Multi-device messaging platform on OCI leverages a three-tier architecture spanning two Availability Domains with redundant Message Routing Service, Device Registry Service, Presence Service, and Notification Orchestrator instances. Client devices (Android, Mac, iPhone, Web) connect through WAF and CDN-backed Flexible Load Balancers to API Gateway, which routes traffic to application services in private subnets across AD1 and AD2. Persistent state flows through Database System (primary/standby), NoSQL Device Sessions replicas, Cache for delivery state, Object Storage for encrypted media, and Vault for identity and session keys, ensuring zero message loss and sub-second delivery at scale. This architecture demonstrates OCI best practices for high-availability messaging: geographic redundancy, encryption at rest and in transit, asynchronous event processing via Event Bus, and offline queue resilience. Fork this diagram on Diagrams.so to customize subnets, add additional regions, or integrate with OCI Functions for serverless notification handlers.