Design a notification system (email, SMS, push).

Summary

Services publish notification requests to a queue; a notification service applies user preferences, templates, deduplication and rate limits, then routes to per-channel workers that call providers (SES, Twilio, APNs/FCM) with retries, recording delivery status.

Requirements

• Send email, SMS, push and in-app notifications
• Respect user preferences and quiet hours
• Templates and localization
• Retries, no duplicates, delivery tracking
• Handle spikes (marketing blasts) without delaying transactional messages

API

POST /api/notifications {"user_id": 42, "type": "order_shipped", "data": {...}, "idempotency_key": "..."}
GET  /api/users/{id}/notifications?unread=true          -- in-app inbox
PUT  /api/users/{id}/notification-preferences

Data Model

notification_requests(id, idempotency_key UNIQUE, user_id, type, payload jsonb, created_at)
deliveries(id, request_id, channel, status, provider_id, attempts, last_error, updated_at)
preferences(user_id, type, channel, enabled, quiet_hours)
device_tokens(user_id, platform, token, last_seen)

Architecture

Producers → API/queue → notification service (validate, dedupe by idempotency key, load preferences, render templates) → per-channel queues with priority (transactional vs bulk) → channel workers calling providers → delivery status updates via provider webhooks. In-app notifications are written to an inbox table and pushed over WebSockets.

Deep Dives

• Priority isolation: separate queues/workers for transactional and marketing traffic
• Retries: exponential backoff; switch providers on sustained failures
• Dedup: idempotency keys and per-user/type rate limits (no 10 identical pushes)
• Token hygiene: remove invalid device tokens reported by APNs/FCM
• Compliance: unsubscribe links, opt-outs, data retention

Scaling

• Horizontal channel workers driven by queue depth
• Batch provider calls where supported
• Throttle to provider rate limits

Comparison / Tradeoffs

What interviewers look for

A structured approach: clarify requirements and scale, sketch the API and data model, draw the high-level architecture, then go deep on the hardest component and discuss trade-offs, bottlenecks and failure modes.

Follow-up questions

• How do you schedule notifications in users' time zones?
• How would you build a digest (batching) feature?