Design a Real-Time Notification System

Focus: scale, async boundaries, and trade-offs.

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Scenario brief

Brief

Goal

Design a real-time notification system: events (likes, messages, alerts) fan out to users quickly via push (WebSocket/SSE) and optional email/push later.

Functional requirements

Producers publish notification events (e.g. “user X liked your post”).
Online users receive near-real-time push on an open connection.
Persist notifications so users can read history when they come back online.