# Error Handling in DDD

This document defines error handling principles that apply across all layers and languages.

## Core Principles

1. **No Silent Failures** - All errors must be explicitly handled or propagated
2. **Layer-Specific Errors** - Each layer defines its own error types
3. **Error Transformation** - Errors are transformed at layer boundaries
4. **Proper Logging** - Original errors logged before transformation
5. **Result Types Over Exceptions** - Prefer Result types (where supported) over exceptions

## Language-Specific Implementation

- **Java**: See [languages/java/error-handling.md](../languages/java/error-handling.md)
- **Go**: Use error returns and custom error types

## Error Hierarchy

```
Domain Errors (business rule violations)
  ↓ transformed at boundary
Application Errors (use case failures)
  ↓ transformed at boundary  
Infrastructure Errors (technical failures)
```

## Logging Strategy

- **ERROR**: Infrastructure failures (database down, network error)
- **WARN**: Business rule violations (insufficient funds, invalid state)
- **INFO**: Normal operations (order placed, account created)
- **DEBUG**: Detailed flow information
- **TRACE**: Original errors when transforming between layers

## Exception Boundary

**Infrastructure Layer** is the exception boundary:
- Infrastructure catches external exceptions (SQL, HTTP, etc.)
- Transforms to domain error types
- Logs original exception at ERROR level
- Returns domain error type

**Domain and Application** layers:
- Never throw exceptions (use Result types or errors)
- Only work with domain/application error types
- No try/catch blocks needed