Designing a scalable notification system to handle millions of notifications per day involves a robust and distributed architecture:

1. Architecture Components:
   • Message Broker: Use a high-throughput, fault-tolerant message broker like Kafka or RabbitMQ to decouple message producers and consumers.
   • Producers: Services that generate notifications, e.g., user actions, system events.
   • Consumers: Multiple worker services that process and send notifications.
   • Notification Service: A microservice architecture to handle different notification channels (email, SMS, push notifications).
   • Database: Use scalable, distributed databases like Cassandra or DynamoDB to store notification logs and user preferences.
2. Flow:
   • Producers publish notification events to the message broker.
   • The message broker ensures reliable delivery of messages to consumers.
   • Consumers process messages and interact with the Notification Service.
   • The Notification Service sends notifications through the appropriate channel.
   • Logs and user preferences are stored and updated in the database.
3. Ensuring Reliability and Performance:
   • Scalability: Use horizontal scaling for producers, consumers, and the Notification Service to handle load increases.
   • Fault Tolerance: Implement retry mechanisms and dead-letter queues to handle failed notifications.
   • Load Balancing: Use load balancers to distribute traffic evenly across consumers and notification services.
   • Monitoring and Alerting: Use tools like Prometheus and Grafana for real-time monitoring, and set up alerts for failures and performance bottlenecks.
   • Caching: Use Redis or Memcached for caching frequent queries to reduce database load.

This architecture ensures high throughput, fault tolerance, and scalability, making the notification system robust and efficient.