# NOWPayments IPN Listener

A TypeScript Express application for receiving and processing Instant Payment Notifications (IPN) from NOWPayments.

## Features

- ✅ Secure IPN signature validation using HMAC SHA512
- ✅ TypeScript for type safety
- ✅ Handles all NOWPayments payment statuses
- ✅ MySQL/MariaDB database integration
- ✅ Automatic payment processing: moves finished/confirmed payments from `pending_orders` to `sales`
- ✅ 10-minute reservation mechanism to prevent overselling
- ✅ Expiration checking and automatic cleanup of expired orders
- ✅ Final inventory validation before creating sales
- ✅ Transaction-safe database operations
- ✅ Idempotent IPN processing (handles duplicate callbacks)
- ✅ Error handling and logging

## Setup

### 1. Install Dependencies

```bash
npm install
```

### 2. Configure Environment Variables

Copy `.env.example` to `.env` and fill in your NOWPayments IPN Secret Key:

```bash
cp .env.example .env
```

Edit `.env` and configure:

**NOWPayments IPN Secret Key:**
- Go to NOWPayments Dashboard → Store Settings → IPN Secret Key
- Generate a new key if you don't have one
- Add it to the `.env` file

**Database Configuration:**
- Set your MySQL/MariaDB connection details:
  - `DB_HOST` - Database host (default: localhost)
  - `DB_PORT` - Database port (default: 3306)
  - `DB_USER` - Database username
  - `DB_PASSWORD` - Database password
  - `DB_NAME` - Database name (default: cbd420)

### 3. Database Migration

The application requires the `expires_at` column in the `pending_orders` table for the 10-minute reservation mechanism. Run the migration:

```bash
mysql -u your_user -p your_database < migrations/add_expires_at_to_pending_orders.sql
```

Or manually add the column:

```sql
ALTER TABLE `pending_orders`
  ADD COLUMN `expires_at` datetime NOT NULL DEFAULT (DATE_ADD(NOW(), INTERVAL 10 MINUTE))
  AFTER `created_at`;

ALTER TABLE `pending_orders`
  ADD INDEX `idx_expires_at` (`expires_at`);
```

### 4. Build the Project

```bash
npm run build
```

### 5. Run the Server

**Development mode (with auto-reload):**
```bash
npm run dev
```

**Production mode:**
```bash
npm start
```

The server will start on port 3000 (or the port specified in your `.env` file).

## Endpoints

### POST `/ipn`
Receives IPN notifications from NOWPayments. This endpoint:
- Validates the request signature
- Processes payment status updates
- Returns 200 OK to acknowledge receipt

### GET `/health`
Health check endpoint to verify the server is running.

## IPN Callback URL Setup

When creating a payment via the NOWPayments API, include the `ipn_callback_url` parameter:

```javascript
{
  "price_amount": 100,
  "price_currency": "usd",
  "pay_currency": "btc",
  "ipn_callback_url": "https://yourdomain.com/ipn",
  // ... other parameters
}
```

## Payment Statuses

The listener handles the following payment statuses:

- `waiting` - Payment is waiting
- `confirming` - Payment is being confirmed on blockchain
- `confirmed` - Payment confirmed on blockchain
- `sending` - Payment is being sent
- `partially_paid` - Payment partially received
- `finished` - Payment completed successfully
- `failed` - Payment failed
- `refunded` - Payment refunded
- `expired` - Payment expired

## Database Integration

The application implements a **10-minute reservation mechanism** to prevent race conditions when multiple buyers attempt to purchase the last available units simultaneously.

### Payment Processing Flow

When a payment status is `finished` or `confirmed`, the system:

1. **Find Pending Order** - Looks up the pending order by `payment_id` or `invoice_id`
2. **Check Expiration** - Verifies the pending order hasn't expired (10-minute window)
3. **Validate Payment Status** - Processes based on status:
   - `finished` or `confirmed` → Proceed to create sale
   - `failed` or `expired` → Delete pending order
   - `waiting`, `confirming` → Acknowledge and wait
4. **Final Inventory Check** - Validates inventory is still available before creating sale
5. **Create Sale Record** - Inserts into `sales` table and deletes from `pending_orders`

All operations are performed within a database transaction to ensure data consistency and prevent race conditions.

### Database Schema

The application expects the following tables (as defined in `cbd420(1).sql` + migration):

- **`pending_orders`** - Stores pending payment orders with 10-minute reservations
  - `payment_id` (unique) - NOWPayments payment/invoice ID
  - `order_id` (unique) - Internal order ID (format: SALE-{timestamp}-{drop_id}-{buyer_id})
  - `drop_id` - Reference to drops table
  - `buyer_id` - Reference to buyers table
  - `size` - Order size (in grams)
  - `price_amount` - Payment amount
  - `price_currency` - Payment currency (default: 'chf')
  - `created_at` - Order creation timestamp
  - `expires_at` - Expiration timestamp (10 minutes from creation) - **REQUIRED**

- **`sales`** - Stores completed sales
  - `drop_id` - Reference to drops table
  - `buyer_id` - Reference to buyers table
  - `size` - Sale size (in grams)
  - `payment_id` - NOWPayments payment ID (matches pending_orders.payment_id)
  - `created_at` - Sale creation timestamp

- **`drops`** - Product drop information
  - `id` - Drop ID
  - `size` - Available size
  - `unit` - Unit of measurement ('g' or 'kg')

### Inventory Calculation

Available inventory is calculated as:
```
Available = drop.size - (SUM(sales.size) + SUM(pending_orders.size WHERE expires_at > NOW()))
```

The system automatically handles unit conversion (kg to grams) when calculating inventory.

### Key Features

- **Expiration Handling**: Pending orders expire after 10 minutes and are automatically excluded from inventory calculations
- **Inventory Validation**: Final inventory check ensures no overselling occurs between payment initiation and completion
- **Idempotency**: IPN callbacks can be processed multiple times safely (checks for existing sales)
- **Transaction Safety**: All database operations use transactions to ensure atomicity

## IPN Callback Processing

The IPN handler follows this flow for each callback:

1. **Find Pending Order** - Searches by `payment_id` or `invoice_id` (tries both)
2. **Check Expiration** - Verifies `expires_at > NOW()` - expired orders are rejected
3. **Payment Status Processing**:
   - `finished` or `confirmed` → Creates sale (after inventory check)
   - `failed` or `expired` → Deletes pending order
   - Other statuses → Acknowledged, waiting for final status
4. **Final Inventory Check** - Re-validates inventory before creating sale (within transaction)
5. **Create Sale** - Atomically creates sale and deletes pending order

### Important Notes

- IPN callbacks may be sent multiple times - the system handles this with idempotency checks
- Expired pending orders are automatically excluded from inventory calculations
- Always returns HTTP 200 to NOWPayments (even on errors) to prevent retries
- Inventory is checked within a database transaction to prevent race conditions

## Security

- All IPN requests are validated using HMAC SHA512 signature verification
- Invalid signatures are rejected with 400 Bad Request
- The IPN secret key should never be committed to version control
- Database transactions ensure data consistency and prevent race conditions

## Testing

You can test the IPN endpoint using tools like:
- Postman
- curl
- NOWPayments test payments

Example curl command (with test signature):
```bash
curl -X POST http://localhost:3000/ipn \
  -H "Content-Type: application/json" \
  -H "x-nowpayments-sig: your_signature_here" \
  -d '{"payment_id":"test123","payment_status":"waiting","price_amount":100,"price_currency":"usd"}'
```

## Documentation

For detailed implementation guide and database schema, refer to the **IPN Callback Integration Guide** provided with this application.

## License

ISC