Introduction
The light-4j service module is a singleton service factory which is an IoC service to allow developers and operators to switch implementations of a specific interface in service.yml config file.
You can have two implementations for one interface and two service.yml file in both main and test folder to use one instance for release and another instance for testing.
Or, you can have a debug implementation in your service and enable it on production when something is wrong, or switch two different data sources on the fly.
For more detail on the module, please take a look service module.