ffsshhttiikk Circuit Breaker Pattern
Overview
The Circuit Breaker Pattern is a resilience design pattern that prevents cascading failures in distributed systems by detecting failures and encapsulating the logic of preventing a failure from constantly recurring. Like electrical circuit breakers that prevent fires by interrupting current flow, this pattern prevents applications from repeatedly trying to execute an operation that's likely to fail, allowing the system to recover and preventing resource exhaustion.
Description
The circuit breaker pattern operates with three states: closed, open, and half-open. In the closed state, requests flow through normally. When failures exceed a threshold, the circuit "opens," immediately rejecting requests without attempting the operation. After a cooldown period, the circuit enters half-open state, allowing a limited number of test requests through. If these succeed, the circuit closes; if they fail, it reopens.
This pattern is essential for building resilient microservices architectures where services depend on each other. Without circuit breakers, a failing service can cause all its callers to hang while waiting for timeouts, leading to thread pool exhaustion and cascading failures throughout the system.
Prerequisites
- Understanding of distributed systems and failure modes
- Knowledge of fallback strategies and graceful degradation
- Familiarity with state machine concepts
- Experience with timeout and retry patterns
- Understanding of resource management in concurrent systems
Core Competencies
- State machine implementation for circuit states
- Failure threshold configuration and tuning
- Cooldown period management
- Half-open state handling and probe requests
- Fallback function implementation
- Thread-safe state transitions
- Metrics collection and monitoring
- Integration with resilience frameworks
Implementation
Python Implementation
import time
import threading
from typing import Any, Callable, Optional, Dict
from dataclasses import dataclass, field
from enum import Enum
from abc import ABC, abstractmethod
import logging
from statistics import mean
from collections import deque
logger = logging.getLogger(__name__)
class CircuitState(Enum):
CLOSED = "closed"
OPEN = "open"
HALF_OPEN = "half_open"
@dataclass
class CircuitConfig:
failure_threshold: int = 5
success_threshold: int = 3
timeout_seconds: float = 60.0
half_open_max_calls: int = 3
sliding_window_size: int = 100
minimum_calls: int = 10
error_rate_threshold: float = 0.5
latency_threshold_ms: float = 1000.0
latency_percentile: float = 0.95
@dataclass
class CircuitMetrics:
state: CircuitState = CircuitState.CLOSED
total_calls: int = 0
successful_calls: int = 0
failed_calls: int = 0
rejected_calls: int = 0
current_consecutive_failures: int = 0
current_consecutive_successes: int = 0
last_failure_time: Optional[float] = None
last_success_time: Optional[float] = None
state_transitions: int = 0
failure_rate: float = 0.0
latency_samples: deque = field(default_factory=lambda: deque(maxlen=100))
def record_call(self, success: bool, latency: float = 0.0):
self.total_calls += 1
self.latency_samples.append((success, latency))
if success:
self.successful_calls += 1
self.current_consecutive_failures = 0
self.current_consecutive_successes += 1
self.last_success_time = time.time()
else:
self.failed_calls += 1
self.current_consecutive_failures += 1
self.current_consecutive_successes = 0
self.last_failure_time = time.time()
if len(self.latency_samples) >= self.minimum_calls:
failures = sum(1 for s, _ in self.latency_samples if not s)
self.failure_rate = failures / len(self.latency_samples)
class CircuitBreaker:
def __init__(self, name: str, config: Optional[CircuitConfig] = None):
self.name = name
self.config = config or CircuitConfig()
self.state = CircuitState.CLOSED
self.metrics = CircuitMetrics()
self.lock = threading.RLock()
self._opened_at: Optional[float] = None
self._half_open_successes: int = 0
self._last_state_change: float = time.time()
def call(
self,
func: Callable[..., Any],
*args,
fallback: Optional[Callable[..., Any]] = None,
**kwargs
) -> Any:
if not self._can_execute():
self.metrics.rejected_calls += 1
if fallback:
return fallback()
raise CircuitOpenError(
f"Circuit '{self.name}' is open. Circuit opened at "
f"{self._opened_at}"
)
start_time = time.time()
try:
result = func(*args, **kwargs)
latency = (time.time() - start_time) * 1000
self._on_success(latency)
return result
except Exception as e:
latency = (time.time() - start_time) * 1000
self._on_failure(latency, e)
if fallback:
return fallback()
raise
def _can_execute(self) -> bool:
with self.lock:
if self.state == CircuitState.CLOSED:
return True
elif self.state == CircuitState.OPEN:
if self._should_attempt_reset():
self._transition_to(CircuitState.HALF_OPEN)
return True
return False
elif self.state == CircuitState.HALF_OPEN:
return self._half_open_successes < self.config.half_open_max_calls
return False
def _should_attempt_reset(self) -> bool:
if self._opened_at is None:
return True
return time.time() - self._opened_at >= self.config.timeout_seconds
def _on_success(self, latency: float):
with self.lock:
self.metrics.record_call(True, latency)
if self.state == CircuitState.HALF_OPEN:
self._half_open_successes += 1
if self._half_open_successes >= self.config.success_threshold:
self._transition_to(CircuitState.CLOSED)
def _on_failure(self, latency: float, error: Exception):
with self.lock:
self.metrics.record_call(False, latency)
logger.warning(f"Circuit '{self.name}' recorded failure: {error}")
if self.state == CircuitState.HALF_OPEN:
self._transition_to(CircuitState.OPEN)
elif self.state == CircuitState.CLOSED:
if self._should_open():
self._transition_to(CircuitState.OPEN)
def _should_open(self) -> bool:
if self.metrics.current_consecutive_failures >= self.config.failure_threshold:
return True
if (
self.metrics.total_calls >= self.config.minimum_calls and
self.metrics.failure_rate >= self.config.error_rate_threshold
):
return True
latency_samples = [lat for _, lat in self.metrics.latency_samples]
if len(latency_samples) >= self.config.minimum_calls:
sorted_latencies = sorted(latency_samples)
percentile_idx = int(len(sorted_latencies) * self.config.latency_percentile)
p99_latency = sorted_latencies[min(percentile_idx, len(sorted_latencies) - 1)]
if p99_latency > self.config.latency_threshold_ms:
return True
return False
def _transition_to(self, new_state: CircuitState):
old_state = self.state
self.state = new_state
self.metrics.state_transitions += 1
self._last_state_change = time.time()
if new_state == CircuitState.OPEN:
self._opened_at = time.time()
self._half_open_successes = 0
elif new_state == CircuitState.CLOSED:
self._opened_at = None
self._half_open_successes = 0
self.metrics.current_consecutive_failures = 0
logger.info(
f"Circuit '{self.name}' transitioned from {old_state.value} "
f"to {new_state.value}"
)
def reset(self):
with self.lock:
self.state = CircuitState.CLOSED
self._opened_at = None
self._half_open_successes = 0
self.metrics = CircuitMetrics()
self.metrics.state = CircuitState.CLOSED
def get_state(self) -> Dict:
with self.lock:
return {
"name": self.name,
"state": self.state.value,
"opened_at": self._opened_at,
"seconds_since_open": (
time.time() - self._opened_at
if self._opened_at else None
),
"metrics": {
"total_calls": self.metrics.total_calls,
"successful_calls": self.metrics.successful_calls,
"failed_calls": self.metrics.failed_calls,
"rejected_calls": self.metrics.rejected_calls,
"failure_rate": self.metrics.failure_rate,
"state_transitions": self.metrics.state_transitions,
"consecutive_failures": self.metrics.current_consecutive_failures,
"consecutive_successes": self.metrics.current_consecutive_successes,
},
}
class CircuitOpenError(Exception):
pass
class CircuitRegistry:
def __init__(self):
self._circuits: Dict[str, CircuitBreaker] = {}
self._lock = threading.Lock()
def register(self, circuit: CircuitBreaker):
with self._lock:
self._circuits[circuit.name] = circuit
def get(self, name: str) -> Optional[CircuitBreaker]:
with self._lock:
return self._circuits.get(name)
def get_all(self) -> Dict[str, CircuitBreaker]:
with self._lock:
return dict(self._circuits)
def get_states(self) -> Dict[str, Dict]:
with self._lock:
return {name: cb.get_state() for name, cb in self._circuits.items()}
class BulkheadCircuitBreaker:
def __init__(
self,
name: str,
circuit_config: Optional[CircuitConfig] = None,
max_concurrent: int = 10,
timeout_seconds: float = 30.0
):
self.name = name
self.circuit = CircuitBreaker(name, circuit_config)
self.semaphore = threading.Semaphore(max_concurrent)
self.timeout_seconds = timeout_seconds
self.lock = threading.Lock()
def call(
self,
func: Callable[..., Any],
*args,
fallback: Optional[Callable[..., Any]] = None,
**kwargs
) -> Any:
acquired = self.semaphore.acquire(timeout=self.timeout_seconds)
if not acquired:
self.circuit.metrics.rejected_calls += 1
if fallback:
return fallback()
raise TimeoutError(
f"Concurrency limit reached for '{self.name}'"
)
try:
return self.circuit.call(func, *args, fallback=fallback, **kwargs)
finally:
self.semaphore.release()
registry = CircuitRegistry()
def get_circuit(name: str) -> CircuitBreaker:
circuit = registry.get(name)
if not circuit:
circuit = CircuitBreaker(name)
registry.register(circuit)
return circuit
def circuit_breaker(
name: str,
failure_threshold: int = 5,
timeout_seconds: float = 60.0
):
def decorator(func: Callable) -> Callable:
def wrapper(*args, **kwargs):
cb = get_circuit(name)
return cb.call(func, *args, **kwargs)
return wrapper
return decorator
Go Implementation
package circuit
import (
"context"
"errors"
"fmt"
"math"
"sort"
"sync"
"time"
)
var ErrCircuitOpen = errors.New("circuit breaker is open")
type State int
const (
StateClosed State = iota
StateOpen
StateHalfOpen
)
func (s State) String() string {
switch s {
case StateClosed:
return "closed"
case StateOpen:
return "open"
case StateHalfOpen:
return "half_open"
default:
return "unknown"
}
}
type Config struct {
FailureThreshold int
SuccessThreshold int
Timeout time.Duration
HalfOpenMaxCalls int
SlidingWindowSize int
MinimumCalls int
ErrorRateThreshold float64
LatencyThresholdMs float64
LatencyPercentile float64
}
type Metrics struct {
TotalCalls int64
SuccessfulCalls int64
FailedCalls int64
RejectedCalls int64
CurrentConsecutiveFails int
CurrentConsecutiveSuccesses int
LastFailureTime time.Time
LastSuccessTime time.Time
StateTransitions int64
FailureRate float64
LatencySamples []float64
}
type CircuitBreaker struct {
name string
config Config
state State
metrics Metrics
openedAt time.Time
lastStateChange time.Time
halfOpenSuccesses int
mu sync.RWMutex
}
func New(name string, config Config) *CircuitBreaker {
return &CircuitBreaker{
name: name,
config: config,
state: StateClosed,
metrics: Metrics{},
lastStateChange: time.Now(),
}
}
func (c *CircuitBreaker) Execute(
ctx context.Context,
fn func(context.Context) error,
fallback func(context.Context) error,
) error {
if !c.canExecute() {
c.mu.Lock()
c.metrics.RejectedCalls++
c.mu.Unlock()
if fallback != nil {
return fallback(ctx)
}
return ErrCircuitOpen
}
start := time.Now()
err := fn(ctx)
latency := time.Since(start).Milliseconds()
c.recordResult(err == nil, float64(latency))
if err != nil {
if fallback != nil {
return fallback(ctx)
}
return err
}
return nil
}
func (c *CircuitBreaker) ExecuteWithResult(
ctx context.Context,
fn func(context.Context) (interface{}, error),
fallback func(context.Context) (interface{}, error),
) (interface{}, error) {
if !c.canExecute() {
c.mu.Lock()
c.metrics.RejectedCalls++
c.mu.Unlock()
if fallback != nil {
return fallback(ctx)
}
return nil, ErrCircuitOpen
}
start := time.Now()
result, err := fn(ctx)
latency := time.Since(start).Milliseconds()
c.recordResult(err == nil, float64(latency))
if err != nil {
if fallback != nil {
return fallback(ctx)
}
return nil, err
}
return result, nil
}
func (c *CircuitBreaker) canExecute() bool {
c.mu.RLock()
defer c.mu.RUnlock()
switch c.state {
case StateClosed:
return true
case StateOpen:
if time.Since(c.openedAt) >= c.config.Timeout {
c.mu.RUnlock()
c.mu.Lock()
c.transitionTo(StateHalfOpen)
c.mu.Unlock()
c.mu.RLock()
return true
}
return false
case StateHalfOpen:
return c.halfOpenSuccesses < c.config.HalfOpenMaxCalls
default:
return false
}
}
func (c *CircuitBreaker) recordResult(success bool, latency float64) {
c.mu.Lock()
defer c.mu.Unlock()
c.metrics.TotalCalls++
c.metrics.LatencySamples = append(c.metrics.LatencySamples, latency)
if len(c.metrics.LatencySamples) > c.config.SlidingWindowSize {
c.metrics.LatencySamples = c.metrics.LatencySamples[len(c.metrics.LatencySamples)-c.config.SlidingWindowSize:]
}
if success {
c.metrics.SuccessfulCalls++
c.metrics.CurrentConsecutiveFails = 0
c.metrics.CurrentConsecutiveSuccesses++
if c.state == StateHalfOpen {
c.halfOpenSuccesses++
if c.halfOpenSuccesses >= c.config.SuccessThreshold {
c.transitionTo(StateClosed)
}
}
} else {
c.metrics.FailedCalls++
c.metrics.CurrentConsecutiveFails++
c.metrics.CurrentConsecutiveSuccesses = 0
c.metrics.LastFailureTime = time.Now()
if c.state == StateHalfOpen {
c.transitionTo(StateOpen)
} else if c.state == StateClosed {
if c.shouldOpen() {
c.transitionTo(StateOpen)
}
}
}
c.updateFailureRate()
}
func (c *CircuitBreaker) shouldOpen() bool {
if c.metrics.CurrentConsecutiveFails >= c.config.FailureThreshold {
return true
}
if c.metrics.TotalCalls >= int64(c.config.MinimumCalls) {
if c.metrics.FailureRate >= c.config.ErrorRateThreshold {
return true
}
}
return c.checkLatencyThreshold()
}
func (c *CircuitBreaker) checkLatencyThreshold() bool {
if len(c.metrics.LatencySamples) < c.config.MinimumCalls {
return false
}
sorted := make([]float64, len(c.metrics.LatencySamples))
copy(sorted, c.metrics.LatencySamples)
sort.Float64s(sorted)
idx := int(float64(len(sorted)) * c.config.LatencyPercentile)
if idx >= len(sorted) {
idx = len(sorted) - 1
}
percentileLatency := sorted[idx]
return percentileLatency > c.config.LatencyThresholdMs
}
func (c *CircuitBreaker) updateFailureRate() {
if c.metrics.TotalCalls == 0 {
c.metrics.FailureRate = 0
return
}
c.metrics.FailureRate = float64(c.metrics.FailedCalls) / float64(c.metrics.TotalCalls)
}
func (c *CircuitBreaker) transitionTo(newState State) {
oldState := c.state
c.state = newState
c.metrics.StateTransitions++
c.lastStateChange = time.Now()
switch newState {
case StateOpen:
c.openedAt = time.Now()
c.halfOpenSuccesses = 0
case StateClosed:
c.openedAt = time.Time{}
c.halfOpenSuccesses = 0
c.metrics.CurrentConsecutiveFails = 0
}
}
func (c *CircuitBreaker) Reset() {
c.mu.Lock()
defer c.mu.Unlock()
c.state = StateClosed
c.openedAt = time.Time{}
c.halfOpenSuccesses = 0
c.metrics = Metrics{}
}
func (c *CircuitBreaker) State() State {
c.mu.RLock()
defer c.mu.RUnlock()
return c.state
}
type CircuitStatus struct {
Name string `json:"name"`
State string `json:"state"`
OpenedAt *time.Time `json:"opened_at,omitempty"`
SecondsSinceOpen float64 `json:"seconds_since_open,omitempty"`
TotalCalls int64 `json:"total_calls"`
SuccessfulCalls int64 `json:"successful_calls"`
FailedCalls int64 `json:"failed_calls"`
RejectedCalls int64 `json:"rejected_calls"`
FailureRate float64 `json:"failure_rate"`
StateTransitions int64 `json:"state_transitions"`
ConsecutiveFailures int `json:"consecutive_failures"`
}
func (c *CircuitBreaker) Status() CircuitStatus {
c.mu.RLock()
defer c.mu.RUnlock()
status := CircuitStatus{
Name: c.name,
State: c.state.String(),
TotalCalls: c.metrics.TotalCalls,
SuccessfulCalls: c.metrics.SuccessfulCalls,
FailedCalls: c.metrics.FailedCalls,
RejectedCalls: c.metrics.RejectedCalls,
FailureRate: c.metrics.FailureRate,
StateTransitions: c.metrics.StateTransitions,
}
if c.openedAt.IsZero() {
status.OpenedAt = nil
} else {
opened := c.openedAt
status.OpenedAt = &opened
status.SecondsSinceOpen = time.Since(c.openedAt).Seconds()
}
return status
}
type Registry struct {
mu sync.RWMutex
circuits map[string]*CircuitBreaker
}
func NewRegistry() *Registry {
return &Registry{
circuits: make(map[string]*CircuitBreaker),
}
}
func (r *Registry) Register(cb *CircuitBreaker) {
r.mu.Lock()
defer r.mu.Unlock()
r.circuits[cb.name] = cb
}
func (r *Registry) Get(name string) (*CircuitBreaker, bool) {
r.mu.RLock()
defer r.mu.RUnlock()
cb, ok := r.circuits[name]
return cb, ok
}
func (r *Registry) MustGet(name string) *CircuitBreaker {
cb, ok := r.Get(name)
if !ok {
panic(fmt.Sprintf("circuit breaker '%s' not found", name))
}
return cb
}
func (r *Registry) AllStatus() []CircuitStatus {
r.mu.RLock()
defer r.mu.RUnlock()
statuses := make([]CircuitStatus, 0, len(r.circuits))
for _, cb := range r.circuits {
statuses = append(statuses, cb.Status())
}
return statuses
}
func DefaultConfig() Config {
return Config{
FailureThreshold: 5,
SuccessThreshold: 3,
Timeout: 60 * time.Second,
HalfOpenMaxCalls: 3,
SlidingWindowSize: 100,
MinimumCalls: 10,
ErrorRateThreshold: 0.5,
LatencyThresholdMs: 1000,
LatencyPercentile: 0.95,
}
}
Use Cases
External API Calls: Protect calls to third-party APIs that may be unreliable or have rate limits.
Database Connections: Prevent application hang when database becomes unavailable by failing fast.
Microservice Dependencies: Isolate failures in downstream services to prevent cascading failures.
Resource-Intensive Operations: Limit concurrent execution of expensive operations.
Legacy System Integration: Add resilience when integrating with older, less reliable systems.
Artifacts
CircuitBreakerclass: Core implementation with three-state machineCircuitConfigdataclass: Configurable thresholds and timeoutsCircuitMetricsdatacslatency trackingCircuitRegistry: Central management of multiple circuit breakersBulkheadCircuitBreaker: Combines circuit breaker with concurrency limits- Decorator utilities for easy integration
Related Skills
- Bulkhead Pattern: Complements circuit breaker for resource isolation
- Retry Pattern: Works with circuit breaker for transient failures
- Timeout Pattern: Essential companion for circuit breaker
- Fallback Pattern: Provides alternative responses when circuit is open
- Health Checks: Monitoring circuit breaker state
- Metrics Collection: Integration with observability platforms