System Design: Multi-Channel Notification Platform

Context

Design a multi-tenant notification platform that delivers messages across email, SMS, and mobile push. The system must support both real-time and scheduled notifications, per-user preferences, and be resilient, scalable, and cost-aware.

Functional Requirements

1. Channels: email, SMS, push notifications.
2. Delivery modes: real-time and scheduled (one-time and recurring).
3. User controls: per-user preferences (opt-in/out per category), quiet hours, locale/time zone.
4. Reliability: deduplication, idempotency, retries with exponential backoff.
5. Throttling: rate limiting per user, per tenant, and per provider.
6. Isolation: strict multi-tenant isolation (configs, limits, data segregation).
7. Content: templating with variables, localization (i18n, l10n), and A/B testing support.
8. Observability: monitoring, alerting, delivery analytics.
9. Compliance: GDPR/data minimization, consent management, auditability.
10. Cost controls: budgets/quotas and provider selection for cost/performance.

Non-Functional Requirements

• High availability; horizontal scalability.
• Message delivery latency targets and throughput (TPS) goals.
• Delivery semantics: discuss exactly-once vs at-least-once.
• Graceful degradation and failure-mode strategies.

Deliverables

• Public APIs (send, schedule, templates, preferences, status).
• Storage schema (entities, key indexes, partitioning).
• Queue/pub-sub design and retry strategy.
• Worker orchestration and provider integration patterns.
• Monitoring/alerting and analytics plan.
• Scalability targets (e.g., p99 latency, TPS) with reasoning.
• Failure modes and mitigations.
• A/B testing strategy.
• GDPR/compliance approach.
• Cost controls and budgets.