# Error Handling Implementation Summary

**Sprint 1 P1 - Cross-cutting Corrections**
**Date:** 2025-12-12

## Overview

Implemented standardized error handling system across all ERP-Suite backends (NestJS and Express).

## Files Created

### Core Error Handling Files

1. **base-error.ts** (1.8 KB)
   - `ErrorResponse` interface - standardized error response structure
   - `BaseError` abstract class - base for all application errors
   - `toResponse()` method for converting errors to HTTP responses

2. **http-errors.ts** (3.5 KB)
   - `BadRequestError` (400)
   - `UnauthorizedError` (401)
   - `ForbiddenError` (403)
   - `NotFoundError` (404)
   - `ConflictError` (409)
   - `ValidationError` (422)
   - `InternalServerError` (500)

3. **error-filter.ts** (6.3 KB)
   - `GlobalExceptionFilter` - NestJS exception filter
   - Handles BaseError, HttpException, and generic errors
   - Automatic request ID extraction
   - Severity-based logging (ERROR/WARN/INFO)

4. **error-middleware.ts** (6.5 KB)
   - `createErrorMiddleware()` - Express error middleware factory
   - `errorMiddleware` - default middleware instance
   - `notFoundMiddleware` - 404 handler
   - `ErrorLogger` interface for custom logging
   - `ErrorMiddlewareOptions` for configuration

5. **index.ts** (1.1 KB)
   - Barrel exports for all error handling components

### Documentation & Examples

6. **README.md** (13 KB)
   - Comprehensive documentation
   - API reference
   - Best practices
   - Migration guide
   - Testing examples

7. **INTEGRATION_GUIDE.md** (8.5 KB)
   - Quick start guide for each backend
   - Step-by-step integration instructions
   - Common patterns
   - Checklist for migration

8. **nestjs-integration.example.ts** (6.7 KB)
   - Complete NestJS integration example
   - Bootstrap configuration
   - Service/Controller examples
   - Request ID middleware
   - Custom domain errors

9. **express-integration.example.ts** (12 KB)
   - Complete Express integration example
   - App setup
   - Router examples
   - Async handler wrapper
   - Custom logger integration

## Updates to Existing Files

### shared-libs/core/index.ts

Added error handling exports (lines 136-153):
```typescript
// Error Handling
export {
  BaseError,
  ErrorResponse,
  BadRequestError,
  UnauthorizedError,
  ForbiddenError,
  NotFoundError,
  ConflictError,
  ValidationError,
  InternalServerError,
  GlobalExceptionFilter,
  createErrorMiddleware,
  errorMiddleware,
  notFoundMiddleware,
  ErrorLogger,
  ErrorMiddlewareOptions,
} from './errors';
```

## Error Response Format

All backends now return errors in this standardized format:

```json
{
  "statusCode": 404,
  "error": "Not Found",
  "message": "User not found",
  "details": {
    "userId": "999"
  },
  "timestamp": "2025-12-12T10:30:00.000Z",
  "path": "/api/users/999",
  "requestId": "550e8400-e29b-41d4-a716-446655440000"
}
```

## Features

### 1. Standardized Error Structure
- Consistent error format across all backends
- HTTP status codes
- Error type strings
- Human-readable messages
- Optional contextual details
- ISO 8601 timestamps
- Request path tracking
- Request ID correlation

### 2. Framework Support
- **NestJS**: Global exception filter with decorator support
- **Express**: Error middleware with custom logger support
- Both frameworks support request ID tracking

### 3. Error Classes
- Base abstract class for extensibility
- 7 built-in HTTP error classes
- Support for custom domain-specific errors
- Type-safe error details

### 4. Logging
- Automatic severity-based logging
- ERROR level for 500+ errors
- WARN level for 400-499 errors
- INFO level for other status codes
- Stack trace inclusion in logs
- Request correlation via request IDs

### 5. Developer Experience
- TypeScript support with full type definitions
- Comprehensive JSDoc documentation
- Extensive examples for both frameworks
- Easy migration path from existing error handling
- No try/catch required in controllers/routes

## Integration Requirements

### For NestJS Backends (gamilit, platform_marketing_content)

1. Add `GlobalExceptionFilter` to main.ts
2. Replace manual error throwing with standardized error classes
3. Remove try/catch blocks from controllers
4. Optional: Add request ID middleware

### For Express Backends (trading-platform)

1. Add error middleware (must be last)
2. Optional: Add 404 middleware
3. Update route handlers to use error classes
4. Use async handler wrapper for async routes
5. Optional: Add request ID middleware

## Testing

Both frameworks support standard testing patterns:

```typescript
// Test example
it('should return 404 when user not found', async () => {
  const response = await request(app)
    .get('/api/users/999')
    .expect(404);

  expect(response.body).toMatchObject({
    statusCode: 404,
    error: 'Not Found',
    message: 'User not found'
  });
});
```

## Benefits

1. **Consistency**: All backends return errors in the same format
2. **Maintainability**: Centralized error handling logic
3. **Debugging**: Request IDs for tracing errors across services
4. **Type Safety**: Full TypeScript support
5. **Extensibility**: Easy to add custom domain errors
6. **Logging**: Automatic structured logging
7. **API Documentation**: Predictable error responses
8. **Client Experience**: Clear, actionable error messages

## Next Steps

1. **gamilit backend**: Integrate GlobalExceptionFilter
2. **trading-platform backend**: Integrate error middleware
3. **platform_marketing_content backend**: Integrate GlobalExceptionFilter
4. Update API documentation with new error format
5. Update client-side error handling
6. Add integration tests for error responses
7. Configure error monitoring/alerting

## File Locations

All files are located in:
```
/home/isem/workspace/projects/erp-suite/apps/shared-libs/core/errors/
```

### Core Files
- `base-error.ts`
- `http-errors.ts`
- `error-filter.ts`
- `error-middleware.ts`
- `index.ts`

### Documentation
- `README.md`
- `INTEGRATION_GUIDE.md`
- `IMPLEMENTATION_SUMMARY.md` (this file)

### Examples
- `nestjs-integration.example.ts`
- `express-integration.example.ts`

## Import Usage

```typescript
// Import from @erp-suite/core
import {
  NotFoundError,
  ValidationError,
  GlobalExceptionFilter,
  createErrorMiddleware,
} from '@erp-suite/core';
```

## Custom Error Example

```typescript
import { BaseError } from '@erp-suite/core';

export class InsufficientBalanceError extends BaseError {
  readonly statusCode = 400;
  readonly error = 'Insufficient Balance';

  constructor(required: number, available: number) {
    super('Insufficient balance for this operation', {
      required,
      available,
      deficit: required - available,
    });
  }
}
```

## Support

For questions or issues:
- See detailed examples in `nestjs-integration.example.ts` and `express-integration.example.ts`
- Refer to `README.md` for comprehensive documentation
- Follow `INTEGRATION_GUIDE.md` for step-by-step integration

---

**Status**: ✅ Implementation Complete
**Ready for Integration**: Yes
**Breaking Changes**: No (additive only)
