Multiple outstanding sent messages with QoS?

[AndrewHeim]

Does this library have outstanding sent messages with QoS?

A bit more background: MQTT has a limit on the max message size - I get the feeling that it may possibly vary a bit from platform to platform, but I've reached the conclusion that an MQTT message is within an order of magnitude of a UDP datagram (65k bytes). I forget at the moment whether it's closer to half or double.

When you have a high-latency link, like say an IoT device, the number of outstanding messages (with QoS) can strongly influence the total effective bandwidth. If each message sent needs to receive acknowledgement before the next one is sent, with 100+ms between each, that puts a damper on the data you can send.

After looking through the code, it gave me the impression that it only had one packet outstanding at a time. It seemed to send a message, and if QoS was enabled it would wait there until receiving acknowledgement before returning (and sending the next message). This would be opposed to a scheme where multiple messages could be sent and are waiting outstanding, pending an acknowledgement. But I could be wrong in my conclusion, depending on how those VIs were called - the architecture is well abstracted and appears very extensible.

Did I understand that right? I'm looking for a library that supports multiple messages outstanding, so I can get a bit higher throughput. Not massive throughput of course, since MQTT isn't the MOST efficient thing out there, but a bit higher than 3-4 messages per second at that size. (It's important to use such a messaging layer with scenarios where a TCP session might break, because those buffers don't tell you exactly what data made it and what didn't - MQTT or such a layer does.)