@Transactional
  public Order createOrder(long customerId, Money orderTotal) {
      ...
      creditManagement.reserveCredit(customerId, order.getId(), orderTotal);

@Service
public class NotificationServiceImpl implements CustomerDomainObserver {

  @PostConstruct
  public void registerCustomerDomainObserver() {
    customerDomainObservers.registerObserver(this);
  }

  @Override
  public void noteCustomerCreated(CustomerInfo customerInfo) {...}

To reduce build-time coupling within a domain module, each domain module consists of multiple Gradle sub-projects. Typically, the sub-projects correspond the elements of the hexagonal architecture - domain or adapter. For example, the Customers domain consists of multiple Gradle sub-projects including customers-domain, customers-web, and customers-persistence. An important benefit of this design is that it reduces build-time coupling. A change, for example, to the web adapter doesn’t require the domain or persistence adapter to be recompiled/retested.

The Customers and Notifications domain modules - specifically their domain logic - are used by the Orders domain module. In order to minimize build-time coupling, each of those domain modules has a Domain API Gradle sub-project, which defines that module’s API. For example, the Customers domain has a customers-api Gradle sub-project, which contains its API classes.

A domain module’s clients only depend on its Domain API Gradle sub-project. Moreover, its clients are tested using mocks of the API. For example, the Orders domain is tested using a mock of the Customers domain’s API. As a result, a domain module’s implementation can be changed without having to recompile/retest its clients.

To further reduce build-time coupling, the Customers domain module uses the Interface Segregation Principle (ISP) to define narrower APIs. It has three Domain Modules API Gradle sub-projects:

• customers-api - defines CRUD operations

• customers-api-credit-management - defines operations for managing credit

• customers-api-observer - for registering observers of the Customers domain

As a result, each client only build-time coupled to those APIs that it’s actually using.

This repository includes a series of Code Tours that explain the design of the application. You can use the code tours in either Visual Studio Code or Github Codespaces.

1. Install the Code Tour extension from the Visual Studio Code Marketplace

2. Use the CodeTour: Start Tour command from the command palette to start the tour

1. Create the codespace

2. If necessary, install the Code Tour extension from the Visual Studio Code Marketplace.

3. Use the CodeTour: Start Tour command from the command palette to start the tour.