Key concepts before diving into code
Use
SignalR is a framework for building asynchronous applications. For web developers, that means I can build applications that break the request->response cycle of the web and move to more of a one-on-one connection that old client server architectures used to offer.
With SignalR, the server can call a JavaScript methods on all the clients by itself when updates are required. The library will handle the connection needed to achieve this: by default WebSocket is used, but it will fallback automatically to older connections types if WebSocket is not available in the browser. The JavaScript can also call the server: this can already be done with AJAX, but if two-way communication is needed it may be easier and cleaner to do it all with SignalR.
So, using a real-time library is the way to go if you want to build an application that requires collaboration between users. Common uses cases includes editors, social networks, chats
Backplane
With a backplane enabled, each application instance sends messages to the backplane, and the backplane forwards them to the other application instances. (In electronics, a backplane is a group of parallel connectors. By analogy, a SignalR backplane connects multiple servers.)
Systems