Design a global real-time notification system
Company: Google
Role: Software Engineer
Category: System Design
Difficulty: medium
Interview Round: Onsite
## 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)
Quick Answer: This question evaluates competency in designing large-scale distributed systems for global real-time notifications, covering scalability, multi-region ingestion and delivery, multi-channel fallback (push, email, SMS), delivery semantics, idempotency/deduplication, latency/throughput SLAs, and observability.