{"id":1,"date":"2024-08-18T15:04:30","date_gmt":"2024-08-18T13:04:30","guid":{"rendered":"https:\/\/codeswarm.io\/?p=1"},"modified":"2026-07-19T13:35:45","modified_gmt":"2026-07-19T11:35:45","slug":"witaj-swiecie","status":"publish","type":"post","link":"https:\/\/codeswarm.io\/?p=1","title":{"rendered":"The Circuit breaker pattern in Java"},"content":{"rendered":"\n

The Circuit Breaker<\/strong> pattern is used to prevent an application from performing an operation that is likely to fail. In microservices architecture, it\u2019s commonly used to handle fault tolerance when calling external services that may fail or take a long time to respond.<\/p>\n\n\n\n

Imagine a scenario where a microservice (Service A<\/code>) calls another service (Service B<\/code>). If Service B<\/code> is down or slow to respond, without a circuit breaker, Service A<\/code> will continue making requests, which may lead to cascading failures or resource exhaustion. The Circuit Breaker pattern \u201copens\u201d and stops calling the failing service for a period to allow it to recover, then \u201ccloses\u201d after the service is healthy again.<\/p>\n\n\n\n

Key states of Circuit Breaker:<\/strong><\/p>\n\n\n\n

    \n
  1. Closed<\/strong>: The circuit breaker allows requests to pass through as normal.<\/li>\n\n\n\n
  2. Open<\/strong>: The circuit breaker prevents requests from being sent, returning a fallback response immediately.<\/li>\n\n\n\n
  3. Half-Open<\/strong>: The circuit breaker allows a limited number of requests to check if the service has recovered.<\/li>\n<\/ol>\n\n\n\n

    Implementation<\/h2>\n\n\n\n

    There are different ways to implement Circuit Breaker pattern. In this case we\u2019ll use Resilience4j<\/code> for implementing the Circuit Breaker in Spring Boot. Add the following dependency to your pom.xml<\/code>:<\/p>\n\n\n\n

    <dependency>\n    <groupId>io.github.resilience4j<\/groupId>\n    <artifactId>resilience4j-spring-boot2<\/artifactId>\n    <version>1.7.0<\/version>\n<\/dependency><\/pre>\n\n\n\n
    Here\u2019s an example of how we might use a RestTemplate<\/code> to call another service:<\/p>\n\n\n\n
    import org.springframework.stereotype.Service;\nimport org.springframework.web.client.RestTemplate;\nimport org.springframework.beans.factory.annotation.Autowired;\n@Service\npublic class ExternalService {\n    @Autowired\n    private RestTemplate restTemplate;\n    public String callExternalService() {\n        String url = \"http:\/\/localhost:8081\/external-api\";\n        return restTemplate.getForObject(url, String.class);\n    }\n}<\/pre>\n\n\n\n
    This method will call an external API, but what happens if this API is down or slow? This is where the Circuit Breaker comes in.<\/p>\n\n\n\n
    To apply the Circuit Breaker, annotate the method that makes the external call using the @CircuitBreaker<\/code> annotation.<\/p>\n\n\n\n
    import io.github.resilience4j.circuitbreaker.annotation.CircuitBreaker;\nimport org.springframework.stereotype.Service;\n@Service\npublic class ExternalService {\n    @Autowired\n    private RestTemplate restTemplate;\n    private static final String EXTERNAL_SERVICE = \"externalService\";\n    @CircuitBreaker(name = EXTERNAL_SERVICE, fallbackMethod = \"fallbackResponse\")\n    public String callExternalService() {\n        String url = \"http:\/\/localhost:8081\/external-api\";\n        return restTemplate.getForObject(url, String.class);\n    }\n    public String fallbackResponse(Throwable t) {\n        return \"External Service is currently unavailable. Please try again later.\";\n    }\n}<\/pre>\n\n\n\n
    @CircuitBreaker(name = \"externalService\")<\/code><\/strong>: This specifies that Resilience4j will manage the circuit breaker with the name externalService<\/code>. <\/p>\n\n\n\n
    fallbackMethod = \"fallbackResponse\"<\/code><\/strong>: If the circuit breaker is in the OPEN<\/code> state, or if the call to the external service fails, this fallback method will be executed.<\/p>\n\n\n\n
    Now we can configure the circuit breaker properties in our application.yml<\/code> file.<\/p>\n\n\n\n
    resilience4j.circuitbreaker:\n  instances:\n    externalService:\n      slidingWindowSize: 5\n      failureRateThreshold: 50\n      waitDurationInOpenState: 10000\n      permittedNumberOfCallsInHalfOpenState: 3<\/pre>\n\n\n\n