Valrix - Backend

A backend system for a general-purpose barter and exchange platform that enables users to trade goods with one another without direct monetary transactions. The platform provides structured item listings, a hybrid trade + chat system with secure transaction-based workflows, and enterprise-grade safeguards against race conditions.

Features

Authentication & Authorization

• User registration and login with password reset functionality
• JWT-based authentication
• Ownership-based access control
• Email service integration for password recovery

Item Management

• Create, update, and delete item listings
• Categorization and condition tracking (electronics, accessories, books, vehicles, etc.)
• Item status tracking (available, traded)
• Ownership validation and transfer

Hybrid Trade + Chat System

Multi-stage Trade Lifecycle:

• Proposed Negotiating Confirmed Completed
• Owner-driven confirmation authority
• Automatic conversation creation on trade request
• Item locking during confirmation phase
• Atomic ownership swaps with MongoDB transactions

Race Condition Prevention:

• Transaction-based multi-document updates
• Atomic state transitions with findOneAndUpdate
• Competing trade rejection on confirmation
• Double-completion prevention

Real-time Chat Integration:

• Conversation auto-linked to trades
• Message read tracking
• Participant-only access control
• Last message tracking

System Design

• RESTful API architecture
• MVC-style modular structure
• MongoDB Atlas with transaction support
• Production-ready error handling
• Atomic database operations

Tech Stack

• Backend: Node.js, Express.js
• Database: MongoDB (Mongoose ODM)
• Authentication: JSON Web Tokens (JWT)
• Architecture: MVC-style modular backend

Project Structure

src/
 controllers/
    authController.js
    itemController.js
    tradeController.js
    conversationController.js
 models/
    User.js
    Item.js
    Trade.js
    Conversation.js
    Message.js
 routes/
    auth.js
    itemRoutes.js
    tradeRoutes.js
    conversationRoutes.js
 middleware/
    authMiddleware.js
 utils/
    emailService.js
 app.js
 server.js

API Endpoints

Authentication (/api/auth)

• POST /register - Create new user account
• POST /login - User login
• POST /forgot-password - Request password reset
• POST /reset-password - Reset password with token

Items (/api/items)

• GET / - List all available items
• GET /:id - Get item details
• POST / - Create new item
• PUT /:id - Update item
• DELETE /:id - Delete item

Trades (/api/trades)

• POST /request - Create trade request (status: proposed)
• GET /myrequests - Get trades initiated by user
• GET /requests-for-me - Get trades received by user
• POST /:id/interest - Express interest (proposed negotiating)
• POST /:id/confirm - Confirm trade (negotiating confirmed, locks items)
• POST /:id/complete - Complete trade (confirmed completed, swaps ownership)
• POST /:id/reject - Reject trade

Conversations (/api/conversations)

• GET / - Get all user conversations
• GET /:id - Get specific conversation
• POST /:id/messages - Send message
• GET /:id/messages - Get conversation messages
• POST /:id/read - Mark messages as read

Data Models

User

• name, email, password (hashed)
• resetPasswordToken, resetPasswordExpires
• profileImage
• timestamps

Item

• title, description, category, condition
• images (array)
• lookingFor (item_swap, credits, flexible)
• status (available, traded)
• owner (ref: User)
• timestamps

Trade

• offeredItem, requestedItem (ref: Item)
• requester, owner (ref: User)
• status (proposed, negotiating, confirmed, completed, rejected)
• completedAt
• timestamps

Conversation

• participants (array of User refs)
• trade (ref: Trade)
• lastMessage (ref: Message)
• timestamps

Message

• conversation (ref: Conversation)
• sender (ref: User)
• text
• readBy (array of User refs)
• timestamps

Trade Workflow

1. Propose Trade - User creates trade request (status: proposed)

   • Validates item availability
   • Prevents self-trading
   • Auto-creates conversation
   • No item locking

2. Express Interest - Owner shows interest (status: negotiating)

   • Only owner can call
   • Opens negotiation phase
   • Users can chat via conversation

3. Confirm Trade - Owner confirms (status: confirmed)

   • Locks both items atomically
   • Rejects competing trades
   • Uses MongoDB transactions
   • Prevents race conditions

4. Complete Trade - Either party completes (status: completed)

   • Swaps item ownership atomically
   • Prevents double completion
   • Items remain in "traded" status

5. Reject Trade - Owner rejects at any time before confirmation

Project Status

Hybrid trade + chat system fully implemented
Transaction-based atomic operations
Race condition prevention
Production-ready error handling
RESTful API with comprehensive endpoints
Conversation system integrated with trades

Future Enhancements

• WebSocket integration for real-time chat
• User reputation and review system
• Image upload service integration
• Push notifications
• Location-based filtering
• Admin moderation dashboard
• Trade history analytics

Security Features

• MongoDB transactions for data integrity
• JWT-based authentication
• Password hashing with bcrypt
• Atomic operations to prevent race conditions
• Authorization checks on all protected routes
• Email token-based password reset
• Participant validation for conversations

Purpose

This project demonstrates:

Advanced Backend Development:

• Transaction-based database operations
• Race condition prevention
• Multi-stage workflow implementation
• Real-time messaging architecture

Production-Ready Practices:

• Proper error handling with rollbacks
• Atomic state transitions
• Authorization and validation
• Scalable RESTful API design

Suitable for senior backend roles, full-stack positions, and demonstrating expertise in complex system design.