System Design: Multi-Channel Notification Service

Goal

Design a company-wide Notification Service that reliably delivers messages via push, SMS, and email.

Functional Requirements

1. Per-recipient personalization and templating (multi-language, per-channel content).
2. Ordering guarantees per user (messages for a user are processed in order of creation).
3. Idempotency and de-duplication across retries and client resubmits.
4. At-least-once delivery with configurable SLAs (deadline/TTL, priorities).
5. Provider failover (multi-provider routing) and multi-region resilience (active-active).
6. Observability: metrics, logs, distributed tracing.
7. Cost controls: budgets/quotas, price-aware routing, graceful degradation.

Deep Dive: 100× Traffic Spike (e.g., flash sale)

Discuss how you would handle sudden spikes 100× above baseline:

• Ingestion protections and API backpressure
• Partitioning strategy
• Queuing and buffering
• Per-user and per-provider rate limiting
• Backpressure propagation end-to-end
• Autoscaling policies
• Scheduling and prioritization (SLA-aware)
• Retry and dead-letter strategy
• Quota enforcement and fairness (tenant/team/user)
• Graceful degradation strategies

Deliverables

• Public APIs (request/response, idempotency)
• Storage schemas (core tables/documents, indexes)
• Capacity estimates and assumptions
• End-to-end data flow
• Potential bottlenecks and mitigations