You can use a direct queue for this. Just have the phone, desktop, laptop, et. al. consume from the same queue. Rabbit gives out messages in a round-robin to the listening consumers, and the first to pickup the message is the only queue to get the message.

Then, let's say there's an error in the phone notification and you want to pass it back and let the next service try. This is where Rabbit's ACK system can come into play: you can setup rabbit to wait for an ACK response from a consumer acknowledging that the message was received and processed. If the consumer errors, you can return the message back to Rabbit and it will be given to the next consumer in line (which is the next consumer not busy).

If a service is not online, then they won't be consuming from the queue, so they won't get the message. If there are no consumers, then the message will wait in the queue until a consumer binds to consume the message.

Hope this makes sense!

You could share one queue amongst several devices, and Rabbit will ensure that only one device gets the message (with proper acking on the subscriber side).

If a message gets sent to an exchange that has no queues attached, typically it will get sent to a dead letter exchange.

Edit: Most Rabbit client libraries have two different modes, one where you automatically acknowledge any message that is received, and another mode where you must explicitly acknowledge that you've received the message.

I was wondering about this also. I'm not sure if there is an acknowledgement system built into RabbitMQ, but it sounds like that is what would be needed for this to happen.

By default any message picked up from a queue by a consumer is removed (acknowledged), but you can configure the exchange so that it waits for an ACK from the consumer before removing the message. This gives the consumer the ability to return the message to the queue, but is dangerous because Rabbit will never stop waiting on the consumer to respond.