A mass notification system is designed for real-time, large-scale communication with high availability, utilizing a microservices architecture to eliminate single points of failure and ensure zero downtime. It features stateless services with shared storage, horizontal scaling, and efficient load balancing to handle peak loads, supported by distributed databases like Cassandra or DynamoDB for high-throughput replication. The system employs an event-driven architecture optimized for low latency and high concurrency, utilizing protocols such as HTTP, WebSockets, and MQTT based on specific real-time needs. It integrates multi-channel notification capabilities, including SMS, email, push, and voice, with an abstraction layer to separate business logic from channel-specific implementations, ensuring flexible routing and consistency. Advanced strategies like adaptive rate control, backpressure handling, and geo-distributed brokers minimize cross-region latency, while end-to-end tracing and analytics enhance system stability and performance. The choice of message queue—Kafka for high throughput, RabbitMQ for transactional messaging, or Redis Streams for real-time fanout—depends on specific requirements. The system supports efficient notification storage using a hybrid database approach, while load balancing at both the application and database layers ensures optimal performance and high availability. Fault tolerance and disaster recovery are prioritized through multi-region deployment, automated failover, and chaos engineering tests. Compliance with regulations such as GDPR, HIPAA, and TCPA is essential, requiring encryption, access controls, and audit trails to protect user data and maintain legal standards.