In Chapter 5, we looked at content definitions as a way of adding support for adding complex external data sources to OpenCms. In OpenCms 5, the developers added a significant extension to the content definition concept. This extension, called Master Module, was intended to alleviate the need to create separate tables in OpenCms for storing non-native data. The Master Module mechanism provides features, such as locking and publishing, that are available to built-in OpenCms types, but not to content definitions. In addition, Master Modules were supposed to be much simpler to create than content definitions. However, rather than becoming less complex, Master Modules added even more complexity to the already complex content definition mechanism.

As a result, OpenCms developers gave up on the idea. They did not supply documentation or even (as far as I know) release any code that used the API. But since the code is open source, external developers have discovered the API and have...