Under the hood, how is a FIFO queue turned into a priority queue in a distributed fashion? Are they actually swapping the underlying datastructure, or is it a "hacked" fix
When a message is published with a priority header, the message with the higher priority value gets placed on the head of the queue. This is done by actually swapping the messages in the queue. This is all done when the message is waiting to be consumed in the queue. In order to allow RabbitMQ to actually prioritise the messages, set the basic.qos of the consumer as low as possible. So if a consumer connects to an empty queue whose basic.qos is not set and to which messages are subsequently published, the messages may not spend any time at all waiting in the queue. In this case, the priority queue will not get any opportunity to prioritise them.
Reference: https://www.rabbitmq.com/priority.html