Design Slack’s Unread Badge · System Design Interview

Design Slack’s Unread BadgeDesign the thing that tells someone which of their channels have something new in them, and how much, the instant they open the app. Cheap writes, per-user state and read amplification are the whole exercise.What it has to do

Opening the app shows, per channel, whether there is anything unread and how many messages

Reading a channel clears its badge on every device that person uses

A message posted to a channel raises the badge for everyone in it

Scale

Daily active users: 30 million

Messages per day: 2 billion

Out of scope

Message storage and history

Delivering the message itself

Email and push notifications

Threads

Practise this challengeAll system design challenges

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