
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Overview Learn how to make your microservice calls resilient using the Circuit Breaker pattern with   and   — complete demo, step-by-step commands, class-by-class explanations, sample outputs, real-world use cases, and production tips. Resilience4j Spring Boot Why This Matters In distributed systems, a failing downstream service can cascade and cause overall system outages. The   pattern: Circuit Breaker 
 
 
 
 
 Detects a failing dependency, Stops sending requests to it (to avoid wasting resources), Returns a graceful fallback response, Periodically tests the dependency and restores normal operation when healthy. This reduces downtime, protects thread pools, keeps user experience reasonable, and prevents retry storms. Demo Summary (What You Have) A two-service Maven demo: 
 
 
 
  (port  ) — simple REST provider that intentionally fails intermittently. hello-service 8081 End point GET /api/hello
 
 
 
 
  (port  ) — calls hello-service using RestTemplate and is protected by   with a fallback. client-service 8080 Resilience4j @CircuitBreaker End point GET /api/get-message
 Run both ( ) hello-service and client-service Then test GET http://localhost:8080/api/get-message
 Architecture Diagram This is a small, focused flow suitable for drawing a diagram Files & Code: Class-By-Class Explanation hello-service HelloServiceApplication.java 
 
 
 
 Standard   bootstrap class. @SpringBootApplication HelloController.java

@RestController
public class HelloController {

    private static int counter = 0;

    @GetMapping("/api/hello")
    public String sayHello() {
        counter++;
        // simulate intermittent failure: fail on every 3rd request
        if (counter % 3 == 0) {
            throw new RuntimeException("Simulated failure from Hello-Service!");
        }
        return "Hello from Hello-Service! (count=" + counter + ")";
    }
}
  This controller intentionally throws a RuntimeException on periodic calls to simulate transient failures you’d see in real systems (DB outage, bad data, timeouts). Explanation: client-service ClientServiceApplication.java 
 
 Standard Spring Boot main class. No special config required. AppConfig.java
@Configuration
public class AppConfig {
    @Bean
    public RestTemplate restTemplate() {
        return new RestTemplate();
    }
}
  Provides a single RestTemplate bean. Ensure RestTemplate is a Spring bean so AOP/resilience proxies can work properly. Explanation: HelloClientService.java
@Service
public class HelloClientService {

    private final RestTemplate restTemplate;

    @Value("${hello.service.url}")
    private String helloServiceUrl;

    public HelloClientService(RestTemplate restTemplate) {
        this.restTemplate = restTemplate;
    }

    @CircuitBreaker(name = "helloService", fallbackMethod = "fallbackHello")
    public String getHelloMessage() {
        System.out.println("Calling hello service: " + helloServiceUrl);
        return restTemplate.getForObject(helloServiceUrl, String.class);
    }

    public String fallbackHello(Throwable t) {
        System.out.println("Fallback triggered: " + t);
        return "Hello Service is currently unavailable. Please try again later.";
    }
}
 Explanation (crucial bits) 
 
 
 
 
 
 @CircuitBreaker(name = "helloService", fallbackMethod = "fallbackHello") wraps the getHelloMessage() call in a circuit breaker. The name links to configuration properties. 
 
 
 
 fallbackHello(Throwable t) is called when the call fails according to the breaker rules. The fallback must:     o    Be in the same class

    o    Have the same return type

    o   Accept the original method parameters (none here) and a final Throwable parameter (or Exception/Throwable compatible with thrown exceptions)
 
 
 Important: The method must be public, and the class must be a Spring bean (@Service), so proxy-based AOP works. ClientController.java
@RestController
public class ClientController {

    private final HelloClientService helloClientService;

    public ClientController(HelloClientService helloClientService) {
        this.helloClientService = helloClientService;
    }

    @GetMapping("/api/get-message")
    public String getMessage() {
        return helloClientService.getHelloMessage();
    }
}
  Simple controller delegating to  . This ensures the call goes through the proxy where the circuit breaker is applied. Explanation: HelloClientService Configuration Used (client-service application.properties) Key configuration used in the demo server.port=8080
spring.application.name=client-service

hello.service.url=http://localhost:8081/api/hello

resilience4j.circuitbreaker.instances.helloService.registerHealthIndicator=true
resilience4j.circuitbreaker.instances.helloService.slidingWindowSize=5
resilience4j.circuitbreaker.instances.helloService.minimumNumberOfCalls=2
resilience4j.circuitbreaker.instances.helloService.failureRateThreshold=50
resilience4j.circuitbreaker.instances.helloService.waitDurationInOpenState=10s

logging.level.io.github.resilience4j.circuitbreaker=DEBUG
 Meaning of important properties 
 
 
 
 
 
  number of calls the breaker monitors for failure percentage. slidingWindowSize:  minimum calls before failure rate is evaluated. minimumNumberOfCalls:  percent failures (e.g., 50) to open the circuit. failureRateThreshold:  how long the circuit stays open before moving to half-open. waitDurationInOpenState:  exposes breaker state via actuator. registerHealthIndicator: Step-By-Step Run & Expected Outputs Start Services 
 
 Start  hello-service Visit: http://localhost:8081/api/hello
  → "Hello from Hello-Service!" (or throws simulated failure) Returns 
 
 Start  client-service Visit: http://localhost:8080/api/get-message
 Test Scenarios & Outputs Scenario A — Hello-Service Healthy Make a request GET http://localhost:8080/api/get-message
 Client logs Calling hello service: http://localhost:8081/api/hello
2025-11-13T11:58:23.366+05:30 DEBUG 32692 --- [client-service] [nio-8080-exec-8] i.g.r.c.i.CircuitBreakerStateMachine : CircuitBreaker 'helloService' succeeded:
2025-11-13T11:58:23.366+05:30 DEBUG 32692 --- [client-service] [nio-8080-exec-8] i.g.r.c.i.CircuitBreakerStateMachine : Event SUCCESS published: 2025-11-13T11:58:23.366634+05:30[Asia/Calcutta]: CircuitBreaker 'helloService' recorded a successful call. Elapsed time: 15 ms
 Response Hello from Hello-Service! (count=4)
 Scenario B — Hello-Service Intermittent Failures If you call repeatedly and hello-service throws   on some requests: RuntimeException ● Successful calls: client returns the hello message. ●  When a downstream call returns HTTP 500/exception:             o    Resilience4j records the failure.

            o    If failure rate exceeds threshold (e.g., 50% over sliding window), the Circuit becomes **OPEN**.

            o    While OPEN, calls are short-circuited; **fallbackHello**() is immediately executed — no network call. \n                       **Client response while fallback active**
 Client Response while active Response Hello Service is currently unavailable. Please try again later.
 Sample client log sequence Calling hello service: http://localhost:8081/api/hello
2025-11-13T12:00:55.842+05:30 DEBUG 32692 --- [client-service] [nio-8080-exec-1] i.g.r.c.i.CircuitBreakerStateMachine : CircuitBreaker 'helloService' recorded an exception as failure:
 Scenario C — Recovery After    waitDurationInOpenState (10s): 
 
 
 
 Circuit goes to  : a few test calls are allowed. HALF_OPEN If test calls succeed, breaker   and normal traffic resumes. CLOSES If tests fail, the breaker goes back to  . OPEN Real-World Use Cases 
 
 
 
 
  A checkout service calling an external bank API: if the bank’s API becomes slow/fails, the circuit breaker returns a user-friendly message and prevents retry storms. Payment Integration -  APIs with limited calls per second — when limits are reached, the circuit opens to avoid hitting quotas further; fallback returns cached data. Third-party rate-limited APIs -  Service A calls B, which calls C. If C is unstable, open breakers protect B and A, preventing system-wide slowdown. Microservice chains inside an enterprise -  When a non-critical feature service fails, return a simpler result to keep core functionality available (e.g., return product list without recommendations). Feature toggles and graceful degradation - Advantages & Business Value 
 
 
 
 
 
 
  — prevents one bad dependency from cascading. Fault isolation  — fails fast instead of waiting on timeouts. Faster failure response  — offers fallback responses instead of full outages. Graceful degradation  — avoids wasting CPU, threads, and network. Resource protection  — automatically returns to normal when the dependency is healthy. Auto recovery  — breaker states and metrics can be exposed via the Actuator and monitored. Observability Sample logs you’ll see (realistic) Calling hello service: http://localhost:8081/api/hello
2025-11-13T12:00:55.842+05:30 DEBUG 32692 --- [client-service] [nio-8080-exec-1] i.g.r.c.i.CircuitBreakerStateMachine : CircuitBreaker 'helloService' recorded an exception as failure
2025-11-13T12:00:55.847+05:30 DEBUG 32692 --- [client-service] [nio-8080-exec-1] i.g.r.c.i.CircuitBreakerStateMachine : Event ERROR published: 2025-11-13T12:00:55.847908200+05:30[Asia/Calcutta]: CircuitBreaker 'helloService' recorded an error: 'org.springframework.web.client.HttpServerErrorException$InternalServerError: 500 : "{"timestamp":"2025-11-13T06:30:55.842+00:00","status":500,"error":"Internal Server Error","path":"/api/hello"}"'. Elapsed time: 8 ms
Fallback triggered: org.springframework.web.client.HttpServerErrorException$InternalServerError: 500 : "{"timestamp":"2025-11-13T06:30:55.842+00:00","status":500,"error":"Internal Server Error","path":"/api/hello"}"


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