Understanding services

Let's begin our journey of understanding services with a concept that you should be very familiar with as a Java developer--polymorphism. It starts with one interface and (possibly multiple) implementations of that interface. Although interfaces are not strictly necessary for services, they are still a good place to start. Let's say you define a service interface called MyServiceInterface that looks like this:

    package service.api; 
    public interface MyServiceInterface { 
      public void runService(); 
    } 

Now you can have multiple modules containing classes that implement this interface. Since all those modules need access to this interface, let's throw this interface into a module of its own, called service.api, and expose the package that the interface MyServiceInterface is in. Then each implementation module can require the service.api module and implement MyServiceInterface.

Consider there are three implementations of MyServiceInterface in three corresponding...