The broker will act as an intermediary between OpenMQTTGateway and your controller, it can be seen as a centralized hub at the heart of your automation system. It carry on messages following a publish / subscribe mechanism.
All the events or commands could pass by the broker.
There is many choice of brokers, here is some of the most populars:
- Mosquitto (opens new window) (Open source)
- Moquette (opens new window) (Open source)
- HiveMQ (opens new window)
- Embedded MQTT brokers (Home Assistant and OpenHAB)
This wikipedia list (opens new window) gives you more details about the different choices you have. This github list (opens new window) seems to be the most exhaustive ones. Here is also some ideas of criteria (opens new window) from HiveMQ.
Once your broker is installed it can be interesting to see the traffic passing to it and to publish data, so as to do that there are several tools available: