Great project! Is there any concept of ack/nack a message? Let's say the subscriber get a message and failed before processing it. Does the message go back to the queue?
However, messages do not disappear from the queue when received. All subscriber requests are idempotent, and can be repeated so long as the message queue is storing the message (which is a configurable parameter). So if a subscriber failed before processing it, it's free to request the same message again.