Circuit Breaker
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Circuit Breaker

Why are they used?

A circuit breaker is used to provide stability and prevent cascading failures in distributed systems. These should be used in conjunction with judicious timeouts at the interfaces between remote systems to prevent the failure of a single component from bringing down all components.

As an example, we have a web application interacting with a remote third party web service. Let's say the third party has oversold their capacity and their database melts down under load. Assume that the database fails in such a way that it takes a very long time to hand back an error to the third party web service. This in turn makes calls fail after a long period of time. Back to our web application, the users have noticed that their form submissions take much longer seeming to hang. Well the users do what they know to do which is use the refresh button, adding more requests to their already running requests. This eventually causes the failure of the web application due to resource exhaustion. This will affect all users, even those who are not using functionality dependent on this third party web service.

Introducing circuit breakers on the web service call would cause the requests to begin to fail-fast, letting the user know that something is wrong and that they need not refresh their request. This also confines the failure behavior to only those users that are using functionality dependent on the third party, other users are no longer affected as there is no resource exhaustion. Circuit breakers can also allow savvy developers to mark portions of the site that use the functionality unavailable, or perhaps show some cached content as appropriate while the breaker is open.

The Akka library provides an implementation of a circuit breaker called akka.pattern.CircuitBreaker which has the behavior described below.

What do they do?

  • During normal operation, a circuit breaker is in the Closed state:
    • Exceptions or calls exceeding the configured callTimeout increment a failure counter
    • Successes reset the failure count to zero
    • When the failure counter reaches a maxFailures count, the breaker is tripped into Open state
  • While in Open state:
    • All calls fail-fast with a CircuitBreakerOpenException
    • After the configured resetTimeout, the circuit breaker enters a Half-Open state
  • In Half-Open state:
    • The first call attempted is allowed through without failing fast
    • All other calls fail-fast with an exception just as in Open state
    • If the first call succeeds, the breaker is reset back to Closed state
    • If the first call fails, the breaker is tripped again into the Open state for another full resetTimeout
  • State transition listeners:
    • Callbacks can be provided for every state entry via onOpen, onClose, and onHalfOpen
    • These are executed in the ExecutionContext provided.
../_images/circuit-breaker-states.png

Examples

Initialization

Here's how a CircuitBreaker would be configured for:
  • 5 maximum failures
  • a call timeout of 10 seconds
  • a reset timeout of 1 minute

Scala

import scala.concurrent.duration._
import akka.pattern.CircuitBreaker
import akka.pattern.pipe
import akka.actor.Actor
import akka.actor.ActorLogging
import scala.concurrent.Future
import akka.event.Logging

class DangerousActor extends Actor with ActorLogging {
  import context.dispatcher

  val breaker =
    new CircuitBreaker(context.system.scheduler,
      maxFailures = 5,
      callTimeout = 10.seconds,
      resetTimeout = 1.minute).onOpen(notifyMeOnOpen())

  def notifyMeOnOpen(): Unit =
    log.warning("My CircuitBreaker is now open, and will not close for one minute")

Java

import akka.actor.UntypedActor;
import scala.concurrent.Future;
import akka.event.LoggingAdapter;
import scala.concurrent.duration.Duration;
import akka.pattern.CircuitBreaker;
import akka.event.Logging;

import static akka.pattern.Patterns.pipe;
import static akka.dispatch.Futures.future;

import java.util.concurrent.Callable;

public class DangerousJavaActor extends UntypedActor {

  private final CircuitBreaker breaker;
  private final LoggingAdapter log = Logging.getLogger(getContext().system(), this);

  public DangerousJavaActor() {
    this.breaker = new CircuitBreaker(
      getContext().dispatcher(), getContext().system().scheduler(),
      5, Duration.create(10, "s"), Duration.create(1, "m"))
      .onOpen(new Runnable() {
        public void run() {
          notifyMeOnOpen();
        }
      });
  }

  public void notifyMeOnOpen() {
    log.warning("My CircuitBreaker is now open, and will not close for one minute");
  }

Call Protection

Here's how the CircuitBreaker would be used to protect an asynchronous call as well as a synchronous one:

Scala

def dangerousCall: String = "This really isn't that dangerous of a call after all"

def receive = {
  case "is my middle name" =>
    breaker.withCircuitBreaker(Future(dangerousCall)) pipeTo sender()
  case "block for me" =>
    sender() ! breaker.withSyncCircuitBreaker(dangerousCall)
}

Java

public String dangerousCall() {
  return "This really isn't that dangerous of a call after all";
}

@Override
public void onReceive(Object message) {
  if (message instanceof String) {
    String m = (String) message;
    if ("is my middle name".equals(m)) {
      final Future<String> f = future(
        new Callable<String>() {
          public String call() {
            return dangerousCall();
          }
        }, getContext().dispatcher());

      pipe(breaker.callWithCircuitBreaker(
        new Callable<Future<String>>() {
          public Future<String> call() throws Exception {
            return f;
          }
        }), getContext().dispatcher()).to(getSender());
    }
    if ("block for me".equals(m)) {
      getSender().tell(breaker
        .callWithSyncCircuitBreaker(
          new Callable<String>() {
            @Override
            public String call() throws Exception {
              return dangerousCall();
            }
          }), getSelf());
    }
  }
}

Note

Using the CircuitBreaker companion object's apply or create methods will return a CircuitBreaker where callbacks are executed in the caller's thread. This can be useful if the asynchronous Future behavior is unnecessary, for example invoking a synchronous-only API.

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