We use RabbitMQ and among others we are using following features:<p>- persistency<p>- high availability<p>- easy clustering<p>- ability to turn on MQTT plugin
We use a janky vpn system where every device connects to a mother server on the vpn and sends messages to a port on a GO server. Looking to transition this to a real MQTT broker soon (hopefully before scaling forces us to). We'll probably use AWS IOT rather than brew up our own. I have used Xively in the past as well. For testing I use HBMQTT (python based open source).