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Exception Handling

The BookWorm application implements a comprehensive exception handling strategy that provides consistent error responses, proper logging, and graceful degradation across all microservices.

Exception Architecture​

Exception Hierarchy​

  • Custom Domain Exceptions - Business rule violations and domain-specific errors
  • Validation Exceptions - Input validation and constraint violations
  • Infrastructure Exceptions - Database, messaging, and external service errors
  • Security Exceptions - Authentication and authorization failures
  • System Exceptions - Unexpected system-level errors

Exception Handling Middleware​

  • Global Exception Handler - Centralized exception processing
  • Error Response Standardization - Consistent error format across APIs
  • Correlation ID Tracking - Error tracing across distributed services
  • Sensitive Data Filtering - Prevent sensitive information leakage

Exception Categories​

Business Exceptions​

  • Domain Rule Violations - Business logic constraint failures
  • Entity Not Found - Resource lookup failures
  • Validation Failures - Business rule validation errors
  • Concurrency Conflicts - Optimistic locking and versioning errors

Technical Exceptions​

  • Database Exceptions - Connection, query, and transaction failures
  • Network Exceptions - Service communication and timeout errors
  • Serialization Exceptions - JSON/XML processing errors
  • Configuration Exceptions - Missing or invalid configuration

Security Exceptions​

  • Authentication Failures - Invalid credentials or expired tokens
  • Authorization Failures - Insufficient permissions for operations
  • Input Validation Failures - Malicious or malformed input detection
  • Rate Limiting Violations - Request throttling and abuse prevention

Error Response Format​

Standardized Error Structure​

  • Error Code - Unique identifier for error type
  • Error Message - Human-readable error description
  • Details - Additional context and troubleshooting information
  • Timestamp - When the error occurred
  • Correlation ID - Request tracking identifier

HTTP Status Code Mapping​

  • 400 Bad Request - Client input validation errors
  • 401 Unauthorized - Authentication required or failed
  • 403 Forbidden - Authorization denied for authenticated user
  • 404 Not Found - Resource not found or access denied
  • 409 Conflict - Business rule violations or concurrency conflicts
  • 422 Unprocessable Entity - Semantic validation errors
  • 429 Too Many Requests - Rate limiting exceeded
  • 500 Internal Server Error - Unexpected system errors
  • 503 Service Unavailable - External dependency failures

Resilience Patterns​

Retry Mechanisms​

  • Exponential Backoff - Progressive retry delays with jitter
  • Circuit Breaker - Prevent cascading failures in distributed systems
  • Timeout Handling - Request timeout configuration and fallback
  • Dead Letter Queue - Failed message handling and reprocessing

Fallback Strategies​

  • Graceful Degradation - Reduced functionality when services are unavailable
  • Default Responses - Sensible defaults when data is unavailable
  • Cache Fallback - Serve stale data when primary sources fail
  • Service Mesh Integration - External resilience patterns

Logging and Monitoring​

Exception Logging​

  • Structured Logging - Consistent exception log format
  • Log Level Classification - Appropriate severity levels for different errors
  • Context Preservation - Maintain request context in exception logs
  • Performance Impact - Minimize logging overhead in high-throughput scenarios

Monitoring and Alerting​

  • Error Rate Metrics - Track exception frequency and trends
  • Error Classification - Categorize errors by type and severity
  • Performance Impact - Monitor exception handling performance
  • Alert Thresholds - Automated alerting for critical error patterns

Validation Framework​

FluentValidation Integration​

  • Input Validation - Request model validation with custom rules
  • Business Rule Validation - Domain-specific validation logic
  • Cross-Field Validation - Complex validation scenarios
  • Localization Support - Multi-language error messages

Validation Patterns​

  • Early Validation - Fail fast at API boundaries
  • Pipeline Validation - Multi-stage validation in CQRS pipelines
  • Conditional Validation - Context-dependent validation rules
  • Async Validation - External service validation (e.g., uniqueness checks)

Best Practices​

Exception Design​

  • Specific Exception Types - Create meaningful exception hierarchies
  • Immutable Exceptions - Exception objects should be immutable
  • Rich Context - Include relevant context in exception messages
  • Localization - Support multiple languages for user-facing errors

Error Handling Guidelines​

  • Don't Swallow Exceptions - Always log and handle exceptions appropriately
  • Use Finally Blocks - Ensure resource cleanup in exception scenarios
  • Avoid Generic Catches - Handle specific exception types when possible
  • Performance Considerations - Exception handling should not impact normal flow performance