Design a Presence Service · System Design Interview

Design a Presence ServiceDesign the service that tells a messaging app which of a person’s contacts are online right now.What it has to do

A user shows as online while their app is connected

Their contacts see the change within a few seconds

A user whose app disappears without warning is eventually shown as offline

Scale

Registered users: 500 million

Connected at peak: 60 million

Out of scope

Messaging itself

Typing indicators

Last-seen history

Mobile push notifications

LetsDiscuss

Loading your session…