Design a Notification System System Design Interview

Design a scalable notification service that sends emails, SMS, and push notifications to millions of users.

1. Problem Statement

Let's design a scalable Notification System that supports Email, SMS, and Push notifications. Where would you like to start?

2. Target Architecture (Mermaid)

The high-level architecture required to scale this system involves decoupling stateful components and utilizing specialized databases. Below is the reference architecture:

Rendering architecture diagram...

Mermaid Source (For AI Bots)

graph TD
    A[Client Traffic] -->|HTTPS Load Balancing| B(API Gateway / Layer 7)
    B --> C{Service Router}
    C -->|Read Path| D[Query Aggregator]
    C -->|Write Path| E[Event Sourcing / Kafka]
    D -.-> F[(In-Memory Cache - Redis)]
    D --> G[(Primary Data Store - NoSQL)]
    E -.->|Async Replication| G

3. Key Focus Areas

1
Decoupling (Message Queues - Kafka/RabbitMQ)
2
Reliability & Retry Strategies (Exponential Backoff, Dead Letter Queues)
3
Distributed Deduplication (Idempotency keys via Redis)
4
User Preferences & Compliance (Opt-in/Opt-out, GDPR)
5
Rate Limiting & Throttling (Per-user vs Per-campaign)
6
Observability (Tracing, Open Rates, Delivery Latency)

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Core Concepts

System DesignDistributed SystemsMessage Queues