Abstract : Planet-scale real-time notification systems must deliver billions of notifications daily to hundreds of millions of users while maintaining strict latency, availability, and durability guarantees. These systems have special difficulties: one event can turn into millions of delivery tasks, there can be huge spikes in traffic during campaigns or emergencies, and there are different limitations for various channels like push notifications (messages sent directly to users' devices), email (electronic mail), SMS (short message service), and webhooks (HTTP callbacks that send real-time data to other applications). This article presents a comprehensive architectural framework addressing these challenges through four key contributions. Firstly, the article presents a formalized taxonomy for fan-out strategies, which analyzes push, pull, hybrid, and hierarchical approaches, along with a quantitative trade-off evaluation. Second, a hierarchical queue architecture provides tenant-region-channel isolation for burst stability and noisy-neighbor prevention. Third, multi-layer back-pressure mechanisms combining admission control, feedback-driven throttling, and retry storm mitigation are used. Fourth, optimize the protocol and infrastructure to ensure cost-efficient delivery at scale. The architectural patterns use simulations and provide engineers with practical advice to build robust notification systems capable of handling significant growth.