A payment gateway is a critical component in any online transaction system. It acts as a bridge between the user, the merchant, and financial institutions by securely processing payment requests, verifying details, and ensuring funds are transferred correctly.

For example, when a customer purchases a product on an e-commerce platform, the payment gateway handles the steps of capturing payment details, validating them, interacting with the bank or wallet provider, and communicating the result (success or failure) to the application.

In this chapter, we will explore the low-level design of a Payment Gateway.

Let’s start by clarifying the requirements:

1. Clarifying Requirements

Designing a payment gateway involves many moving parts. Before diving into the implementation, it's critical to clarify the scope and constraints of the system we are expected to design.

After gathering the details, we can summarize the key system requirements.

1.1 Functional Requirements

• The system supports multiple payment methods, including Credit Card, PayPal, and UPI.
• The system processes the request using the appropriate payment processor.
• If processing fails, the system retries the request up to a maximum number of times (e.g., 3).
• The system should notify interested parties (e.g., customer, merchant) upon payment status updates.

1.2 Non-Functional Requirements

• The design should follow object-oriented principles and be modular to support future extensions (e.g., new payment methods).
• The system should be testable, extensible, and simulate real-world payment flows without actual external API calls.

After the requirements are clear, the next step is to identify the core entities that we will form the foundation of our design.

2. Identifying Core Entities