Billing Application

Overview

The Billing App calculates the final bill amount for a user after applying various discounts based on their user type (Employee, Affiliate, or Customer) and discount on bill amount. Additionally, the service supports real time currency conversion for bills by integrating with an external exchange rate API.

Key Features

• Dynamic discount calculation based on user type using the Strategy Design Pattern.
• Additional discounts on bills over certain amounts.
• Realtime currency conversion for the final bill amount using an external API.
• Easily extendable architecture to add new user type, discount strategies or currency conversion logic.

High-Level Design Details

1. Billing Controller & Service:

   • BillingController handles billing API request.
   • BillingService does the overall billing process, including discounts and currency conversion.

2. Discount Strategy Factory:

   • Implements the Strategy Design Pattern to dynamically choose the correct discount strategy based on the user type.
   • User types supported: EMPLOYEE, AFFILIATE, and CUSTOMER.
   • Using Factory and Strategy design pattern enables scalability of new user type and discount rules.

3. Discount Strategies:

   • EmployeeDiscount: 30% discount on non-grocery items.
   • AffiliateDiscount: 10% discount on non-grocery items.
   • CustomerLoyaltyDiscount: 5% discount on non-grocery items for customers associated for more than 2 years.

4. Currency Exchange Rate Service:

   • Integrates with an ExchangeRate-API to fetch currency conversion rates.
   • Used Pair API to get conversion rate between two currencies.

5. Resilient and Modular Design:

   • Ensures separation of concerns between billing logic, discount calculation, and currency conversion.
   • Log information for debugging and monitoring purposes.

6. Authentication & Authorization:

   • Just for this demo application implemented ImMemoryUserDetail providing three users USER, ADMIN, and GUEST.
   • API is accessible only for USER, ADMIN users only.
   • GUEST user will get 403 Unauthorised error (Added to demonstrate authorization).

Design Details

Design Patterns Used

1. Strategy Pattern:

   • Used for selecting the discount calculation logic dynamically based on the user type.

2. Factory Pattern:

   • Encapsulates the creation logic of discount strategies in DiscountStrategyFactory.

3. Separation of Concerns:

   • The project cleanly separates concerns across services and components to ensure maintainability and testability.

Technologies Used

• Java 17
• Spring Boot 3.0
• Spring Security for authentication and authorization
• Lombok for reducing boilerplate code
• JUnit 5 and Mockito for testing
• SLF4J for logging

Prerequisites

• Java 17 or later
• An IDE (e.g., IntelliJ IDEA, Eclipse, or VS Code)
• Postman or cURL for testing APIs

How to Run the Project

1. Clone the repository:

   git clone https://github.com/avidee007/billing-app
   cd billing-app

2. Update application properties:
   • Configure the following property in the src/main/resources/application.properties file:

     app.currency.exchange.api.key=<API_KEY>

3. Build the project:

   ./gradlew build

4. Run the application:

   ./gradlew bootRun

The application will be available at http://localhost:8080/billing-service.

API Endpoints

1. Calculate Final Bill

Endpoint: POST /api/v1/calculate

Request Body:

{
  "userType": "EMPLOYEE",
  "tenure": 4,
  "totalAmount": 700,
  "originalCurrency": "AED",
  "targetCurrency": "INR",
  "items": [
    {
      "category": "OTHER",
      "price": 500
    },
    {
      "category": "GROCERIES",
      "price": 200
    }
  ]
}

Response:

{
  "finalAmount": 12104.251,
  "currency": "INR"
}

Testing the API

1. Using Postman

• Import the Postman collection into Postman.
• Send the request to the /api/v1/calculate endpoint.

2. Using cURL

curl -X POST http://localhost:8080/billing-service/api/v1/calculate \
-H "Content-Type: application/json" \
-H "Authorization: Basic dXNlcjpwYXNzd29yZA=="
-d '{
  "userType": "EMPLOYEE",
  "tenure": 1,
  "totalAmount": 700,
  "originalCurrency": "AED",
  "targetCurrency": "INR",
  "items": [
    {
      "category": "OTHER",
      "price": 500
    },
    {
      "category": "GROCERIES",
      "price": 200
    },
  ]
}'

3. Unit Tests

• Before running tests update api key else one integration test will fail.
• Run unit tests using Gradlew:

  ./gradlew test

UML Class Diagram

UML class diagram and plantUml code for UML diagram can be found in here.

Code Coverage & Sonar report

Code Coverage report

Sonar Report