Design a global real-time notification system

Q: Design a global real-time notification system

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

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Question

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Problem

Design a global, real-time notification system that can deliver messages via:

Mobile/Web push
Email
SMS

The system supports 100M+ users and must provide multi-channel fallback (e.g., try push, then email/SMS if not delivered).

Requirements to clarify

Message types: transactional vs marketing; priority classes
Latency SLA (p50/p99), throughput, burstiness
Delivery semantics: at-most-once vs at-least-once; user-visible deduplication
User preferences: opt-in/out, quiet hours, locale, templates
Idempotency: retries without duplicate user notifications
Global: multi-region ingestion and delivery

Expected output

Propose the architecture, data model, and pipelines for:

ingestion → fan-out → per-channel delivery
retry policies + DLQ
dedup/idempotency strategy
observability and experimentation (gradual rollout/A-B)

Design a global real-time notification system

Problem

Requirements to clarify

Expected output

Solution

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