Design the real-time delivery backend for a chat app - persistent client connections fanned out across a sharded server fleet.
Design the real-time message delivery backend for a chat application (modeled on how Slack delivers messages). Each connected client holds a persistent connection to one of several backend server instances, and a message posted to a channel must be fanned out to every member currently connected, wherever they're connected. No schema or REST contract is required - just the component diagram.