Book: System Design Pocket Guide: Fundamentals Also by me: Database Playbook My project: Hermes IDE | GitHub — an IDE for developers who ship with Claude Code and other AI coding tools Me: xgabriel.com | GitHub A user updates their profile photo. The POST /me returns 200. The page reloads, and the old photo is back. Three seconds later it flips to the new one. The bug ticket says "intermittent UI flicker on profile save." The actual cause is async replication, and the ticket will get reopened twice before someone names it. The senior interview question that exposes this is some variant of "you've added a read replica, what breaks?" Junior answers stop at "we offload reads." Senior answers map out the read-your-writes problem, propose three different mitigations with their trade-offs, sketch monitoring, and only then write code. The split between those two answers is what the panel scores.…