System Design Interview Question: Design a Notification System

A popular question that combines APIs, queues, third-party integrations and reliability.

Requirements

• Channels: mobile push, email, SMS, in-app.
• Triggered by other services ("order shipped") and by scheduled campaigns.
• Respect user preferences and quiet hours; no duplicates.
• Scale: millions of notifications per day, with bursts during campaigns.
• Soft real-time: transactional messages within seconds; marketing can be slower.

Architecture

Services --> [Notification API] --> validate, dedupe, check preferences
                                 --> [Queue per channel + priority]
                                        |        |        |
                                 [Push workers][Email][SMS workers]
                                        |        |        |
                                      APNs/FCM  Email   SMS provider
                                               provider
             [Notification log DB] <-- status updates, delivery receipts

Key components

• Notification API: accepts { userId, type, data, idempotencyKey }; returns immediately.
• Preferences service: per-user, per-type, per-channel opt-ins, quiet hours and locale.
• Templates: stored and versioned per type, channel and language; rendered by workers.
• Separate queues per channel and priority, so a marketing blast doesn't delay password-reset emails.
• Workers call providers, retry with backoff on transient failures, and move permanent failures to a dead-letter queue.

Reliability details

• Idempotency keys to avoid double sends on retries.
• Rate limits per user (don't spam) and per provider (respect their quotas).
• Fallbacks: if push fails or the device token is invalid, consider email; clean up stale device tokens.
• Provider failover: a second email/SMS provider for critical messages.

Tracking

Store each notification's status (queued, sent, delivered, opened, failed) from provider callbacks. This powers debugging ("why didn't I get the OTP?"), analytics and the in-app notification inbox.