💳 Payment Gateway Simulator (v2.0: With Persistence)

🤔 Why did I build this?

Have you ever wondered what actually happens in those few seconds after you hit the "Pay" button on a website? As someone aiming to dive into the Fintech world and Backend development, I didn't want to just use a library—I wanted to build the logic from scratch.

This project simulates the "brain" behind a payment processor, handling validation, logic, and storage.

🚀 🆕 Update v2.0: The "Mid-Level" Upgrade

I recently refactored the project to move from a basic script to a robust backend system.

• Real Persistence (SQL): Integrated SQLite and SQLAlchemy. Transactions are now permanently saved in a database file (payments.db), meaning data survives server restarts (unlike the previous in-memory version).
• Security & Privacy (PCI-DSS principles): Implemented Data Masking. The database never stores the full credit card number, only the last 4 digits (e.g., 4444). This mimics real-world GDPR and security compliance.
• Audit Endpoint: Added GET /history to query the persistent database.

⚙️ How it works (The Business Logic)

I've programmed real-world constraints to mimic how actual banks operate:

1. The Gatekeeper (Validation): Before even touching the database, the system checks if the data makes sense using Pydantic.
2. The Banker (Decision Logic):
   • ✅ Approved: Valid card + reasonable amount.
   • ⛔ Insufficient Funds: Charges over 2,000€ are automatically rejected.
3. The Vault (Storage): If approved, the transaction is hashed and safely stored in the SQL database for future auditing.

🛠️ Tech Stack (The "Why")

• Python 3.10: My core language.
• FastAPI: Chosen for high performance and automatic documentation.
• SQLAlchemy ORM: Used to manage database transactions professionally, avoiding raw SQL queries.
• SQLite: A serverless SQL engine for local persistence.
• Pydantic: For strict data validation.

🚀 Take it for a spin

1. Clone the repo:

   git clone [https://github.com/Anguis400/payment-gateway-api.git](https://github.com/Anguis400/payment-gateway-api.git)
   cd payment-gateway-api

2. Install dependencies (Updated):

   pip install fastapi uvicorn sqlalchemy

3. Fire up the engine:

   uvicorn main:app --reload

   Note: This will automatically generate the payments.db file in your folder.

4. Interactive Testing: Go to http://127.0.0.1:8000/docs.

   • Try POST /process-payment to make a transaction.
   • Restart the server.
   • Try GET /history to see that your data survived!

🧠 What I learned

Building v2.0 taught me the importance of Data Persistence. It's easy to make a script work in RAM, but connecting an API to a SQL database via an ORM (Object Relational Mapper) is what makes an application "production-ready". Also, learning to hide sensitive data (Masking) made me realize how critical privacy is in Fintech.

---

Built with ⌨️, code, and caffeine by Anguis400.