Akka Project

Where does the name Akka come from?

It is the name of a beautiful Swedish mountain up in the northern part of Sweden called Laponia. The mountain is also sometimes called ‘The Queen of Laponia’.

Akka is also the name of a goddess in the Sámi (the native Swedish population) mythology. She is the goddess that stands for all the beauty and good in the world. The mountain can be seen as the symbol of this goddess.

Also, the name AKKA is the a palindrome of letters A and K as in Actor Kernel.

Akka is also:

• the name of the goose that Nils traveled across Sweden on in The Wonderful Adventures of Nils by the Swedish writer Selma Lagerlöf.
• the finnish word for ‘nasty elderly woman’ and the word for ‘elder sister’ in the Indian languages Tamil, Telugu, Kannada and Marathi.
• the word for ‘earlier’ in Korean, written like this: 아까
• a town in Morocco
• a near-earth asteroid

Actors in General

My actors send replies, but they are lost, and the sender is deadLetters

This happens most easily when using Java: a tell() operation needs to be told what the sending actor’s reference is, so that this reference can be passed to the target actor together with the message. When within an actor, always add getSelf() as second argument:

otherActor.tell(msg, getSelf());

For Scala users this can only happen outside of actors, since the ! operator picks up the sender reference implicitly from the surrounding actor’s context.

How can I get compile time errors for missing messages in receive?

One solution to help you get a compile time warning for not handling a message that you should be handling is to define your actors input and output messages implementing base traits, and then do a match that the will be checked for exhaustiveness.

Here is an example where the compiler will warn you that the match in receive isn’t exhaustive:

object MyActor {
  // these are the messages we accept
  sealed abstract trait Message
  case class FooMessage(foo: String) extends Message
  case class BarMessage(bar: Int) extends Message

  // these are the replies we send
  sealed abstract trait Reply
  case class BazMessage(foo: String) extends Reply
}

class MyActor extends Actor {
  import MyActor._
  def receive = {
    case message: Message ⇒ message match {
      case BarMessage(bar) => sender ! BazMessage("Got " + bar)
    }
  }
}

Remoting

I want to send to a remote system but it does not do anything

Make sure that you have remoting enabled on both ends: client and server. Both do need hostname and port configured, and you will need to know the port of the server; the client can use an automatic port in most cases (i.e. configure port zero). If both systems are running on the same network host, their ports must be different

If you still do not see anything, look at what the logging of remote life-cycle events tells you (normally logged at INFO level) or switch on logging of all sent and received messages (logged at DEBUG level).

Which options shall I enable when debugging remoting issues?

Have a look at the reference.conf for akka-remote, the typical candidates are:

• akka.remote.log-sent-messages
• akka.remote.log-received-messages
• akka.remote.log-remote-lifecycle-events (this also includes deserialization errors)

What is the name of a remote actor?

When you want to send messages to an actor on a remote host, you need to know its full path, which is of the form

akka://system@host:1234/user/my/actor/hierarchy/path

Observe all the parts you need here:

• system is the remote system’s name (must match exactly, case-sensitive!)

• host is the remote system’s IP address or DNS name, and it must match that system’s configuration (i.e. akka.remote.netty.hostname)

• 1234 is the port number on which the remote system is listening for connections and receiving messages

• /user/my/actor/hierarchy/path is the absolute path of the remote actor in the remote system’s supervision hierarchy, including the system’s guardian (i.e. /user, there are others e.g. /system which hosts loggers, /temp which keeps temporary actor refs used with ask(), /remote which enables remote deployment, etc.); this matches how the actor prints its own self reference on the remote host, e.g. in log output.

Why are replies not received from a remote actor?

The most common reason is that the local system’s name (i.e. the system@host:1234 part in the answer above) is not reachable from the remote system’s network location, e.g. because host was configured to be 0.0.0.0, localhost or a NAT’ed IP address.

sender/getSender() disappears when I use Future in my Actor, why?

When using future callbacks, 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 breaks 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.

Read more about it in the docs for Actors and the JMM

Microkernel

Why doesn’t sbt dist copy my subproject into the kernel distribution?

For this to happen, that project’s name (in the project’s settings) and id (argument to the Project factory) must match exactly.

How do I use Play2 Mini with Akka?

In case you need to start Play2 Mini programatically you can use::

play.core.server.NettyServer.main(Array())

Read more about it in the docs for Http

Debugging

How do I turn on debug logging?

To turn on debug logging in your actor system add the following to your configuration:

akka.loglevel = DEBUG

To enable different types of debug logging add the following to your configuration:

• akka.actor.debug.receive will log all messages sent to an actor if that actors receive method is a LoggingReceive

• akka.actor.debug.autoreceive will log all special messages like Kill, PoisonPill e.t.c. sent to all actors

• akka.actor.debug.lifecycle will log all actor lifecycle events of all actors

Read more about it in the docs for Testing Actor Systems