Notification System — Mock Interview

Medium

A multi-channel notification pipeline: fan one event out to push, SMS, and email at scale, deliver it reliably, and never spam the user.

Interviewer

You're designing a notification system for a live system-design interview. A single internal event — an OTP request, an order update, a social 'like', or a marketing campaign — must reach the right users on the right channels (push, SMS, email, in-app) reliably and without spamming them.

Scale assumptions:

100M registered users; ~20M active/day receiving ~10 notifications each → ~2×10⁸ notifications/day.
Channel mix ≈ 70% push, 20% email, 8% SMS, 2% in-app; ~2k average / ~10k peak dispatch QPS.
Broadcasts fan one trigger out to up to ~10M recipients in a burst.
Targets: transactional (OTP) delivered in seconds at 99.9%+; marketing may take minutes; strictly honor user preferences and quiet hours.

We'll move through requirements, estimation + data model, architecture, a fan-out + reliable-delivery deep dive, and trade-offs. Think out loud and state your assumptions.

The interviewer grades each stage against a hidden rubric and asks a probing follow-up, mirroring a real system design round. Sessions are capped at 30 turns.