Design a Notification Service

Design the routing and orchestration layer that sits above a company's email, SMS, mobile push, in-app, and web-push delivery providers. The service ingests notification events from hundreds of internal producers (order updates, security alerts, marketing campaigns, social pings), selects the right channel(s) based on user preferences, quiet hours, frequency caps, and urgency tier, renders localized templates, fans out to per-channel workers that call external providers (SendGrid, Twilio, APNs, FCM), handles bounces and provider outages with retries and fallback chains, and emits per-notification delivery telemetry for observability and compliance.

This is a top-10 interview question and is distinct from a single-channel push notification service: the hard part is channel selection, preference enforcement, cross-channel deduplication, and isolating slow providers so one degraded channel doesn't take down the others. Must handle 100K+ notifications per second at peak (campaign blasts), maintain idempotency across retries, and respect compliance rules (GDPR unsubscribe, CAN-SPAM, TCPA quiet hours).