Introduction

Every day, billions of people send messages using applications like WhatsApp, Telegram, Signal, Messenger, and iMessage.

Sending a simple "Hello" looks easy from the user's perspective.

Behind the scenes, however, the system has to solve problems like:

• Deliver messages in real time
• Support offline users
• Keep messages ordered
• Synchronize multiple devices
• Handle group chats with thousands of members
• Encrypt messages end-to-end
• Scale to billions of users

This series explains the architecture commonly used by modern messaging platforms. While the examples are inspired by WhatsApp, the concepts apply to most large-scale messaging systems.

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