Update README.md

README.md

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 ## Design goals
 
-The soda machine is a web app, which utilizes the method of Domain-Driven Design (DDD) of Eric Evans. The goal of this project is to show how the out-of-the-box tools of Spring can be used to make a proper clean domain model in DDD. 
+The soda machine is a web app, which utilizes the method of Domain-Driven Design (DDD) of Eric Evans. The goal of this project is to show how the out-of-the-box tools of Spring can be used to make a proper clean domain model in DDD. DDD is a modeling technique, where a domain model is the edge of the application.
 
 ### In detail
 
@@ -28,6 +28,32 @@ Soda machine is based on the hexagonal architecture. This architecture style use
 
 ![Hexagonal architecture is used for the application](hexagonal.png)
 
+### Domain model
+
+The application is organized by the following aggregates:
+
+* **Automaton**: On each start an automaton is created, which can be switched on and off
+* **Recipe**: Holds the recipes with its ingredients, e.g. black coffee, white coffee, milk and soda
+* **Price**: Hold the prices of each of the recipes in Cent, which is dependant on a separate priceId
+* **Selection**: Holds the selected recipes of each automaton and if selecting recipes is possible
+* **Pay**: Holds the inserted coins in a money slot and the register of the automaton
+* **Mixing**: Holds the Mixer state and the inventory boxes with their fills (like a coffee, milk or soda box)
+* **Display**: Holds the output messages of each automaton, which is displayed to the UI
+* **Trace**: Traces all the events in the application and prints it to the console
+
+E.g. a `Display Message` aggregate looks like this. It extends the `AbstractAggregateRoot` interface, is annotated by `@Entity` and has a message Id annotated by `@EmbeddedId`. It contains domain methods like `sendToWebsocketPort(Port)` and registers events via `registerEvent(Event)`, which are fired on persistency.
+
+```java
+@Entity
+@Data
+@EqualsAndHashCode(callSuper = true)
+public class DisplayMessage extends AbstractAggregateRoot<Message> {
+    @EmbeddedId
+    private MessageId id;
+    private String message;
+}
+```
+
 ### Event model
 
 The event flow was created using the event-storming method of Alberto Brandolini, which helps to properly design the application events upfront in an unobtrusive way by using events as the first-class citizen.