README

A lightweight PHP Circuit Breaker implementation to prevent cascading failures in distributed systems.

Installation

composer require chiragagg5k/circuit-breaker

What is a Circuit Breaker?

A Circuit Breaker is a design pattern used to detect failures and prevent an application from constantly trying to execute an operation that's likely to fail. This helps prevent cascading failures in distributed systems.

Features

✅ Proper state management (CLOSED, OPEN, HALF_OPEN)
✅ Half-open state with gradual recovery
✅ Configurable failure and success thresholds
✅ Automatic state transitions
✅ State inspection methods
✅ Simple and lightweight implementation
✅ Optional Redis and Swoole Table cache adapters for shared state
✅ Native telemetry metrics through optional utopia-php/telemetry adapters
✅ PSR-4 autoloading compatible
✅ PHP 8.2+ support with enums

Usage

Basic Example

use ChiragAgg5k\CircuitBreaker;

$breaker = new CircuitBreaker(
    threshold: 3,         // Open circuit after 3 failures
    timeout: 30,          // Try half-open after 30 seconds
    successThreshold: 2   // Require 2 successes to close circuit
);

$result = $breaker->call(
    open: fn() => "Service unavailable - circuit is open",
    close: fn() => makeExternalApiCall(),
    halfOpen: fn() => makeExternalApiCall() // Optional: called during recovery testing
);

Using All Three States

use ChiragAgg5k\CircuitBreaker;

$breaker = new CircuitBreaker(threshold: 3, timeout: 30, successThreshold: 2);

$result = $breaker->call(
    open: function() {
        // Circuit is OPEN - service is down
        logger()->warning('Circuit breaker is OPEN - using fallback');
        return getCachedData() ?? ['error' => 'Service unavailable'];
    },
    close: function() {
        // Circuit is CLOSED - normal operation
        return apiClient()->fetchData();
    },
    halfOpen: function() {
        // Circuit is HALF_OPEN - testing recovery
        logger()->info('Circuit breaker testing recovery...');
        return apiClient()->fetchData(['timeout' => 5]); // Use shorter timeout
    }
);

Real-world Example

use ChiragAgg5k\CircuitBreaker;

$breaker = new CircuitBreaker(threshold: 5, timeout: 60, successThreshold: 2);

$data = $breaker->call(
    open: function() {
        // Fallback when circuit is open
        return cache()->get('user_data') ?? ['error' => 'Service temporarily unavailable'];
    },
    close: function() {
        // Primary operation
        $response = Http::get('https://api.example.com/users');

        if (!$response->successful()) {
            throw new \Exception('API request failed');
        }

        return $response->json();
    }
);

Shared Cache State

By default, each CircuitBreaker instance keeps state in memory. To share circuit state between PHP workers, pass a cache adapter and a stable cacheKey.

Redis

use ChiragAgg5k\CircuitBreaker\Adapter\Redis as RedisAdapter;
use ChiragAgg5k\CircuitBreaker;

$redis = new \Redis();
$redis->connect('127.0.0.1');

$breaker = new CircuitBreaker(
    threshold: 5,
    timeout: 60,
    successThreshold: 2,
    cache: new RedisAdapter($redis),
    cacheKey: 'users-api'
);

Swoole Table

Use the Swoole adapter when workers need to share state through Swoole shared memory.

use ChiragAgg5k\CircuitBreaker\Adapter\SwooleTable;
use ChiragAgg5k\CircuitBreaker;

$table = SwooleTable::createTable(size: 1024);
$cache = new SwooleTable($table);

$breaker = new CircuitBreaker(
    threshold: 5,
    timeout: 60,
    successThreshold: 2,
    cache: $cache,
    cacheKey: 'users-api'
);

Telemetry

Telemetry is opt-in. The telemetry constructor argument defaults to null, which emits no metrics and does not require utopia-php/telemetry at runtime. Install utopia-php/telemetry and pass any adapter to emit counters and gauges for calls, fallbacks, callback failures, transitions, state, failure counts, success counts, active calls, and transition/probe events.

composer require utopia-php/telemetry

use ChiragAgg5k\CircuitBreaker;
use Utopia\Telemetry\Adapter\OpenTelemetry;

$telemetry = new OpenTelemetry(
    'http://otel-collector:4318/v1/metrics',
    'backend',
    'orders',
    gethostname() ?: 'local'
);

$breaker = new CircuitBreaker(
    threshold: 5,
    timeout: 60,
    successThreshold: 2,
    cacheKey: 'orders-api',
    telemetry: $telemetry,
    metricPrefix: 'backend'
);

$result = $breaker->call(
    open: fn () => ['fallback' => true],
    close: fn () => $client->request('/orders')
);

$telemetry->collect();

By default, metrics are emitted as breaker.*. Pass metricPrefix to namespace those metric names for a host application; for example metricPrefix: 'backend' emits backend.breaker.calls.

You can also attach or replace the adapter after construction:

$breaker = new CircuitBreaker(metricPrefix: 'backend');
$breaker->setTelemetry($telemetry);

How it Works

The circuit breaker operates in three states:

CLOSED State (Normal Operation)
- Requests pass through to the protected service
- Failures are counted
- When failures >= threshold, transitions to OPEN
OPEN State (Blocking Requests)
- All requests are immediately rejected (calls open callback)
- After timeout period, transitions to HALF_OPEN
- Prevents overwhelming a failing service
HALF_OPEN State (Testing Recovery)
- Allows requests through to test if service recovered
- Calls halfOpen callback if provided, otherwise uses close callback
- Counts successful requests
- If successes >= successThreshold: transitions to CLOSED
- If any failure occurs: immediately transitions back to OPEN

This gradual recovery mechanism prevents overwhelming a service that's just starting to recover. The optional halfOpen callback lets you apply different behavior during testing (e.g., shorter timeouts, reduced payload, logging).

Configuration

Constructor Parameters

threshold (int, default: 3): Number of failures before opening the circuit
timeout (int, default: 30): Seconds to wait before transitioning to half-open state
successThreshold (int, default: 2): Number of consecutive successes required to close the circuit from half-open state
cache (?ChiragAgg5k\CircuitBreaker\Adapter, default: null): Optional shared cache adapter
cacheKey (string, default: default): Cache namespace for one circuit's state
telemetry (?Utopia\Telemetry\Adapter, default: null): Optional telemetry adapter
metricPrefix (string, default: ''): Optional prefix for telemetry metric names, such as edge

Call Method Parameters

$breaker->call(
    open: callable,      // Required: Called when circuit is OPEN
    close: callable,     // Required: Called when circuit is CLOSED (or HALF_OPEN if no halfOpen callback)
    halfOpen: ?callable  // Optional: Called when circuit is HALF_OPEN for recovery testing
);

State Inspection Methods

// Check current state
$state = $breaker->getState();  // Returns CircuitState enum

// Boolean checks
$breaker->isOpen();      // true if circuit is open
$breaker->isClosed();    // true if circuit is closed
$breaker->isHalfOpen();  // true if circuit is half-open

// Get metrics
$breaker->getFailureCount();  // Current failure count
$breaker->getSuccessCount();  // Current success count (in half-open state)

Requirements

PHP 8.2 or higher
Optional: utopia-php/telemetry, ext-opentelemetry, and ext-protobuf for OpenTelemetry metrics and the local telemetry demo
Optional: ext-redis for ChiragAgg5k\CircuitBreaker\Adapter\Redis
Optional: ext-swoole for ChiragAgg5k\CircuitBreaker\Adapter\SwooleTable

Testing

Unit tests avoid Redis and Swoole runtime dependencies:

composer test

E2E tests run Redis and a PHP runtime with Redis/Swoole extensions through Docker:

composer test:e2e:docker

Local Telemetry Demo

Run the local demo stack to start Redis, an instrumented PHP demo server, OpenTelemetry Collector, Prometheus, and Grafana:

composer telemetry:up

Demo UI: http://localhost:8080
Grafana: http://localhost:3030/d/circuit-breaker/circuit-breaker-telemetry
Prometheus: http://localhost:9090

Preview from a five-minute checkout-api scenario:

Populate the dashboard with the same scenario:

composer telemetry:scenario

Stop the stack and remove local volumes:

composer telemetry:down

License

MIT License

Contributing

Contributions are welcome! Please feel free to submit a Pull Request.

chiragagg5k / circuit-breaker

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