a messaging platform — balancing accuracy, latency, privacy, and the network chatter cost of keeping millions of clients continuously informed. 01 · Introduction02 ·…

Real-Time Account Takeover Detection A production-grade, interview-ready system design walkthrough for engineers building the intelligence layer that catches attackers the moment they walk in wearing someone else’s identity — combining device, network, behavior, and transaction signals into a millisecond-latency risk decision. 01 · Introduction 02 · History 03 · Why…