This is the repository for the Sycamore backend task. I have leveraged a modern tech stack to ensure data precision, system reliability, and maintainability through clean architecture.

This project is built with a focus on long-term scalability and fault tolerance. Here are my key design choices:

• Domain-Driven Design (DDD): I organized the codebase by feature modules (User, Wallet, Transfer, Interest). This ensures that business logic is encapsulated within its own domain, making the system easier to navigate and scale.

• Repository Pattern: I used repositories to abstract the data access layer (Sequelize). By injecting repositories into my services rather than models directly, I decoupled business logic from the ORM, facilitating easier unit testing and providing the flexibility to switch data sources in the future.

• Eventual Consistency via Message Queues: For transaction processing, I opted for Bull Queue (Redis). Instead of processing heavy financial transfers in a single request-response cycle, I queue them. This ensures:

• Scalability: The system can handle spikes in transaction volume without crashing.

• Reliability: Built-in retry mechanisms with exponential backoff handle transient database or network failures.

• User Experience: The API responds immediately, while the heavy lifting happens in the background.

• Precision Engineering: In fintech, floating-point math is a liability. I used decimal.js for all balance and interest calculations to ensure zero rounding errors.

A standout feature of this wallet is the Automated Interest Accrual.

• Rate: 27.5% APR (Annual Percentage Rate).

• Formula: Interest is calculated daily using the following formula:

• Logic: The service automatically detects leap years (366 days) vs. non-leap years (365 days) to ensure absolute accuracy.

• Docker and Docker Compose
• Node.js 18+ (if running locally)

1. Clone the Repository:

git clone https://github.com/Bolu1/simple-wallet.git
cd simple-wallet

2. Environment Configuration: There is an .env.example file included in the root directory. Copy it to create your local environment file:

cp .env.example .env

Update the variables in .env if you have specific port preferences. 3. Start the Application: You can run the entire stack (PostgreSQL, Redis, NestJS) using Docker:

docker-compose up -d --build

Alternatively, for local development, ensure Postgres and Redis are running, then use npm run start:dev.

To populate your environment with test data (Users and Wallets), use the following command to reach inside the running container:

docker exec -it simple-wallet-app npm run seed:prod

The API follows standard RESTful principles.

• Endpoint: GET /wallets
• Description: Retrieves all wallets associated with the authenticated user. Use this to get the Wallet IDs needed for transfers.

• Endpoint: POST /v1/transfer
• Header: x-idempotency-key: <UUID> (Mandatory to prevent double-spending/duplicate transactions).
• Body:

{
    "fromWalletId": "2f970b6d-9d8b-475f-859a-4e85d57bc92b",
    "toWalletId": "332ebe85-2a12-4b03-adfb-c830587b128e",
    "amount": 100
}

The project includes a comprehensive test suite using Jest, covering precision math, leap year edge cases, and queue processing.

# Run all tests
npm test

# Run tests with coverage
npm run test:cov