# IPN Callback Integration Guide

## Overview

This document describes the race condition prevention system implemented for the 420Deals.ch collective drop platform. The system uses a **10-minute reservation mechanism** via the `pending_orders` table to prevent overselling when multiple buyers attempt to purchase the last available units simultaneously.

## Database Schema

### `pending_orders` Table

```sql
CREATE TABLE `pending_orders` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `payment_id` varchar(255) NOT NULL,           -- NOWPayments invoice/payment ID
  `order_id` varchar(255) NOT NULL,            -- Internal order ID (format: SALE-{timestamp}-{drop_id}-{buyer_id})
  `drop_id` int(11) NOT NULL,                   -- Foreign key to drops table
  `buyer_id` int(11) NOT NULL,                  -- Foreign key to buyers table
  `size` int(11) NOT NULL,                      -- Quantity in grams
  `price_amount` decimal(10,2) NOT NULL,       -- Price amount
  `price_currency` varchar(10) NOT NULL DEFAULT 'chf',
  `created_at` datetime NOT NULL DEFAULT current_timestamp(),
  `expires_at` datetime NOT NULL,              -- Expiration time (10 minutes from creation)
  PRIMARY KEY (`id`),
  UNIQUE KEY `payment_id` (`payment_id`),
  UNIQUE KEY `order_id` (`order_id`),
  KEY `drop_id` (`drop_id`),
  KEY `buyer_id` (`buyer_id`),
  KEY `idx_expires_at` (`expires_at`),         -- Index for cleanup queries
  FOREIGN KEY (`drop_id`) REFERENCES `drops` (`id`) ON DELETE CASCADE,
  FOREIGN KEY (`buyer_id`) REFERENCES `buyers` (`id`) ON DELETE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
```

### `sales` Table

```sql
CREATE TABLE `sales` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `drop_id` int(11) NOT NULL,
  `buyer_id` int(11) NOT NULL,
  `size` int(11) NOT NULL DEFAULT 1,           -- Quantity in grams
  `payment_id` text NOT NULL DEFAULT '',       -- NOWPayments payment ID (matches pending_orders.payment_id)
  `created_at` datetime NOT NULL DEFAULT current_timestamp(),
  PRIMARY KEY (`id`),
  KEY `drop_id` (`drop_id`),
  KEY `buyer_id` (`buyer_id`),
  FOREIGN KEY (`drop_id`) REFERENCES `drops` (`id`) ON DELETE CASCADE,
  FOREIGN KEY (`buyer_id`) REFERENCES `buyers` (`id`) ON DELETE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
```

## How the System Works

### 1. Purchase Flow

When a buyer initiates a purchase:

1. **Inventory Check**: System checks available inventory = `drop.size - (sales.total + non_expired_pending_orders.total)`
2. **Reservation**: If inventory available, creates `pending_order` with:
   - `expires_at` = NOW() + 10 minutes
   - Inventory is now "on hold"
3. **Invoice Creation**: Creates NOWPayments invoice with `invoice_timeout: 600` (10 minutes)
4. **Transaction**: All of the above happens atomically in a database transaction

### 2. Inventory Calculation

**Available Inventory** = `drop.size - (SUM(sales.size) + SUM(pending_orders.size WHERE expires_at > NOW()))`

- `sales.size`: Confirmed purchases (permanent)
- `pending_orders.size`: Temporary reservations (expire after 10 minutes)

### 3. Expiration

- Pending orders expire 10 minutes after creation (`expires_at < NOW()`)
- Expired orders should be cleaned up periodically (recommended: every 1-2 minutes)
- Cleanup endpoint: `POST /api/payments/cleanup-expired`
- Expired orders are automatically excluded from inventory calculations

## IPN Callback Handling

### NOWPayments IPN Callback Format

The system expects IPN callbacks with the following structure:

```json
{
  "payment_id": "string",        // NOWPayments payment/invoice ID
  "invoice_id": "string",        // Alternative field name (used if payment_id not present)
  "order_id": "string",          // Internal order ID (format: SALE-{timestamp}-{drop_id}-{buyer_id})
  "payment_status": "string",    // Status: waiting, confirming, confirmed, finished, failed, expired, etc.
  "pay_amount": "number",
  "pay_currency": "string",
  "price_amount": "number",
  "price_currency": "string"
}
```

### IPN Callback Processing Logic

Your IPN callback handler should follow this flow:

#### Step 1: Find Pending Order

```sql
SELECT * FROM pending_orders 
WHERE payment_id = ? OR payment_id = ? 
-- Try both payment_id and invoice_id from callback
```

**Important**: The system uses `payment_id` OR `invoice_id` to find pending orders. Check both fields.

#### Step 2: Check Expiration

```sql
-- Verify order hasn't expired
SELECT * FROM pending_orders 
WHERE id = ? AND expires_at > NOW()
```

**Action if expired**: 
- Delete the pending order
- Return error response (don't create sale)
- Log the expiration

#### Step 3: Validate Payment Status

Process based on `payment_status`:

- **`finished`** or **`confirmed`**: Payment successful → proceed to Step 4
- **`failed`** or **`expired`**: Payment failed → delete pending order, return success
- **`waiting`**, **`confirming`**: Payment in progress → return success, wait for final status

#### Step 4: Final Inventory Check (Before Creating Sale)

```sql
-- Get drop details
SELECT * FROM drops WHERE id = ?

-- Calculate current inventory
SELECT COALESCE(SUM(size), 0) as total_sales 
FROM sales WHERE drop_id = ?

-- Calculate other pending orders (excluding current one)
SELECT COALESCE(SUM(size), 0) as total_pending 
FROM pending_orders 
WHERE drop_id = ? 
  AND id != ? 
  AND expires_at > NOW()

-- Check availability
-- Available = drop.size - (total_sales + total_pending)
-- If pending_order.size > Available: REJECT
```

**Important**: Always check inventory again before creating the sale, as other buyers may have reserved inventory in the meantime.

#### Step 5: Create Sale Record

If inventory is available:

```sql
-- Create sale
INSERT INTO sales (drop_id, buyer_id, size, payment_id) 
VALUES (?, ?, ?, ?)

-- Delete pending order
DELETE FROM pending_orders WHERE id = ?
```

**Important**: 
- Use the same `payment_id` from pending_order for the sale record
- Delete the pending order after creating the sale
- This should be done in a transaction to ensure atomicity

#### Step 6: Handle Idempotency

Before creating a sale, check if it already exists:

```sql
SELECT * FROM sales WHERE payment_id = ?
```

If sale exists, return success (idempotent operation).

### Example IPN Callback Handler (Pseudocode)

```javascript
async function handleIPNCallback(callbackData) {
  const { payment_id, invoice_id, order_id, payment_status } = callbackData;
  
  // Step 1: Find pending order
  const paymentIdToFind = invoice_id || payment_id;
  const pendingOrder = await db.query(
    'SELECT * FROM pending_orders WHERE payment_id = ?',
    [paymentIdToFind]
  );
  
  if (!pendingOrder) {
    // Check if sale already exists (idempotency)
    const existingSale = await db.query(
      'SELECT * FROM sales WHERE payment_id = ?',
      [paymentIdToFind]
    );
    if (existingSale) {
      return { status: 'ok' }; // Already processed
    }
    return { error: 'Pending order not found' };
  }
  
  // Step 2: Check expiration
  if (new Date(pendingOrder.expires_at) < new Date()) {
    await db.query('DELETE FROM pending_orders WHERE id = ?', [pendingOrder.id]);
    return { error: 'Order expired' };
  }
  
  // Step 3: Process payment status
  if (payment_status === 'finished' || payment_status === 'confirmed') {
    // Step 4: Final inventory check
    const drop = await db.query('SELECT * FROM drops WHERE id = ?', [pendingOrder.drop_id]);
    const sales = await db.query(
      'SELECT COALESCE(SUM(size), 0) as total FROM sales WHERE drop_id = ?',
      [pendingOrder.drop_id]
    );
    const otherPending = await db.query(
      'SELECT COALESCE(SUM(size), 0) as total FROM pending_orders WHERE drop_id = ? AND id != ? AND expires_at > NOW()',
      [pendingOrder.drop_id, pendingOrder.id]
    );
    
    const totalReserved = sales.total + otherPending.total;
    const available = drop.size - totalReserved;
    
    if (pendingOrder.size > available) {
      await db.query('DELETE FROM pending_orders WHERE id = ?', [pendingOrder.id]);
      return { error: 'Inventory no longer available' };
    }
    
    // Step 5: Create sale
    await db.transaction(async (tx) => {
      await tx.query(
        'INSERT INTO sales (drop_id, buyer_id, size, payment_id) VALUES (?, ?, ?, ?)',
        [pendingOrder.drop_id, pendingOrder.buyer_id, pendingOrder.size, pendingOrder.payment_id]
      );
      await tx.query('DELETE FROM pending_orders WHERE id = ?', [pendingOrder.id]);
    });
    
    return { status: 'ok' };
  } else if (payment_status === 'failed' || payment_status === 'expired') {
    await db.query('DELETE FROM pending_orders WHERE id = ?', [pendingOrder.id]);
    return { status: 'ok' };
  }
  
  return { status: 'ok' }; // Payment still in progress
}
```

## Important Considerations

### 1. Database Transactions

- Always use transactions when creating sales and deleting pending orders
- This ensures atomicity and prevents race conditions

### 2. Expiration Handling

- Expired pending orders should be excluded from inventory calculations
- Clean up expired orders periodically (every 1-2 minutes recommended)
- The main application has a cleanup endpoint: `POST /api/payments/cleanup-expired`

### 3. Unit Conversion

- All `size` values in `sales` and `pending_orders` are stored in **grams**
- `drops.size` and `drops.unit` may be in different units (g or kg)
- When calculating inventory, convert to the drop's unit:
  ```javascript
  if (drop.unit === 'kg') {
    sizeInDropUnit = sizeInGrams / 1000;
  }
  ```

### 4. Idempotency

- IPN callbacks may be sent multiple times
- Always check if a sale already exists before creating a new one
- Use `payment_id` to check for existing sales

### 5. Error Handling

- Always return HTTP 200 to NOWPayments, even on errors
- Log errors for debugging
- Don't retry failed operations indefinitely

### 6. Inventory Availability

- Inventory is calculated as: `drop.size - (sales + non_expired_pending_orders)`
- Always re-check inventory before creating a sale
- Other buyers may have reserved inventory between payment initiation and confirmation

## API Endpoints Reference

### Cleanup Expired Orders

```
POST /api/payments/cleanup-expired
Authorization: Bearer {CLEANUP_API_TOKEN} (optional)

Response:
{
  "message": "Cleaned up X expired pending orders",
  "cleaned": 5,
  "total": 5
}
```

### Check Expired Orders Count

```
GET /api/payments/cleanup-expired

Response:
{
  "expired_orders_count": 3,
  "message": "There are 3 expired pending orders that need cleanup"
}
```

## Testing Checklist

When implementing your IPN callback handler, test:

1. ✅ Payment success → Sale created, pending order deleted
2. ✅ Payment failure → Pending order deleted, no sale created
3. ✅ Payment expiration → Pending order deleted, no sale created
4. ✅ Expired pending order → Rejected, no sale created
5. ✅ Insufficient inventory → Pending order deleted, no sale created
6. ✅ Duplicate IPN callbacks → Idempotent (sale not created twice)
7. ✅ Race condition → Only one sale created when multiple payments complete simultaneously

## Database Queries Reference

### Find Pending Order by Payment ID
```sql
SELECT * FROM pending_orders 
WHERE payment_id = ? OR payment_id = ?
```

### Check if Pending Order is Expired
```sql
SELECT * FROM pending_orders 
WHERE id = ? AND expires_at > NOW()
```

### Calculate Available Inventory
```sql
-- Sales
SELECT COALESCE(SUM(size), 0) as total_sales 
FROM sales WHERE drop_id = ?

-- Non-expired pending orders
SELECT COALESCE(SUM(size), 0) as total_pending 
FROM pending_orders 
WHERE drop_id = ? AND expires_at > NOW()

-- Available = drop.size - (total_sales + total_pending)
```

### Create Sale and Delete Pending Order (Transaction)
```sql
START TRANSACTION;

INSERT INTO sales (drop_id, buyer_id, size, payment_id) 
VALUES (?, ?, ?, ?);

DELETE FROM pending_orders WHERE id = ?;

COMMIT;
```

### Check for Existing Sale (Idempotency)
```sql
SELECT * FROM sales WHERE payment_id = ?
```

## Environment Variables

The main application uses these environment variables (for reference):

- `IPN_CALLBACK_URL`: URL where NOWPayments sends IPN callbacks
- `CLEANUP_API_TOKEN`: (Optional) Token for cleanup endpoint authentication
- `NOWPAYMENTS_API_KEY`: NOWPayments API key
- `NOWPAYMENTS_TESTNET`: Set to 'true' for sandbox environment

## Support

For questions or issues with the IPN callback integration, refer to:
- Database schema: `cbd420.sql`
- Race condition fix documentation: `RACE_CONDITION_FIX.md`

**Note**: The main application does not include an IPN callback handler. All IPN callbacks must be handled by your external service using the logic described in this document.