5.3 Pub/Sub for Real-Time Notifications

A user's browser holds a WebSocket connection to exactly one application server, server A. When something happens that the user should see, such as a new chat message or a price change, the code that detects it may run on server B. Server B knows about the event, but only server A can push it to the user. Redis Pub/Sub solves this routing problem: every server subscribes to channels, any server can publish, and the message reaches whichever server holds the connection.