Akka Extensions

If you want to add features to Akka, there is a very elegant, but powerful mechanism for doing so. It’s called Akka Extensions and is comprised of 2 basic components: an Extension and an ExtensionId.

Extensions will only be loaded once per ActorSystem, which will be managed by Akka. You can choose to have your Extension loaded on-demand or at ActorSystem creation time through the Akka configuration. Details on how to make that happens are below, in the “Loading from Configuration” section.

Warning

Since an extension is a way to hook into Akka itself, the implementor of the extension needs to ensure the thread safety of his/her extension.

Building an Extension

So let’s create a sample extension that just lets us count the number of times something has happened.

First, we define what our Extension should do:

Scala
import akka.actor.Extension

class CountExtensionImpl extends Extension {
  //Since this Extension is a shared instance
  // per ActorSystem we need to be threadsafe
  private val counter = new AtomicLong(0)

  //This is the operation this Extension provides
  def increment() = counter.incrementAndGet()
}
Java
import akka.actor.*;
import java.util.concurrent.atomic.AtomicLong;

public class CountExtensionImpl implements Extension {
  //Since this Extension is a shared instance
  // per ActorSystem we need to be threadsafe
  private final AtomicLong counter = new AtomicLong(0);

  //This is the operation this Extension provides
  public long increment() {
    return counter.incrementAndGet();
  }
}

Then we need to create an ExtensionId for our extension so we can grab a hold of it.

Scala
import akka.actor.ActorSystem
import akka.actor.ExtensionId
import akka.actor.ExtensionIdProvider
import akka.actor.ExtendedActorSystem

object CountExtension
  extends ExtensionId[CountExtensionImpl]
  with ExtensionIdProvider {
  //The lookup method is required by ExtensionIdProvider,
  // so we return ourselves here, this allows us
  // to configure our extension to be loaded when
  // the ActorSystem starts up
  override def lookup = CountExtension

  //This method will be called by Akka
  // to instantiate our Extension
  override def createExtension(system: ExtendedActorSystem) = new CountExtensionImpl

  /**
   * Java API: retrieve the Count extension for the given system.
   */
  override def get(system: ActorSystem): CountExtensionImpl = super.get(system)
}
Java
import akka.actor.*;
import java.util.concurrent.atomic.AtomicLong;

public class CountExtension extends AbstractExtensionId<CountExtensionImpl>
  implements ExtensionIdProvider {
  //This will be the identifier of our CountExtension
  public final static CountExtension CountExtensionProvider = new CountExtension();

  private CountExtension() {}

  //The lookup method is required by ExtensionIdProvider,
  // so we return ourselves here, this allows us
  // to configure our extension to be loaded when
  // the ActorSystem starts up
  public CountExtension lookup() {
    return CountExtension.CountExtensionProvider; //The public static final
  }

  //This method will be called by Akka
  // to instantiate our Extension
  public CountExtensionImpl createExtension(ExtendedActorSystem system) {
    return new CountExtensionImpl();
  }
}

Wicked! Now all we need to do is to actually use it:

Scala
CountExtension(system).increment
Java
// typically you would use static import of the
// CountExtension.CountExtensionProvider field
CountExtension.CountExtensionProvider.get(system).increment();

Or from inside of an Akka Actor:

Scala

class MyActor extends Actor { def receive = { case someMessage ⇒ CountExtension(context.system).increment() } }
Java
public class MyActor extends AbstractActor {
  @Override
  public Receive createReceive() {
    return receiveBuilder()
      .matchAny(msg -> {
        // typically you would use static import of the
        // CountExtension.CountExtensionProvider field
        CountExtension.CountExtensionProvider.get(getContext().getSystem()).increment();
      })
      .build();
  }
}

You can also hide extension behind traits:


trait Counting { self: Actor ⇒ def increment() = CountExtension(context.system).increment() } class MyCounterActor extends Actor with Counting { def receive = { case someMessage ⇒ increment() } }

That’s all there is to it!

Loading from Configuration

To be able to load extensions from your Akka configuration you must add FQCNs of implementations of either ExtensionId or ExtensionIdProvider in the akka.extensions section of the config you provide to your ActorSystem.

Scala
akka {
  extensions = ["docs.extension.CountExtension"]
}
Java
akka {
  extensions = ["docs.extension.ExtensionDocTest.CountExtension"]
}

Applicability

The sky is the limit! By the way, did you know that Akka’s Typed Actors, Serialization and other features are implemented as Akka Extensions?

Application specific settings

The configuration can be used for application specific settings. A good practice is to place those settings in an Extension.

Sample configuration:

myapp {
  db {
    uri = "mongodb://example1.com:27017,example2.com:27017"
  }
  circuit-breaker {
    timeout = 30 seconds
  }
}

The Extension:

Scala
import akka.actor.ActorSystem
import akka.actor.Extension
import akka.actor.ExtensionId
import akka.actor.ExtensionIdProvider
import akka.actor.ExtendedActorSystem
import scala.concurrent.duration.Duration
import com.typesafe.config.Config
import java.util.concurrent.TimeUnit

class SettingsImpl(config: Config) extends Extension {
  val DbUri: String = config.getString("myapp.db.uri")
  val CircuitBreakerTimeout: Duration =
    Duration(
      config.getMilliseconds("myapp.circuit-breaker.timeout"),
      TimeUnit.MILLISECONDS)
}
object Settings extends ExtensionId[SettingsImpl] with ExtensionIdProvider {

  override def lookup = Settings

  override def createExtension(system: ExtendedActorSystem) =
    new SettingsImpl(system.settings.config)

  /**
   * Java API: retrieve the Settings extension for the given system.
   */
  override def get(system: ActorSystem): SettingsImpl = super.get(system)
}
Java
import akka.actor.Extension;
import akka.actor.AbstractExtensionId;
import akka.actor.ExtensionIdProvider;
import akka.actor.ActorSystem;
import akka.actor.ExtendedActorSystem;
import scala.concurrent.duration.Duration;
import com.typesafe.config.Config;
import java.util.concurrent.TimeUnit;

public class SettingsImpl implements Extension {

  public final String DB_URI;
  public final Duration CIRCUIT_BREAKER_TIMEOUT;

  public SettingsImpl(Config config) {
    DB_URI = config.getString("myapp.db.uri");
    CIRCUIT_BREAKER_TIMEOUT =
      Duration.create(config.getDuration("myapp.circuit-breaker.timeout", 
        TimeUnit.MILLISECONDS), TimeUnit.MILLISECONDS);
  }

}

public class Settings extends AbstractExtensionId<SettingsImpl>
  implements ExtensionIdProvider {
  public final static Settings SettingsProvider = new Settings();

  private Settings() {}

  public Settings lookup() {
    return Settings.SettingsProvider;
  }

  public SettingsImpl createExtension(ExtendedActorSystem system) {
    return new SettingsImpl(system.settings().config());
  }
}

Use it:

Scala

class MyActor extends Actor { val settings = Settings(context.system) val connection = connect(settings.DbUri, settings.CircuitBreakerTimeout)
Java
public class MyActor extends AbstractActor {
  // typically you would use static import of the Settings.SettingsProvider field
  final SettingsImpl settings =
    Settings.SettingsProvider.get(getContext().getSystem());
  Connection connection =
    connect(settings.DB_URI, settings.CIRCUIT_BREAKER_TIMEOUT);

}

Library extensions

A third part library may register it’s extension for auto-loading on actor system startup by appending it to akka.library-extensions in its reference.conf.

akka.library-extensions += "docs.extension.ExampleExtension"

As there is no way to selectively remove such extensions, it should be used with care and only when there is no case where the user would ever want it disabled or have specific support for disabling such sub-features. One example where this could be important is in tests.

Warning

Theakka.library-extensions must never be assigned (= ["Extension"]) instead of appending as this will break the library-extension mechanism and make behavior depend on class path ordering.

The source code for this page can be found here.