When a remote service is down the Circuit Breaker pattern prevents a cascade of failures. After a number of failed attempts, we can consider that the service is unavailable/overloaded and reject all subsequent requests to it. The Circuit Breaker acts like a switch that opens or closes a circuit.

In this post we’ll talk about the resilience4j library that allows us to apply this pattern.

Dependencies

Add the next dependencies to your pom.xml

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<properties>
    <resilience4j.version>0.13.2</resilience4j.version>
</properties>

<dependencies>
    <dependency>
        <groupId>io.github.resilience4j</groupId>
        <artifactId>resilience4j-spring-boot</artifactId>
        <version>${resilience4j.version}</version>
    </dependency>
    <dependency>
        <groupId>io.github.resilience4j</groupId>
        <artifactId>resilience4j-metrics</artifactId>
        <version>${resilience4j.version}</version>
    </dependency>
</dependencies>

Configuration

For the next configuration we have defined two circuit breaker configurations, default and serviceA.

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resilience4j.circuitbreaker:
  backends:
    default:
      ringBufferSizeInClosedState: 3
      ringBufferSizeInHalfOpenState: 3
      waitInterval: 1000
      failureRateThreshold: 20
    serviceA:
      ringBufferSizeInClosedState: 2
      ringBufferSizeInHalfOpenState: 2
      waitInterval: 1000
      failureRateThreshold: 50
#      registerHealthIndicator: false
#      recordExceptions:
#      - org.springframework.web.client.HttpServerErrorException
#      ignoreExceptions:
#      - org.springframework.web.client.HttpClientErrorException

• ringBufferSizeInClosedState: The size of ring buffer when the CircuitBreaker is closed
• ringBufferSizeInHalfOpenState: The size of ring buffer when the CircuitBreaker is half open
• waitInterval: The wait duration in millis which defines how long the CircuitBreaker should stay open before it switches to half open
• failureRateThreshold: The failure rate threshold above which the CircuitBreaker opens and starts short-circuiting calls

Example of Circuit Breaker

The test

We’ll call to a Consumer that iterate 5 times executing a method that will throw an exception. We will log:

• The begin of method “Entering in service …”
• The exception that will return the service “Exception in method”
• When a CircuitBreaker is opened “Circuit breaker applied”

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private static final Consumer<Runnable> consumer = runnable -> IntStream.range(0, 5).forEach(value -> {
    try {
        runnable.run();
    } catch (CircuitBreakerOpenException e) {
        log.warn("Circuit breaker applied");
    } catch (Exception e) {
        log.warn("Exception in method");
    }
});

Circuit Breaker using annotations

You could annotate the method or the entire class with @CircuitBreaker annotation. Example for method annotation with serviceA configuration.

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@CircuitBreaker(name = "serviceA")
public void runWithCircuitBreaker() {
    // This code will throw an exception and will not be executed when applies Circuit Breaker
}

An example of 5 invocations to a method that always throws exception, the Circuit Breaker mechanism will be applied. The method will be invoked 2 times and the next 3 times

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2019-01-02 20:15:26.936  INFO ... : Running with circuit breaker using annotations ...
2019-01-02 20:15:26.943  INFO ... : Entering in service ...
2019-01-02 20:15:27.431  WARN ... : Exception in method
2019-01-02 20:15:27.432  INFO ... : Entering in service ...
2019-01-02 20:15:28.106  WARN ... : Exception in method
2019-01-02 20:15:28.107  WARN ... : Circuit breaker applied
2019-01-02 20:15:28.107  WARN ... : Circuit breaker applied
2019-01-02 20:15:28.107  WARN ... : Circuit breaker applied

Circuit Breaker with direct invocation

You can decorate any Supplier / Runnable / Function or CheckedRunnable / CheckedFunction function with CircuitBreaker.decorateCheckedSupplier(), CircuitBreaker.decorateCheckedRunnable() or CircuitBreaker.decorateCheckedFunction()

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CircuitBreaker defaultCircuitBreaker = circuitBreakerRegistry.circuitBreaker("default");
Runnable decoratedRunnable = CircuitBreaker.decorateRunnable(defaultCircuitBreaker, () -> circuitBreakerTestService.run(false));

The same example of the previous case, 5 invocations but applying the default configuration:

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2019-01-02 20:15:28.107  INFO ... : Running with default circuit breaker using manual invocation ...
2019-01-02 20:15:28.108  INFO ... : Entering in service ...
2019-01-02 20:15:28.298  WARN ... : Exception in method
2019-01-02 20:15:28.298  INFO ... : Entering in service ...
2019-01-02 20:15:28.485  WARN ... : Exception in method
2019-01-02 20:15:28.486  INFO ... : Entering in service ...
2019-01-02 20:15:28.670  WARN ... : Exception in method
2019-01-02 20:15:28.670  WARN ... : Circuit breaker applied
2019-01-02 20:15:28.670  WARN ... : Circuit breaker applied

References

[1] Link to the project in Github

[2] CircuitBreaker of Martin Fowler Circuit Breaker

[3] Resilience4j in Baeldung Resilience4j

[4] Resilience4j in Github Resilience4j Github