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318"""Circuit breaker pattern for fault tolerance and cascading failure prevention.
Implements the circuit breaker pattern to prevent repeated calls to failing services.
When a service repeatedly fails, the circuit breaker "opens" to prevent further
attempts, giving the service time to recover. After a timeout, it transitions to
"half-open" to test if the service has recovered.
States:
- CLOSED: Normal operation, requests pass through
- OPEN: Service is failing, requests blocked immediately
- HALF_OPEN: Testing recovery, limited requests allowed
Typical usage:
>>> breaker = CircuitBreaker(name="s3-upload", failure_threshold=5)
>>>
>>> # Before operation
>>> if breaker.is_open:
>>> raise CircuitBreakerOpenError("S3 circuit breaker open")
>>>
>>> try:
>>> result = await some_s3_operation()
>>> breaker.record_success()
>>> except Exception:
>>> breaker.record_failure()
>>> raise
"""
import logging
import time
from dataclasses import dataclass
from enum import Enum
from threading import Lock
from typing import Any
logger = logging.getLogger(__name__)
class CircuitState(str, Enum):
"""Circuit breaker states."""
CLOSED = "closed" # Normal operation
OPEN = "open" # Blocking requests (service failing)
HALF_OPEN = "half_open" # Testing recovery
class CircuitBreakerOpenError(Exception):
"""Exception raised when circuit breaker is open (blocking requests)."""
pass
@dataclass
class CircuitBreakerConfig:
"""Configuration for circuit breaker behavior.
Attributes:
failure_threshold: Number of consecutive failures to open circuit (default: 5)
success_threshold: Number of consecutive successes in half-open to close circuit (default: 2)
timeout: Seconds to wait before transitioning from open to half-open (default: 60)
half_open_max_calls: Max concurrent calls allowed in half-open state (default: 1)
"""
failure_threshold: int = 5
success_threshold: int = 2
timeout: float = 60.0
half_open_max_calls: int = 1
class CircuitBreaker:
"""Circuit breaker for preventing cascading failures.
Thread-safe implementation that tracks failure/success counts and manages
state transitions to protect downstream services from repeated failures.
Args:
name: Human-readable name for logging and metrics
config: Configuration object (uses defaults if not provided)
failure_threshold: Override failure threshold
success_threshold: Override success threshold
timeout: Override timeout in seconds
Example:
>>> breaker = CircuitBreaker("s3-upload")
>>>
>>> # Check state before operation
>>> breaker.check_state() # Raises CircuitBreakerOpenError if open
>>>
>>> try:
>>> result = await s3_client.put_object(...)
>>> breaker.record_success()
>>> except Exception:
>>> breaker.record_failure()
>>> raise
"""
def __init__(
self,
name: str,
config: CircuitBreakerConfig | None = None,
failure_threshold: int | None = None,
success_threshold: int | None = None,
timeout: float | None = None
):
"""Initialize circuit breaker.
Args:
name: Circuit breaker name for logging
config: Full configuration object
failure_threshold: Override config failure threshold
success_threshold: Override config success threshold
timeout: Override config timeout
"""
self.name = name
# Use provided config or create default
self.config = config or CircuitBreakerConfig()
# Allow individual overrides
if failure_threshold is not None:
self.config.failure_threshold = failure_threshold
if success_threshold is not None:
self.config.success_threshold = success_threshold
if timeout is not None:
self.config.timeout = timeout
# State tracking
self._state = CircuitState.CLOSED
self._failure_count = 0
self._success_count = 0
self._last_failure_time: float | None = None
self._half_open_calls = 0
# Thread safety
self._lock = Lock()
logger.info(
f"Circuit breaker '{name}' initialized: "
f"failure_threshold={self.config.failure_threshold}, "
f"success_threshold={self.config.success_threshold}, "
f"timeout={self.config.timeout}s"
)
@property
def state(self) -> CircuitState:
"""Get current circuit state (thread-safe)."""
with self._lock:
self._update_state()
return self._state
@property
def is_open(self) -> bool:
"""Check if circuit is open (blocking requests)."""
return self.state == CircuitState.OPEN
@property
def is_closed(self) -> bool:
"""Check if circuit is closed (normal operation)."""
return self.state == CircuitState.CLOSED
@property
def is_half_open(self) -> bool:
"""Check if circuit is half-open (testing recovery)."""
return self.state == CircuitState.HALF_OPEN
def check_state(self) -> None:
"""Check circuit state and raise exception if open.
Raises:
CircuitBreakerOpenError: If circuit is open or half-open limit reached
"""
with self._lock:
self._update_state()
if self._state == CircuitState.OPEN:
raise CircuitBreakerOpenError(
f"Circuit breaker '{self.name}' is open. "
f"Service has failed {self._failure_count} times. "
f"Retry after {self.config.timeout}s."
)
if self._state == CircuitState.HALF_OPEN:
if self._half_open_calls >= self.config.half_open_max_calls:
raise CircuitBreakerOpenError(
f"Circuit breaker '{self.name}' is half-open with max concurrent calls reached."
)
self._half_open_calls += 1
def record_success(self) -> None:
"""Record successful operation.
- In CLOSED: Resets failure count
- In HALF_OPEN: Increments success count, may transition to CLOSED
"""
with self._lock:
if self._state == CircuitState.HALF_OPEN:
self._success_count += 1
self._half_open_calls = max(0, self._half_open_calls - 1)
logger.debug(
f"Circuit breaker '{self.name}': success in half-open state "
f"({self._success_count}/{self.config.success_threshold})"
)
# Transition to CLOSED if enough successes
if self._success_count >= self.config.success_threshold:
self._transition_to_closed()
elif self._state == CircuitState.CLOSED:
# Reset failure count on success
if self._failure_count > 0:
logger.debug(
f"Circuit breaker '{self.name}': success, resetting failure count "
f"(was {self._failure_count})"
)
self._failure_count = 0
def record_failure(self) -> None:
"""Record failed operation.
- In CLOSED: Increments failure count, may transition to OPEN
- In HALF_OPEN: Immediately transitions back to OPEN
"""
with self._lock:
self._failure_count += 1
self._last_failure_time = time.time()
if self._state == CircuitState.HALF_OPEN:
logger.warning(
f"Circuit breaker '{self.name}': failure in half-open state, reopening circuit"
)
self._half_open_calls = max(0, self._half_open_calls - 1)
self._transition_to_open()
elif self._state == CircuitState.CLOSED:
logger.debug(
f"Circuit breaker '{self.name}': failure recorded "
f"({self._failure_count}/{self.config.failure_threshold})"
)
# Transition to OPEN if threshold reached
if self._failure_count >= self.config.failure_threshold:
self._transition_to_open()
def reset(self) -> None:
"""Manually reset circuit breaker to CLOSED state.
Useful for testing or manual intervention.
"""
with self._lock:
logger.info(f"Circuit breaker '{self.name}': manual reset")
self._transition_to_closed()
def _update_state(self) -> None:
"""Update state based on timeout (OPEN → HALF_OPEN transition).
Must be called while holding self._lock.
"""
if self._state == CircuitState.OPEN and self._last_failure_time is not None:
elapsed = time.time() - self._last_failure_time
if elapsed >= self.config.timeout:
logger.info(
f"Circuit breaker '{self.name}': timeout elapsed ({elapsed:.1f}s), "
f"transitioning to half-open"
)
self._transition_to_half_open()
def _transition_to_open(self) -> None:
"""Transition to OPEN state (must hold lock)."""
self._state = CircuitState.OPEN
self._success_count = 0
self._half_open_calls = 0
logger.warning(
f"Circuit breaker '{self.name}': OPEN - service has failed "
f"{self._failure_count} times (threshold: {self.config.failure_threshold})"
)
def _transition_to_half_open(self) -> None:
"""Transition to HALF_OPEN state (must hold lock)."""
self._state = CircuitState.HALF_OPEN
self._success_count = 0
self._failure_count = 0
self._half_open_calls = 0
logger.info(f"Circuit breaker '{self.name}': HALF_OPEN - testing recovery")
def _transition_to_closed(self) -> None:
"""Transition to CLOSED state (must hold lock)."""
self._state = CircuitState.CLOSED
self._failure_count = 0
self._success_count = 0
self._last_failure_time = None
self._half_open_calls = 0
logger.info(f"Circuit breaker '{self.name}': CLOSED - service recovered")
def get_stats(self) -> dict[str, Any]:
"""Get current circuit breaker statistics.
Returns:
Dictionary with state, counts, and timing info
"""
with self._lock:
self._update_state()
return {
'name': self.name,
'state': self._state.value,
'failure_count': self._failure_count,
'success_count': self._success_count,
'last_failure_time': self._last_failure_time,
'half_open_calls': self._half_open_calls,
'config': {
'failure_threshold': self.config.failure_threshold,
'success_threshold': self.config.success_threshold,
'timeout': self.config.timeout,
'half_open_max_calls': self.config.half_open_max_calls
}
}