Design a follower push-notification system

Q: Design a follower push-notification system

This question evaluates understanding of designing scalable, reliable push-notification services and related competencies in distributed systems, high-throughput messaging and fan-out strategies, data modeling for follower graphs, rate limiting, user preferences, deduplication, retries, and observability.

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Question

Design a notification system for a social product:

When a user publishes a new post, the system should send push notifications to that user’s followers.
Notifications are primarily mobile push (e.g., APNs/FCM), but you may mention extensibility to other channels.

Cover:

Core requirements and non-functional goals (latency, reliability, scale).
High-level architecture and main components.
How to handle large fan-out (users with many followers).
Data model / storage choices for follower graph and notification records.
Rate limiting, user preferences (opt-out, quiet hours), deduplication, retries.
Observability and failure handling.

Design a follower push-notification system

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