[Feature]: Implement Logging and Structured Error Handling of the Payment Gateway system

[adityacosmos24]

So, what is it about?

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Title: Implement Logging and Structured Error Handling

Issue Description

Current Behavior 📝

The current implementation of the Payment Gateway system directly prints transaction status updates to the standard output (std::cout). While this is useful for real-time monitoring during a run, it lacks persistence and structure. If the application closes or crashes, all transaction history is lost.

Furthermore, error handling is managed through simple boolean return types. When a step like validatePayment or initiatePayment fails, the system only knows that it failed, but not why. For example, the PaytmGateway::validatePayment function returns false if the amount is invalid or if the currency is not "INR", but the calling function cannot distinguish between these two different error conditions.

Expected Behavior ✅

To make the system more robust and production-ready, two key enhancements are needed:

1. File-Based Logging: All transaction events should be logged to a file (e.g., payment_gateway.log). Each log entry should be timestamped and should clearly describe the event (e.g., validation start, initiation failure, retry attempt, final success/failure). This creates a persistent audit trail for debugging and analysis.

2. Structured Error Handling: Instead of returning simple booleans, the payment processing functions should provide specific details about errors. This can be achieved by:

   • Throwing custom exceptions (e.g., ValidationException, InitiationException).
   • Returning a struct or std::pair that contains both a success status and an error message string.

This will allow the system to log meaningful error messages, such as "Validation failed: Currency must be INR" instead of a generic "[PaymentGateway] Validation failed".

Motivation for Change 💡

Implementing logging and structured error handling are crucial steps in moving this project from a proof-of-concept to a more mature and maintainable application. These changes will:

• Improve Debugging: A persistent log file makes it possible to trace the lifecycle of a transaction and diagnose issues after the fact.
• Enhance Maintainability: Specific error messages make it easier for developers to understand and fix bugs.
• Provide Auditing Capabilities: Logs serve as a record of all payment activities, which is essential for any financial application.

Suggested Implementation 🛠️

1. Create a Logger Class: Introduce a singleton Logger class that can write formatted messages to a file.

   class Logger {
   public:
       static Logger& getInstance();
       void log(const std::string& message);
   private:
       // Private constructor, destructor, etc.
       // std::ofstream file_handle;
   };

2. Integrate Logging: Call the Logger from within the PaymentGateway, BankingSystem, and PaymentGatewayProxy classes to record every significant step of the payment process.

3. Refactor Error Handling: Modify the methods in the PaymentGateway hierarchy. For example, change validatePayment to throw an exception on failure.

   // In PaytmGateway.h
   class ValidationException : public std::exception {
   public:
       const char* what() const noexcept override {
           return message.c_str();
       }
   private:
       std::string message;
   };
   
   // In PaytmGateway.cpp
   bool PaytmGateway::validatePayment(PaymentRequest* request) {
       // ...
       if (request->currency != "INR") {
           throw ValidationException("Invalid currency. Must be INR.");
       }
       // ...
       return true;
   }

4. Add Exception Handling: Update the PaymentGateway::processPayment template method and the PaymentController to include try...catch blocks to handle these exceptions gracefully and log the specific error messages.

Code of Conduct

• I agree to follow this project's Code of Conduct

[adityacosmos24]

@prajwal3114 please assign this to me