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Notification System Design
Visual Problem Diagram
Scenario
Flash sale push goes to 40M devices in one minute—APNs/FCM rate limits bite while email backlog grows in a queue that loses ordering for password resets. Multi-channel delivery, preferences, and idempotent templates separate real notification systems from "call FCM in the API.
Design a notification system delivering push, email, and SMS based on user events and preferences. Providers impose hard rate limits; users expect transactional messages to win over marketing blasts.
You should support event ingestion, templating, multi-channel routing, preferences, retries, and delivery tracking. Be ready to explain queue sharding, idempotency, and priority.
Constraints
Ingest events, template render, push/email/SMS send, user preferences, schedule, delivery status
Transactional < 30s p95 delivery, at-least-once with dedupe, 99.9% availability, respect opt-out
100M users, millions notifications/min peak, thousands templates, global providers
Stages ahead
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