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 = ["docs.extension.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:
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)
}
Use it:
class MyActor extends Actor {
val settings = Settings(context.system)
val connection = connect(settings.DbUri, settings.CircuitBreakerTimeout)
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