System design interview question
Design a Real-Time Notification Delivery System for 500M Users Across Multiple Regions
Design a global notification delivery platform that handles 2M notifications per second across web, mobile, and email channels with sub-second latency guarantees. The system must support user preferences (do-not-disturb windows, channel preferences, frequency caps), handle delivery retries with exponential backoff, provide exactly-once semantics, and maintain per-user ordered delivery. Users are geographically distributed across NA, EU, and APAC regions. The platform must scale elastically during traffic spikes (5x peak load during major events), provide monitoring/alerting for delivery SLAs (99.9% within 5 seconds), and persist audit logs for compliance. Consider both synchronous (in-app) and asynchronous (email/SMS) delivery pipelines with different latency requirements.
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