Build Decision Service Logic

In this lab you are going to create a new decision logic from scratch. The main goal is to learn how to build a decision logic taking advantage of the powerful DMN notation.

Before proceeding, you can also review the following tutorials to get a first introduction to the DMN basis:

Learn DMN in 15 minutes

DMN Tutorial - Video

This hands on is not going to give you a step by step guidance on how to craft the DMN, it’s structured as a puzzle that you have to solve with some hints.

Challenge description

A Cart contains multiple items, an item is defined by a category and a price. The outcome of the decision is the Final Price of the cart. A first discount is evaluated on every item in the cart depending on the category: for the food category, the discount is 5% if the price is more than 19 otherwise 2%; for all other categories the discount is 10% if the price is more than 15. Finally, if the overall amount of the cart (after the item discount) is more than 70 then apply an extra discount of 10%.

Start creating the file: discount.dmn inside the project created in the previous hands on.

The folder for DMN and BPMN artefacts is: src/main/resources/.

Hints

Create the Data Types

Add:

The type for the item: tItem with category and price
The type for the list of item: tItemList
The type for the list of numbers

Create the Decision Requirement Graph

The input is the Cart which is a list of item.
The output is a decision called Final Price which is a number.
Create a Business Knowledge Model called Discount, which goal is to compute the discount for an item.

Implement the Decision Nodes

Open the Discount BKM:

the input parameter is item typed tItem
the output is a number
the core of the BKM can be implemented as a decision table:
1. Input clause item.category (string)
2. Input clause item.price (number)
3. Output clause Discount (number)

Open the Final Price decision node:

Select context as logic type
Implement the following entries:
discounted prices the list with the discounted prices
discounted total the sum of the discounted prices

FEEL syntax reminder:

A BKM is function, the syntax is: Discount(item)

The for loop expression iterates over lists of elements or ranges of numbers. The general syntax is:

for i 1 in ic return e

where ic stands for iteration context and can be:

a list
a range e.g. 1..10

The function sum return sum of numbers:

list = [1,2,3]
sum(list) -> 6

Run locally the Decision Service

Start quarkus in dev mode:
```
mvn quarkus:dev
```

Create the file: src/test/resources/dmn-test.http with the following content:

POST http://localhost:8080/discount
Accept: application/json
Content-Type: application/json

{
   "Cart": [
      {
      "category": "electronics",
      "price": 80
      },
      {
      "category": "food",
      "price": 20
      }
   ]
}

Click on the Send request link to probe the DMN service. In the result page you should spot: "Final Price": 81.9

If you haven’t installed the Rest Client plug-in, you can probe the DMN service through the Swagger UI: http://localhost:8080/q/swagger-ui/

Create a Test Scenario

Add to the pom.xml file the following dependency:

<dependency>
   <groupId>org.kie.kogito</groupId>
   <artifactId>kogito-scenario-simulation</artifactId>
   <scope>test</scope>
</dependency>

Create the Java class KogitoScenarioJUnitActivatorTest.java in the folder src/test/java/testscenario

Add the annotation @RunWith as in the snippet:

import org.junit.runner.RunWith;
import org.kogito.scenariosimulation.runner.KogitoJunitActivator;

@RunWith(KogitoJunitActivator.class)
public class KogitoScenarioJunitActivatorTest {

Create the file DiscountTS.scesim in the folder src/test/resources/
In the dialogue box Create Test Scenario
- Select DMN, then discount.dmn and Create button.
- You should get a quite simple table with a column named GIVEN and another named EXPECTED.
In the GIVEN column, double click on the last cell (there is a place holder text: Insert value)
- In the pop up window, select Add list value
- Type "food" in the category field (make sure to type the double quote ")
- Type 20 in the price field
- Click the confirmation button (check mark)
- Click the Save button
In the EXPECTED column, double click on the last cell (there is a place holder text: Insert value)
- Type 0
From a terminal launch the tests: mvn clean test
You should get a failure message: KogitoScenarioJunitActivatorTest #1: Failed in "Final Price": The expected value is "0" but the actual one is "19.00" (DiscountTS)
Change the test scenario accordingly and run the test again to check the successful execution.
Optionally, add other rows to the test scenarios.

Appendix: a possible solution

DON’T READ THIS SECTION try before to solve the challenge with hints provided above. It’s not important if your solution does not work as expected or it’s different from the following: the philosophy of this hands on is that you can learn more from a failed attempt than from a "guided" success.

Data types

Decision Requirement Graph

Decision Nodes