Modular Monolith Exercise

Uses Spring Modulith https://spring.io/projects/spring-modulith/

Module Overview

• catalog: product details management & search
• order: order workflow
• inventory: stock and reservations
• customer data
• shipping

Spring Modulith Intro

A module can only reference by default the classes in the top-level package of other modules. A class in any subpackage (eg. impl) is considered implementation detail, and it should NOT be accessed from outside that module. Extra packages can be exposed via @NamedInterface

Modules cannot form dependency cycles.

index.adoc contains a diagram of the module interactions generated at each test run.

Exercises

Expand the hints if needed, and keep running ArchSpringModulithTest:

1. In GetProductApi return the stock in the response.

• catalog should not access any internal class of inventory module (run tests).
• Enable and Pass GetProductE2ETest.
• Hint

  Retrieve the stock item number via a call to a new method in `InventoryInternalApi`

2. Extract payment-related classes out of order module into a new payment module.

• Fix the encapsulation violation and the dependency cycle introduced.
  • Hint to fix 'non-exposed..':

    Code having to do with the `order` internals should stay in `order`.
  • Hint to fix cycle:Solution#1

    Have a `PaymentCompletedEvent` thrown from payment back into order
  • Hint to fix cycle:Solution#2

    Introduce an interface in one of the modules implemented in the other (aka Dependency Inversion). Which module should hold the interface?
• Encapsulate the new payment module: hide as many classes exposing as few classes to other modules
  • Hint

    Move classes in a subpackage, like 'impl'

3. SearchProductApi should only return products in stock

   • Test SearchProductsApiTest should pass.
   1. Option

      Search for matching products and join in-memory with all stock. Or vice-versa.
   2. Option

      JOIN Stock via SQL/JPQL😐
   3. Option

      Replicate stock item number at every change via events from `inventory`
   4. Option

      Publish `OutOfStockEvent` and `BackInStockEvent` from `inventory`, updating a `Product.inStock` boolean;
   5. Option

      Join the Product with the StockView @Entity exposed by `inventory`

4. Notifications

   • When payment is confirmed

   • Hint - code snippet

      public void onOrderStatusChanged(OrderStatusChangedEvent event) {
        String customerEmail = customerModule.getCustomer(event.customerId()).email();
        if (event.status() == OrderStatus.PAYMENT_APPROVED) {
          sendPaymentConfirmedEmail(event, customerEmail);
        }
        if (event.status() == OrderStatus.SHIPPING_IN_PROGRESS) {
          sendOrderShippedEmail(event, customerEmail);
        }
      }
     

5. Move Reviews-related stuff out of catalog module

   • tests should pass