Spring Boot 4 Microservices & REST APIs · Lesson

Circuit Breakers with Resilience4j

Apply the circuit breaker pattern using Resilience4j to prevent cascading failures in your microservices.

Circuit Breakers with Resilience4j is a free Spring Boot 4 Microservices & REST APIs lesson on CoddyKit — lesson 1 of 3. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Spring Boot 4 Microservices & REST APIs learning path, one of 3 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Building Resilient Microservices

Microservices are great, but they also bring new challenges. When one service fails, it shouldn't take down the whole system. This is where resilience comes in.

Building resilient systems means they can recover gracefully from failures and continue to function, even if in a degraded mode. It's crucial for maintaining service availability and user experience.

Avoiding Cascading Failures

Imagine a chain of services: Service A calls B, which calls C. If Service C becomes slow or unresponsive, Service B will wait, consuming resources. Then Service A will wait for B, and so on.

Eventually, all services in the chain might exhaust their resources (like threads or connections) and fail, leading to a complete system outage. This is a cascading failure, a common problem in distributed systems.

The Circuit Breaker Pattern

The circuit breaker pattern helps prevent cascading failures. It's like an electrical circuit breaker in your house: if there's an overload, it 'trips' and cuts off the power to prevent damage.

In software, a circuit breaker wraps a function call to a potentially failing service. If calls to that service repeatedly fail, the circuit breaker 'opens,' preventing further calls to the failing service and returning an error immediately.

Resilience4j: Our Tool

Resilience4j is a lightweight, easy-to-use fault-tolerance library for Java and Kotlin. It provides several patterns to make your applications more resilient, including:

Circuit Breaker: Prevents repeated calls to failing services.
Retry: Automatically retries failed operations.
Rate Limiter: Controls the rate of requests to a service.

In this lesson, we'll focus specifically on the Circuit Breaker pattern.

Setup: Adding Dependency

First, let's add the necessary dependency to our Spring Boot project. For Maven, you'd add this to your pom.xml:

<dependency> <groupId>io.github.resilience4j</groupId> <artifactId>resilience4j-spring-boot2</artifactId> <version>2.2.0</version>  </dependency>

This dependency brings in the core Resilience4j modules and its Spring Boot integration for easy configuration.

Configuring Circuit Breaker

We can configure circuit breakers in our application.yml or application.properties. Here's a basic example for a service named 'myExternalService':

resilience4j.circuitbreaker: instances: myExternalService: failureRateThreshold: 50 waitDurationInOpenState: 5s slidingWindowType: COUNT_BASED slidingWindowSize: 10

failureRateThreshold: If 50% of calls fail, the circuit opens.
waitDurationInOpenState: How long the circuit stays open (5 seconds).
slidingWindowType & slidingWindowSize: Metrics are collected over the last 10 calls.

Implementing Circuit Breaker

Now, let's protect a service call using the @CircuitBreaker annotation. This tells Spring to apply the configured circuit breaker named "myExternalService" to this method.

Try running this example. If callExternalService() throws an exception frequently (simulated here), the circuit will eventually open and reject calls immediately.

package com.coddykit.resilience;

import io.github.resilience4j.circuitbreaker.annotation.CircuitBreaker;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

import java.util.Random;

@SpringBootApplication
@RestController
public class CircuitBreakerDemoApplication {

    private Random random = new Random();

    public static void main(String[] args) {
        SpringApplication.run(CircuitBreakerDemoApplication.class, args);
    }

    @GetMapping("/call-external")
    @CircuitBreaker(name = "myExternalService")
    public String callExternalService() {
        System.out.println("Attempting external service call...");
        if (random.nextBoolean()) { // Simulate 50% failure rate
            System.out.println("External service failed!");
            throw new RuntimeException("External Service Unavailable!");
        }
        System.out.println("External service call successful!");
        return "External Service Data";
    }
}

Circuit Breaker States

A circuit breaker operates in three main states:

CLOSED: Normal operation. Calls go through. If failures exceed a threshold, it transitions to OPEN.
OPEN: Calls are immediately rejected with an error. After a waitDurationInOpenState, it transitions to HALF_OPEN.
HALF_OPEN: A limited number of test calls are allowed through. If these succeed, it transitions back to CLOSED. If they fail, it returns to OPEN.

This cycle prevents overwhelming a struggling service while allowing it to recover.

Graceful Fallbacks

When a circuit breaker is OPEN, or if a call fails for other reasons, we don't want to just return a generic error. We can provide a fallback method to return a default response or perform alternative logic.

Resilience4j's @CircuitBreaker annotation allows you to specify a fallback method that will be called if the primary method fails or the circuit is open. This ensures a more graceful degradation of service.

Circuit Breaker with Fallback

Let's enhance our previous example with a fallback method. When the circuit is open or callExternalService() fails, fallbackMethod() will be executed instead.

Run this and try to trigger the circuit breaker. Notice how the fallback message is returned when the external service fails or the circuit is open, providing a better user experience.

package com.coddykit.resilience;

import io.github.resilience4j.circuitbreaker.annotation.CircuitBreaker;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

import java.util.Random;

@SpringBootApplication
@RestController
public class CircuitBreakerWithFallbackApplication {

    private Random random = new Random();

    public static void main(String[] args) {
        SpringApplication.run(CircuitBreakerWithFallbackApplication.class, args);
    }

    @GetMapping("/call-external-with-fallback")
    @CircuitBreaker(name = "myExternalService", fallbackMethod = "fallbackMethod")
    public String callExternalService() {
        System.out.println("Attempting external service call...");
        if (random.nextBoolean()) { // Simulate 50% failure rate
            System.out.println("External service failed!");
            throw new RuntimeException("External Service Unavailable!");
        }
        System.out.println("External service call successful!");
        return "External Service Data";
    }

    // The fallback method must have the same return type and can accept a Throwable parameter
    public String fallbackMethod(Throwable t) {
        System.out.println("Fallback method called: " + t.getMessage());
        return "Fallback: Default Data (Service Currently Unavailable)";
    }
}

Circuit Breaker Check

Consider a circuit breaker configured with failureRateThreshold: 70 and slidingWindowSize: 10. If 8 out of the last 10 consecutive calls fail while the circuit is CLOSED, what is the most likely immediate next state of the circuit breaker?

Recap: Circuit Breakers

We've learned how circuit breakers, especially with Resilience4j, are vital for building resilient microservices. They prevent cascading failures by stopping repeated calls to failing services.

Key takeaways:

Circuit breakers protect services from overload and unresponsiveness.
They operate in CLOSED, OPEN, and HALF_OPEN states.
Fallbacks provide graceful degradation when a service is unavailable.

Next, we'll explore other resilience patterns like implementing fallbacks and timeouts more deeply.

Frequently asked questions

Is the “Circuit Breakers with Resilience4j” lesson free?

Yes — the full text of “Circuit Breakers with Resilience4j” is free to read here on the web, and the Spring Boot 4 Microservices & REST APIs course includes 3 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Spring Boot 4 Microservices & REST APIs course, upgrade to CoddyKit PRO.

What will I learn in “Circuit Breakers with Resilience4j”?

Apply the circuit breaker pattern using Resilience4j to prevent cascading failures in your microservices. You practise Spring Boot 4 Microservices & REST APIs with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Spring Boot 4 Microservices & REST APIs?

No prior experience is required. Spring Boot 4 Microservices & REST APIs on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 3, so you can start here or from the beginning and move at your own pace.

How long does the “Circuit Breakers with Resilience4j” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Spring Boot 4 Microservices & REST APIs lesson?

Yes. Every Spring Boot 4 Microservices & REST APIs lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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