The document provides an overview of circuit breakers in Java, discussing their states (closed, open, half-open) and how they manage failures in remote APIs to maintain application resilience. It includes implementations using libraries like Failsafe, Javaslang, Netflix Hystrix, and Vert.x, along with sample code for each. The focus is on ensuring resources are efficiently utilized and recovery is facilitated in overloaded systems.

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Circuit Breakers for Java: Failsafe,
Javaslang-CB, Hystrix and Vert.x
Resilient Architecture in Practice

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Why?
● Remote-APIs might hang / have latency
issues / throw exceptions / behave
unresponsive
● Our application blocks resources, allocates
memory for threads and bound objects
● Spamming requests to an overloaded remote
API might prevent its recovery

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States

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Closed-State
● The circuit-breaker executes operations as
usual
● If a failure occurs, the circuit-breaker writes it
down
● If a specified error threshold (number of failures
or frequency of failures) is reached, it trips and
opens the circuit (transitions to the open-state)

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Open-State
● Calls to the circuit-breaker in the open state fail
immediately
● No call to the underlying operation is executed
● After a specified timeout is reached, the circuit-
breaker transitions to the half-open state.

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Half-Open-State
● In this state, one call is allowed to call the
underlying operation
● If this call fails, the circuit-breaker transitions to
the open-state again until another timeout is
reached
● If it succeeds, the circuit-breaker resets and
transitions to the closed-state.

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Implementations for Java
● Failsafe
● Javaslang-Circuitbreaker
● Netflix Hystrix
● Vert.x Circuitbreaker

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Failsafe - Example
UnstableApplication app = new UnstableApplication();
CircuitBreaker breaker = new CircuitBreaker().withFailureThreshold(2).withSuccessThreshold(5).withDelay(1,
TimeUnit.SECONDS);
RetryPolicy retryPolicy = new RetryPolicy().withDelay(2, TimeUnit.SECONDS).withMaxDuration(60,
TimeUnit.SECONDS)
.withBackoff(4, 40, TimeUnit.SECONDS);
System.out.printf("circuit-breaker state is: %sn", breaker.getState());
for (int i = 0; i < 10; i++) {
Thread.sleep(1000);
try {
String id = Failsafe.with(breaker).with(retryPolicy)
.onFailedAttempt((a, b) -> System.err.printf(
"failed with exception: '%s' at '%s', circuit-breaker state is: '%s'n", b,
ZonedDateTime.now(), breaker.getState()))
.onSuccess((a, b) -> System.out.printf("call succeeded, circuit-breaker state is: '%s'n",
breaker.getState()))
.get(app::generateId);
System.out.printf("FailsafeExample: id '%s' received at '%s'n", id, ZonedDateTime.now());
} catch (CircuitBreakerOpenException e) {
System.out.printf("circuit-breaker is open (state %s), time is '%s'n", breaker.getState(),
ZonedDateTime.now());
}
}

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Javaslang ExampleUnstableApplication app = new UnstableApplication();
CircuitBreakerConfig breakerConfig =
CircuitBreakerConfig.custom().ringBufferSizeInClosedState(2)
.ringBufferSizeInHalfOpenState(2).failureRateThreshold(50)
.waitDurationInOpenState(Duration.ofMillis(1000)).build();
CircuitBreaker breaker = CircuitBreaker.of("unstableAppBreaker",
breakerConfig);
Try.CheckedSupplier<String> decoratedSupplier =
Decorators.ofCheckedSupplier(app::generateId)
.withCircuitBreaker(breaker).decorate();
System.out.printf("circuit-breaker state is: %sn", breaker.getState());
for (int i = 0; i < 10; i++) {
Thread.sleep(1000);
Try<String> result = Try.of(decoratedSupplier)
.onSuccess((a) -> System.out.printf("call succeeded, circuit-
breaker state is: '%s'n",
breaker.getState()))
.onFailure(e -> System.err.printf(
"failed with exception: '%s' at '%s', circuit-breaker
state is: '%s'n", e,
ZonedDateTime.now(), breaker.getState()));
if (!result.isEmpty()) {
System.out.printf("JavaslangExample: id '%s' received at '%s'n",
result.get(), ZonedDateTime.now());
}
}

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Hystrix Example (I)
class IdGeneratingCommand extends HystrixObservableCommand<String> {
private final UnstableApplication app;
public IdGeneratingCommand(HystrixObservableCommand.Setter setter,
UnstableApplication app) {
super(setter);
this.app = app;
}
@Override
protected Observable<String> construct() {
return Observable.create(observer -> {
try {
if (!observer.isUnsubscribed()) {
observer.onNext(app.generateId());
observer.onCompleted();
}
} catch (SampleException e) {
observer.onError(e);
}
});
}
};

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Hystrix Example (II)UnstableApplication app = new UnstableApplication();
HystrixObservableCommand.Setter setter = HystrixObservableCommand.Setter
.withGroupKey(HystrixCommandGroupKey.Factory.asKey("unstableAppCmdGroup"))
.andCommandPropertiesDefaults(HystrixCommandProperties.Setter().withCircuitBreakerEnabled(t
rue)
.withCircuitBreakerErrorThresholdPercentage(50)
.withCircuitBreakerSleepWindowInMilliseconds(1000).withCircuitBreakerRequestVolumeThreshold
(1));
for (int i = 0; i < 10; i++) {
CountDownLatch l = new CountDownLatch(1);
IdGeneratingCommand cmd = new IdGeneratingCommand(setter, app);
final HealthCounts healthCounts = cmd.getMetrics().getHealthCounts();
System.out.printf("circuit-breaker state is open: %s, %d errors of %d
requestsn",
cmd.isCircuitBreakerOpen(), healthCounts.getErrorCount(),
healthCounts.getTotalRequests());
Observable<String> observable = cmd.observe();
observable.subscribe(s -> {
System.out.printf("HystrixExample: id '%s' received at '%s'n", s,
ZonedDateTime.now());
} , t -> {
System.err.printf("HystrixExample: error %s, circuit-breaker state is open:
%sn", t,
cmd.isCircuitBreakerOpen());
} , () -> {
l.countDown();
});
l.await(4, TimeUnit.SECONDS);
}

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Vert.x ExampleUnstableApplication app = new UnstableApplication();
Vertx vertx = Vertx.vertx();
CircuitBreaker breaker = CircuitBreaker
.create("unstableAppBreaker", vertx,
new
CircuitBreakerOptions().setMaxFailures(2).setTimeout(2000).setFallbackOnFailure(true)
.setResetTimeout(2000))
.openHandler(h -> System.err.println("circuit-breaker opened"))
.closeHandler(h -> System.out.println("circuit-breaker closed"))
.halfOpenHandler(h -> System.err.println("circuit-breaker half-opened"));
System.out.printf("circuit-breaker state is: %sn", breaker.state());
for (int i = 0; i < 10; i++) {
Thread.sleep(1000);
breaker.<String> execute(future -> {
try {
final String id = app.generateId();
future.complete(id);
} catch (SampleException e) {
future.fail(e);
}
if (future.failed()) {
System.err.printf("failed with exception: '%s' at '%s', circuit-breaker
state is: '%s'n",
future.cause(), ZonedDateTime.now(), breaker.state());
}
}).setHandler(id -> {
if (id.succeeded())
System.out.printf("VertxExample: id '%s' received at '%s'n", id,
ZonedDateTime.now());
});
}
vertx.close();

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The End
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