Scalable WebSockets with NestJS and Redis

In the realm of application development, especially for young projects, scalability is often overlooked in favor of rapid feature implementation. However, ensuring scalability from the onset is crucial, as highlighted by the integration of WebSockets into a NestJS application. This integration posed challenges, particularly in transitioning a stateless application to one with state, which complicates scalability. Stateless applications benefit from technologies like Docker and Kubernetes, allowing for easy creation and disposal of instances. Introducing WebSockets requires a state to manage open connections, leading to issues such as events dispatched by one instance not reaching clients connected to another. The solution involves using Redis' publish/subscribe mechanism to share open connections across instances, ensuring all clients receive events. The article explains setting up Redis in a Docker container, using ioredis for communication, and creating a RedisService to manage event propagation. It also discusses creating a socket state management system to track user connections and a custom adapter for socket.io within NestJS. This approach maintains scalability and ensures events are correctly propagated across multiple instances, emphasizing the importance of early scalability considerations for future-proofing applications.