The ask-pattern implements the initiator side of a request–reply protocol. The ? operator is pronounced as "ask" (and a convenience symbolic operation kept since the previous ask API, if unsure which one to use, prefer the non-symbolic method as it leads to fewer surprises with the scope of the replyTo function)

Note that if you are inside of an actor you should prefer ActorContext.ask as that provides better safety.

The party that asks may be within or without an Actor, since the implementation will fabricate a (hidden) ActorRef that is bound to a scala.concurrent.Promise. This ActorRef will need to be injected in the message that is sent to the target Actor in order to function as a reply-to address, therefore the argument to the ask / ? operator is not the message itself but a function that given the reply-to address will create the message.

case class Request(msg: String, replyTo: ActorRef[Reply])
case class Reply(msg: String)

implicit val system = ...
implicit val timeout = Timeout(3.seconds)
val target: ActorRef[Request] = ...
val f: Future[Reply] = target ? (replyTo => Request("hello", replyTo))

Note: it is preferrable to use the non-symbolic ask method as it easier allows for wildcards for the replyTo: ActorRef.

The ask-pattern implements the initiator side of a request–reply protocol.

Note that if you are inside of an actor you should prefer ActorContext.ask as that provides better safety.

The party that asks may be within or without an Actor, since the implementation will fabricate a (hidden) ActorRef that is bound to a scala.concurrent.Promise. This ActorRef will need to be injected in the message that is sent to the target Actor in order to function as a reply-to address, therefore the argument to the ask / ? operator is not the message itself but a function that given the reply-to address will create the message.

case class Request(msg: String, replyTo: ActorRef[Reply])
case class Reply(msg: String)

implicit val system = ...
implicit val timeout = Timeout(3.seconds)
val target: ActorRef[Request] = ...
val f: Future[Reply] = target.ask(replyTo => Request("hello", replyTo))
// alternatively
val f2: Future[Reply] = target.ask(Request("hello", _))

The same as ask but only for requests that result in a response of type akka.pattern.StatusReply. If the response is a akka.pattern.StatusReply.Success the returned future is completed successfully with the wrapped response. If the status response is a akka.pattern.StatusReply.Error the returned future will be failed with the exception in the error (normally a akka.pattern.StatusReply.ErrorMessage).