Design a scalable notification system for spikes

Q: Design a scalable notification system for spikes

This is a System Design interview question from DoorDash for Software Engineer roles. View the full question and solution on PracHub.

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Question

System Design: Multi-Channel Notification Service

Goal

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

Functional Requirements

Per-recipient personalization and templating (multi-language, per-channel content).
Ordering guarantees per user (messages for a user are processed in order of creation).
Idempotency and de-duplication across retries and client resubmits.
At-least-once delivery with configurable SLAs (deadline/TTL, priorities).
Provider failover (multi-provider routing) and multi-region resilience (active-active).
Observability: metrics, logs, distributed tracing.
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