Integrating Circuit Breakers in Spring Boot

Circuit breakers help your Spring Boot applications handle failures gracefully when calling external services. By using libraries like Resilience4j, you can add circuit breaker functionality with minimal configuration.

Adding Resilience4j to Your Project

Add the following dependency to your pom.xml if you are using Maven:

<dependency>
    <groupId>io.github.resilience4j</groupId>
    <artifactId>resilience4j-spring-boot2</artifactId>
    <version>1.7.1</version>
</dependency>

Using Circuit Breaker Annotations

You can use the @CircuitBreaker annotation to protect methods that call external services. The annotation automatically monitors the method for failures and opens the circuit if errors exceed a threshold.

import io.github.resilience4j.circuitbreaker.annotation.CircuitBreaker;

@Service
public class ExternalService {
    @CircuitBreaker(name = "externalApi", fallbackMethod = "fallbackResponse")
    public String callExternalApi() {
        // Simulate external API call
        return restTemplate.getForObject("https://api.example.com/data", String.class);
    }

    public String fallbackResponse(Exception ex) {
        return "Default response due to error";
    }
}

• The name attribute identifies the circuit breaker instance.
• The fallbackMethod attribute specifies a method to call if the circuit is open or an error occurs.

Setting Custom Circuit Breaker Properties

You can customize how your circuit breaker behaves by adding properties to your application.yml:

resilience4j:
  circuitbreaker:
    instances:
      externalApi:
        registerHealthIndicator: true
        slidingWindowSize: 10
        failureRateThreshold: 50
        waitDurationInOpenState: 10000

• slidingWindowSize: number of calls to consider for failure rate calculation;
• failureRateThreshold: percentage of failures to open the circuit;
• waitDurationInOpenState: time in milliseconds before attempting to close the circuit.

Defining Fallback Methods

Fallback methods provide a safe response when the main method fails or the circuit is open. A fallback method must have the same parameters as the main method, plus an optional Throwable parameter.

public String fallbackResponse(Exception ex) {
    return "Default response due to error";
}

Monitoring Circuit Breaker Status

You can monitor circuit breaker metrics using Spring Boot Actuator. Add the dependency:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-actuator</artifactId>
</dependency>

Enable actuator endpoints in your application.yml:

management:
  endpoints:
    web:
      exposure:
        include: resilience4j.circuitbreakers

Access circuit breaker status at:

http://localhost:8080/actuator/resilience4j/circuitbreakers

You will see real-time data about each circuit breaker, including state, failure rate, and number of calls.

By integrating circuit breakers with Spring Boot, you can make your applications more robust and reliable when dealing with unreliable external systems.