Service types

There are two types of services:

1. Stateless.
2. Stateful.

In general, services should be stateless but there are some exceptions from the rule. When working in traditional environment, state is not the problem since each request-response happens in separate environment.

When working with [RoadRunner](https://github.com/yiisoft/docs/blob/master/guide/en/tutorial/usi

Provide possibility to create instances with dependencies provided from other modules.

class RepositoriesModule extends Module {
  RepositoriesModule () {
    // PersonRepository depends of PersonWebService
    factory(DependentRequestsRepository(inject()), () =>
        DependentRequestsRepository(inject()));
  }
}

class WebServicesModules extends Module {
    WebServicesMod

When a field's or method's class is not present it is virtually impossible to find from which class it comes. This needs to be improved by e.g. introducing a logging mechanism – configurable so it can be disabled, or that a custom logger can be set?