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Messaging & Realtime · Medium

Design a Real-Time Notification System

Mini Map

Drag a component from the left onto the canvas

Scenario 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.

Non-functional requirements

  • Low latency for online push (seconds matter).
  • Handle fan-out: one event may notify many recipients.
  • Don’t block the main API write path on slow delivery — use async.

Scale assumptions

Assume ~5M DAU, ~20 notifications generated per user/day on average → ~100M notifications/day. Peaks several× higher.

Your canvas task

Show async fan-out: Client → Gateway/LB → App → Queue; Client → Realtime Gateway for online push; DB stores history.

Capacity check (optional)

Optional — 2 quick estimates from the scale above. Skip anytime; skipping does not fail you, it only skips a small bonus.

1.~5M DAU × ~20 notifications/user/day. About how many notifications/day?

2.~100M notifications/day averaged over 86,400s. About how many notifications/second?