To use Actors, you must add the following dependency in your project:
@@dependency[sbt,Maven,Gradle] { group="com.typesafe.akka" artifact="akka-actor_$scala.binary_version$" version="$akka.version$" }
The Actor Model provides a higher level of abstraction for writing concurrent and distributed systems. It alleviates the developer from having to deal with explicit locking and thread management, making it easier to write correct concurrent and parallel systems. Actors were defined in the 1973 paper by Carl Hewitt but have been popularized by the Erlang language, and used for example at Ericsson with great success to build highly concurrent and reliable telecom systems.
The API of Akka’s Actors is similar to Scala Actors which has borrowed some of its syntax from Erlang.
@@@ note
Since Akka enforces parental supervision every actor is supervised and (potentially) the supervisor of its children, it is advisable that you familiarize yourself with @ref:Actor Systems and @ref:supervision and it may also help to read @ref:Actor References, Paths and Addresses.
@@@
@@@ div { .group-scala }
Actors are implemented by extending the Actor
base trait and implementing the
receive
method. The receive
method should define a series of case
statements (which has the type PartialFunction[Any, Unit]
) that defines
which messages your Actor can handle, using standard Scala pattern matching,
along with the implementation of how the messages should be processed.
@@@
@@@ div { .group-java }
Actor classes are implemented by extending the AbstractActor
class and setting
the “initial behavior” in createReceive
method.
createReceive
method has no arguments and returns AbstractActor.Receive
. It
defines which messages your Actor can handle, along with the implementation of how
the messages should be processed. You can build such behavior with a builder named
ReceiveBuilder
. This build has convenient factory in AbstractActor
called receiveBuilder
.
@@@
Here is an example:
Scala : @@snip ActorDocSpec.scala { #imports1 #my-actor }
Java : @@snip MyActor.java { #imports #my-actor }
Please note that the Akka Actor @scala[receive
] message loop is exhaustive, which
is different compared to Erlang and the late Scala Actors. This means that you
need to provide a pattern match for all messages that it can accept and if you
want to be able to handle unknown messages then you need to have a default case
as in the example above. Otherwise an akka.actor.UnhandledMessage(message, sender, recipient)
will be published to the ActorSystem
's
EventStream
.
Note further that the return type of the behavior defined above is Unit
; if
the actor shall reply to the received message then this must be done explicitly
as explained below.
The result of the @scala[receive
method is a partial function object, which is]
@java[createReceive
method is AbstractActor.Receive
which is a wrapper around partial
scala function object. It is] stored within the actor as its “initial behavior”,
see Become/Unbecome for
further information on changing the behavior of an actor after its
construction.
@@@ div { .group-scala }
@@fiddle ActorDocSpec.scala { #fiddle_code height=400px extraParams=theme=light&layout=v75&passive cssStyle=width:100%; }
@@@
Props
is a configuration class to specify options for the creation
of actors, think of it as an immutable and thus freely shareable recipe for
creating an actor including associated deployment information (e.g. which
dispatcher to use, see more below). Here are some examples of how to create a
Props
instance.
Scala : @@snip ActorDocSpec.scala { #creating-props }
Java : @@snip ActorDocTest.java { #import-props #creating-props }
The second variant shows how to pass constructor arguments to the
Actor
being created, but it should only be used outside of actors as
explained below.
The last line shows a possibility to pass constructor arguments regardless of
the context it is being used in. The presence of a matching constructor is
verified during construction of the Props
object, resulting in an
IllegalArgumentException
if no or multiple matching constructors are
found.
@@@ note { .group-scala }
The recommended approach to create the actor Props
is not supported
for cases when the actor constructor takes value classes as arguments.
@@@
Scala : @@snip ActorDocSpec.scala { #creating-props-deprecated }
Java : @@snip ActorDocTest.java { #creating-props-deprecated }
This method is not recommended to be used within another actor because it
encourages to close over the enclosing scope, resulting in non-serializable
Props
and possibly race conditions (breaking the actor encapsulation).
On the other hand using this variant in a Props
factory in
the actor’s companion object as documented under “Recommended Practices” below
is completely fine.
There were two use-cases for these methods: passing constructor arguments to
the actor—which is solved by the newly introduced
@scala[Props.apply(clazz, args)
] @java[Props.create(clazz, args)
] method above or the recommended practice
below—and creating actors “on the spot” as anonymous classes. The latter should
be solved by making these actors named classes instead (if they are not
declared within a top-level object
then the enclosing instance’s this
reference needs to be passed as the first argument).
@@@ warning
Declaring one actor within another is very dangerous and breaks actor
encapsulation. Never pass an actor’s this
reference into Props
!
@@@
@@@ div { .group-scala }
There are two edge cases in actor creation with Props
:
- An actor with
AnyVal
arguments.
@@snip PropsEdgeCaseSpec.scala { #props-edge-cases-value-class }
@@snip PropsEdgeCaseSpec.scala { #props-edge-cases-value-class-example }
- An actor with default constructor values.
@@snip PropsEdgeCaseSpec.scala { #props-edge-cases-default-values }
In both cases an IllegalArgumentException
will be thrown stating
no matching constructor could be found.
The next section explains the recommended ways to create Actor
props in a way,
which simultaneously safe-guards against these edge cases.
@@@
It is a good idea to provide @scala[factory methods on the companion object of each
Actor
] @java[static factory methods for each Actor
] which help keeping the creation of
suitable Props
as close to the actor definition as possible. This also avoids the pitfalls
associated with using the @scala[Props.apply(...)
method which takes a by-name
argument, since within a companion object] @java[ Props.create(...)
method which takes
arguments as constructor parameters, since within static method]
the given code block will not retain a reference to its enclosing scope:
Scala : @@snip ActorDocSpec.scala { #props-factory }
Java : @@snip ActorDocTest.java { #props-factory }
Another good practice is to declare what messages an Actor can receive @scala[in the companion object of the Actor] @java[as close to the actor definition as possible (e.g. as static classes inside the Actor or using other suitable class)], which makes easier to know what it can receive:
Scala : @@snip ActorDocSpec.scala { #messages-in-companion }
Java : @@snip ActorDocTest.java { #messages-in-companion }
Actors are created by passing a Props
instance into the
actorOf
factory method which is available on ActorSystem
and
ActorContext
.
Scala : @@snip ActorDocSpec.scala { #system-actorOf }
Java : @@snip ActorDocTest.java { #import-actorRef }
Using the ActorSystem
will create top-level actors, supervised by the
actor system’s provided guardian actor, while using an actor’s context will
create a child actor.
Scala : @@snip ActorDocSpec.scala { #context-actorOf }
Java : @@snip ActorDocTest.java { #context-actorOf }
It is recommended to create a hierarchy of children, grand-children and so on such that it fits the logical failure-handling structure of the application, see @ref:Actor Systems.
The call to actorOf
returns an instance of ActorRef
. This is a
handle to the actor instance and the only way to interact with it. The
ActorRef
is immutable and has a one to one relationship with the Actor
it represents. The ActorRef
is also serializable and network-aware.
This means that you can serialize it, send it over the wire and use it on a
remote host and it will still be representing the same Actor on the original
node, across the network.
The name parameter is optional, but you should preferably name your actors,
since that is used in log messages and for identifying actors. The name must
not be empty or start with $
, but it may contain URL encoded characters
(eg. %20
for a blank space). If the given name is already in use by
another child to the same parent an InvalidActorNameException
is thrown.
Actors are automatically started asynchronously when created.
@@@ div { .group-scala }
The recommended way to instantiate actor props uses reflection at runtime to determine the correct actor constructor to be invoked and due to technical limitations is not supported when said constructor takes arguments that are value classes. In these cases you should either unpack the arguments or create the props by calling the constructor manually:
@@snip ActorDocSpec.scala { #actor-with-value-class-argument }
@@@
If your Actor
has a constructor that takes parameters then those need to
be part of the Props
as well, as described above. But there
are cases when a factory method must be used, for example when the actual
constructor arguments are determined by a dependency injection framework.
Scala : @@snip ActorDocSpec.scala { #creating-indirectly }
Java : @@snip DependencyInjectionDocTest.java { #import #creating-indirectly }
@@@ warning
You might be tempted at times to offer an IndirectActorProducer
which always returns the same instance, e.g. by using a @scala[lazy val
.] @java[static field.] This is
not supported, as it goes against the meaning of an actor restart, which is
described here: @ref:What Restarting Means.
When using a dependency injection framework, actor beans MUST NOT have singleton scope.
@@@
Techniques for dependency injection and integration with dependency injection frameworks are described in more depth in the Using Akka with Dependency Injection guideline and the Akka Java Spring tutorial.
When writing code outside of actors which shall communicate with actors, the
ask
pattern can be a solution (see below), but there are two things it
cannot do: receiving multiple replies (e.g. by subscribing an ActorRef
to a notification service) and watching other actors’ lifecycle. For these
purposes there is the Inbox
class:
Scala : @@snip ActorDSLSpec.scala { #inbox }
Java : @@snip InboxDocTest.java { #inbox }
@@@ div { .group-scala }
There is an implicit conversion from inbox to actor reference which means that in this example the sender reference will be that of the actor hidden away within the inbox. This allows the reply to be received on the last line. Watching an actor is quite simple as well:
@@snip ActorDSLSpec.scala { #watch }
@@@
@@@ div { .group-java }
The send
method wraps a normal tell
and supplies the internal
actor’s reference as the sender. This allows the reply to be received on the
last line. Watching an actor is quite simple as well:
@@snip InboxDocTest.java { #watch }
@@@
@scala[The Actor
trait defines only one abstract method, the above mentioned
receive
, which implements the behavior of the actor.]
@java[The AbstractActor
class defines a method called createReceive
,
that is used to set the “initial behavior” of the actor.]
If the current actor behavior does not match a received message,
unhandled
is called, which by default publishes an
akka.actor.UnhandledMessage(message, sender, recipient)
on the actor
system’s event stream (set configuration item
akka.actor.debug.unhandled
to on
to have them converted into
actual Debug messages).
In addition, it offers:
-
@scala[
self
] @java[getSelf()
] reference to theActorRef
of the actor -
@scala[
sender
] @java[getSender()
] reference sender Actor of the last received message, typically used as described in @scala[Actor.Reply] @java[LambdaActor.Reply] -
@scala[
supervisorStrategy
] @java[supervisorStrategy()
] user overridable definition the strategy to use for supervising child actorsThis strategy is typically declared inside the actor in order to have access to the actor’s internal state within the decider function: since failure is communicated as a message sent to the supervisor and processed like other messages (albeit outside of the normal behavior), all values and variables within the actor are available, as is the
sender
reference (which will be the immediate child reporting the failure; if the original failure occurred within a distant descendant it is still reported one level up at a time). -
@scala[
context
] @java[getContext()
] exposes contextual information for the actor and the current message, such as:- factory methods to create child actors (
actorOf
) - system that the actor belongs to
- parent supervisor
- supervised children
- lifecycle monitoring
- hotswap behavior stack as described in @scala[Actor.HotSwap] @java[Become/Unbecome]
- factory methods to create child actors (
@@@ div { .group-scala }
You can import the members in the context
to avoid prefixing access with context.
@@snip ActorDocSpec.scala { #import-context }
@@@
The remaining visible methods are user-overridable life-cycle hooks which are described in the following:
Scala : @@snip Actor.scala { #lifecycle-hooks }
Java : @@snip ActorDocTest.java { #lifecycle-callbacks }
The implementations shown above are the defaults provided by the @scala[Actor
trait.] @java[AbstractActor
class.]
A path in an actor system represents a "place" which might be occupied
by a living actor. Initially (apart from system initialized actors) a path is
empty. When actorOf()
is called it assigns an incarnation of the actor
described by the passed Props
to the given path. An actor incarnation is
identified by the path and a UID.
It is worth noting about the difference between:
- restart
- stop, followed by re-creation of actor
as explained below.
A restart only swaps the Actor
instance defined by the Props
but the incarnation and hence the UID remains
the same.
As long as the incarnation is same, you can keep using the same ActorRef
.
Restart is handled by the @ref:Supervision Strategy of actor's parent actor,
and there is more discussion about @ref:what restart means.
The lifecycle of an incarnation ends when the actor is stopped. At
that point the appropriate lifecycle events are called and watching actors
are notified of the termination. After the incarnation is stopped, the path can
be reused again by creating an actor with actorOf()
. In this case the
name of the new incarnation will be the same as the previous one but the
UIDs will differ. An actor can be stopped by the actor itself, another actor
or the ActorSystem
(see Stopping actors).
@@@ note
It is important to note that Actors do not stop automatically when no longer referenced, every Actor that is created must also explicitly be destroyed. The only simplification is that stopping a parent Actor will also recursively stop all the child Actors that this parent has created.
@@@
An ActorRef
always represents an incarnation (path and UID) not just a
given path. Therefore if an actor is stopped and a new one with the same
name is created an ActorRef
of the old incarnation will not point
to the new one.
ActorSelection
on the other hand points to the path (or multiple paths
if wildcards are used) and is completely oblivious to which incarnation is currently
occupying it. ActorSelection
cannot be watched for this reason. It is
possible to resolve the current incarnation's ActorRef
living under the
path by sending an Identify
message to the ActorSelection
which
will be replied to with an ActorIdentity
containing the correct reference
(see ActorSelection). This can also be done with the resolveOne
method of the ActorSelection
, which returns a Future
of the matching
ActorRef
.
In order to be notified when another actor terminates (i.e. stops permanently,
not temporary failure and restart), an actor may register itself for reception
of the Terminated
message dispatched by the other actor upon
termination (see Stopping Actors). This service is provided by the
DeathWatch
component of the actor system.
Registering a monitor is easy:
Scala : @@snip ActorDocSpec.scala { #watch }
Java : @@snip ActorDocTest.java { #import-terminated #watch }
It should be noted that the Terminated
message is generated
independent of the order in which registration and termination occur.
In particular, the watching actor will receive a Terminated
message even if the
watched actor has already been terminated at the time of registration.
Registering multiple times does not necessarily lead to multiple messages being
generated, but there is no guarantee that only exactly one such message is
received: if termination of the watched actor has generated and queued the
message, and another registration is done before this message has been
processed, then a second message will be queued, because registering for
monitoring of an already terminated actor leads to the immediate generation of
the Terminated
message.
It is also possible to deregister from watching another actor’s liveliness
using context.unwatch(target)
. This works even if the Terminated
message has already been enqueued in the mailbox; after calling unwatch
no Terminated
message for that actor will be processed anymore.
Right after starting the actor, its preStart
method is invoked.
Scala : @@snip ActorDocSpec.scala { #preStart }
Java : @@snip ActorDocTest.java { #preStart }
This method is called when the actor is first created. During restarts it is
called by the default implementation of postRestart
, which means that
by overriding that method you can choose whether the initialization code in
this method is called only exactly once for this actor or for every restart.
Initialization code which is part of the actor’s constructor will always be
called when an instance of the actor class is created, which happens at every
restart.
All actors are supervised, i.e. linked to another actor with a fault handling strategy. Actors may be restarted in case an exception is thrown while processing a message (see @ref:supervision). This restart involves the hooks mentioned above:
- The old actor is informed by calling
preRestart
with the exception which caused the restart and the message which triggered that exception; the latter may beNone
if the restart was not caused by processing a message, e.g. when a supervisor does not trap the exception and is restarted in turn by its supervisor, or if an actor is restarted due to a sibling’s failure. If the message is available, then that message’s sender is also accessible in the usual way (i.e. by callingsender
). This method is the best place for cleaning up, preparing hand-over to the fresh actor instance, etc. By default it stops all children and callspostStop
. - The initial factory from the
actorOf
call is used to produce the fresh instance. - The new actor’s
postRestart
method is invoked with the exception which caused the restart. By default thepreStart
is called, just as in the normal start-up case.
An actor restart replaces only the actual actor object; the contents of the
mailbox is unaffected by the restart, so processing of messages will resume
after the postRestart
hook returns. The message
that triggered the exception will not be received again. Any message
sent to an actor while it is being restarted will be queued to its mailbox as
usual.
@@@ warning
Be aware that the ordering of failure notifications relative to user messages is not deterministic. In particular, a parent might restart its child before it has processed the last messages sent by the child before the failure. See @ref:Discussion: Message Ordering for details.
@@@
After stopping an actor, its postStop
hook is called, which may be used
e.g. for deregistering this actor from other services. This hook is guaranteed
to run after message queuing has been disabled for this actor, i.e. messages
sent to a stopped actor will be redirected to the deadLetters
of the
ActorSystem
.
As described in @ref:Actor References, Paths and Addresses, each actor has a unique logical path, which
is obtained by following the chain of actors from child to parent until
reaching the root of the actor system, and it has a physical path, which may
differ if the supervision chain includes any remote supervisors. These paths
are used by the system to look up actors, e.g. when a remote message is
received and the recipient is searched, but they are also useful more directly:
actors may look up other actors by specifying absolute or relative
paths—logical or physical—and receive back an ActorSelection
with the
result:
Scala : @@snip ActorDocSpec.scala { #selection-local }
Java : @@snip ActorDocTest.java { #selection-local }
@@@ note
It is always preferable to communicate with other Actors using their ActorRef instead of relying upon ActorSelection. Exceptions are
- sending messages using the @ref:At-Least-Once Delivery facility
- initiating first contact with a remote system
In all other cases ActorRefs can be provided during Actor creation or initialization, passing them from parent to child or introducing Actors by sending their ActorRefs to other Actors within messages.
@@@
The supplied path is parsed as a java.net.URI
, which means
that it is split on /
into path elements. If the path starts with /
, it
is absolute and the look-up starts at the root guardian (which is the parent of
"/user"
); otherwise it starts at the current actor. If a path element equals
..
, the look-up will take a step “up” towards the supervisor of the
currently traversed actor, otherwise it will step “down” to the named child.
It should be noted that the ..
in actor paths here always means the logical
structure, i.e. the supervisor.
The path elements of an actor selection may contain wildcard patterns allowing for broadcasting of messages to that section:
Scala : @@snip ActorDocSpec.scala { #selection-wildcard }
Java : @@snip ActorDocTest.java { #selection-wildcard }
Messages can be sent via the ActorSelection
and the path of the
ActorSelection
is looked up when delivering each message. If the selection
does not match any actors the message will be dropped.
To acquire an ActorRef
for an ActorSelection
you need to send
a message to the selection and use the @scala[sender()
] @java[getSender()
] reference of the reply from
the actor. There is a built-in Identify
message that all Actors will
understand and automatically reply to with a ActorIdentity
message
containing the ActorRef
. This message is handled specially by the
actors which are traversed in the sense that if a concrete name lookup fails
(i.e. a non-wildcard path element does not correspond to a live actor) then a
negative result is generated. Please note that this does not mean that delivery
of that reply is guaranteed, it still is a normal message.
Scala : @@snip ActorDocSpec.scala { #identify }
Java : @@snip ActorDocTest.java { #import-identify #identify }
You can also acquire an ActorRef
for an ActorSelection
with
the resolveOne
method of the ActorSelection
. It returns a Future
of the matching ActorRef
if such an actor exists. @java[(see also
@ref:Java 8 Compatibility for Java compatibility).] It is completed with
failure akka.actor.ActorNotFound
if no such actor exists or the identification
didn't complete within the supplied timeout
.
Remote actor addresses may also be looked up, if @ref:remoting is enabled:
Scala : @@snip ActorDocSpec.scala { #selection-remote }
Java : @@snip ActorDocTest.java { #selection-remote }
An example demonstrating actor look-up is given in @ref:Remoting Sample.
@@@ warning { title=IMPORTANT }
Messages can be any kind of object but have to be immutable. @scala[Scala] @java[Akka] can’t enforce immutability (yet) so this has to be by convention. @scala[Primitives like String, Int, Boolean are always immutable. Apart from these the recommended approach is to use Scala case classes which are immutable (if you don’t explicitly expose the state) and works great with pattern matching at the receiver side.]
@@@
Here is an @scala[example:] @java[example of an immutable message:]
Scala : @@snip ActorDocSpec.scala { #immutable-message-definition #immutable-message-instantiation }
Java : @@snip ImmutableMessage.java { #immutable-message }
Messages are sent to an Actor through one of the following methods.
- @scala[
!
] @java[tell
] means “fire-and-forget”, e.g. send a message asynchronously and return immediately. @scala[Also known astell
.] - @scala[
?
] @java[ask
] sends a message asynchronously and returns aFuture
representing a possible reply. @scala[Also known asask
.]
Message ordering is guaranteed on a per-sender basis.
@@@ note
There are performance implications of using ask
since something needs to
keep track of when it times out, there needs to be something that bridges
a Promise
into an ActorRef
and it also needs to be reachable through
remoting. So always prefer tell
for performance, and only ask
if you must.
@@@
@@@ div { .group-java }
In all these methods you have the option of passing along your own ActorRef
.
Make it a practice of doing so because it will allow the receiver actors to be able to respond
to your message, since the sender reference is sent along with the message.
@@@
This is the preferred way of sending messages. No blocking waiting for a message. This gives the best concurrency and scalability characteristics.
Scala : @@snip ActorDocSpec.scala { #tell }
Java : @@snip ActorDocTest.java { #tell }
@@@ div { .group-scala }
If invoked from within an Actor, then the sending actor reference will be
implicitly passed along with the message and available to the receiving Actor
in its sender(): ActorRef
member method. The target actor can use this
to reply to the original sender, by using sender() ! replyMsg
.
If invoked from an instance that is not an Actor the sender will be
deadLetters
actor reference by default.
@@@
@@@ div { .group-java }
The sender reference is passed along with the message and available within the
receiving actor via its getSender()
method while processing this
message. Inside of an actor it is usually getSelf()
who shall be the
sender, but there can be cases where replies shall be routed to some other
actor—e.g. the parent—in which the second argument to tell
would be a
different one. Outside of an actor and if no reply is needed the second
argument can be null
; if a reply is needed outside of an actor you can use
the ask-pattern described next..
@@@
The ask
pattern involves actors as well as futures, hence it is offered as
a use pattern rather than a method on ActorRef
:
Scala : @@snip ActorDocSpec.scala { #ask-pipeTo }
Java : @@snip ActorDocTest.java { #import-ask #ask-pipe }
This example demonstrates ask
together with the pipeTo
pattern on
futures, because this is likely to be a common combination. Please note that
all of the above is completely non-blocking and asynchronous: ask
produces
a Future
, @scala[three] @java[two] of which are composed into a new future using the
@scala[for-comprehension and then pipeTo
installs an onComplete
-handler on the future to affect]
@java[Futures.sequence
and map
methods and then pipe
installs an onComplete
-handler on the future to effect]
the submission of the aggregated Result
to another actor.
Using ask
will send a message to the receiving Actor as with tell
, and
the receiving actor must reply with @scala[sender() ! reply
] @java[getSender().tell(reply, getSelf())
] in order to
complete the returned Future
with a value. The ask
operation involves creating
an internal actor for handling this reply, which needs to have a timeout after
which it is destroyed in order not to leak resources; see more below.
@@@ note { .group-java }
A variant of the ask
pattern that returns a CompletionStage
instead of a Scala Future
is available in the akka.pattern.PatternsCS
object.
@@@
@@@ warning
To complete the future with an exception you need to send an akka.actor.Status.Failure
message to the sender.
This is not done automatically when an actor throws an exception while processing a message.
Please note that Scala's Try
sub types scala.util.Failure
and scala.util.Success
are not treated
specially, and would complete the ask Future with the given value - only the akka.actor.Status
messages
are treated specially by the ask pattern.
@@@
Scala : @@snip ActorDocSpec.scala { #reply-exception }
Java : @@snip ActorDocTest.java { #reply-exception }
If the actor does not complete the future, it will expire after the timeout period,
@scala[completing it with an AskTimeoutException
. The timeout is taken from one of the following locations in order of precedence:]
@java[specified as parameter to the ask
method; this will complete the Future
with an AskTimeoutException
.]
@@@ div { .group-scala }
-
explicitly given timeout as in:
@@snip ActorDocSpec.scala { #using-explicit-timeout }
-
implicit argument of type
akka.util.Timeout
, e.g.@@snip ActorDocSpec.scala { #using-implicit-timeout }
@@@
See @ref:Futures for more information on how to await or query a future.
The onComplete
, onSuccess
, or onFailure
methods of the Future
can be
used to register a callback to get a notification when the Future completes, giving
you a way to avoid blocking.
@@@ warning
When using future callbacks, @scala[such as onComplete
, onSuccess
, and onFailure
,]
inside actors you need to carefully avoid closing over
the containing actor’s reference, 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.
See also: @ref:Actors and shared mutable state
@@@
You can forward a message from one actor to another. This means that the original sender address/reference is maintained even though the message is going through a 'mediator'. This can be useful when writing actors that work as routers, load-balancers, replicators etc.
Scala : @@snip ActorDocSpec.scala { #forward }
Java : @@snip ActorDocTest.java { #forward }
An Actor has to
@scala[implement the receive
method to receive messages:]
@java[define its initial receive behavior by implementing the createReceive
method in the AbstractActor
:]
Scala : @@snip Actor.scala { #receive }
Java : @@snip ActorDocTest.java { #createReceive }
@@@ div { .group-scala }
This method returns a PartialFunction
, e.g. a ‘match/case’ clause in
which the message can be matched against the different case clauses using Scala
pattern matching. Here is an example:
@@@
@@@ div { .group-java }
The return type is AbstractActor.Receive
that defines which messages your Actor can handle,
along with the implementation of how the messages should be processed.
You can build such behavior with a builder named ReceiveBuilder
. Here is an example:
@@@
Scala : @@snip ActorDocSpec.scala { #imports1 #my-actor }
Java : @@snip MyActor.java { #imports #my-actor }
@@@ div { .group-java }
In case you want to provide many match
cases but want to avoid creating a long call
trail, you can split the creation of the builder into multiple statements as in the example:
@@snip GraduallyBuiltActor.java { #imports #actor }
Using small methods is a good practice, also in actors. It's recommended to delegate the
actual work of the message processing to methods instead of defining a huge ReceiveBuilder
with lots of code in each lambda. A well structured actor can look like this:
@@snip ActorDocTest.java { #well-structured }
That has benefits such as:
- easier to see what kind of messages the actor can handle
- readable stack traces in case of exceptions
- works better with performance profiling tools
- Java HotSpot has a better opportunity for making optimizations
The Receive
can be implemented in other ways than using the ReceiveBuilder
since it in the
end is just a wrapper around a Scala PartialFunction
. In Java, you can implement PartialFunction
by
extending AbstractPartialFunction
. For example, one could implement an adapter
to Vavr Pattern Matching DSL. See the @extrefAkka Vavr sample project for more details.
If the validation of the ReceiveBuilder
match logic turns out to be a bottleneck for some of your
actors you can consider to implement it at lower level by extending UntypedAbstractActor
instead
of AbstractActor
. The partial functions created by the ReceiveBuilder
consist of multiple lambda
expressions for every match statement, where each lambda is referencing the code to be run. This is something
that the JVM can have problems optimizing and the resulting code might not be as performant as the
untyped version. When extending UntypedAbstractActor
each message is received as an untyped
Object
and you have to inspect and cast it to the actual message type in other ways, like this:
@@snip ActorDocTest.java { #optimized }
@@@
If you want to have a handle for replying to a message, you can use
@scala[sender()
] @java[getSender()
], which gives you an ActorRef. You can reply by sending to
that ActorRef with @scala[sender() ! replyMsg
.] @java[getSender().tell(replyMsg, getSelf())
.] You can also store the ActorRef
for replying later, or passing on to other actors. If there is no sender (a
message was sent without an actor or future context) then the sender
defaults to a 'dead-letter' actor ref.
Scala : @@snip ActorDocSpec.scala { #reply-without-sender }
Java : @@snip MyActor.java { #reply }
The ActorContext
setReceiveTimeout
defines the inactivity timeout after which
the sending of a ReceiveTimeout
message is triggered.
When specified, the receive function should be able to handle an akka.actor.ReceiveTimeout
message.
1 millisecond is the minimum supported timeout.
Please note that the receive timeout might fire and enqueue the ReceiveTimeout
message right after
another message was enqueued; hence it is not guaranteed that upon reception of the receive
timeout there must have been an idle period beforehand as configured via this method.
Once set, the receive timeout stays in effect (i.e. continues firing repeatedly after inactivity
periods). Pass in Duration.Undefined
to switch off this feature.
Scala : @@snip ActorDocSpec.scala { #receive-timeout }
Java : @@snip ActorDocTest.java { #receive-timeout }
Messages marked with NotInfluenceReceiveTimeout
will not reset the timer. This can be useful when
ReceiveTimeout
should be fired by external inactivity but not influenced by internal activity,
e.g. scheduled tick messages.
Messages can be scheduled to be sent at a later point by using the @ref:Scheduler directly, but when scheduling periodic or single messages in an actor to itself it's more convenient and safe to use the support for named timers. The lifecycle of scheduled messages can be difficult to manage when the actor is restarted and that is taken care of by the timers.
Scala : @@snip ActorDocSpec.scala { #timers }
Java : @@snip ActorDocTest.java { #timers }
Each timer has a key and can be replaced or cancelled. It's guaranteed that a message from the previous incarnation of the timer with the same key is not received, even though it might already be enqueued in the mailbox when it was cancelled or the new timer was started.
The timers are bound to the lifecycle of the actor that owns it, and thus are cancelled
automatically when it is restarted or stopped. Note that the TimerScheduler
is not thread-safe,
i.e. it must only be used within the actor that owns it.
Actors are stopped by invoking the stop
method of a ActorRefFactory
,
i.e. ActorContext
or ActorSystem
. Typically the context is used for stopping
the actor itself or child actors and the system for stopping top level actors. The actual
termination of the actor is performed asynchronously, i.e. stop
may return before
the actor is stopped.
Scala : @@snip ActorDocSpec.scala { #stoppingActors-actor }
Java : @@snip MyStoppingActor.java { #my-stopping-actor }
Processing of the current message, if any, will continue before the actor is stopped,
but additional messages in the mailbox will not be processed. By default these
messages are sent to the deadLetters
of the ActorSystem
, but that
depends on the mailbox implementation.
Termination of an actor proceeds in two steps: first the actor suspends its
mailbox processing and sends a stop command to all its children, then it keeps
processing the internal termination notifications from its children until the last one is
gone, finally terminating itself (invoking postStop
, dumping mailbox,
publishing Terminated
on the DeathWatch, telling
its supervisor). This procedure ensures that actor system sub-trees terminate
in an orderly fashion, propagating the stop command to the leaves and
collecting their confirmation back to the stopped supervisor. If one of the
actors does not respond (i.e. processing a message for extended periods of time
and therefore not receiving the stop command), this whole process will be
stuck.
Upon ActorSystem.terminate()
, the system guardian actors will be
stopped, and the aforementioned process will ensure proper termination of the
whole system.
The postStop()
hook is invoked after an actor is fully stopped. This
enables cleaning up of resources:
Scala : @@snip ActorDocSpec.scala { #postStop }
Java : @@snip ActorDocTest.java { #postStop }
@@@ note
Since stopping an actor is asynchronous, you cannot immediately reuse the
name of the child you just stopped; this will result in an
InvalidActorNameException
. Instead, watch()
the terminating
actor and create its replacement in response to the Terminated
message which will eventually arrive.
@@@
You can also send an actor the akka.actor.PoisonPill
message, which will
stop the actor when the message is processed. PoisonPill
is enqueued as
ordinary messages and will be handled after messages that were already queued
in the mailbox.
Scala : @@snip ActorDocSpec.scala { #poison-pill }
Java : @@snip ActorDocTest.java { #poison-pill }
You can also "kill" an actor by sending a Kill
message. Unlike PoisonPill
this will cause
the actor to throw a ActorKilledException
, triggering a failure. The actor will
suspend operation and its supervisor will be asked how to handle the failure,
which may mean resuming the actor, restarting it or terminating it completely.
See @ref:What Supervision Means for more information.
Use Kill
like this:
Scala : @@snip ActorDocSpec.scala { #kill }
Java : @@snip ActorDocTest.java { #kill }
In general though it is not recommended to overly rely on either PoisonPill
or Kill
in
designing your actor interactions, as often times a protocol-level message like PleaseCleanupAndStop
which the actor knows how to handle is encouraged. The messages are there for being able to stop actors
over which design you do not have control over.
gracefulStop
is useful if you need to wait for termination or compose ordered
termination of several actors:
Scala : @@snip ActorDocSpec.scala { #gracefulStop}
Java : @@snip ActorDocTest.java { #import-gracefulStop #gracefulStop}
When gracefulStop()
returns successfully, the actor’s postStop()
hook
will have been executed: there exists a happens-before edge between the end of
postStop()
and the return of gracefulStop()
.
In the above example a custom Manager.Shutdown
message is sent to the target
actor to initiate the process of stopping the actor. You can use PoisonPill
for
this, but then you have limited possibilities to perform interactions with other actors
before stopping the target actor. Simple cleanup tasks can be handled in postStop
.
@@@ warning
Keep in mind that an actor stopping and its name being deregistered are
separate events which happen asynchronously from each other. Therefore it may
be that you will find the name still in use after gracefulStop()
returned. In order to guarantee proper deregistration, only reuse names from
within a supervisor you control and only in response to a Terminated
message, i.e. not for top-level actors.
@@@
There is an extension named CoordinatedShutdown
that will stop certain actors and
services in a specific order and perform registered tasks during the shutdown process.
The order of the shutdown phases is defined in configuration akka.coordinated-shutdown.phases
.
The default phases are defined as:
@@snip reference.conf { #coordinated-shutdown-phases }
More phases can be added in the application's configuration if needed by overriding a phase with an
additional depends-on
. Especially the phases before-service-unbind
, before-cluster-shutdown
and
before-actor-system-terminate
are intended for application specific phases or tasks.
The default phases are defined in a single linear order, but the phases can be ordered as a directed acyclic graph (DAG) by defining the dependencies between the phases. The phases are ordered with topological sort of the DAG.
Tasks can be added to a phase with:
Scala : @@snip ActorDocSpec.scala { #coordinated-shutdown-addTask }
Java : @@snip ActorDocTest.java { #coordinated-shutdown-addTask }
The returned @scala[Future[Done]
] @java[CompletionStage<Done>
] should be completed when the task is completed. The task name parameter
is only used for debugging/logging.
Tasks added to the same phase are executed in parallel without any ordering assumptions. Next phase will not start until all tasks of previous phase have been completed.
If tasks are not completed within a configured timeout (see @ref:reference.conf)
the next phase will be started anyway. It is possible to configure recover=off
for a phase
to abort the rest of the shutdown process if a task fails or is not completed within the timeout.
Tasks should typically be registered as early as possible after system startup. When running the coordinated shutdown tasks that have been registered will be performed but tasks that are added too late will not be run.
To start the coordinated shutdown process you can invoke @scala[run
] @java[runAll
] on the CoordinatedShutdown
extension:
Scala : @@snip ActorDocSpec.scala { #coordinated-shutdown-run }
Java : @@snip ActorDocTest.java { #coordinated-shutdown-run }
It's safe to call the @scala[run
] @java[runAll
] method multiple times. It will only run once.
That also means that the ActorSystem
will be terminated in the last phase. By default, the
JVM is not forcefully stopped (it will be stopped if all non-daemon threads have been terminated).
To enable a hard System.exit
as a final action you can configure:
akka.coordinated-shutdown.exit-jvm = on
When using @ref:Akka Cluster the CoordinatedShutdown
will automatically run
when the cluster node sees itself as Exiting
, i.e. leaving from another node will trigger
the shutdown process on the leaving node. Tasks for graceful leaving of cluster including graceful
shutdown of Cluster Singletons and Cluster Sharding are added automatically when Akka Cluster is used,
i.e. running the shutdown process will also trigger the graceful leaving if it's not already in progress.
By default, the CoordinatedShutdown
will be run when the JVM process exits, e.g.
via kill SIGTERM
signal (SIGINT
ctrl-c doesn't work). This behavior can be disabled with:
akka.coordinated-shutdown.run-by-jvm-shutdown-hook=off
If you have application specific JVM shutdown hooks it's recommended that you register them via the
CoordinatedShutdown
so that they are running before Akka internal shutdown hooks, e.g.
those shutting down Akka Remoting (Artery).
Scala : @@snip ActorDocSpec.scala { #coordinated-shutdown-jvm-hook }
Java : @@snip ActorDocTest.java { #coordinated-shutdown-jvm-hook }
For some tests it might be undesired to terminate the ActorSystem
via CoordinatedShutdown
.
You can disable that by adding the following to the configuration of the ActorSystem
that is
used in the test:
# Don't terminate ActorSystem via CoordinatedShutdown in tests
akka.coordinated-shutdown.terminate-actor-system = off
akka.coordinated-shutdown.run-by-jvm-shutdown-hook = off
akka.cluster.run-coordinated-shutdown-when-down = off
Akka supports hotswapping the Actor’s message loop (e.g. its implementation) at
runtime: invoke the context.become
method from within the Actor.
become
takes a @scala[PartialFunction[Any, Unit]
] @java[PartialFunction<Object, BoxedUnit>
] that implements the new
message handler. The hotswapped code is kept in a Stack which can be pushed and
popped.
@@@ warning
Please note that the actor will revert to its original behavior when restarted by its Supervisor.
@@@
To hotswap the Actor behavior using become
:
Scala : @@snip ActorDocSpec.scala { #hot-swap-actor }
Java : @@snip ActorDocTest.java { #hot-swap-actor }
This variant of the become
method is useful for many different things,
such as to implement a Finite State Machine (FSM, for an example see @scala[Dining
Hakkers).] @java[Dining
Hakkers).] It will replace the current behavior (i.e. the top of the behavior
stack), which means that you do not use unbecome
, instead always the
next behavior is explicitly installed.
The other way of using become
does not replace but add to the top of
the behavior stack. In this case care must be taken to ensure that the number
of “pop” operations (i.e. unbecome
) matches the number of “push” ones
in the long run, otherwise this amounts to a memory leak (which is why this
behavior is not the default).
Scala : @@snip ActorDocSpec.scala { #swapper }
Java : @@snip ActorDocTest.java { #swapper }
See this @extrefUnnested receive example.
The @scala[Stash
trait] @java[AbstractActorWithStash
class] enables an actor to temporarily stash away messages
that can not or should not be handled using the actor's current
behavior. Upon changing the actor's message handler, i.e., right
before invoking @scala[context.become
or context.unbecome
] @java[getContext().become()
or getContext().unbecome()
], all
stashed messages can be "unstashed", thereby prepending them to the actor's
mailbox. This way, the stashed messages can be processed in the same
order as they have been received originally. @java[An actor that extends
AbstractActorWithStash
will automatically get a deque-based mailbox.]
@@@ note { .group-scala }
The trait Stash
extends the marker trait
RequiresMessageQueue[DequeBasedMessageQueueSemantics]
which
requests the system to automatically choose a deque based
mailbox implementation for the actor. If you want more
control over the
mailbox, see the documentation on mailboxes: @ref:Mailboxes.
@@@
@@@ note { .group-java }
The abstract class AbstractActorWithStash
implements the marker
interface RequiresMessageQueue<DequeBasedMessageQueueSemantics>
which requests the system to automatically choose a deque based
mailbox implementation for the actor. If you want more
control over the mailbox, see the documentation on mailboxes: @ref:Mailboxes.
@@@
Here is an example of the @scala[Stash
] @java[AbstractActorWithStash
class] in action:
Scala : @@snip ActorDocSpec.scala { #stash }
Java : @@snip ActorDocTest.java { #stash }
Invoking stash()
adds the current message (the message that the
actor received last) to the actor's stash. It is typically invoked
when handling the default case in the actor's message handler to stash
messages that aren't handled by the other cases. It is illegal to
stash the same message twice; to do so results in an
IllegalStateException
being thrown. The stash may also be bounded
in which case invoking stash()
may lead to a capacity violation,
which results in a StashOverflowException
. The capacity of the
stash can be configured using the stash-capacity
setting (an Int
) of the
mailbox's configuration.
Invoking unstashAll()
enqueues messages from the stash to the
actor's mailbox until the capacity of the mailbox (if any) has been
reached (note that messages from the stash are prepended to the
mailbox). In case a bounded mailbox overflows, a
MessageQueueAppendFailedException
is thrown.
The stash is guaranteed to be empty after calling unstashAll()
.
The stash is backed by a scala.collection.immutable.Vector
. As a
result, even a very large number of messages may be stashed without a
major impact on performance.
@@@ warning { .group-scala }
Note that the Stash
trait must be mixed into (a subclass of) the
Actor
trait before any trait/class that overrides the preRestart
callback. This means it's not possible to write
Actor with MyActor with Stash
if MyActor
overrides preRestart
.
@@@
Note that the stash is part of the ephemeral actor state, unlike the
mailbox. Therefore, it should be managed like other parts of the
actor's state which have the same property. The @scala[Stash
trait’s] @java[AbstractActorWithStash
]
implementation of preRestart
will call unstashAll()
, which is
usually the desired behavior.
@@@ note
If you want to enforce that your actor can only work with an unbounded stash,
then you should use the @scala[UnboundedStash
trait] @java[AbstractActorWithUnboundedStash
class] instead.
@@@
It can happen that while a message is being processed by an actor, that some kind of exception is thrown, e.g. a database exception.
If an exception is thrown while a message is being processed (i.e. taken out of its mailbox and handed over to the current behavior), then this message will be lost. It is important to understand that it is not put back on the mailbox. So if you want to retry processing of a message, you need to deal with it yourself by catching the exception and retry your flow. Make sure that you put a bound on the number of retries since you don't want a system to livelock (so consuming a lot of cpu cycles without making progress).
If an exception is thrown while a message is being processed, nothing happens to the mailbox. If the actor is restarted, the same mailbox will be there. So all messages on that mailbox will be there as well.
If code within an actor throws an exception, that actor is suspended and the supervision process is started (see @ref:supervision). Depending on the supervisor’s decision the actor is resumed (as if nothing happened), restarted (wiping out its internal state and starting from scratch) or terminated.
@@@ div { .group-scala }
Sometimes it can be useful to share common behavior among a few actors, or compose one actor's behavior from multiple smaller functions.
This is possible because an actor's receive
method returns an Actor.Receive
, which is a type alias for PartialFunction[Any,Unit]
,
and partial functions can be chained together using the PartialFunction#orElse
method. You can chain as many functions as you need,
however you should keep in mind that "first match" wins - which may be important when combining functions that both can handle the same type of message.
For example, imagine you have a set of actors which are either Producers
or Consumers
, yet sometimes it makes sense to
have an actor share both behaviors. This can be achieved without having to duplicate code by extracting the behaviors to
traits and implementing the actor's receive
as combination of these partial functions.
@@snip ActorDocSpec.scala { #receive-orElse }
Instead of inheritance the same pattern can be applied via composition - compose the receive method using partial functions from delegates.
@@@
The rich lifecycle hooks of Actors provide a useful toolkit to implement various initialization patterns. During the
lifetime of an ActorRef
, an actor can potentially go through several restarts, where the old instance is replaced by
a fresh one, invisibly to the outside observer who only sees the ActorRef
.
Initialization might be necessary every time an actor is instantiated,
but sometimes one needs initialization to happen only at the birth of the first instance when the
ActorRef
is created. The following sections provide patterns for different initialization needs.
Using the constructor for initialization has various benefits. First of all, it makes it possible to use val
fields to store
any state that does not change during the life of the actor instance, making the implementation of the actor more robust.
The constructor is invoked when an actor instance is created calling actorOf
and also on restart, therefore the internals of the actor can always assume
that proper initialization happened. This is also the drawback of this approach, as there are cases when one would
like to avoid reinitializing internals on restart. For example, it is often useful to preserve child actors across
restarts. The following section provides a pattern for this case.
The method preStart()
of an actor is only called once directly during the initialization of the first instance, that
is, at creation of its ActorRef
. In the case of restarts, preStart()
is called from postRestart()
, therefore
if not overridden, preStart()
is called on every restart. However, by overriding postRestart()
one can disable
this behavior, and ensure that there is only one call to preStart()
.
One useful usage of this pattern is to disable creation of new ActorRefs
for children during restarts. This can be
achieved by overriding preRestart()
. Below is the default implementation of these lifecycle hooks:
Scala : @@snip InitializationDocSpec.scala { #preStartInit }
Java : @@snip InitializationDocTest.java { #preStartInit }
Please note, that the child actors are still restarted, but no new ActorRef
is created. One can recursively apply
the same principles for the children, ensuring that their preStart()
method is called only at the creation of their
refs.
For more information see @ref:What Restarting Means.
There are cases when it is impossible to pass all the information needed for actor initialization in the constructor,
for example in the presence of circular dependencies. In this case the actor should listen for an initialization message,
and use become()
or a finite state-machine state transition to encode the initialized and uninitialized states
of the actor.
Scala : @@snip InitializationDocSpec.scala { #messageInit }
Java : @@snip InitializationDocTest.java { #messageInit }
If the actor may receive messages before it has been initialized, a useful tool can be the Stash
to save messages
until the initialization finishes, and replaying them after the actor became initialized.
@@@ warning
This pattern should be used with care, and applied only when none of the patterns above are applicable. One of
the potential issues is that messages might be lost when sent to remote actors. Also, publishing an ActorRef
in
an uninitialized state might lead to the condition that it receives a user message before the initialization has been
done.
@@@