I Was Asked to Design WhatsApp in a Senior Interview. Here's the Full Breakdown — From the Mobile Architect's Lens. By Subrata Kumar — Cross-Platform Mobile Architect, React Native | 15 min read The interviewer said four words: "Design a messaging app." Then waited. It was a senior mobile architect interview. I'd prepared. I'd read the usual system design resources. And then the interviewer said — casually, like it was nothing — "Walk me through how you'd design a messaging system. Like WhatsApp." The problem wasn't that I didn't know anything. The problem was that I knew about WhatsApp. I knew what it did. What I wasn't prepared for was the moment the interviewer asked: "Okay — if User A and User B are on different servers, how does the message get from one to the other?" Silence. Not the confident, thinking-out-loud kind. The kind where you feel the room change. That question — that specific routing problem — is where most answers fall apart.…