kivicko / spring-modulith

Modular Monolith Coding Kata

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

Domain Overview

Catalog

• Search product (REST)
• Get product details (REST)

Order Management

• Place order (REST): returns a redirect url to a payment page
• calls shipping provider with customer.address to get a shipping number

Payment

• Calls Payment Gateway to get payment redirect URL
• Is called back by Payment Gateway to report result

Shipping

Inventory

• Add stock (REST)
• API get stock bulk [pid,pid...]
• API reserve stock (orderid, items:[{},{},{}])
• publish OutOfStockEvent/BackInStockEvent
• listen OrderConfirmedEvent(orderid) to effect the stock reservation

Customer

• API get customer address (needed at shipping)

Spring Modulith Intro

A module can only reference by default the top-level package of other modules. Any class in a subpackage (eg. impl) is considered implementation detail and it's unaccessible from outside that module.

Modules cannot form cycles.

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

Exercises

Taking small baby-steps, try to implement the following changes in code:

1. Pull payment logic out of order module into a separate payment module.
   • Check there are no illegal internal calls or cycles with ArchitectureTest
   • Hint

     Have an event thrown from payment back into order
   • Hint2

     That event can be named PaymentCompletedEvent and should be placed in the payment module
   • Encapsulate the new 'payment' module: hide its internals exposing the least amount of public stuff
   • Hint

     Use an 'impl' package for simplicity
2. Return the number of items in stock in product page (see GetProductApi)
   • Respect the encapsulation of inventory module (ArchitectureTest should pass)
   • Hint

     Retrieve the stock item number via a call to a new method in `InventoryModule`
3. Search should only return products in stock (see SearchProductApi)
   • What options can you identify. Tradeoffs of each?
   1. Option

      Find all products and join in-memory with all stock. Or vice-versa.
   2. Option

      Force a JOIN via SQL/JPQL
   3. Option

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

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

      A separate ElasticSearch engine to which you publish product{id,name} and stock{items} (just imagine)
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);
        }
      }