This object contains implementation details of the “ask” pattern.
This is what is used to complete a Future that is returned from an ask/? call, when it times out.
This actor can be used to supervise a child actor and start it again after a back-off duration if the child actor is stopped.
This actor can be used to supervise a child actor and start it again after a back-off duration if the child actor is stopped.
This is useful in situations where the re-start of the child actor should be delayed e.g. in order to give an external resource time to recover before the child actor tries contacting it again (after being restarted).
Specifically this pattern is useful for for persistent actors, which are stopped in case of persistence failures. Just restarting them immediately would probably fail again (since the data store is probably unavailable). It is better to try again after a delay.
It supports exponential back-off between the given minBackoff
and
maxBackoff
durations. For example, if minBackoff
is 3 seconds and
maxBackoff
30 seconds the start attempts will be delayed with
3, 6, 12, 24, 30, 30 seconds. The exponential back-off counter is reset
if the actor is not terminated within the minBackoff
duration.
In addition to the calculated exponential back-off an additional
random delay based the given randomFactor
is added, e.g. 0.2 adds up to 20%
delay. The reason for adding a random delay is to avoid that all failing
actors hit the backend resource at the same time.
You can retrieve the current child ActorRef
by sending BackoffSupervisor.GetCurrentChild
message to this actor and it will reply with akka.pattern.BackoffSupervisor.CurrentChild
containing the ActorRef
of the current child, if any.
The BackoffSupervisor
forwards all other messages to the child, if it is currently running.
The child can stop itself and send a akka.actor.PoisonPill to the parent supervisor if it wants to do an intentional stop.
Provides circuit breaker functionality to provide stability when working with "dangerous" operations, e.g.
Provides circuit breaker functionality to provide stability when working with "dangerous" operations, e.g. calls to remote systems
Transitions through three states:
- In *Closed* state, calls pass through until the maxFailures
count is reached. This causes the circuit breaker
to open. Both exceptions and calls exceeding callTimeout
are considered failures.
- In *Open* state, calls fail-fast with an exception. After resetTimeout
, circuit breaker transitions to
half-open state.
- In *Half-Open* state, the first call will be allowed through, if it succeeds the circuit breaker will reset to
closed state. If it fails, the circuit breaker will re-open to open state. All calls beyond the first that
execute while the first is running will fail-fast with an exception.
Exception thrown when Circuit Breaker is open.
Companion object providing factory methods for Circuit Breaker which runs callbacks in caller's thread
Returns a scala.concurrent.Future that will be completed with the success or failure of the provided value after the specified duration.
Returns a scala.concurrent.Future that will be completed with the success or failure of the provided value after the specified duration.
Sends a message asynchronously and returns a scala.concurrent.Future
holding the eventual reply message; this means that the target actor
needs to send the result to the sender
reference provided.
Sends a message asynchronously and returns a scala.concurrent.Future
holding the eventual reply message; this means that the target actor
needs to send the result to the sender
reference provided. The Future
will be completed with an akka.pattern.AskTimeoutException after the
given timeout has expired; this is independent from any timeout applied
while awaiting a result for this future (i.e. in
Await.result(..., timeout)
).
Warning: When using future callbacks, inside actors you need to carefully avoid closing over the containing actor’s object, i.e. do not call methods or access mutable state on the enclosing actor from within the callback. This would break the actor encapsulation and may introduce synchronization bugs and race conditions because the callback will be scheduled concurrently to the enclosing actor. Unfortunately there is not yet a way to detect these illegal accesses at compile time.
Recommended usage:
val f = ask(worker, request)(timeout) f.map { response => EnrichedMessage(response) } pipeTo nextActor
Import this implicit conversion to gain ?
and ask
methods on
akka.actor.ActorSelection, which will defer to the
ask(actorSelection, message)(timeout)
method defined here.
Import this implicit conversion to gain ?
and ask
methods on
akka.actor.ActorSelection, which will defer to the
ask(actorSelection, message)(timeout)
method defined here.
import akka.pattern.ask val future = selection ? message // => ask(selection, message) val future = selection ask message // => ask(selection, message) val future = selection.ask(message)(timeout) // => ask(selection, message)(timeout)
All of the above use an implicit akka.util.Timeout.
Sends a message asynchronously and returns a scala.concurrent.Future
holding the eventual reply message; this means that the target actor
needs to send the result to the sender
reference provided.
Sends a message asynchronously and returns a scala.concurrent.Future
holding the eventual reply message; this means that the target actor
needs to send the result to the sender
reference provided. The Future
will be completed with an akka.pattern.AskTimeoutException after the
given timeout has expired; this is independent from any timeout applied
while awaiting a result for this future (i.e. in
Await.result(..., timeout)
).
Warning: When using future callbacks, inside actors you need to carefully avoid closing over the containing actor’s object, i.e. do not call methods or access mutable state on the enclosing actor from within the callback. This would break the actor encapsulation and may introduce synchronization bugs and race conditions because the callback will be scheduled concurrently to the enclosing actor. Unfortunately there is not yet a way to detect these illegal accesses at compile time.
Recommended usage:
val f = ask(worker, request)(timeout) f.map { response => EnrichedMessage(response) } pipeTo nextActor
Import this implicit conversion to gain ?
and ask
methods on
akka.actor.ActorRef, which will defer to the
ask(actorRef, message)(timeout)
method defined here.
Import this implicit conversion to gain ?
and ask
methods on
akka.actor.ActorRef, which will defer to the
ask(actorRef, message)(timeout)
method defined here.
import akka.pattern.ask val future = actor ? message // => ask(actor, message) val future = actor ask message // => ask(actor, message) val future = actor.ask(message)(timeout) // => ask(actor, message)(timeout)
All of the above use an implicit akka.util.Timeout.
Returns a scala.concurrent.Future that will be completed with success (value true
) when
existing messages of the target actor has been processed and the actor has been
terminated.
Returns a scala.concurrent.Future that will be completed with success (value true
) when
existing messages of the target actor has been processed and the actor has been
terminated.
Useful when you need to wait for termination or compose ordered termination of several actors, which should only be done outside of the ActorSystem as blocking inside Actors is discouraged.
IMPORTANT NOTICE: the actor being terminated and its supervisor being informed of the availability of the deceased actor’s name are two distinct operations, which do not obey any reliable ordering. Especially the following will NOT work:
def receive = { case msg => Await.result(gracefulStop(someChild, timeout), timeout) context.actorOf(Props(...), "someChild") // assuming that that was someChild’s name, this will NOT work }
If the target actor isn't terminated within the timeout the scala.concurrent.Future is completed with failure akka.pattern.AskTimeoutException.
If you want to invoke specialized stopping logic on your target actor instead of PoisonPill, you can pass your stop command as a parameter:
gracefulStop(someChild, timeout, MyStopGracefullyMessage).onComplete {
// Do something after someChild being stopped
}
Import this implicit conversion to gain the pipeTo
method on scala.concurrent.Future:
Import this implicit conversion to gain the pipeTo
method on scala.concurrent.Future:
import akka.pattern.pipe
Future { doExpensiveCalc() } pipeTo nextActor
or
pipe(someFuture) to nextActor
The successful result of the future is sent as a message to the recipient, or the failure is sent in a akka.actor.Status.Failure to the recipient.
Commonly Used Patterns With Akka
This package is used as a collection point for usage patterns which involve actors, futures, etc. but are loosely enough coupled to (multiple of) them to present them separately from the core implementation. Currently supported are:
In Scala the recommended usage is to import the pattern from the package object:
For Java the patterns are available as static methods of the akka.pattern.Patterns class:
import static akka.pattern.Patterns.ask; ask(actor, message);