Circuit_breaker

Chapter 21: Circuit Breaker Pattern

Protecting Systems from Cascading Failures

---

21.1 What is Circuit Breaker?

The Circuit Breaker pattern prevents cascading failures by detecting when a service is failing and stopping requests to that service temporarily.

    Circuit Breaker Analogy
    =======================

    Electrical Circuit Breaker:
    +----------------------------------+
    |                                   |
    |  Power ---> [Breaker] ---> Appliance |
    |                                   |
    |  Too much current -> Breaker trips |
    |  Appliance protected!            |
    +----------------------------------+

    Software Circuit Breaker:
    +----------------------------------+
    |                                   |
    | Request ---> [Breaker] ---> Service |
    |                                   |
    | Too many failures -> Circuit opens |
    | Fail fast, protect system!        |
    +----------------------------------+

---

21.2 How Circuit Breaker Works

Three States

    Circuit Breaker States
    =====================

    +----------+       +----------+       +----------+
    |  CLOSED  | ---->|  OPEN    | ---->| HALF-OPEN |
    +----------+       +----------+       +----------+
         ^                |                |
         |                v                |
         |            +----------+          |
         +----------- |  RESET   | <--------+
                      +----------+

    CLOSED:
    - Normal operation
    - Requests pass through
    - Failures counted

    OPEN:
    - Service considered failing
    - Requests fail immediately
    - No calls to failing service

    HALF-OPEN:
    - Testing if service recovered
    - Limited requests allowed
    - If success -> Close circuit
    - If fail -> Open again

---

21.3 Implementation

Configuration

    Circuit Breaker Parameters
    ==========================

    +--------------------+--------------------------------+
    | Parameter          | Description                    |
    +--------------------+--------------------------------+
    | Failure Threshold  | Failures before opening       |
    |                    | e.g., 5 failures in 10 sec   |
    +--------------------+--------------------------------+
    | Success Threshold  | Successes to close circuit  |
    |                    | e.g., 3 successes           |
    +--------------------+--------------------------------+
    | Timeout            | Time before half-open        |
    |                    | e.g., 30 seconds             |
    +--------------------+--------------------------------+
    | Half-open requests | Requests allowed in half-open|
    |                    | e.g., 3 requests             |
    +--------------------+--------------------------------+

Code Example

class CircuitBreaker:
    def __init__(self, failure_threshold=5, timeout=30):
        self.failure_threshold = failure_threshold
        self.timeout = timeout
        self.state = CLOSED
        self.failures = 0
        self.last_failure_time = None

    def call(self, func):
        if self.state == OPEN:
            if time.time() - self.last_failure_time > self.timeout:
                self.state = HALF_OPEN
            else:
                raise CircuitOpenException()

        try:
            result = func()
            self.on_success()
            return result
        except Exception as e:
            self.on_failure()
            raise e

    def on_success(self):
        self.failures = 0
        self.state = CLOSED

    def on_failure(self):
        self.failures += 1
        self.last_failure_time = time.time()
        if self.failures >= self.failure_threshold:
            self.state = OPEN

---

21.4 Benefits

Why Use Circuit Breaker?

Benefit	Description
Fail Fast	Don’t wait for timeouts
Protect System	Prevent cascading failures
Recovery	Allow services to recover
Resilience	Handle partial outages
Visibility	Monitor service health

---

21.5 Integration with Libraries

Popular Libraries

Language	Library
Java	Resilience4j, Hystrix
.NET	Polly
Node.js	opossum
Python	pybreaker
Go	circuitbreaker

---

Summary

Key circuit breaker concepts:

1. Three states - Closed, Open, Half-Open
2. Fail fast - Don’t wait for timeouts
3. Auto-recovery - Test if service is healthy
4. Prevents cascading - Isolate failures
5. Monitor - Track circuit state
6. Configure thresholds - Tune for your system

---

Next: Chapter 22: Rate Limiting & Throttling