Example project for multi-module library architecture that tries to follow SOLID principles.
The idea is to have small modules responsible for a strict thing that you can reuse across your stack. If a bug emerges — it will only need to be fixed in that one place. Your codebase will be super-easy to rearrange and you will stand victorious against the vicissitudes of your project requirements.
Feel free to fork it and use it in anyway you want!
As you can see, the main layer is an interfaces-api module, which is being maintained as a separate library.
It only contains interfaces of all objects used across the stack.
The concrete implementation lies in separate modules that implement the interfaces — like cloud-module,
or core-module for example. In such way, every module needs only to declare a dependency on interfaces-api.
This will enable them to access other parts of your stack via those interfaces,
thus making them independent from concrete implementation.
I used here RxJava because the combo RX+Kotlin is so powerful :)