"How would you design a push notification system?"
Push notifications are critical for user engagement but complex to implement reliably. This tests your system design skills in a mobile context.
Cover the architecture: notification service, provider integration (APNs/FCM), delivery tracking.
Discuss reliability: retries, fallbacks, delivery guarantees.
Address user experience: frequency, relevance, opt-out.
Mention analytics and A/B testing.
I would design a three-layer system. The notification service receives triggers from business logic, applies user preferences and rate limits, then queues messages. The delivery layer integrates with APNs for iOS and FCM for Android, handling token management, retry logic, and delivery receipts. The analytics layer tracks open rates, dismissal rates, and downstream conversions. Key design decisions: use a message queue (SQS/Kafka) for reliability, implement per-user rate limiting to prevent notification fatigue, support rich notifications with images and action buttons, and store notification history for in-app display. I also implement a preference center where users control notification categories. In my experience, apps that respect user preferences see 40% higher engagement than those that blast everything.
Emphasize user respect: frequency limits, relevant content, easy opt-out.
Discuss token management — tokens expire and change, this is often overlooked.
Mention silent notifications for background data updates.
APNs (Apple Push Notification service) is for iOS, FCM (Firebase Cloud Messaging) is for Android. Both require device tokens and have different payload formats and size limits.
Implement retry with exponential backoff, track delivery status, clean up invalid tokens, and use fallback channels (in-app notifications, email) for critical messages.
Research shows 2-5 per day is optimal for most apps. More than that increases uninstall rates. Always let users control frequency through preferences.
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