How WhatsApp Works: A Step-by-Step System Design Walkthrough

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A DEV Community article documents a real adaptive learning session in which two developers, Pedro and João, collaboratively designed a WhatsApp-like messaging system from first principles. Rather than presenting concepts upfront, João guided Pedro through a series of engineering problems, letting solutions emerge organically from each challenge. The session covered key topics including WebSockets for persistent connections, load balancers for distributing traffic across multiple servers, and Redis as a shared state layer to track user-to-server mappings. Each solved problem revealed a new one, such as how to handle stale state in Redis when a user unexpectedly loses connectivity. The article aims to preserve this problem-driven learning process as a reusable educational resource for engineers studying distributed systems.

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