lcbp3/.agents/rules/10-error-handling.md

trigger

trigger
always_on

ADR-007 Error Handling Strategy

CRITICAL RULES

• ALWAYS use layered error classification (Validation, Business, System)
• NEVER expose technical details to end users
• ALWAYS provide user-friendly error messages with recovery guidance
• ALWAYS log technical details for debugging
• NEVER use generic error messages without context

Error Classification

Error Type	Description	User Message	Technical Log
Validation	Input validation failures	Clear field-level errors	Full validation details
Business	Business rule violations	Actionable guidance	Business context + user ID
System	Infrastructure failures	Generic "try again"	Full stack trace + metrics

Backend Pattern (NestJS)

// Custom Exception Hierarchy
export class BusinessException extends HttpException {
  constructor(
    message: string,
    userMessage: string,
    recoveryAction?: string,
    errorCode?: string
  ) {
    super({ message, userMessage, recoveryAction, errorCode }, 400);
  }
}

// Global Exception Filter
@Catch()
export class GlobalExceptionFilter implements ExceptionFilter {
  catch(exception: unknown, host: ArgumentsHost) {
    // Classify error and provide appropriate response
    // Log technical details
    // Return user-friendly message
  }
}

Frontend Pattern (Next.js)

// Error Display Component
const ErrorDisplay = ({ error, onRetry }) => {
  const userMessage = error.userMessage || 'เกิดข้อผิดพลาด';
  const recoveryAction = error.recoveryAction;

  return (
    <div>
      <p>{userMessage}</p>
      {recoveryAction && <p>{recoveryAction}</p>}
      {onRetry && <button onClick={onRetry}>ลองใหม่</button>}
    </div>
  );
};

Required Implementation

• Global Exception Filter with layered classification
• Custom exception hierarchy (Validation, Business, System)
• Standardized error response DTOs
• Frontend error display components
• Error recovery mechanisms where applicable

Related Documents

• specs/06-Decision-Records/ADR-007-error-handling-strategy.md
• specs/06-Decision-Records/ADR-010-logging-monitoring-strategy.md