What is a Plugin ?
A plugin adds a special feature or functionality to the broker. Common use cases for plugins are custom authentication & authorization or implementation of your own business logic and integration into your existing environment.
Which plugins are available ?
How do I build my own plugin ?
See the Plugin Developer Guide for detailed Information on how to develop custom HiveMQ plugins.
How hard it is to build my own plugin ?
If you are accustomed to Java, it is pretty simple and straightforward.
With the additional tooling (including maven plugin and archetype) which is part of the Plugin SDK it is very easy to get started and very simple to set up a sophisticated development environment, including debugging in your favorite IDE, in a matter of minutes.
You can also take a look at the example plugins on github to get a good starting point for your own plugin.