DDD with Kotlin

This is a simple demo project to show a Domain Driven Design with Kotlin.

Note that there is a similar version of this demo which is using event sourcing at event-sourcing-with-kotlin.

Run it

./gradlew bootRun

Then simulate an incoming event:

POST to http://localhost:8080/master_data_update
POST to http://localhost:8080/media_data_update

Example Use Case

The subject of this demo is the Product Service. The Product Service is located between five other components. The Master Data Service will push new products to the Product Service. This is the beginning of our business process. The Product Service will register each new product at the Media Data Service. After a product is registered, the Product Service will receive updates for this product from the Media Data Service. After an update was received, the Product Service will:

• Update the CDN if media data has changed
• Update the Shop and search index if master data has changed

+-------------+
| Master Data |
|   Service   |
+-------------+
   |                     << DEMO >>   
   +------------------► +---------+
    1: update product   | Product |
                        | Service |---------+---------+
               +------► +---------+         |         |
      3:       |             |      2:      |         |        5:
 push updates  |             | register new |         | update if master 
               |             |   product    |         | data has changed
   +------------+            |              |         |
   | Media Data | ◄----------+              |         +----------+
   |  Service   |                           |         |          |
   +------------+        4: update if media |         |          |
                          data has changed  |         |          |
                                            ▼         ▼          ▼
                                         +-----+  +------+  +--------+ 
                                         | CDN |  | Shop |  | Search |
                                         +-----+  +------+  +--------+

What to see

• A domain entity called Product.kt which encapsulates business logic and throws domain events.
• A process flow with events ("something has happened") and commands ("now do something").
• Value objects such as ProductNumber.kt or ProductInformation.kt.
• A ports-and-adapters package layout.
• An anti-corruption layer for external events - they will be transformed to internal commands.

Resources

• https://martinfowler.com/tags/domain%20driven%20design.html
  • A collection of articles by Martin Fowler. Each article enlightens a different aspect of DDD.
• https://docs.microsoft.com/en-us/dotnet/standard/microservices-architecture/microservice-ddd-cqrs-patterns/ddd-oriented-microservice
  • First article of series by Microsoft on how to use DDD for microservices. Although the series is using C#, the examples are easy to understand.