System Design: Multi-Channel Notification Platform

Context

Design a production-grade notification platform that sends messages via email, SMS, and push for both transactional and campaign use cases. The system must support high scale, multi-tenancy, compliance, and multi-region high availability.

Functional Requirements

1. Public APIs to publish notifications (immediate) and schedule future sends.
2. Preference and subscription management per user and per channel.
3. Idempotency and deduplication to avoid duplicate sends.
4. Per-user and per-channel rate limiting and quotas (also per-tenant).
5. Retries with exponential backoff and jitter; dead-letter handling.
6. Scheduling: immediate and future (one-time and recurring) sends.
7. Fan-out to millions of users (campaigns) with audience targeting.
8. Provider selection and failover across multiple vendors (email/SMS/push).
9. Delivery status tracking and webhooks ingestion from providers.
10. GDPR/CCPA-compliant opt-outs, consent, suppression, and deletion.
11. Multi-tenant isolation (data, quotas, rate limits, RBAC, keys).
12. Observability: metrics, traces, logs, and audit trails.
13. Multi-region, highly available, low latency operations.

Non-Functional Requirements to Address

• High-level architecture and data model.
• Storage, queues/streams, workers and scaling choices.
• Ordering guarantees where required.
• Message semantics (exactly-once vs at-least-once) and idempotency.
• Sharding strategy and backpressure handling.
• Capacity estimates and SLAs/SLOs.
• Failure modes and load-testing strategies.