This doc describes my simple approach to building a payment gateway using a bank simulator using Java/SpringBoot and maven build.

Process a user's card payment via acquiring bank.

URL : /payment/process

Method : POST

Auth required : None

Permissions required : None

Request body example

{
  "cardNumber": "1234567893618822",
  "cvv": "222",
  "expiry": "05/28",
  "amount": "200.0",
  "currency": "GBP"
}

Code : 200 OK

Response example

{
    "message": "Money retrieved successfully!",
    "originalAmount": 200.0,
    "status": "successful",
    "totalAmount": 200.02,
    "currency": "GBP",
    "transactionId": "ca859365-4bcd-4a84-a6c0-ea972fef04f4"
}

Code : 200 OK

Response examples

{
  "message": "Failed to retrieve money. Please check your card details or available balance.",
  "originalAmount": 200.0,
  "status": "successful",
  "totalAmount": 200.02,
  "transactionId": "ca859365-4bcd-4a84-a6c0-ea972fef04f4"
}

{
    "message": "Invalid payment request.",
    "status": "failed"
}

Provides details of a previous card transaction.

URL : /payment/{transactionId}

Method : GET

Auth required : None

Permissions required : None

Code : 200 OK

Response example

{
  "message": "Money retrieved successfully!",
  "originalAmount": 200.0,
  "status": "successful",
  "totalAmount": 200.02,
  "transactionId": "d14299ad-5051-47c0-ae88-daee7fd2e05e"
}

Code : 200 OK

Response example

{
  "status": 404,
  "error": "Not Found",
  "message": "Payment details not found for transaction : e9185820-5fb7-4652-880c-f84d100db0d5",
  "path": "/payment/e9185820-5fb7-4652-880c-f84d100db0d5"
}

• The BankSimulator acts as the acquiring bank. It's a dummy class containing some necessary validations in the process of retrieving funds from a user's card.
• The PaymentService handles the payment processing via the acquiring bank simulator when called by the `merchant.
• The PaymentTransactionService class handles storage and retrieval of payment transaction data.

• Install an IDE for Java/SpringBoot, I used IntelliJ
• Clone the GitHub repo
• Open the project from your IDE. Using IntelliJ, click Run then Edit Configuration and select the PaymentGatewayApplication under the Application tab (you can change the Unnamed title).
• Click apply then OK
• Select the right pointer button beside the application (Unnamed or your chosen name above)
• The application should run at http://localhost:8888 (you can change the port in src/main/resources/application.properties)
• To test the APIs, you can use an API platform like Postman and follow the API docs above.

1. Compliance and Data Protection: I suggest proper card tokenization and hashing algorithms to protect sensitive user data e.g. card pan, pin, cvv, expiry.

2. Distributed Database: Integrating with a distributed database system like SQL, to manage storage and retrieval of transaction data

3. Input Validation: Implement thorough input validation to ensure that the API handles and rejects invalid or malicious input gracefully.

4. Error Handling and Logging: Enhance the error handling mechanism by implementing a centralized exception handling mechanism to handle exceptions and return meaningful error responses to the client.

5. Authentication and Authorization: Implement authentication and authorization mechanisms to secure the APIs. e.g. OAuth 2.0 or JSON Web Tokens (JWT) to authenticate clients and authorize access to protected resources.

6. Unit and Integration Testing: Expand the test coverage by writing comprehensive unit and integration tests.

7. Performance Optimization: Conduct performance testing and optimization to ensure that the APIs can respond efficiently and handle high load.

8. Security Considerations: Encrypt sensitive data at rest and use secure protocols (e.g., HTTPS).

9. Scalability and High Availability: Improve the software architecture to be scalable e.g. using horizontal scaling by deploying multiple instances of the application behind a load balancer to handle increased traffic.

10. Monitoring and Metrics: Tools like Prometheus, Grafana, or Kibana can collect and visualize relevant metrics and logs, establish alerts and monitoring thresholds.

I'd suggest using platforms like Azure and AWS because they implement concepts like containerization (e.g. Docker) and orchestration frameworks (e.g. Kubernetes) to manage app deployments and high availability.

For example, Azure has a simple interface, offers more scalability for virtual machines and offers cost-savings compared to other clouds through discounts on licensing across several regions. Azure is compatible with Java and .Net-based applications, provides robust security and offers more flexibility for hybrid cloud architecture.

Using cloud technologies like Azure also ensure that we adhere to regulations like GDPR and PCI DSS when storing sensitive customer data in payment systems under the required regions (e.g. UK, US, EU, ASIA, etc.)