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 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
- All calls fail-fast with a
-
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 and the resetTimeout is reset
- If the first call fails, the breaker is tripped again into the Open state (as for exponential backoff circuit breaker, the resetTimeout is multiplied by the exponential backoff factor)
-
State transition listeners:
- Callbacks can be provided for every state entry via
onOpen
,onClose
, andonHalfOpen
- These are executed in the
ExecutionContext
provided.
- Callbacks can be provided for every state entry via
-
Calls result listeners:
- Callbacks can be used eg. to collect statistics about all invocations or to react on specific call results like success, failures or timeouts.
- Supported callbacks are:
onCallSuccess
,onCallFailure
,onCallTimeout
,onCallBreakerOpen
. - These are executed in the
ExecutionContext
provided.
Examples
Initialization
Here’s how a named CircuitBreaker
is configured with the name data-access
:
- 5 maximum failures
- a call timeout of 10 seconds
- a reset timeout of 1 minute
sourceakka.circuit-breaker.data-access {
max-failures = 5
call-timeout = 10s
reset-timeout = 1m
}
The circuit breaker is created on first access with the same name, subsequent lookups will return the same circuit breaker instance. Looking up the circuit breaker and using it looks like this:
Future & Synchronous based API
Once a circuit breaker actor has been initialized, interacting with that actor is done by either using the Future based API or the synchronous API. Both of these APIs are considered Call Protection
because whether synchronously or asynchronously, the purpose of the circuit breaker is to protect your system from cascading failures while making a call to another service.
In the future based API, we use the withCircuitBreaker
which takes an asynchronous method (some method wrapped in a Future
), for instance a call to retrieve data from a service, and we pipe the result back to the sender. If for some reason the service in this example isn’t responding, or there is another issue, the circuit breaker will open and stop trying to hit the service again and again until the timeout is reached.
- Scala
-
source
class DataAccess( context: ActorContext[DataAccess.Command], id: String, service: ThirdPartyWebService, circuitBreaker: CircuitBreaker) { import DataAccess._ private def active(): Behavior[Command] = { Behaviors.receiveMessagePartial { case Handle(value, replyTo) => val futureResult: Future[Done] = circuitBreaker.withCircuitBreaker { service.call(id, value) } context.pipeToSelf(futureResult) { case Success(_) => HandleSuceeded(replyTo) case Failure(exception) => HandleFailed(replyTo, exception) } Behaviors.same case HandleSuceeded(replyTo) => replyTo ! StatusReply.Ack Behaviors.same case HandleFailed(replyTo, exception) => context.log.warn("Failed to call web service", exception) replyTo ! StatusReply.error("Dependency service not available") Behaviors.same } } }
- Java
The Synchronous API would also wrap your call with the circuit breaker logic, however, it uses the withSyncCircuitBreaker
and receives a method that is not wrapped in a Future
.
The CircuitBreaker
will execute all callbacks on the default system dispatcher.
Control failure count explicitly
By default, the circuit breaker treats Exception
as failure in synchronized API, or failed Future
as failure in future based API. On failure, the failure count will increment. If the failure count reaches the maxFailures, the circuit breaker will be opened. However, some applications may require certain exceptions to not increase the failure count. In other cases one may want to increase the failure count even if the call succeeded. Akka circuit breaker provides a way to achieve such use cases: withCircuitBreaker
and withSyncCircuitBreaker
.
All methods above accept an argument defineFailureFn
Type of defineFailureFn
: Try[T]
=> Boolean
This is a function which takes in a Try[T]
and returns a Boolean
. The Try[T]
correspond to the Future[T]
of the protected call. This function should return true
if the result of a call should increase the failure count, or false
to not affect the count.
- Scala
-
source
val evenNumberAsFailure: Try[Int] => Boolean = { case Success(n) => n % 2 == 0 case Failure(_) => true } val breaker = CircuitBreaker("dangerous-breaker") // this call will return 8888 and increase failure count at the same time breaker.withCircuitBreaker(Future(8888), evenNumberAsFailure) - Java
Low level API
Instead of looking up a configured circuit breaker by name, it is also possible to construct it in the source code:
- Scala
-
source
import akka.actor.typed.scaladsl.adapter._ val breaker = new CircuitBreaker( context.system.scheduler.toClassic, maxFailures = 5, callTimeout = 10.seconds, resetTimeout = 1.minute).onOpen(context.self ! BreakerOpen)
- Java
This also allows for creating the circuit breaker with a specific execution context to run its callbacks on.
The low-level API allows you to describe the behavior of the CircuitBreaker
in detail, including deciding what to return to the calling Actor
in case of success or failure. This is especially useful when expecting the remote call to send a reply. CircuitBreaker
doesn’t support Tell Protection
(protecting against calls that expect a reply) natively at the moment. Thus, you need to use the low-level power-user APIs, succeed
and fail
methods, as well as isClosed
, isOpen
, isHalfOpen
to implement it.
As can be seen in the examples below, a Tell Protection
pattern could be implemented by using the succeed
and fail
methods, which would count towards the CircuitBreaker
counts. In the example, a call is made to the remote service if the breaker is closed or half open. Once a response is received, the succeed
method is invoked, which tells the CircuitBreaker
to keep the breaker closed. On the other hand, if an error or timeout is received we trigger a fail
, and the breaker accrues this failure towards its count for opening the breaker.
- Scala
-
source
object CircuitBreakingIntermediateActor { sealed trait Command case class Call(payload: String, replyTo: ActorRef[StatusReply[Done]]) extends Command private case class OtherActorReply(reply: Try[Done], originalReplyTo: ActorRef[StatusReply[Done]]) extends Command private case object BreakerOpen extends Command def apply(recipient: ActorRef[OtherActor.Command]): Behavior[Command] = Behaviors.setup { context => implicit val askTimeout: Timeout = 11.seconds import context.executionContext import akka.actor.typed.scaladsl.adapter._ val breaker = new CircuitBreaker( context.system.scheduler.toClassic, maxFailures = 5, callTimeout = 10.seconds, resetTimeout = 1.minute).onOpen(context.self ! BreakerOpen) Behaviors.receiveMessage { case Call(payload, replyTo) => if (breaker.isClosed || breaker.isHalfOpen) { context.askWithStatus(recipient, OtherActor.Call(payload, _))(OtherActorReply(_, replyTo)) } else { replyTo ! StatusReply.error("Service unavailable") } Behaviors.same case OtherActorReply(reply, originalReplyTo) => if (reply.isSuccess) breaker.succeed() else breaker.fail() originalReplyTo ! StatusReply.fromTry(reply) Behaviors.same case BreakerOpen => context.log.warn("Circuit breaker open") Behaviors.same } } }
- Java
This example always makes remote calls when the state is HalfOpen. Using the power-user APIs, it is your responsibility to judge when to make remote calls in HalfOpen.