(Resilence4j- Circuit Breaker and retry)

What is @CircuitBreaker Annotation?

@CircuitBreaker is used in Spring Boot (with Resilience4j) to stop calling a failing service after a certain number of failures and return a fallback response instead.

It helps in fault tolerance and prevents cascading failures.

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✅ Syntax

@CircuitBreaker(name = "serviceName", fallbackMethod = "fallbackMethod")

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1️⃣ Simple Example

@Service
public class OrderService {

    @CircuitBreaker(name = "orderCB", fallbackMethod = "orderFallback")
    public String placeOrder() {
        // external service call
        throw new RuntimeException("Service down");
    }

    public String orderFallback(Exception ex) {
        return "Order service is temporarily unavailable";
    }
}

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2️⃣ How Annotation Works Internally

• Spring AOP proxy intercepts the method call

• Resilience4j tracks:

  • failures

  • slow calls

  • success rate

• Based on config, circuit moves between:

  • Closed

  • Open

  • Half-Open

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3️⃣ Configuration (application.yml)

resilience4j:
  circuitbreaker:
    instances:
      orderCB:
        failureRateThreshold: 50
        minimumNumberOfCalls: 5
        slidingWindowSize: 10
        waitDurationInOpenState: 10s
        permittedNumberOfCallsInHalfOpenState: 2

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4️⃣ Circuit States with Annotation

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5️⃣ Fallback Method Rules ❗ (Very Important)

✔ Same return type
✔ Same class
✔ Last parameter must be Exception or Throwable

Valid:

public String orderFallback(Exception ex)

Invalid ❌:

public void orderFallback()

1️⃣ Closed → Open

• Failure threshold is reached

• Circuit goes to OPEN

• Message to callers:

  ❌ “Don’t call me now — service is down”

• All requests go directly to fallback

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2️⃣ Open State (Wait period)

• Circuit stays OPEN for 5 seconds
  (waitDurationInOpenState = 5s)

• No real calls allowed

• Fallback is always returned

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3️⃣ Open → Half-Open

• After 5 seconds, circuit moves to HALF-OPEN

• Circuit says:

  🧪 “Service might be up now — you can try”

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4️⃣ Half-Open State (Test calls)

• Allows 2 real calls
  (permittedNumberOfCallsInHalfOpenState = 2)

• These calls hit the actual service

Outcomes:

• ❌ If even 1 call fails

  • Circuit → OPEN again

  • Wait another 5 seconds

• ✅ If both calls succeed

  • Circuit → CLOSED

  • Normal traffic resumes

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🔁 Final Flow Diagram

CLOSED
  ↓ (failures exceed threshold)
OPEN (wait 5 sec, fallback only)
  ↓
HALF-OPEN (allow 2 test calls)
  ↓
❌ fail → OPEN again
✅ success → CLOSED

Retry Mechanism (Resilence4j)

@Retry automatically retries a failed method call a fixed number of times before giving up.

It is used when:

• Failures are temporary (network glitch, timeout)

• Service is likely to recover quickly

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✅ Syntax

@Retry(name = "serviceRetry", fallbackMethod = "fallbackMethod")

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1️⃣ Simple Example

@Service
public class InventoryService {

    @Retry(name = "inventoryRetry", fallbackMethod = "inventoryFallback")
    public String checkStock() {
        System.out.println("Calling inventory service...");
        throw new RuntimeException("Temporary failure");
    }

    public String inventoryFallback(Exception ex) {
        return "Inventory service unavailable after retries";
    }
}

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2️⃣ Retry Configuration (application.yml)

resilience4j:
  retry:
    instances:
      inventoryRetry:
        maxAttempts: 3
        waitDuration: 2s

Meaning:

• maxAttempts: 3 → 1 original + 2 retries

• waitDuration: 2s → wait between retries

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3️⃣ How Retry Works (Flow)

Call 1 → fail
   ↓
Retry 1 → fail
   ↓
Retry 2 → fail
   ↓
Fallback method called

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4️⃣ Retry vs Circuit Breaker (Very Important)