Circuit Breaker

A Circuit Breaker is a software design pattern used to prevent system failures when a service is experiencing high failures or slow responses. It helps improve system resilience by stopping requests to an unhealthy service and allowing recovery once it stabilizes.

How It Works

1. Closed State:

   • Requests are allowed to pass normally.
   • Failures are counted.
   • If failures exceed a defined threshold, the circuit switches to Open state.

2. Open State:

   • Requests are blocked immediately to prevent overload.
   • The circuit stays open for a timeout period before moving to Half-Open.

3. Half-Open State:

   • Allows a limited number of requests to test service recovery.
   • If requests succeed, the circuit resets to Closed.
   • If requests fail, the circuit returns to Open.

Benefits of Using a Circuit Breaker

✅ Prevents cascading failures in microservices.
✅ Improves system reliability by avoiding repeated failed requests.
✅ Reduces load on struggling services and allows recovery.

Install

go get -u github.com/gofiber/fiber/v2
go get -u github.com/gofiber/contrib/circuitbreaker

Signature

circuitbreaker.New(config ...circuitbreaker.Config) *circuitbreaker.Middleware 

Config

Property	Type	Description	Default
FailureThreshold	int	Number of consecutive errors required to open the circuit	5
Timeout	time.Duration	Timeout for the circuit breaker	10 * time.Second
SuccessThreshold	int	Number of successful requests required to close the circuit	5
HalfOpenMaxConcurrent	int	Max concurrent requests in half-open state	1
IsFailure	func(error) bool	Custom function to determine if an error is a failure	Status >= 500
OnOpen	func(*fiber.Ctx)	Callback function when the circuit is opened	503 response
OnClose	func(*fiber.Ctx)	Callback function when the circuit is closed	Continue request
OnHalfOpen	func(*fiber.Ctx)	Callback function when the circuit is half-open	429 response

Circuit Breaker Usage in Fiber (Example)

This guide explains how to use a Circuit Breaker in a Fiber application at different levels, from basic setup to advanced customization.

1. Basic Setup

A global Circuit Breaker protects all routes.

Example: Applying Circuit Breaker to All Routes

package main

import (
	"github.com/gofiber/fiber/v2"
	"github.com/gofiber/contrib/circuitbreaker"
)

func main() {
	app := fiber.New()
	
	// Create a new Circuit Breaker with custom configuration
	cb := circuitbreaker.New(circuitbreaker.Config{
		FailureThreshold: 3,               // Max failures before opening the circuit
		Timeout:          5 * time.Second, // Wait time before retrying
		SuccessThreshold: 2,               // Required successes to move back to closed state
	})

	// Apply Circuit Breaker to ALL routes
	app.Use(circuitbreaker.Middleware(cb))

	// Sample Route
	app.Get("/", func(c *fiber.Ctx) error {
		return c.SendString("Hello, world!")
	})

	// Optional: Expose health check endpoint
	app.Get("/health/circuit", cb.HealthHandler())

	// Optional: Expose metrics about the circuit breaker:
	app.Get("/metrics/circuit", func(c *fiber.Ctx) error {
  		return c.JSON(cb.GetStateStats())
	})

	app.Listen(":3000")

	// In your application shutdown logic
	app.Shutdown(func() {
		// Make sure to stop the circuit breaker when your application shuts down:
		cb.Stop()
	})
}

2. Route & Route-Group Specific Circuit Breaker

Apply the Circuit Breaker only to specific routes.

app.Get("/protected", circuitbreaker.Middleware(cb), func(c *fiber.Ctx) error {
	return c.SendString("Protected service running")
})

Apply the Circuit Breaker only to specific routes groups.

app := route.Group("/api")
app.Use(circuitbreaker.Middleware(cb))

// All routes in this group will be protected
app.Get("/users", getUsersHandler)
app.Post("/users", createUserHandler)

3. Circuit Breaker with Custom Failure Handling

Customize the response when the circuit opens.

cb := circuitbreaker.New(circuitbreaker.Config{
	FailureThreshold: 3,
	Timeout:   10 * time.Second,
	OnOpen: func(c *fiber.Ctx) error {
		return c.Status(fiber.StatusServiceUnavailable).
			JSON(fiber.Map{"error": "Circuit Open: Service unavailable"})
	},
	OnHalfOpen: func(c *fiber.Ctx) error {
		return c.Status(fiber.StatusTooManyRequests).
			JSON(fiber.Map{"error": "Circuit Half-Open: Retrying service"})
	},
	OnClose: func(c *fiber.Ctx) error {
		return c.Status(fiber.StatusOK).
			JSON(fiber.Map{"message": "Circuit Closed: Service recovered"})
	},
})

// Apply to a specific route
app.Get("/custom", circuitbreaker.Middleware(cb), func(c *fiber.Ctx) error {
	return c.SendString("This service is protected by a Circuit Breaker")
})

✅ Now, when failures exceed the threshold, *custom error responses will be sent.

4. Circuit Breaker for External API Calls

Use a Circuit Breaker when calling an external API.

app.Get("/external-api", circuitbreaker.Middleware(cb), func(c *fiber.Ctx) error {
	// Simulating an external API call
	resp, err := fiber.Get("https://example.com/api")
	if err != nil {
		return fiber.NewError(fiber.StatusInternalServerError, "External API failed")
	}
	return c.SendString(resp.Body())
})

✅ If the external API fails repeatedly, the circuit breaker prevents further calls.

5. Circuit Breaker with Concurrent Requests Handling

Use a semaphore-based approach to limit concurrent requests.

cb := circuitbreaker.New(circuitbreaker.Config{
	FailureThreshold:  3,
	Timeout:           5 * time.Second,
	SuccessThreshold:  2,
	HalfOpenSemaphore: make(chan struct{}, 2), // Allow only 2 concurrent requests
})

app.Get("/half-open-limit", circuitbreaker.Middleware(cb), func(c *fiber.Ctx) error {
	time.Sleep(2 * time.Second) // Simulating slow response
	return c.SendString("Half-Open: Limited concurrent requests")
})

✅ When in half-open state, only 2 concurrent requests are allowed.

6. Circuit Breaker with Custom Metrics

Integrate Prometheus metrics and structured logging.

cb := circuitbreaker.New(circuitbreaker.Config{
	FailureThreshold: 5,
	Timeout:   10 * time.Second,
	OnOpen: func(c *fiber.Ctx) error {
		log.Println("Circuit Breaker Opened!")
		prometheus.Inc("circuit_breaker_open_count")
		return c.Status(fiber.StatusServiceUnavailable).JSON(fiber.Map{"error": "Service Down"})
	},
})

✅ Logs when the circuit opens & increments Prometheus metrics.

7. Advanced: Multiple Circuit Breakers for Different Services

Use different Circuit Breakers for different services.

dbCB := circuitbreaker.New(circuitbreaker.Config{FailureThreshold: 5, Timeout: 10 * time.Second})
apiCB := circuitbreaker.New(circuitbreaker.Config{FailureThreshold: 3, Timeout: 5 * time.Second})

app.Get("/db-service", circuitbreaker.Middleware(dbCB), func(c *fiber.Ctx) error {
	return c.SendString("DB service request")
})

app.Get("/api-service", circuitbreaker.Middleware(apiCB), func(c *fiber.Ctx) error {
	return c.SendString("External API service request")
})

✅ Each service has its own failure threshold & timeout.