CHAPTER 17

Design a Notification System

Multi-channel delivery with priorities, preferences, retries and provider failover

Source: PDF p. 54

Core PDF content
Functional requirementsNon-functional requirements and scale
- Send push (iOS/Android), SMS, email and in-app notifications - Templates, localisation, scheduling - Respect user preferences, opt-outs, quiet hours - Delivery tracking and analytics- Critical messages (OTP) delivered within seconds; bulk can lag - At-least-once with dedupe; never spam the same user twice - 100 M notifications/day (~1,200/s avg, bursts 50x for campaigns) - Resilient to third-party provider outages
Figure 38. Notification pipeline: one service enforces policy; per-channel queues and workers isolate failures of third-party providers. · PDF p. 54
QuantityEstimateDesign implication
Throughput100 M/day = ~1,200/s average; a campaign to 20 M users in 10 min = ~33,000/sQueues absorb bursts; scale workers on lag
PriorityOTP/security vs transactional vs marketingSeparate topics or priority lanes so bulk never delays OTP
Provider limitsFCM/APNs/SMS gateways throttle per senderPer-provider rate limiter and batching

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References

System Design & AI System Design: The Complete Guide — 2026 Edition, p. 54; figures 38.

Ch. 16Ch. 18: Design a Video Streaming Platform

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