Modular Monolith Exercise

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

API Overview

• Catalog: Search product
• Catalog: Get product details
• Order: Place order, returning a redirect url to a payment page
• Inventory: Add stock
• Inventory: Get stock bulk
• Inventory: Reserve stock (confirmed via OrderConfirmedEvent)
• Customer: Get customer address

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.

Modules cannot form dependency cycles.

A diagram overviewing the module interactions is auto-generated at each test run. See index.adoc

Exercises

Taking small baby-steps, implement the changes below. Expand the hints if needed, and keep running ArchitectureTest:

1. Return the number of items currently in stock from GetProductApi

• catalog should not access any internal class of inventory module (run tests).
• GetProductReturnsStockE2ETest should pass.
• Hint

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

2. Pull payment-related classes out of order module into a separate payment module.

   • Check there are no illegal internal calls or cycles with ArchitectureTest
   • 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 the least amount of public stuff
   • Hint

     Move classes in a subpackage, like 'impl'

3. SearchProductApi should only return products in stock

   • What options you see? Tradeoffs of each?
   1. Option

      Find all 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

   • E2E Test should still pass