Actor discovery
You are viewing the documentation for the new actor APIs, to view the Pekko Classic documentation, see Classic Actors.
Dependency
To use Pekko Actor Typed, you must add the following dependency in your project:
- sbt
val PekkoVersion = "1.0.3" libraryDependencies += "org.apache.pekko" %% "pekko-actor-typed" % PekkoVersion
- Maven
<properties> <scala.binary.version>2.13</scala.binary.version> </properties> <dependencyManagement> <dependencies> <dependency> <groupId>org.apache.pekko</groupId> <artifactId>pekko-bom_${scala.binary.version}</artifactId> <version>1.0.3</version> <type>pom</type> <scope>import</scope> </dependency> </dependencies> </dependencyManagement> <dependencies> <dependency> <groupId>org.apache.pekko</groupId> <artifactId>pekko-actor-typed_${scala.binary.version}</artifactId> </dependency> </dependencies>
- Gradle
def versions = [ ScalaBinary: "2.13" ] dependencies { implementation platform("org.apache.pekko:pekko-bom_${versions.ScalaBinary}:1.0.3") implementation "org.apache.pekko:pekko-actor-typed_${versions.ScalaBinary}" }
Obtaining Actor references
There are two general ways to obtain Actor references: by creating actors and by discovery using the Receptionist.
You can pass actor references between actors as constructor parameters or part of messages.
Sometimes, you need something to bootstrap the interaction, for example when actors are running on different nodes in the Cluster or when “dependency injection” with constructor parameters is not applicable.
Receptionist
When an actor needs to be discovered by another actor but you are unable to put a reference to it in an incoming message, you can use the Receptionist
Receptionist
. It supports both local and cluster(see cluster). You register the specific actors that should be discoverable from each node in the local Receptionist
instance. The API of the receptionist is also based on actor messages. This registry of actor references is then automatically distributed to all other nodes in the case of a cluster. You can lookup such actors with the key that was used when they were registered. The reply to such a Find
request is a Listing
, which contains a Set
of actor references that are registered for the key. Note that several actors can be registered to the same key.
The registry is dynamic. New actors can be registered during the lifecycle of the system. Entries are removed when registered actors are stopped, manually deregistered or the node they live on is removed from the Cluster. To facilitate this dynamic aspect you can also subscribe to changes with the Receptionist.Subscribe
message. It will send Listing
messages to the subscriber, first with the set of entries upon subscription, then whenever the entries for a key are changed.
These imports are used in the following example:
- Scala
-
source
import org.apache.pekko import pekko.actor.typed.ActorRef import pekko.actor.typed.Behavior import pekko.actor.typed.receptionist.Receptionist import pekko.actor.typed.receptionist.ServiceKey import pekko.actor.typed.scaladsl.Behaviors
- Java
-
source
import org.apache.pekko.actor.typed.ActorRef; import org.apache.pekko.actor.typed.Behavior; import org.apache.pekko.actor.typed.javadsl.ActorContext; import org.apache.pekko.actor.typed.javadsl.Behaviors; import org.apache.pekko.actor.typed.receptionist.Receptionist; import org.apache.pekko.actor.typed.receptionist.ServiceKey;
First we create a PingService
actor and register it with the Receptionist
against a ServiceKey
ServiceKey
that will later be used to lookup the reference:
- Scala
-
source
object PingService { val PingServiceKey = ServiceKey[Ping]("pingService") final case class Ping(replyTo: ActorRef[Pong.type]) case object Pong def apply(): Behavior[Ping] = { Behaviors.setup { context => context.system.receptionist ! Receptionist.Register(PingServiceKey, context.self) Behaviors.receiveMessage { case Ping(replyTo) => context.log.info("Pinged by {}", replyTo) replyTo ! Pong Behaviors.same } } } }
- Java
-
source
public class PingService { public static final ServiceKey<Ping> pingServiceKey = ServiceKey.create(Ping.class, "pingService"); public static class Pong {} public static class Ping { private final ActorRef<Pong> replyTo; public Ping(ActorRef<Pong> replyTo) { this.replyTo = replyTo; } } public static Behavior<Ping> create() { return Behaviors.setup( context -> { context .getSystem() .receptionist() .tell(Receptionist.register(pingServiceKey, context.getSelf())); return new PingService(context).behavior(); }); } private final ActorContext<Ping> context; private PingService(ActorContext<Ping> context) { this.context = context; } private Behavior<Ping> behavior() { return Behaviors.receive(Ping.class).onMessage(Ping.class, this::onPing).build(); } private Behavior<Ping> onPing(Ping msg) { context.getLog().info("Pinged by {}", msg.replyTo); msg.replyTo.tell(new Pong()); return Behaviors.same(); } }
Then we have another actor that requires a PingService
to be constructed:
- Scala
-
source
object Pinger { def apply(pingService: ActorRef[PingService.Ping]): Behavior[PingService.Pong.type] = { Behaviors.setup { context => pingService ! PingService.Ping(context.self) Behaviors.receiveMessage { _ => context.log.info("{} was ponged!!", context.self) Behaviors.stopped } } } }
- Java
-
source
public class Pinger { private final ActorContext<PingService.Pong> context; private final ActorRef<PingService.Ping> pingService; private Pinger(ActorContext<PingService.Pong> context, ActorRef<PingService.Ping> pingService) { this.context = context; this.pingService = pingService; } public static Behavior<PingService.Pong> create(ActorRef<PingService.Ping> pingService) { return Behaviors.setup( ctx -> { pingService.tell(new PingService.Ping(ctx.getSelf())); return new Pinger(ctx, pingService).behavior(); }); } private Behavior<PingService.Pong> behavior() { return Behaviors.receive(PingService.Pong.class) .onMessage(PingService.Pong.class, this::onPong) .build(); } private Behavior<PingService.Pong> onPong(PingService.Pong msg) { context.getLog().info("{} was ponged!!", context.getSelf()); return Behaviors.stopped(); } }
Finally in the guardian actor we spawn the service as well as subscribing to any actors registering against the ServiceKey
ServiceKey
. Subscribing means that the guardian actor will be informed of any new registrations via a Listing
message:
- Scala
-
source
object Guardian { def apply(): Behavior[Nothing] = { Behaviors .setup[Receptionist.Listing] { context => context.spawnAnonymous(PingService()) context.system.receptionist ! Receptionist.Subscribe(PingService.PingServiceKey, context.self) Behaviors.receiveMessagePartial[Receptionist.Listing] { case PingService.PingServiceKey.Listing(listings) => listings.foreach(ps => context.spawnAnonymous(Pinger(ps))) Behaviors.same } } .narrow } }
- Java
-
source
public class Guardian { public static Behavior<Void> create() { return Behaviors.setup( (ActorContext<Receptionist.Listing> context) -> { context .getSystem() .receptionist() .tell( Receptionist.subscribe( PingService.pingServiceKey, context.getSelf().narrow())); context.spawnAnonymous(PingService.create()); return new Guardian(context).behavior(); }) .unsafeCast(); // Void } private final ActorContext<Receptionist.Listing> context; private Guardian(ActorContext<Receptionist.Listing> context) { this.context = context; } private Behavior<Receptionist.Listing> behavior() { return Behaviors.receive(Receptionist.Listing.class) .onMessage(Receptionist.Listing.class, this::onListing) .build(); } private Behavior<Receptionist.Listing> onListing(Receptionist.Listing msg) { msg.getServiceInstances(PingService.pingServiceKey) .forEach(pingService -> context.spawnAnonymous(Pinger.create(pingService))); return Behaviors.same(); } }
Each time a new (which is just a single time in this example) PingService
is registered the guardian actor spawns a Pinger
for each currently known PingService
. The Pinger
sends a Ping
message and when receiving the Pong
reply it stops.
In above example we used Receptionist.Subscribe
, but it’s also possible to request a single Listing
of the current state without receiving further updates by sending the Receptionist.Find
message to the receptionist. An example of using Receptionist.Find
:
- Scala
-
source
object PingManager { sealed trait Command case object PingAll extends Command private case class ListingResponse(listing: Receptionist.Listing) extends Command def apply(): Behavior[Command] = { Behaviors.setup[Command] { context => val listingResponseAdapter = context.messageAdapter[Receptionist.Listing](ListingResponse.apply) context.spawnAnonymous(PingService()) Behaviors.receiveMessagePartial { case PingAll => context.system.receptionist ! Receptionist.Find(PingService.PingServiceKey, listingResponseAdapter) Behaviors.same case ListingResponse(PingService.PingServiceKey.Listing(listings)) => listings.foreach(ps => context.spawnAnonymous(Pinger(ps))) Behaviors.same } } } }
- Java
-
source
public class PingManager { interface Command {} enum PingAll implements Command { INSTANCE } private static class ListingResponse implements Command { final Receptionist.Listing listing; private ListingResponse(Receptionist.Listing listing) { this.listing = listing; } } public static Behavior<Command> create() { return Behaviors.setup(context -> new PingManager(context).behavior()); } private final ActorContext<Command> context; private final ActorRef<Receptionist.Listing> listingResponseAdapter; private PingManager(ActorContext<Command> context) { this.context = context; this.listingResponseAdapter = context.messageAdapter(Receptionist.Listing.class, ListingResponse::new); context.spawnAnonymous(PingService.create()); } private Behavior<Command> behavior() { return Behaviors.receive(Command.class) .onMessage(PingAll.class, notUsed -> onPingAll()) .onMessage(ListingResponse.class, response -> onListing(response.listing)) .build(); } private Behavior<Command> onPingAll() { context .getSystem() .receptionist() .tell(Receptionist.find(PingService.pingServiceKey, listingResponseAdapter)); return Behaviors.same(); } private Behavior<Command> onListing(Receptionist.Listing msg) { msg.getServiceInstances(PingService.pingServiceKey) .forEach(pingService -> context.spawnAnonymous(Pinger.create(pingService))); return Behaviors.same(); } }
Also note how a messageAdapter
messageAdapter
is used to convert the Receptionist.Listing
to a message type that the PingManager
understands.
If a server no longer wish to be associated with a service key it can deregister using the command Receptionist.Deregister
which will remove the association and inform all subscribers.
The command can optionally send an acknowledgement once the local receptionist has removed the registration. The acknowledgement does not guarantee that all subscribers has seen that the instance has been removed, it may still receive messages from subscribers for some time after this.
- Scala
-
source
context.system.receptionist ! Receptionist.Deregister(PingService.PingServiceKey, context.self)
- Java
-
source
context .getSystem() .receptionist() .tell(Receptionist.deregister(PingService.pingServiceKey, context.getSelf()));
Cluster Receptionist
The Receptionist
also works in a cluster, an actor registered to the receptionist will appear in the receptionist of the other nodes of the cluster.
The state for the receptionist is propagated via distributed data which means that each node will eventually reach the same set of actors per ServiceKey
.
Subscription
s and Find
queries to a clustered receptionist will keep track of cluster reachability and only list registered actors that are reachable. The full set of actors, including unreachable ones, is available through Listing.allServiceInstances
Listing.getAllServiceInstances
.
One important difference from local only receptions are the serialization concerns, all messages sent to and back from an actor on another node must be serializable, see serialization.
Receptionist Scalability
The receptionist does not scale up to any number of services or very high turnaround of services. It will likely handle up to thousands or tens of thousands of services. Use cases with higher demands the receptionist for initial contact between actors on the nodes while the actual logic of those is up to the applications own actors.