Error Handling and Validation

# Implement Comprehensive Error Handling and Validation

## Overview

Implement a consistent and centralized error-handling strategy across the RESTO POS system to improve system reliability, user experience, debugging, and data integrity.

As the system continues to expand beyond basic online ordering into a more complete **Restaurant Management & E-Commerce Platform**, proper error handling is necessary to ensure that unexpected conditions do not result in broken pages, invalid transactions, data corruption, or unclear messages to users and staff.

## Objectives

* Prevent unexpected application crashes and broken pages.
* Provide clear and user-friendly error messages.
* Validate user input before processing requests.
* Handle database and server-side errors safely.
* Prevent sensitive technical information from being exposed to users.
* Ensure failed operations do not result in incomplete or inconsistent data.
* Establish a consistent error-handling pattern across the entire system.
* Improve debugging and maintenance for future development.

## Scope

### 1. Input Validation

Implement validation for all user-submitted data, including:

* Required fields
* Email addresses
* Passwords
* Contact information
* Quantities
* Prices
* Menu item selections
* Order information
* Delivery information
* Date and time values
* Authentication-related inputs

Invalid input should be rejected before being processed or stored in the database.

### 2. Authentication & Authorization Errors

Handle authentication-related failures such as:

* Invalid login credentials
* Missing authentication sessions
* Expired sessions
* Unauthorized access
* Attempting to access admin/staff pages without permission
* Invalid or manipulated user sessions

Users should receive an appropriate message without exposing sensitive authentication details.

### 3. Database Error Handling

Handle database-related failures, including:

* Failed database connections
* Failed queries
* Missing records
* Duplicate records
* Foreign key violations
* Invalid data types
* Transaction failures
* Unexpected database errors

Database errors should be logged for developers while displaying a safe, user-friendly message to the end user.

### 4. Order Processing Errors

Implement error handling for the ordering workflow, including:

* Invalid menu items
* Unavailable menu items
* Invalid quantities
* Invalid prices
* Empty carts
* Failed order creation
* Failed order item insertion
* Failed order status updates
* Invalid order IDs
* Duplicate order submissions

Order operations should maintain data consistency even when an error occurs.

### 5. Payment Error Handling

Prepare the system for payment-related failures, especially with future e-payment/API integration.

Examples include:

* Payment request failure
* Payment timeout
* Failed payment
* Cancelled payment
* Invalid payment response
* Duplicate payment requests
* Payment status mismatch
* API communication failure
* Unexpected payment gateway response

Payment failures must not incorrectly mark an order as `Paid`.

### 6. API & External Service Errors

Implement handling for external API failures, including:

* Connection timeouts
* Invalid API responses
* Authentication failures
* Rate limits
* Service unavailable errors
* Unexpected response formats

External API errors should not expose API keys, tokens, credentials, or internal implementation details.

### 7. File & Resource Errors

Handle errors involving:

* Missing files
* Failed file includes
* Missing configuration files
* Invalid uploads
* Missing images/assets
* File permission errors

The system should fail gracefully instead of displaying raw PHP warnings or server paths.

### 8. Frontend Error Handling

Improve client-side handling for:

* Failed AJAX/fetch requests
* Network errors
* Invalid server responses
* Form validation errors
* Session expiration
* Failed CRUD operations
* Unexpected application states

Users should receive clear feedback instead of silent failures.

### 9. HTTP Error Pages

Implement appropriate responses/pages for common HTTP errors:

* `400 Bad Request`
* `401 Unauthorized`
* `403 Forbidden`
* `404 Not Found`
* `405 Method Not Allowed`
* `500 Internal Server Error`

Error pages should maintain the RESTO POS interface and provide users with an appropriate next action.

### 10. Logging & Debugging

Implement server-side error logging for unexpected errors.

Logs should contain useful debugging information such as:

* Timestamp
* Error type
* Error message
* Request/action involved
* Relevant system context

**Do not log sensitive information**, including:

* Passwords
* Authentication tokens
* API keys
* Payment credentials
* Sensitive customer information

## Error Response Standard

Establish a consistent response structure for backend operations.

Example:

```json
{
"success": false,
"message": "Unable to process the request.",
"error_code": "ORDER_PROCESSING_FAILED"
}
```

Successful operations should follow a consistent structure as well:

```json
{
"success": true,
"message": "Order created successfully.",
"data": {}
}
```

Detailed technical error information should remain available to developers through server-side logs rather than being exposed to end users.

## PHP Error Handling

Review the current PHP implementation and replace direct exposure of errors such as:

```text
Warning: include(...)
Fatal error: Uncaught mysqli_sql_exception...
```

with controlled application-level error handling.

Development and production environments should use different error-display configurations.

### Development

Detailed errors may be displayed for debugging.

### Production

Detailed PHP warnings, stack traces, file paths, SQL errors, and server information should not be displayed to users.

## Database Transactions

Critical multi-step operations should use database transactions where appropriate.

For example:

```text
Create Order
↓
Create Order Items
↓
Update Related Records
↓
Commit Transaction
```

If any step fails:

```text
Rollback Transaction
↓
Return Error
```

This prevents partially completed orders from being stored.

## Acceptance Criteria

* [ ] User input is validated before processing.
* [ ] Authentication and authorization failures are handled safely.
* [ ] Database failures are handled without exposing SQL errors.
* [ ] Order-processing failures are handled consistently.
* [ ] Payment failures cannot incorrectly mark orders as paid.
* [ ] External API failures are handled gracefully.
* [ ] Missing files/resources do not expose server paths.
* [ ] Frontend requests handle network/server failures.
* [ ] Appropriate HTTP error responses/pages are implemented.
* [ ] Server-side error logging is implemented.
* [ ] Sensitive information is excluded from logs and error responses.
* [ ] Production mode does not expose PHP warnings or stack traces.
* [ ] Critical database operations use transactions where necessary.
* [ ] Error messages are understandable to non-technical users.
* [ ] Error-handling behavior follows a consistent pattern throughout the system.

## Priority

**High**

## Suggested Labels

`enhancement` `backend` `security` `validation` `error-handling` `database`

## Future Considerations

As RESTO POS expands toward a more comprehensive restaurant management and e-commerce platform, the error-handling system should be designed to support future modules such as:

* Online payments
* Google Authentication
* Customer accounts
* Order tracking
* Inventory management
* Notifications
* Reporting and analytics
* External APIs
* Additional staff/admin roles

A centralized and consistent error-handling strategy should be established before these modules significantly increase the system's complexity.

No files, tests, or entry points are identified, so begin by mapping the PHP backend, frontend request handlers, database operations, and HTTP error pages before narrowing the work. Done means the scoped error cases are validated, safely reported and logged, sensitive data is excluded, and critical operations preserve data consistency.

Written by the indexing model from the issue text.