Communicating with a huge number of clients requires a simple, fast and reliable solution. Message queues seem to solve this problem in a very efficient way. There is a reason why many people treat MQTT as the "language of the Internet-of-Things".
One (or many) of my future projects - for example: a private telegram bot - will need a central always-on device. So I had to find a good device for this use-case. Some of my criteria were: "powerful"/modern CPU, enough RAM to do some tasks in a parallel way, Ethernet or Wireless LAN, at least one USB-port, small and reasonable! But the most important feature: It should be able to run Node.js!