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We're using RabbitMQ in a new project. We'll have IoT devices communicating with queues.

For the devices to send info to the cloud we don't see any issues, however sometimes we need to deliver messages from our backend to the IoT devices. For this we let the devices open an exclusive queue. This works perfectly, as long as the devices are online. When they aren't, the queue is closed and no messages can be send to it anymore.

Is there a way to keep the queue open, so messages are kept until the IoT device comes back online?

Vice-versa: Is there some way to have guaranteed delivery starting at the IoT device. For example: energy measurements every 15 minutes. If the connection drops, messages should be stored on disk (to prevent message loss in case of power cut). They are sent later on when the connection comes back online. Does a service or client library exist that implements this or do we need to develop this ourselves?

Is there a way to keep the queue open, so messages are kept until the
IoT device comes back online?

Use a regular queue, and make sure it's durable if you'd like it to survive RabbitMQ restarts.

Is there some way to have guaranteed delivery starting at the IoT
device.

That depends on the library you are using, but you don't tell us what library nor what protocol you're using (AMQP vs MQTT, for instance).

Some libraries offer automatic reconnect and re-creation of topology (queues, exchanges, etc) but I'm not aware of any that offer local storage of messages until the broker is available again. You'll have to code that yourself.

Please carefully read the documentation with regard to publisher confirmations and consumer acknowledgements, as those are both necessary for reliable messaging link.

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