💸 Paytm-like Wallet System

A full-stack wallet platform built using Next.js, Express, and MongoDB, designed to mimic Paytm-style wallet operations — user authentication, balance management, and secure wallet-to-wallet transfers.

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🚀 Overview

This project demonstrates a production-grade monorepo architecture using Turborepo and PNPM workspaces, combining:

• A Next.js frontend for user interaction.
• An Express.js API for backend logic and data handling.
• Shared Mongoose models and configuration packages for scalability and reusability.

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🏗️ Monorepo Architecture

apps/
  api/                  → Express + TypeScript REST API
  web/                  → Next.js frontend (App Router)
packages/
  db/                   → Shared Mongoose models & MongoDB connection
  ui/                   → Shared React UI components
  tailwind-config/      → Centralized TailwindCSS setup
  eslint-config/        → Common ESLint configuration
  typescript-config/    → Shared TS references

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⚙️ Tech Stack

Layer	Technologies
Frontend	Next.js 15 (App Router), React 19, TailwindCSS 4
Backend	Express.js, TypeScript, JWT, bcrypt
Database	MongoDB, Mongoose, MongoDB Sessions (Transactions)
Tooling	Turborepo, PNPM Workspaces, ESLint, Prettier, TypeScript Project References

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✨ Features

• 🔐 User Authentication: Secure signup/signin using hashed passwords (bcrypt) and JWT-based session management.

• 💳 Wallet Management: Automatic wallet creation on signup; protected balance retrieval endpoint.

• 🔄 Atomic Transactions: Peer-to-peer wallet transfers implemented with MongoDB sessions ensuring atomic debit/credit operations.

• 🧩 Monorepo Modularity: Shared Mongoose models and configuration across multiple apps via PNPM workspaces.

• 🛠️ Developer Experience: Unified build, lint, and type-check scripts across all packages using Turborepo pipelines.

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🔑 API Endpoints

Base URL: http://localhost:4173

Method	Endpoint	Description
POST	/signup	Register new user and auto-create wallet
POST	/signin	Authenticate user and return JWT
GET	/wallet	Fetch wallet balance (JWT required)
POST	/transactions	Perform atomic wallet-to-wallet transfer

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🧠 Data Models

User Model

{
  userName: string,
  email: string,
  password: string,  // hashed with bcrypt
  connections?: ObjectId[]
}

Wallet Model

{
  userId: ObjectId,
  balance: number
}

Transaction Model

{
  fromUser: ObjectId,
  toUser: ObjectId,
  debit?: number,
  credit?: number
}

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🔒 Authentication Flow

1. User signs up with username, email, and password.
2. Password is hashed with bcrypt before storing in MongoDB.
3. On successful signin, a JWT token is issued.
4. Token is validated in middleware before accessing protected routes like /wallet or /transactions.

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🔁 Transaction Workflow

1. POST /transactions is called with { from, to, amount }.
2. MongoDB session-based transaction begins.
3. Balance is debited from sender’s wallet and credited to receiver’s wallet atomically.
4. Any failure triggers rollback, ensuring data integrity.

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⚡ Local Development

Prerequisites

• Node.js ≥ 18
• PNPM
• MongoDB (local or Atlas)

Setup

# Install dependencies
pnpm install

# Start API
pnpm --filter api dev

# Start Web
pnpm --filter web dev

Default Ports:

• API → 4173
• Web → 4005

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🧾 Environment Variables

Create a .env file inside apps/api:

MONGODB_URI=mongodb://localhost:27017/paytm-wallet
JWT_SECRET=your_jwt_secret
PORT=4173

You can also use a cloud MongoDB URI (Atlas).

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📦 Scripts

Root Scripts

pnpm dev           # run all apps in parallel (turbo run dev)
pnpm build         # build all packages
pnpm lint          # lint all apps
pnpm check-types   # type-check all projects

Individual Apps

pnpm --filter api dev
pnpm --filter web dev

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🧩 Example API Usage

Signup

curl -X POST http://localhost:4173/signup \
  -H "Content-Type: application/json" \
  -d '{"username":"alice","email":"[email protected]","password":"pass"}'

Signin

curl -X POST http://localhost:4173/signin \
  -H "Content-Type: application/json" \
  -d '{"email":"[email protected]","password":"pass"}'
# → { "token": "<JWT>" }

Get Wallet Balance

curl http://localhost:4173/wallet \
  -H "Authorization: <JWT>"

Transfer Funds

curl -X POST http://localhost:4173/transactions \
  -H "Content-Type: application/json" \
  -d '{"from":"alice","to":"bob","amount":200}'

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🧩 Roadmap

• Add transaction history and audit logs
• Integrate input validation with Zod/Valibot
• Enhance frontend wallet dashboard and transfer UI
• Add Docker setup for containerized deployment
• Implement rate limiting and CORS

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🧠 Key Learnings

• Hands-on with Turborepo monorepo structure and shared configurations.
• Implemented atomic DB operations with MongoDB transactions.
• Gained deep understanding of secure authentication and route protection using JWT.
• Achieved code scalability through modular shared packages.