Notification System

A notification system delivers messages to users through push, SMS, and email on behalf of many internal services. The hard parts are reliability through flaky third-party providers, respecting user preferences and rate caps, and handling huge bursts like a marketing campaign to 50M users.

Clarify requirements

Functional

• Internal services send notifications to a user (or a segment) with a template and data.
• Channels: mobile push (APNs/FCM), SMS, email, in-app.
• User preferences: opt-out per channel and category; quiet hours; locale.
• Priorities: transactional (OTP, payment receipt) versus marketing.
• Scheduling (send at time T) and deduplication.
• Track status: sent, delivered, opened, failed.

Non-functional

• 100M users; 500M notifications/day; marketing bursts of 50M in an hour.
• Transactional notifications delivered within seconds; marketing within the hour.
• At-least-once delivery with deduplication so users rarely see duplicates.
• Highly available ingestion; provider outages should not lose notifications.