System Design: Building a Notification Service
Notifications are deceptively complex. You need reliability, speed, and user control, all without spamming anyone.
Core Requirements
- Deliver across channels (email, push, SMS).
- Allow user preferences.
- Support retries and deduplication.
High-Level Architecture
- Producer service emits events.
- Queue buffers delivery jobs.
- Workers send messages.
- Status stored for retries.
Idempotency Matters
Duplicate deliveries feel like bugs. Use an idempotency key per notification and store delivery status.
Retry Strategy
- Exponential backoff.
- Dead letter queue after max retries.
- Alert on permanent failures.
Preference Management
Store preferences per user and enforce them before enqueueing. This avoids accidental delivery.
Final Thought
A notification system is a reliability system. Start with correctness, then optimize throughput.
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