In these sorts of interviews there’s an extended back-and-forth since the interviewer really drills down to understand how you solve things like this structurally. System design interviews like this one are particularly crucial to nail correctly since they expect you to demonstrate technical familiarity across the entire range of architectural decisions for scale. Could be worth trying out one of the FB TPM folks on Prepfully - all their experts are currently FB TPMs so they know what the process is like and simulate a proper/real experience. prepfully.com/practice-interviews