System Design: Multi-Channel Notification Platform

Design a scalable Notification System that supports real-time and scheduled messages across multiple channels (email, push, SMS). Assume this is for a high-traffic consumer platform with millions of users and mission-critical notifications.

Assumptions (you may adjust if you state them explicitly):

• Traffic: up to 50k notifications/second at peak, 1B/day worst case during major events.
• Channels: email, mobile push, SMS; multiple third-party providers per channel for redundancy.
• Scheduling: immediate and future delivery (seconds to 30 days ahead), with cancel/reschedule.
• Global audience with time zones, quiet hours, user preferences, and regulatory constraints.

Deliverables

1. Components: Enumerate core services and infra.
2. Data flow: Real-time and scheduled paths end-to-end.
3. Storage: Schemas/data models for templates, preferences, schedules, logs, idempotency.
4. Failure handling: Retries, deduplication, provider failover, DLQs, backpressure.
5. Scaling strategy: Partitioning, autoscaling, rate limiting, multi-region.

Also provide:

• APIs: send now, schedule, cancel, update preferences, provider callbacks.
• Observability: metrics, tracing, alerting, and SLOs.
• Capacity planning example.
• Security and compliance considerations.

Project Architecture Walkthrough (Past Project)

Pick one substantial system you have built or owned. Draw and explain the end-to-end architecture:

• Services and their responsibilities.
• Databases/storage choices and schemas.
• Communication patterns (sync vs async, queues, streams, webhooks), data consistency strategy (e.g., outbox/Saga).
• Scaling, availability, and failure modes.
• Tradeoffs and lessons learned.

Be prepared for deep, domain-specific follow-ups (e.g., exact-once semantics, time-zone correctness, multi-tenant throttling, provider outage strategy, replay/backfill, GDPR deletion).