Akka in Scala, exclamation mark and question mark

Question

What is the difference between exclamation mark (!) and question mark (?) when sending messages to Actors?

myActor ! Hello(value1)
myActor ? Hello(value1)

Answer 1

From the recipient's point of view, it sees tell and ask messages the same way. However when receiving a tell the value of sender will be the reference of the actor who sent the message, whereas for an ask, the sender is set up such that any reply goes to the Future created in the actor who did the asking.

There is an advantage in ask, that it is easy to know that the response you're receiving was definitely a result of the message you asked, whereas with Tell, you may need to use unique IDs to achieve a similar result. However with ask you need to set a timeout after which the Future will fail if no response is received.

In the code below, the same effect is achieved with a tell and and ask.

import akka.actor.{Props, Actor}
import scala.concurrent.duration._
import akka.pattern.ask

class TellActor extends Actor {

  val recipient = context.actorOf(Props[ReceiveActor])

  def receive = {
    case "Start" =>
      recipient ! "Hello" // equivalent to recipient.tell("hello", self)

    case reply => println(reply)
  }
} 

class AskActor extends Actor {

  val recipient = context.actorOf(Props[ReceiveActor])

  def receive = {
    case "Start" =>
      implicit val timeout = 3 seconds
      val replyF = recipient ? "Hello" // equivalent to recipient.ask("Hello")
      replyF.onSuccess{
        case reply => println(reply)
      }
  }
}

class ReceiveActor extends Actor {

  def receive = {
    case "Hello" => sender ! "And Hello to you!"
  }
}

Answer 2

Shamelessly copied [awesome] official doc (look Send messages section for more):

Messages are sent to an Actor through one of the following methods.

! means “fire-and-forget”, e.g. send a message asynchronously and return immediately. Also known as tell.

? sends a message asynchronously and returns a Future representing a possible reply. Also known as ask.