Drupal node update detection
In the classic HTTP model, the server is passive and can only return information to the client when it request it. So, in this model, to alert the users when a node is updated, you have to periodically poll the server the ask it if the node has been updated. If you have a lot of clients waiting for the update of a lot of nodes, this can quickly overload your web server with too much request to handle.

Today, the best solution for this kind of requirements is to use WebSockets or a similar full-duplex technologies. Drupal and PHP are not designed to serve a WebSockets. Technologies like node.js and Socket.IO,  or APE are more suited.

The Node.js integration module integrates node.js and Socket.IO in a easy to use solution. It provides an API to easily implements your need.