System design interviews are the biggest hurdle for developers moving from Junior to Senior roles. It’s not about writing code; it’s about trade-offs. This item breaks down the standard "Design a Chat App" question into a repeatable framework: Requirements Clarification: Functional vs. Non-functional (CAP Theorem). High-Level Design: Load balancers, API Gateway, Database choice (SQL vs. NoSQL). Deep Dive: How to handle real-time messaging (WebSockets vs. Long Polling). The goal here is to provide a cheat sheet that helps you structure your 45-minute discussion without getting lost in the weeds. We also include common pitfalls, like forgetting to discuss scalability limits or single points of failure. This is essential reading for anyone targeting L4/L5 roles in Big Tech.