Design WhatsApp Message Fan-Out · System Design Interview

Design WhatsApp Message Fan-OutDesign the delivery path of a messenger at WhatsApp’s size, where the interesting parts are that most recipients are offline and the server is not allowed to read anything. Availability, delivery guarantees and multi-device are the whole exercise.What it has to do

A message reaches every recipient device exactly once, in order, however long that device stays offline

The server routes and stores messages it cannot read

A user’s several devices all receive the message, and all show the same conversation

Scale

Monthly users: 2 billion

Messages per day: 100 billion

Devices connected at peak: 500 million

Out of scope

The encryption protocol itself

Voice and video calls

Media transcoding

Status and channels

Moderation

Practise this challengeAll system design challenges

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