💳 LedgerLoom: High-Concurrency Transaction Engine

A specialized backend engine for atomic financial transfers, designed to solve the Double-Spending Problem in high-traffic fintech environments.

"In a system processing millions of transactions, a 0.01% race condition is a 100% failure in trust."

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🎯 Focus

This is NOT a full banking system. This is a focused demonstration of handling concurrent financial transactions safely.

Key Challenge Solved: Preventing double-spending when multiple transfer requests hit the server simultaneously.

✨ Core Features

• Wallet Provisioning: Secure creation and funding of digital wallets.
• Atomic Transfers: Guaranteed "all-or-nothing" fund movements between users.
• Concurrency Control: Automatic detection and rejection of simultaneous conflicting transactions.
• Real-time Balance: Instant retrieval of current account state.

🔐 Transaction Safety (ACID Compliance)

🛡️ Optimistic Locking

Uses JPA @Version to detect concurrent modifications:

• If User A and User B both try to transfer from the same wallet at the same millisecond, the first transaction succeeds.
• The second transaction fails immediately (Fail-fast) to prevent a "Lost Update."
• Benefit: Prevents double-spending without the performance bottleneck of heavy database locks.

⚛️ Atomic Transactions

The @Transactional annotation ensures that both wallet updates (Deduct from sender → Add to receiver) succeed or both fail:

• If ANY step fails, the entire transaction rolls back.
• Benefit: No partial transfers or "missing" money in the ledger.

🏗️ Tech Stack

• Java 17 & Spring Boot 3.4+
• Spring Data JPA (Hibernate)
• PostgreSQL (Chosen for robust transaction support)
• Spring Security & JWT (Stateless Authentication)
• Docker & Docker Compose (Containerized Environment)

📡 API Endpoints

Method	Endpoint	Description
POST	/api/auth/register	Create a new user and wallet
POST	/api/auth/login	Authenticate and receive JWT
POST	/api/wallet/deposit	Add funds to current user's wallet
POST	/api/wallet/transfer	Execute atomic transfer to another user
GET	/api/wallet/balance	View current balance

🚀 Quick Start

# Clone the repository
git clone https://github.com

# Start the PostgreSQL database
docker compose up -d

# Run the application
mvn spring-boot:run