Book: System Design Pocket Guide: Interviews

Also by me: Event-Driven Architecture Pocket Guide

My project: Hermes IDE | GitHub — an IDE for developers who ship with Claude Code and other AI coding tools

Me: xgabriel.com | GitHub

Picture walking into a system design loop at a major messaging company. The interviewer skips the usual "design a chat app" prompt and asks something narrower: design the three little dots. The typing indicator. The small grey ellipsis that appears under your contact's name and disappears half a second after they stop tapping.

It sounds like a junior question. It is not. The typing indicator is the cleanest separation of concerns in a chat product, and it tests four things at once: pubsub vs persistent connection, transport choice for server-to-client push, ephemeral vs durable storage, and backpressure when a million people start typing at the same time. If you can design the indicator, you can design the rest of WhatsApp.…