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# Error Handling & Resilience Skill — v1.0
---
## 1. Exception Design Principles
### 1.1 Custom Exception Hierarchy
```java
// Java: Create custom exceptions with clear semantics
/**
* Base exception for all domain errors in the order system.
*
* <p>All domain-specific exceptions inherit from this, making it easy to catch
* all business errors while letting system errors propagate.</p>
*/
public abstract class DomainException extends RuntimeException {
private final String errorCode;
public DomainException(String errorCode, String message) {
super(message);
this.errorCode = errorCode;
}
public DomainException(String errorCode, String message, Throwable cause) {
super(message, cause);
this.errorCode = errorCode;
}
public String getErrorCode() {
return errorCode;
}
}
/**
* Thrown when a required entity is not found.
*/
public class EntityNotFoundException extends DomainException {
public EntityNotFoundException(String entityType, Object id) {
super("ENTITY_NOT_FOUND",
String.format("%s not found: %s", entityType, id));
}
}
/**
* Thrown when business rule validation fails.
*/
public class ValidationException extends DomainException {
public ValidationException(String message) {
super("VALIDATION_ERROR", message);
}
}
/**
* Thrown when operation cannot proceed due to resource constraints.
*/
public class InsufficientResourcesException extends DomainException {
public InsufficientResourcesException(String resource) {
super("INSUFFICIENT_RESOURCES",
String.format("Insufficient %s available", resource));
}
}
/**
* Thrown when operation conflicts with existing state.
*/
public class ConflictException extends DomainException {
public ConflictException(String message) {
super("CONFLICT", message);
}
}
```plaintext
```python
# Python: Create custom exception hierarchy
class DomainException(Exception):
"""
Base exception for all domain errors.
All business logic exceptions inherit from this.
"""
def __init__(self, error_code: str, message: str, details: dict | None = None):
"""
Initialize domain exception.
Args:
error_code: Machine-readable error code (e.g., "ENTITY_NOT_FOUND")
message: Human-readable error message
details: Optional additional error details (dict)
"""
super().__init__(message)
self.error_code = error_code
self.details = details or {}
class EntityNotFoundException(DomainException):
"""Raised when a required entity is not found."""
def __init__(self, entity_type: str, entity_id):
super().__init__(
error_code="ENTITY_NOT_FOUND",
message=f"{entity_type} not found: {entity_id}"
)
class ValidationException(DomainException):
"""Raised when business rule validation fails."""
def __init__(self, message: str, details: dict | None = None):
super().__init__(
error_code="VALIDATION_ERROR",
message=message,
details=details
)
class InsufficientResourcesException(DomainException):
"""Raised when operation cannot proceed due to resource constraints."""
def __init__(self, resource: str):
super().__init__(
error_code="INSUFFICIENT_RESOURCES",
message=f"Insufficient {resource} available"
)
```plaintext
---
## 2. Try-Catch Patterns
### 2.1 Java — Specific Exceptions First
```java
/**
* Process an order with comprehensive error handling.
*
* @param orderId the order ID to process
* @return processing result
* @throws DomainException if business validation fails
*/
public OrderProcessResult processOrder(Long orderId) {
try {
// 1. Try specific, recoverable exceptions first
Order order = findOrderById(orderId); // throws EntityNotFoundException
if (order.getStatus() == OrderStatus.CANCELLED) {
throw new ValidationException("Cannot process cancelled order");
}
// 2. Perform business operation
BigDecimal totalAmount = calculateTotal(order);
if (totalAmount.compareTo(BigDecimal.ZERO) <= 0) {
throw new ValidationException("Order total must be positive");
}
// 3. External service call with retry
PaymentResult paymentResult = retryPaymentService(() ->
paymentGateway.processPayment(order.getPaymentToken(), totalAmount)
);
if (!paymentResult.isSuccessful()) {
throw new ConflictException("Payment processing failed: " +
paymentResult.getErrorMessage());
}
// 4. Persist changes
order.setStatus(OrderStatus.CONFIRMED);
orderRepository.save(order);
return OrderProcessResult.success(order);
} catch (EntityNotFoundException e) {
// Handle not found — client error
log.warn("Order not found: {}", orderId);
return OrderProcessResult.notFound(orderId);
} catch (ValidationException e) {
// Handle validation errors — client error
log.warn("Validation failed: {}", e.getMessage());
return OrderProcessResult.validationFailed(e.getMessage());
} catch (ConflictException e) {
// Handle conflict — retry-able error
log.warn("Conflict detected: {}", e.getMessage());
return OrderProcessResult.conflict(e.getMessage());
} catch (IOException e) {
// Handle I/O errors — infrastructure issue
log.error("I/O error during order processing", e);
return OrderProcessResult.systemError("Communication error occurred");
} catch (Exception e) {
// Catch-all for unexpected errors
log.error("Unexpected error processing order {}", orderId, e);
return OrderProcessResult.systemError("Unexpected error occurred");
} finally {
// Cleanup resources (connections, file handles, etc.)
closeAnyOpenResources();
}
}
```plaintext
### 2.2 Python — Try-Except-Else Pattern
```python
async def process_order(order_id: int) -> OrderProcessResult:
"""
Process an order with comprehensive error handling.
Args:
order_id: The order ID to process.
Returns:
OrderProcessResult indicating success or failure.
"""
try:
# Try to retrieve and validate order
order = await order_service.get_order_by_id(order_id)
if not order:
raise EntityNotFoundException("Order", order_id)
if order.status == OrderStatus.CANCELLED:
raise ValidationException(
"Cannot process cancelled order",
details={"order_id": order_id, "status": order.status}
)
# Validate order total
total_amount = calculate_total(order)
if total_amount <= 0:
raise ValidationException(
"Order total must be positive",
details={"total": total_amount}
)
# Process payment with retries
payment_result = await retry_operation(
operation=lambda: payment_gateway.process_payment(
order.payment_token,
total_amount
),
max_retries=3,
backoff_factor=2
)
if not payment_result.is_successful:
raise ConflictException(
f"Payment processing failed: {payment_result.error_message}"
)
except EntityNotFoundException as e:
logger.warning("Order not found: %s", e.message)
return OrderProcessResult.not_found(order_id)
except ValidationException as e:
logger.warning("Validation failed: %s", e.message)
return OrderProcessResult.validation_failed(
message=e.message,
details=e.details
)
except ConflictException as e:
logger.warning("Conflict detected: %s", e.message)
return OrderProcessResult.conflict(e.message)
except (IOError, ConnectionError) as e:
logger.error("I/O error during order processing", exc_info=True)
return OrderProcessResult.system_error("Communication error occurred")
except Exception as e:
logger.error("Unexpected error processing order %s", order_id, exc_info=True)
return OrderProcessResult.system_error("Unexpected error occurred")
else:
# Executes only if no exception was raised
order.status = OrderStatus.CONFIRMED
await order_service.save_order(order)
return OrderProcessResult.success(order)
finally:
# Cleanup: always executes
await cleanup_resources()
```plaintext
---
## 3. Logging Best Practices
### 3.1 Log Levels and What to Log
```java
@Slf4j
public class OrderProcessor {
public void processOrder(Long orderId) {
// DEBUG: Detailed diagnostic info (heavy, disabled in prod)
log.debug("Starting order processing for orderId={}, timestamp={}",
orderId, Instant.now());
try {
Order order = orderRepository.findById(orderId)
.orElseThrow(() -> new EntityNotFoundException("Order", orderId));
// INFO: Significant business events
log.info("Retrieved order: orderId={}, customerId={}, status={}",
orderId, order.getCustomerId(), order.getStatus());
// Process order
PaymentResult result = paymentGateway.processPayment(order);
if (result.isSuccessful()) {
log.info("Payment processed successfully: orderId={}, amount={}",
orderId, result.getAmount());
}
} catch (EntityNotFoundException e) {
// WARN: Recoverable problems (expected error conditions)
log.warn("Order not found: orderId={}, cause={}", orderId, e.getMessage());
} catch (PaymentGatewayException e) {
// ERROR: Unexpected failures that need attention
log.error("Payment gateway error: orderId={}, retryable={}, details={}",
orderId, e.isRetryable(), e.getDetails(), e);
} catch (Exception e) {
// ERROR + stack trace: Unexpected errors
log.error("Unexpected error processing order: orderId={}", orderId, e);
}
}
}
```plaintext
```python
import logging
logger = logging.getLogger(__name__)
async def process_order(order_id: int) -> None:
"""
Process an order with appropriate logging at each step.
Args:
order_id: The order ID to process.
"""
try:
# DEBUG: Detailed diagnostic info
logger.debug("Starting order processing for order_id=%s, timestamp=%s",
order_id, datetime.utcnow().isoformat())
order = await order_service.get_order_by_id(order_id)
if not order:
raise EntityNotFoundException("Order", order_id)
# INFO: Significant business events
logger.info("Retrieved order: order_id=%s, customer_id=%s, status=%s",
order_id, order.customer_id, order.status.value)
# Process payment
payment_result = await payment_gateway.process_payment(order)
if payment_result.is_successful:
logger.info("Payment processed successfully: order_id=%s, amount=%.2f",
order_id, payment_result.amount)
except EntityNotFoundException as e:
# WARN: Expected, recoverable errors
logger.warning("Order not found: order_id=%s, error=%s",
order_id, e.message)
except PaymentGatewayException as e:
# ERROR: Unexpected failures requiring attention
logger.error("Payment gateway error: order_id=%s, retryable=%s",
order_id, e.is_retryable, exc_info=e.is_critical)
except Exception as e:
# ERROR: Unexpected errors with full stack trace
logger.error("Unexpected error processing order: order_id=%s",
order_id, exc_info=True)
```plaintext
---
## 4. Retry Logic with Exponential Backoff
```java
/**
* Retry helper for resilient operations.
*/
public class RetryPolicy<T> {
private final int maxAttempts;
private final long initialBackoffMs;
private final double backoffMultiplier;
public RetryPolicy(int maxAttempts, long initialBackoffMs, double backoffMultiplier) {
this.maxAttempts = maxAttempts;
this.initialBackoffMs = initialBackoffMs;
this.backoffMultiplier = backoffMultiplier;
}
/**
* Execute operation with retry logic.
*
* @param operation the operation to retry
* @return the operation result
* @throws Exception if all retries are exhausted
*/
public <T> T execute(Callable<T> operation) throws Exception {
long backoffMs = initialBackoffMs;
for (int attempt = 1; attempt <= maxAttempts; attempt++) {
try {
return operation.call();
} catch (TransientException e) {
if (attempt >= maxAttempts) {
throw e; // Last attempt failed, give up
}
log.warn("Transient error on attempt {}/{}: {}. Retrying in {}ms",
attempt, maxAttempts, e.getMessage(), backoffMs);
Thread.sleep(backoffMs);
backoffMs = (long) (backoffMs * backoffMultiplier);
} catch (Exception e) {
throw e; // Non-transient error, fail immediately
}
}
throw new IllegalStateException("Unreachable code");
}
}
// Usage
RetryPolicy<PaymentResult> policy = new RetryPolicy<>(3, 100, 2.0);
PaymentResult result = policy.execute(() ->
paymentGateway.processPayment(order)
);
```plaintext
```python
import asyncio
from typing import Callable, TypeVar, Any
T = TypeVar('T')
async def retry_operation(
operation: Callable,
max_retries: int = 3,
initial_backoff_ms: int = 100,
backoff_multiplier: float = 2.0,
retryable_exceptions: tuple = (ConnectionError, TimeoutError),
) -> Any:
"""
Execute operation with exponential backoff retry logic.
Args:
operation: Async function to execute.
max_retries: Maximum number of retry attempts.
initial_backoff_ms: Initial backoff in milliseconds.
backoff_multiplier: Multiplier for exponential backoff.
retryable_exceptions: Tuple of exceptions that trigger retry.
Returns:
Operation result.
Raises:
The last exception if all retries are exhausted.
"""
backoff_ms = initial_backoff_ms
for attempt in range(1, max_retries + 1):
try:
return await operation()
except retryable_exceptions as e:
if attempt >= max_retries:
raise # Last attempt failed
logger.warning(
"Transient error on attempt %d/%d: %s. Retrying in %dms",
attempt, max_retries, str(e), backoff_ms
)
await asyncio.sleep(backoff_ms / 1000)
backoff_ms = int(backoff_ms * backoff_multiplier)
except Exception as e:
raise # Non-retryable error, fail immediately
raise RuntimeError("Unreachable code")
# Usage
result = await retry_operation(
operation=lambda: payment_gateway.process_payment(order),
max_retries=3,
initial_backoff_ms=100,
backoff_multiplier=2.0
)
```plaintext
---
## 5. Error Recovery Strategies
### 5.1 Graceful Degradation
```java
/**
* Fetch order details with graceful degradation.
*
* If caching service fails, fall back to primary database.
*/
public Order getOrderWithFallback(Long orderId) {
try {
// Try cache first (fast but unreliable)
return cacheService.getOrder(orderId);
} catch (CacheException e) {
log.warn("Cache service failed, falling back to database: {}", e.getMessage());
try {
return orderRepository.findById(orderId)
.orElseThrow(() -> new EntityNotFoundException("Order", orderId));
} catch (DatabaseException e2) {
log.error("Database error, cannot retrieve order", e2);
throw new SystemUnavailableException("Order service unavailable");
}
}
}
```plaintext
### 5.2 Circuit Breaker Pattern
```java
/**
* Circuit breaker for external service calls.
*
* Fails fast when service is down instead of retrying endlessly.
*/
public class CircuitBreaker<T> {
enum State { CLOSED, OPEN, HALF_OPEN }
private State state = State.CLOSED;
private int failureCount = 0;
private long lastFailureTime = 0;
private final int failureThreshold = 5;
private final long timeout = 60_000; // 1 minute
public synchronized T execute(Callable<T> operation) throws Exception {
if (state == State.OPEN) {
if (System.currentTimeMillis() - lastFailureTime > timeout) {
state = State.HALF_OPEN;
} else {
throw new CircuitBreakerOpenException("Circuit breaker is OPEN");
}
}
try {
T result = operation.call();
onSuccess();
return result;
} catch (Exception e) {
onFailure();
throw e;
}
}
private void onSuccess() {
failureCount = 0;
state = State.CLOSED;
}
private void onFailure() {
failureCount++;
lastFailureTime = System.currentTimeMillis();
if (failureCount >= failureThreshold) {
state = State.OPEN;
}
}
}
```plaintext
---
## 6. Error Handling Checklist
✅ Create custom exception hierarchy (extends common base)
✅ Catch specific exceptions before general ones
✅ Log at appropriate levels (DEBUG, INFO, WARN, ERROR)
✅ Include context in log messages (IDs, status, amounts)
✅ Implement retry logic for transient errors
✅ Use exponential backoff to avoid thundering herd
✅ Fail fast for non-retryable errors
✅ Implement graceful degradation (fallbacks)
✅ Use circuit breaker for external service calls
✅ Clean up resources in finally blocks
✅ Return consistent error responses to clients
✅ Never swallow exceptions silently
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