Primary and Secondary: How WhatsApp's Multi-Device Architecture Works

This Habr article explores the architectural differences between primary and secondary devices within the WhatsApp ecosystem. The author explains that linking an app to a laptop or web browser is not a traditional account login, but rather the creation of a 'companion device.' WhatsApp's internal logic distinguishes between these device types at the code level, which dictates their functionality, authentication methods, and behavior when disconnected. The primary device (the phone) acts as the control center, while linked devices rely on it for data synchronization. The analysis focuses on the conceptual interaction model, explaining how the application's binary code switches between these two operating modes to ensure the security and stability of the multi-device system.