pekko/akka-docs/src/main/paradox/scala/mailboxes.md

Mailboxes

An Akka Mailbox holds the messages that are destined for an Actor. Normally each Actor has its own mailbox, but with for example a BalancingPool all routees will share a single mailbox instance.

Mailbox Selection

Requiring a Message Queue Type for an Actor

It is possible to require a certain type of message queue for a certain type of actor by having that actor @scala[extend]@java[implement] the parameterized @scala[trait]@java[interface] RequiresMessageQueue. Here is an example:

Scala

@@snip DispatcherDocSpec.scala { #required-mailbox-class }

Java

@@snip MyBoundedActor.java { #my-bounded-untyped-actor }

The type parameter to the RequiresMessageQueue @scala[trait]@java[interface] needs to be mapped to a mailbox in configuration like this:

@@snip DispatcherDocSpec.scala { #bounded-mailbox-config #required-mailbox-config }

Now every time you create an actor of type MyBoundedActor it will try to get a bounded mailbox. If the actor has a different mailbox configured in deployment, either directly or via a dispatcher with a specified mailbox type, then that will override this mapping.

@@@ note

The type of the queue in the mailbox created for an actor will be checked against the required type in the @scala[trait]@java[interface] and if the queue doesn't implement the required type then actor creation will fail.

@@@

Requiring a Message Queue Type for a Dispatcher

A dispatcher may also have a requirement for the mailbox type used by the actors running on it. An example is the BalancingDispatcher which requires a message queue that is thread-safe for multiple concurrent consumers. Such a requirement is formulated within the dispatcher configuration section like this:

my-dispatcher {
  mailbox-requirement = org.example.MyInterface
}

The given requirement names a class or interface which will then be ensured to be a supertype of the message queue’s implementation. In case of a conflict—e.g. if the actor requires a mailbox type which does not satisfy this requirement—then actor creation will fail.

How the Mailbox Type is Selected

When an actor is created, the ActorRefProvider first determines the dispatcher which will execute it. Then the mailbox is determined as follows:

1. If the actor’s deployment configuration section contains a mailbox key then that names a configuration section describing the mailbox type to be used.
2. If the actor’s Props contains a mailbox selection—i.e. withMailbox was called on it—then that names a configuration section describing the mailbox type to be used.
3. If the dispatcher’s configuration section contains a mailbox-type key the same section will be used to configure the mailbox type.
4. If the actor requires a mailbox type as described above then the mapping for that requirement will be used to determine the mailbox type to be used; if that fails then the dispatcher’s requirement—if any—will be tried instead.
5. If the dispatcher requires a mailbox type as described above then the mapping for that requirement will be used to determine the mailbox type to be used.
6. The default mailbox akka.actor.default-mailbox will be used.

Default Mailbox

When the mailbox is not specified as described above the default mailbox is used. By default it is an unbounded mailbox, which is backed by a java.util.concurrent.ConcurrentLinkedQueue.

SingleConsumerOnlyUnboundedMailbox is an even more efficient mailbox, and it can be used as the default mailbox, but it cannot be used with a BalancingDispatcher.

Configuration of SingleConsumerOnlyUnboundedMailbox as default mailbox:

akka.actor.default-mailbox {
  mailbox-type = "akka.dispatch.SingleConsumerOnlyUnboundedMailbox"
}

Which Configuration is passed to the Mailbox Type

Each mailbox type is implemented by a class which extends MailboxType and takes two constructor arguments: a ActorSystem.Settings object and a Config section. The latter is computed by obtaining the named configuration section from the actor system’s configuration, overriding its id key with the configuration path of the mailbox type and adding a fall-back to the default mailbox configuration section.

Builtin Mailbox Implementations

Akka comes shipped with a number of mailbox implementations:

• UnboundedMailbox (default)
  • The default mailbox
  • Backed by a java.util.concurrent.ConcurrentLinkedQueue
  • Blocking: No
  • Bounded: No
  • Configuration name: "unbounded" or "akka.dispatch.UnboundedMailbox"
• SingleConsumerOnlyUnboundedMailbox This queue may or may not be faster than the default one depending on your use-case—be sure to benchmark properly!
  • Backed by a Multiple-Producer Single-Consumer queue, cannot be used with BalancingDispatcher
  • Blocking: No
  • Bounded: No
  • Configuration name: "akka.dispatch.SingleConsumerOnlyUnboundedMailbox"
• NonBlockingBoundedMailbox
  • Backed by a very efficient Multiple-Producer Single-Consumer queue
  • Blocking: No (discards overflowing messages into deadLetters)
  • Bounded: Yes
  • Configuration name: "akka.dispatch.NonBlockingBoundedMailbox"
• UnboundedControlAwareMailbox
  • Delivers messages that extend akka.dispatch.ControlMessage with higher priority
  • Backed by two java.util.concurrent.ConcurrentLinkedQueue
  • Blocking: No
  • Bounded: No
  • Configuration name: "akka.dispatch.UnboundedControlAwareMailbox"
• UnboundedPriorityMailbox
  • Backed by a java.util.concurrent.PriorityBlockingQueue
  • Delivery order for messages of equal priority is undefined - contrast with the UnboundedStablePriorityMailbox
  • Blocking: No
  • Bounded: No
  • Configuration name: "akka.dispatch.UnboundedPriorityMailbox"
• UnboundedStablePriorityMailbox
  • Backed by a java.util.concurrent.PriorityBlockingQueue wrapped in an akka.util.PriorityQueueStabilizer
  • FIFO order is preserved for messages of equal priority - contrast with the UnboundedPriorityMailbox
  • Blocking: No
  • Bounded: No
  • Configuration name: "akka.dispatch.UnboundedStablePriorityMailbox"

Other bounded mailbox implementations which will block the sender if the capacity is reached and configured with non-zero mailbox-push-timeout-time.

@@@ note

The following mailboxes should only be used with zero mailbox-push-timeout-time.

@@@

• BoundedMailbox
  • Backed by a java.util.concurrent.LinkedBlockingQueue
  • Blocking: Yes if used with non-zero mailbox-push-timeout-time, otherwise No
  • Bounded: Yes
  • Configuration name: "bounded" or "akka.dispatch.BoundedMailbox"
• BoundedPriorityMailbox
  • Backed by a java.util.PriorityQueue wrapped in an akka.util.BoundedBlockingQueue
  • Delivery order for messages of equal priority is undefined - contrast with the BoundedStablePriorityMailbox
  • Blocking: Yes if used with non-zero mailbox-push-timeout-time, otherwise No
  • Bounded: Yes
  • Configuration name: "akka.dispatch.BoundedPriorityMailbox"
• BoundedStablePriorityMailbox
  • Backed by a java.util.PriorityQueue wrapped in an akka.util.PriorityQueueStabilizer and an akka.util.BoundedBlockingQueue
  • FIFO order is preserved for messages of equal priority - contrast with the BoundedPriorityMailbox
  • Blocking: Yes if used with non-zero mailbox-push-timeout-time, otherwise No
  • Bounded: Yes
  • Configuration name: "akka.dispatch.BoundedStablePriorityMailbox"
• BoundedControlAwareMailbox
  • Delivers messages that extend akka.dispatch.ControlMessage with higher priority
  • Backed by two java.util.concurrent.ConcurrentLinkedQueue and blocking on enqueue if capacity has been reached
  • Blocking: Yes if used with non-zero mailbox-push-timeout-time, otherwise No
  • Bounded: Yes
  • Configuration name: "akka.dispatch.BoundedControlAwareMailbox"

Mailbox configuration examples

PriorityMailbox

How to create a PriorityMailbox:

Scala

@@snip DispatcherDocSpec.scala { #prio-mailbox }

Java

@@snip DispatcherDocTest.java { #prio-mailbox }

And then add it to the configuration:

@@snip DispatcherDocSpec.scala { #prio-dispatcher-config }

And then an example on how you would use it:

Scala

@@snip DispatcherDocSpec.scala { #prio-dispatcher }

Java

@@snip DispatcherDocTest.java { #prio-dispatcher }

It is also possible to configure a mailbox type directly like this:

Scala

@@snip DispatcherDocSpec.scala { #prio-mailbox-config #mailbox-deployment-config }

Java

@@snip DispatcherDocSpec.scala { #prio-mailbox-config-java #mailbox-deployment-config }

And then use it either from deployment like this:

Scala

@@snip DispatcherDocSpec.scala { #defining-mailbox-in-config }

Java

@@snip DispatcherDocTest.java { #defining-mailbox-in-config }

Or code like this:

Scala

@@snip DispatcherDocSpec.scala { #defining-mailbox-in-code }

Java

@@snip DispatcherDocTest.java { #defining-mailbox-in-code }

ControlAwareMailbox

A ControlAwareMailbox can be very useful if an actor needs to be able to receive control messages immediately no matter how many other messages are already in its mailbox.

It can be configured like this:

@@snip DispatcherDocSpec.scala { #control-aware-mailbox-config }

Control messages need to extend the ControlMessage trait:

Scala

@@snip DispatcherDocSpec.scala { #control-aware-mailbox-messages }

Java

@@snip DispatcherDocTest.java { #control-aware-mailbox-messages }

And then an example on how you would use it:

Scala

@@snip DispatcherDocSpec.scala { #control-aware-dispatcher }

Java

@@snip DispatcherDocTest.java { #control-aware-dispatcher }

Creating your own Mailbox type

An example is worth a thousand quacks:

Scala

@@snip MyUnboundedMailbox.scala { #mailbox-marker-interface }

Java

@@snip MyUnboundedMessageQueueSemantics.java { #mailbox-marker-interface }

Scala

@@snip MyUnboundedMailbox.scala { #mailbox-implementation-example }

Java

@@snip MyUnboundedMailbox.java { #mailbox-implementation-example }

And then you just specify the FQCN of your MailboxType as the value of the "mailbox-type" in the dispatcher configuration, or the mailbox configuration.

@@@ note

Make sure to include a constructor which takes akka.actor.ActorSystem.Settings and com.typesafe.config.Config arguments, as this constructor is invoked reflectively to construct your mailbox type. The config passed in as second argument is that section from the configuration which describes the dispatcher or mailbox setting using this mailbox type; the mailbox type will be instantiated once for each dispatcher or mailbox setting using it.

@@@

You can also use the mailbox as a requirement on the dispatcher like this:

@@snip DispatcherDocSpec.scala { #custom-mailbox-config-java }

Or by defining the requirement on your actor class like this:

Scala

@@snip DispatcherDocSpec.scala { #require-mailbox-on-actor }

Java

@@snip DispatcherDocTest.java { #require-mailbox-on-actor }

Special Semantics of system.actorOf

In order to make system.actorOf both synchronous and non-blocking while keeping the return type ActorRef (and the semantics that the returned ref is fully functional), special handling takes place for this case. Behind the scenes, a hollow kind of actor reference is constructed, which is sent to the system’s guardian actor who actually creates the actor and its context and puts those inside the reference. Until that has happened, messages sent to the ActorRef will be queued locally, and only upon swapping the real filling in will they be transferred into the real mailbox. Thus,

Scala

@@@vars

val props: Props = ...
// this actor uses MyCustomMailbox, which is assumed to be a singleton
system.actorOf(props.withDispatcher("myCustomMailbox")) ! "bang"
assert(MyCustomMailbox.instance.getLastEnqueuedMessage == "bang")

@@@

Java

@@@vars

final Props props = ...
// this actor uses MyCustomMailbox, which is assumed to be a singleton
system.actorOf(props.withDispatcher("myCustomMailbox").tell("bang", sender);
assert(MyCustomMailbox.getInstance().getLastEnqueued().equals("bang"));

@@@

will probably fail; you will have to allow for some time to pass and retry the check à la TestKit.awaitCond.