I am working on a project with Paho on Java. Messages received from the broker are sent to an API that processes them, but this process might fail or take to long. I would like to take advantage of MQTT acknowledgement system and, in such cases, prevent the ACK to be sent so the broker resends the same message after a while. Is this possible with Paho?
Regards.
There is no way to guarantee that this will work. Message retries are implementation specific.
Section 4.4 of the MQTT v3.1.1 spec states
When a Client reconnects with CleanSession set to 0, both the Client and Server MUST re-send any unacknowledged PUBLISH Packets (where QoS 0) and PUBREL Packets using their original Packet Identifiers [MQTT-4.4.0-1]. This is the only circumstance where a Client or Server is REQUIRED to redeliver messages.
The emphasis is mine.
The trend I've seen is for implementations to remove message retries except for when reconnecting.