# Race Condition Fix - Implementation Summary

## Problem
When multiple buyers attempt to purchase the last available units simultaneously, a race condition occurs where inventory can be oversold.

## Solution
Implemented a 10-minute reservation system using the `pending_orders` table to temporarily hold inventory during checkout.

## Changes Made

### 1. Database Migration
- **File**: `migrations/add_expires_at_to_pending_orders.sql`
- Added `expires_at` column to `pending_orders` table
- Added index on `expires_at` for efficient cleanup queries

**To apply the migration:**
```sql
-- Run the migration file
SOURCE migrations/add_expires_at_to_pending_orders.sql;
```

### 2. Create Invoice Endpoint (`app/api/payments/create-invoice/route.ts`)
- **Atomic Inventory Reservation**: Uses database transactions to atomically check and reserve inventory
- **Pending Orders Check**: Includes non-expired pending orders when calculating available inventory
- **Expiration Time**: Sets `expires_at` to 10 minutes from creation
- **NOWPayments Integration**: Sets `invoice_timeout` to 600 seconds (10 minutes) when creating invoice

### 3. Active Drop Endpoint (`app/api/drops/active/route.ts`)
- **Fill Calculation**: Includes non-expired pending orders in fill calculation
- **UI Display**: Progress bar now shows total reserved inventory (sales + pending orders)

### 4. Cleanup Endpoint (`app/api/payments/cleanup-expired/route.ts`)
- **Automatic Cleanup**: Removes expired pending orders
- **NOWPayments Status Check**: Verifies payment status before cleanup
- **Monitoring**: GET endpoint to check count of expired orders

### 5. UI Updates (`app/components/Drop.tsx`)
- **Visual Indicator**: Added note that items are held for 10 minutes during checkout
- **Progress Bar**: Automatically reflects reserved inventory (including pending orders)

## Setup Instructions

### 1. Run Database Migration
```bash
mysql -u your_user -p your_database < migrations/add_expires_at_to_pending_orders.sql
```

### 2. Set Up Cleanup Job
The cleanup endpoint should be called periodically (recommended: every 1-2 minutes) to remove expired pending orders.

#### Option A: Cron Job (Linux/Mac)
```bash
# Add to crontab (crontab -e)
*/2 * * * * curl -X POST https://your-domain.com/api/payments/cleanup-expired -H "Authorization: Bearer YOUR_CLEANUP_TOKEN"
```

#### Option B: Vercel Cron (if using Vercel)
Add to `vercel.json`:
```json
{
  "crons": [{
    "path": "/api/payments/cleanup-expired",
    "schedule": "*/2 * * * *"
  }]
}
```

#### Option C: Node.js Cron Library
```javascript
const cron = require('node-cron');
const fetch = require('node-fetch');

cron.schedule('*/2 * * * *', async () => {
  await fetch('http://localhost:3000/api/payments/cleanup-expired', {
    method: 'POST',
    headers: {
      'Authorization': `Bearer ${process.env.CLEANUP_API_TOKEN}`
    }
  });
});
```

### 3. Environment Variables
Add to your `.env` file:
```
# Required: URL for external IPN handler service
IPN_CALLBACK_URL=http://your-ipn-service.com/api/payments/ipn-callback

# Optional: Token for cleanup endpoint security
CLEANUP_API_TOKEN=your-secure-random-token
```

**Note**: IPN callbacks are handled by an external service. The `IPN_CALLBACK_URL` must point to your external IPN handler that processes payment confirmations and creates sales. See `IPN_INTEGRATION_README.md` for integration details.

## How It Works

1. **Purchase Initiation**: When a buyer clicks "Join Drop":
   - System checks available inventory (sales + non-expired pending orders)
   - If available, creates pending order with 10-minute expiration
   - Creates NOWPayments invoice with 10-minute timeout
   - Inventory is now "on hold" for 10 minutes

2. **During Checkout**:
   - Progress bar shows reserved inventory (including pending orders)
   - Other buyers see reduced availability
   - If buyer completes payment within 10 minutes → sale is created
   - If buyer doesn't complete payment within 10 minutes → order expires

3. **Expiration**:
   - Cleanup job runs every 1-2 minutes
   - Expired pending orders are removed
   - Inventory becomes available again
   - NOWPayments invoice expires on their end

4. **Payment Confirmation** (handled by external IPN service):
   - External IPN handler receives payment notification from NOWPayments
   - Checks if pending order is expired
   - Validates final inventory availability
   - Creates sale record if valid
   - Deletes pending order

## Testing

1. **Test Race Condition**:
   - Open two browser windows
   - Try to purchase the last available unit simultaneously
   - Only one should succeed
   - The other should see "Not enough inventory remaining"

2. **Test Expiration**:
   - Create a pending order
   - Wait 10+ minutes
   - Run cleanup endpoint manually
   - Verify order is removed

3. **Test Payment Flow**:
   - Create pending order
   - Complete payment within 10 minutes
   - Verify sale is created
   - Verify pending order is deleted

## Notes

- Pending orders are automatically included in inventory calculations
- The 10-minute hold prevents overselling while giving buyers time to complete payment
- NOWPayments invoices also expire after 10 minutes
- Cleanup job should run frequently (every 1-2 minutes) to free up inventory quickly