I had the opportunity to chat with Florian Raschbichler and Anja Helmbrecht-Schaar from the company dc-square GmbH, who are developers of the HiveMQ Broker.

If you are new to MQTT they have the ultimate kickstart for MQTT beginners. They also maintain an MQTT Client Library Encyclopedia

Links

https://www.hivemq.com/

https://www.dc-square.de/en/

https://en.wikipedia.org/wiki/Comparison_of_MQTT_Implementations

https://en.wikipedia.org/wiki/MQTT

https://www.hivemq.com/blog/mqtt-essentials/page/2/

https://github.com/hivemq

https://www.hivemq.com/mqtt-client-library-encyclopedia