Akka Extensions (Scala)

Akka Extensions (Scala)

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:

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()
}

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

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
}

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

CountExtension(system).increment

Or from inside of an Akka Actor:

class MyActor extends Actor {
  def receive = {
    case someMessage 
      CountExtension(context.system).increment()
  }
}

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.

akka {
  extensions = ["akka.docs.extension.CountExtension$"]
}

Note that in this case CountExtension is an object and therefore the class name ends with $.

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:

import akka.actor.Extension
import akka.actor.ExtensionId
import akka.actor.ExtensionIdProvider
import akka.actor.ExtendedActorSystem
import akka.util.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)
}

Use it:

class MyActor extends Actor {
  val settings = Settings(context.system)
  val connection = connect(settings.DbUri, settings.CircuitBreakerTimeout)

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