
#4 Scaling Chat to 50 Million Users
In this episode of Tech Interview Prep, we apply our 4-step system design framework to one of the most common and challenging interview questions: designing a global chat system like WhatsApp or Facebook Messenger. We begin by establishing a design scope that supports one-on-one messaging, small group chats, and online presence indicators for 50 million daily active users. We explore the high-level architecture required for real-time communication, explaining why WebSockets are preferred over traditional HTTP for bi-directional messaging. You will learn about the unique challenges of managing stateful chat servers and how service discovery tools like Apache Zookeeper are used to coordinate connections and avoid server overloading. In our technical deep dive, we break down the end-to-end message flow—from unique ID generation to message synchronization across multiple devices. We also discuss the "insider" details of presence indicators, including the use of heartbeat mechanisms to handle unstable internet connections and the publish-subscribe model for updating friend status. Finally, we analyze the critical trade-off of choosing NoSQL key-value stores over relational databases to handle the enormous "long tail" of chat history data. Join us to master the blueprint for building low-latency, distributed communication systems.