Error Recovery Patterns

Overview

When systems fail, how you recover determines the difference between a minor hiccup and a catastrophic outage. This skill provides systematic approaches to error recovery, failure handling, and resilience patterns.

When to use this skill:

• Task execution fails with errors
• Timeouts or external service failures detected
• Database transaction failures occur
• Risk of cascading failures across systems
• Need to maintain data consistency during failures
• Implementing resilience for distributed systems

---

Quick Diagnosis Framework

1. Classify the Failure

Transient vs Permanent:

• Transient: Temporary issue, likely to succeed on retry (network blip, timeout, rate limit)
• Permanent: Fundamental issue, won't succeed without intervention (invalid input, missing resource, configuration error)

Recoverable vs Fatal:

• Recoverable: Can restore to consistent state (retry, rollback, compensating transaction)
• Fatal: Cannot recover automatically (data corruption, invariant violation, critical dependency failure)

Scope:

• Local: Single operation failure
• Distributed: Multiple service failure
• Cascading: Failure spreading across system boundaries

2. Select Recovery Strategy

Failure Type	Recovery Strategy	Pattern to Use
Transient + Recoverable	Retry with backoff	Exponential backoff, jitter
Transient + Service failure	Circuit breaker	Open/half-open/closed states
Permanent + Recoverable	Rollback or compensate	Transaction rollback, saga pattern
Permanent + Fatal	Fail gracefully	Graceful degradation, fallback
Distributed + Transient	Retry with timeout	Distributed timeout, bulkhead
Distributed + Permanent	Compensating transaction	Saga pattern, event sourcing
Cascading risk	Prevent cascade	Circuit breaker, rate limiting

---

Core Recovery Patterns

Pattern 1: Retry with Exponential Backoff

When to use:

• Transient failures (network timeouts, temporary service unavailability)
• Rate limiting (429 responses)
• Database connection pool exhaustion

Implementation approach:

# Exponential backoff with jitter
def retry_with_backoff(func, max_retries=5, base_delay=1, max_delay=60):
    """
    Retry with exponential backoff and jitter.

    Args:
        func: Function to retry
        max_retries: Maximum retry attempts (default 5)
        base_delay: Initial delay in seconds (default 1)
        max_delay: Maximum delay in seconds (default 60)
    """
    for attempt in range(max_retries):
        try:
            return func()
        except TransientError as e:
            if attempt == max_retries - 1:
                raise  # Final attempt, re-raise

            # Exponential backoff: 1s, 2s, 4s, 8s, 16s...
            delay = min(base_delay * (2 ** attempt), max_delay)

            # Add jitter (±25%) to prevent thundering herd
            jitter = delay * 0.25 * (2 * random.random() - 1)
            actual_delay = delay + jitter

            logger.warning(f"Transient error on attempt {attempt + 1}, "
                         f"retrying in {actual_delay:.2f}s: {e}")
            time.sleep(actual_delay)

Key principles:

• Exponential backoff: Double delay each retry (1s → 2s → 4s → 8s)
• Max delay cap: Prevent excessive wait times (typically 60s)
• Jitter: Add randomness (±25%) to prevent thundering herd problem
• Max retries: Limit attempts (typically 3-5) to prevent infinite loops
• Idempotency: Ensure retries are safe (use idempotency keys)

When NOT to use:

• Permanent failures (invalid input, missing resources)
• User-interactive operations (don't make users wait)
• Write operations without idempotency guarantees

---

Pattern 2: Circuit Breaker

When to use:

• Repeated failures to external service
• Protecting downstream services from overload
• Preventing cascading failures
• Fast-failing when service is known to be down

Implementation approach:

class CircuitBreaker:
    """
    Circuit breaker pattern with three states:
    - CLOSED: Normal operation, requests pass through
    - OPEN: Failures exceeded threshold, requests fail fast
    - HALF_OPEN: Testing if service recovered
    """
    def __init__(self, failure_threshold=5, timeout=60, success_threshold=2):
        self.failure_threshold = failure_threshold  # Failures before opening
        self.timeout = timeout  # Seconds before trying again (half-open)
        self.success_threshold = success_threshold  # Successes to close

        self.failure_count = 0
        self.success_count = 0
        self.state = "CLOSED"
        self.opened_at = None

    def call(self, func):
        """Execute function through circuit breaker."""
        if self.state == "OPEN":
            # Check if timeout elapsed
            if time.time() - self.opened_at >= self.timeout:
                self.state = "HALF_OPEN"
                self.success_count = 0
                logger.info("Circuit breaker entering HALF_OPEN state")
            else:
                raise CircuitBreakerOpenError("Circuit breaker is OPEN")

        try:
            result = func()
            self._on_success()
            return result
        except Exception as e:
            self._on_failure()
            raise

    def _on_success(self):
        """Handle successful call."""
        self.failure_count = 0

        if self.state == "HALF_OPEN":
            self.success_count += 1
            if self.success_count >= self.success_threshold:
                self.state = "CLOSED"
                logger.info("Circuit breaker CLOSED (service recovered)")

    def _on_failure(self):
        """Handle failed call."""
        self.failure_count += 1

        if self.state == "HALF_OPEN":
            # Failure in half-open state, reopen circuit
            self.state = "OPEN"
            self.opened_at = time.time()
            logger.warning("Circuit breaker OPEN (half-open test failed)")

        elif self.failure_count >= self.failure_threshold:
            # Threshold exceeded, open circuit
            self.state = "OPEN"
            self.opened_at = time.time()
            logger.error(f"Circuit breaker OPEN ({self.failure_count} failures)")

Key principles:

• CLOSED state: Normal operation, track failures
• OPEN state: Fast-fail without calling service (save resources)
• HALF_OPEN state: Test if service recovered (limited requests)
• Failure threshold: Open after N consecutive failures (typically 5-10)
• Timeout: Try again after N seconds (typically 30-60s)
• Success threshold: Close after N successes in half-open (typically 2-3)

Monitoring:

• Track state transitions (CLOSED → OPEN is critical alert)
• Monitor failure rate before opening
• Alert on circuit breaker open events
• Dashboard: show circuit breaker states across services

---

Pattern 3: Transaction Rollback

When to use:

• Database transaction failures
• Multi-step operations where consistency required
• ACID transactions (single database)
• Failures during data modification

Implementation approach:

def transfer_funds(from_account, to_account, amount):
    """
    Transfer funds with automatic rollback on failure.
    Demonstrates transaction rollback pattern.
    """
    with db.transaction():  # Automatically rolls back on exception
        try:
            # Step 1: Debit source account
            from_balance = db.get_balance(from_account)
            if from_balance < amount:
                raise InsufficientFundsError(f"Balance {from_balance} < {amount}")

            db.debit(from_account, amount)
            logger.info(f"Debited {amount} from {from_account}")

            # Step 2: Credit destination account
            db.credit(to_account, amount)
            logger.info(f"Credited {amount} to {to_account}")

            # Step 3: Record transaction
            db.record_transaction(from_account, to_account, amount, timestamp=now())
            logger.info(f"Recorded transaction {from_account} → {to_account}")

            # All steps succeeded, commit
            return TransactionResult(success=True, transaction_id=...)

        except Exception as e:
            # Any failure triggers automatic rollback
            logger.error(f"Transaction failed, rolling back: {e}")
            # with statement handles rollback automatically
            raise TransactionFailedError(f"Transfer failed: {e}") from e

Key principles:

• All-or-nothing: Either all steps succeed or none do
• Automatic rollback: Use database transaction context managers
• Idempotency: Safe to retry entire transaction
• Logging: Log each step for debugging rolled-back transactions
• Isolation: Prevent dirty reads during transaction

When NOT to use:

• Distributed systems (multiple databases) - use saga pattern instead
• Long-running operations (locks held too long)
• Operations with external side effects (APIs, file writes)

---

Pattern 4: Compensating Transaction (Saga Pattern)

When to use:

• Distributed systems (multiple services/databases)
• Long-running business processes
• Cannot use traditional transactions (no ACID across services)
• Need eventual consistency

Implementation approach:

class BookingWorkflow:
    """
    Hotel booking saga with compensating transactions.
    Steps: Reserve hotel → Charge payment → Send confirmation
    Compensations: Cancel reservation ← Refund payment ← Cancel email
    """
    def execute(self, booking_request):
        """Execute saga with automatic compensation on failure."""
        executed_steps = []

        try:
            # Step 1: Reserve hotel room
            reservation = self.hotel_service.reserve_room(
                booking_request.hotel_id,
                booking_request.dates
            )
            executed_steps.append(("reserve_room", reservation.id))
            logger.info(f"Reserved room {reservation.id}")

            # Step 2: Charge payment
            payment = self.payment_service.charge(
                booking_request.card,
                booking_request.amount
            )
            executed_steps.append(("charge_payment", payment.id))
            logger.info(f"Charged payment {payment.id}")

            # Step 3: Send confirmation email
            email = self.email_service.send_confirmation(
                booking_request.email,
                reservation,
                payment
            )
            executed_steps.append(("send_email", email.id))
            logger.info(f"Sent confirmation {email.id}")

            # All steps succeeded
            return BookingResult(success=True, reservation_id=reservation.id)

        except Exception as e:
            # Failure occurred, compensate executed steps in reverse order
            logger.error(f"Booking failed at step {len(executed_steps)}, "
                        f"compensating: {e}")

            self._compensate(executed_steps)

            raise BookingFailedError(f"Booking failed: {e}") from e

    def _compensate(self, executed_steps):
        """Execute compensating transactions in reverse order."""
        for step_name, step_id in reversed(executed_steps):
            try:
                if step_name == "reserve_room":
                    self.hotel_service.cancel_reservation(step_id)
                    logger.info(f"Compensated: cancelled reservation {step_id}")

                elif step_name == "charge_payment":
                    self.payment_service.refund(step_id)
                    logger.info(f"Compensated: refunded payment {step_id}")

                elif step_name == "send_email":
                    self.email_service.send_cancellation(step_id)
                    logger.info(f"Compensated: sent cancellation {step_id}")

            except Exception as comp_error:
                # Compensation failed - critical error requiring manual intervention
                logger.critical(f"COMPENSATION FAILED for {step_name} {step_id}: "
                              f"{comp_error}")
                # Alert operations team
                self.alert_service.critical_alert(
                    f"Manual intervention required: compensation failed",
                    details={"step": step_name, "id": step_id, "error": comp_error}
                )

Key principles:

• Forward steps: Execute business logic steps sequentially
• Track execution: Record each completed step for compensation
• Reverse compensation: Undo steps in reverse order on failure
• Semantic compensation: Cancel reservation, refund payment (not database rollback)
• Idempotent compensations: Safe to retry compensation
• Handle compensation failures: Critical alerts for manual intervention

Saga orchestration patterns:

• Orchestration: Central coordinator (shown above)
• Choreography: Services coordinate via events (more decoupled)

---

Pattern 5: Graceful Degradation

When to use:

• Non-critical service failures
• Maintaining core functionality during partial outages
• User experience over complete accuracy
• Fallback to cached/simplified responses

Implementation approach:

def get_user_recommendations(user_id):
    """
    Get personalized recommendations with graceful degradation.
    Fallback chain: ML model → collaborative filtering → popular items
    """
    try:
        # Primary: ML-based personalized recommendations
        recommendations = ml_service.get_recommendations(user_id, timeout=2.0)
        logger.info(f"Returned ML recommendations for user {user_id}")
        return recommendations

    except (TimeoutError, ServiceUnavailableError) as e:
        logger.warning(f"ML service unavailable, falling back to collaborative: {e}")

        try:
            # Fallback 1: Collaborative filtering (faster, simpler)
            recommendations = collab_filter_service.get_similar_users_items(
                user_id,
                timeout=1.0
            )
            logger.info(f"Returned collaborative filtering for user {user_id}")
            return recommendations

        except (TimeoutError, ServiceUnavailableError) as e2:
            logger.warning(f"Collaborative filtering failed, falling back to popular: {e2}")

            try:
                # Fallback 2: Popular items (cached, always available)
                recommendations = cache.get_popular_items(category=user.category)
                logger.info(f"Returned popular items for user {user_id}")
                return recommendations

            except Exception as e3:
                # Final fallback: Empty recommendations (degraded but functional)
                logger.error(f"All recommendation sources failed, returning empty: {e3}")
                return []  # Graceful degradation: site still works, just no recommendations

Key principles:

• Fallback chain: Primary → Fallback 1 → Fallback 2 → Minimal functionality
• Decreasing quality: Each fallback is simpler/less accurate but faster/more reliable
• Always functional: Never completely fail (return empty/cached data)
• Timeouts: Use aggressive timeouts (1-2s) to fail fast and try fallback
• Monitoring: Track fallback rate (high rate = investigate primary service)

Fallback strategies:

• Cached data: Stale but available
• Simplified logic: Faster, less accurate algorithm
• Default values: Reasonable defaults (popular items, average values)
• Degraded UI: Hide non-critical features

---

Data Consistency Strategies

Strategy 1: Immediate Consistency (Single Database)

Pattern: Use database transactions for ACID guarantees

When to use:

• Single database operations
• Strong consistency required
• Operations must be atomic

Implementation: Transaction rollback pattern (see Pattern 3 above)

---

Strategy 2: Eventual Consistency (Distributed Systems)

Pattern: Accept temporary inconsistency, guarantee eventual convergence

When to use:

• Distributed systems (multiple databases/services)
• High availability more important than immediate consistency
• Can tolerate temporary stale data

Implementation:

# Step 1: Write to primary database (source of truth)
db.create_order(order_id, user_id, items, total)

# Step 2: Publish event for eventual consistency
event_bus.publish(OrderCreatedEvent(
    order_id=order_id,
    user_id=user_id,
    items=items,
    total=total,
    timestamp=now()
))

# Other services consume event asynchronously:
# - Inventory service decrements stock
# - Analytics service records purchase
# - Email service sends confirmation
# Each service eventually becomes consistent

Key principles:

• Source of truth: One service owns the data
• Event-driven: Publish events for others to react
• Idempotent consumers: Handle duplicate events safely
• Monitoring: Track lag between write and propagation
• Conflict resolution: Define merge strategy for concurrent updates

---

Strategy 3: Causal Consistency

Pattern: Preserve cause-effect relationships while allowing other operations to be reordered

When to use:

• Social media feeds (comment after post)
• Collaborative editing (see your own changes)
• Chat applications (messages in order per conversation)

Implementation: Use vector clocks or logical timestamps to track causality

---

Failure Prevention Patterns

Bulkhead Pattern

Purpose: Isolate failures to prevent cascade

Implementation:

• Connection pools: Separate pools per service (failure in service A doesn't exhaust pool for service B)
• Thread pools: Separate threads per operation type
• Resource limits: CPU/memory limits per service (cgroups, Docker limits)

---

Timeout Pattern

Purpose: Prevent indefinite waiting

Implementation:

# Set aggressive timeouts
response = requests.get(
    external_api_url,
    timeout=(3.0, 10.0)  # (connection timeout, read timeout)
)

# Async with timeout
async with asyncio.timeout(5.0):
    result = await long_running_operation()

Guidelines:

• Connection timeout: 1-3s (how long to establish connection)
• Read timeout: 5-10s (how long to wait for response)
• User-facing: 2-5s max (prevent frustration)
• Background jobs: 30-60s (allow more time)

---

Rate Limiting

Purpose: Prevent overload from excessive requests

Implementation:

• Client-side: Throttle requests to prevent hitting limits
• Server-side: Reject requests above threshold
• Adaptive: Reduce rate during degraded performance

---

Monitoring & Alerting for Recovery

Key Metrics

1. Error rate: Percentage of failed requests (alert if >1%)
2. Retry rate: How often retries happen (high rate = transient failures)
3. Circuit breaker state: Alert when circuits open
4. Recovery time: How long to recover from failure (MTTR)
5. Compensation rate: How often sagas compensate (should be low)

Alerting Strategy

Symptom-based alerts (what users experience):

• High error rate (>1% for 5 minutes)
• High latency (p99 > 1s)
• Circuit breaker open

Cause-based logging (for debugging):

• Which specific operation failed
• Transient vs permanent failure
• Retry attempts and outcomes
• Compensation steps executed

---

Testing Recovery Patterns

Chaos Engineering

Inject failures in test/staging:

• Kill random services
• Introduce network latency
• Cause database timeouts
• Fill disk space
• Exhaust connection pools

Verify:

• System recovers automatically
• No data corruption
• Appropriate fallbacks triggered
• Monitoring alerts fire correctly

Recovery Testing Checklist

• Retry logic works (verified with network failures)
• Circuit breaker opens/closes correctly
• Transaction rollback prevents partial writes
• Saga compensation executes in correct order
• Graceful degradation provides minimal functionality
• Timeouts prevent indefinite hangs
• Monitoring captures recovery events
• Alerts fire for critical failures

---

Decision Framework

When task fails, ask:

1. Is failure transient or permanent?

   • Transient → Retry with exponential backoff
   • Permanent → Don't retry, log error

2. Is system distributed?

   • Single database → Transaction rollback
   • Multiple services → Saga pattern

3. Is service external/unreliable?

   • Yes → Circuit breaker + fallback
   • No → Standard retry

4. Can I degrade gracefully?

   • Yes → Fallback chain (ML → simple → cached → empty)
   • No → Fast fail with error message

5. What consistency guarantees needed?

   • Strong (financial) → ACID transactions
   • Eventual (social) → Event-driven updates
   • Causal (chat) → Vector clocks

---

Common Anti-Patterns

❌ Retry Forever

Problem: Infinite retry loop exhausts resources Solution: Max retries (3-5), exponential backoff, circuit breaker

❌ No Jitter

Problem: Thundering herd (all clients retry simultaneously) Solution: Add jitter (±25% randomness) to backoff

❌ Retry Non-Idempotent Operations

Problem: Duplicate charges, duplicate emails Solution: Use idempotency keys, check before retrying

❌ Ignore Compensation Failures

Problem: Half-completed saga leaves inconsistent state Solution: Alert operations team, manual intervention required

❌ No Timeouts

Problem: Indefinite hangs consume resources Solution: Set timeouts on all external calls (3-10s)

❌ Synchronous Saga

Problem: Long-running saga blocks user Solution: Asynchronous saga (immediate response, poll for completion)

---

Real-World Examples

Example 1: Payment Processing Failure

Scenario: Credit card charge fails during checkout

Recovery strategy:

1. Classify: Transient (network) or permanent (declined card)?
2. If transient: Retry with exponential backoff (3 attempts)
3. If permanent: Show user error, don't retry
4. If retry exhausted: Circuit breaker opens, use backup payment processor

Example 2: Microservice Dependency Down

Scenario: Recommendation service unavailable

Recovery strategy:

1. Circuit breaker opens after 5 failures
2. Fast-fail for next 60 seconds (don't call service)
3. Graceful degradation: Show cached popular items
4. After 60s: Half-open state, test if service recovered
5. If recovered: Close circuit, resume normal operation

Example 3: Database Transaction Failure

Scenario: Transfer between accounts fails mid-transaction

Recovery strategy:

1. Transaction rollback: Undo debit from source account
2. Log failure for investigation
3. Return error to user: "Transfer failed, please try again"
4. User retries: Idempotent (check if already processed)

---

Summary Checklist

When implementing error recovery:

• Classify failure: Transient vs permanent, recoverable vs fatal
• Select pattern: Retry, circuit breaker, rollback, saga, or graceful degradation
• Implement idempotency: Use keys to prevent duplicate operations
• Add timeouts: Prevent indefinite hangs (3-10s)
• Monitor recovery: Track error rates, retry rates, circuit breaker states
• Test failures: Chaos engineering, inject faults, verify recovery
• Alert on critical: Circuit open, compensation failures, high error rates
• Document runbooks: How to manually intervene when automation fails

Remember: The goal is to recover automatically when possible, fail gracefully when not, and always maintain data consistency.