System Design: Low-Latency, Multi-Channel Notification Platform

You are asked to design a scalable, reliable notification system that can send messages to millions of users with low latency across multiple channels (email, SMS, push).

Assume a large, consumer-facing product operating globally with both transactional (real-time) and bulk/marketing use cases.

Cover the following:

1. Requirements Gathering
   • Functional, non-functional, traffic assumptions, SLAs/latency targets, compliance.
2. High-Level Architecture
   • Core components and data flow for real-time, scheduled, and bulk sends.
3. Data Model
   • Key entities (templates, preferences, messages, attempts, providers, etc.).
4. API Design
   • Producer APIs, admin APIs, idempotency, status callbacks/webhooks.
5. Message Prioritization
   • Priority levels, queueing, fairness, rate limits, quotas.
6. Deduplication
   • Idempotency keys, content-based dedup, time windows.
7. Retries and Failure Handling
   • Backoff, dead-letter queues, poison-pill handling, fallback channels, circuit breaking.
8. Scaling Strategies
   • Partitioning, horizontal scaling, multi-region, autoscaling triggers.
9. Monitoring and Alerting
   • SLIs/SLOs, metrics, logs, traces, runbooks.
10. Cost Considerations

• Unit economics by channel, routing, batching, budgets, frequency caps.

State reasonable assumptions where needed and explain trade-offs.