I am a little confused by how I am supposed to get a reference to my Actor once it has been crerated in the Akka system. Say I have the following:
class MyActor(val x: Int) extends Actor {
def receive = {
case Msg => doSth()
}
}
And at some stage I would create the actor as follows:
val actorRef = system.actorOf(Props(new MyActor(2), name = "Some actor")
But when I need to refer to the MyActor object I cannot see any way to get it from the actor ref?
Thanks
Des
I'm adding my comment as an answer since it seems to be viewed as something worth putting in as an answer.
Not being able to directly access the actor is the whole idea behind actor systems. You are not supposed to be able to get at the underlying actor class instance. The actor ref is a lightweight proxy to your actor class instance. Allowing people to directly access the actor instance could lead down the path of mutable data issues/concurrent state update issues and that's a bad path to go down. By forcing you to go through the ref (and thus the mailbox), state and data will always be safe as only one message is processed at a time.
I think cmbaxter has a good answer, but I want to make it just a bit more clear. The ActorRef is your friend for the following reasons:
You cannot ever access the underlying actor. Actors receive their thread of execution from the Dispatcher given to them when they are created. They operate on one mailbox message at a time, so you never have to worry about concurrency inside of the actor unless YOU introduce it (by handling a message asynchronously with a Future or another Actor instance via delegation, for example). If someone had access to the underlying class behind the ActorRef, they could easily call into the actor via a method using another thread, thus negating the point of using the Actor to avoid concurrency in the first place.
ActorRefs provide Location Transparency. By this, I mean that the Actor instance could exist locally on the same JVM and machine as the actor from which you would like to send it a message, or it could exist on a different JVM, on a different box, in a different data center. You don't know, you don't care, and your code is not littered with the details of HOW the message will be sent to that actor, thus making it more declarative (focused on the what you want to do business-wise, not how it will be done). When you start using Remoting or Clusters, this will prove very valuable.
ActorRefs mask the instance of the actor behind it when failure occurs, as well. With the old Scala Actors, you had no such proxy and when an actor "died" (threw an Exception) that resulted in a new instance of the Actor type being created in its place. You then had to propagate that new instance reference to anyone who needed it. With ActorRef, you don't care that the Actor has been reincarnated - it's all transparent.
There is one way to get access to the underlying actor when you want to do testing, using TestActorRef.underlyingActor. This way, you can write unit tests against functionality written in methods inside the actor. Useful for checking basic business logic without worrying about Akka dynamics.
Hope that helps.
Related
It is a general question about developing with akka actor system.
I know, it sacrifices static type checking for greater flexibility, that is not the problem. Java does the same thing all the way.
But I'd like at least to check compatibility of ActorRefs dynamically. I searched for some method like actorRef.asInstanceOf[ActorType]. Such method should provide validation for an actorRef passed through messages. And it would allow safe application development. But I've found no method to do any kind of type check. Its even impossible to check if an actorRef correspond to given Props.
How this task typically solved in akka application? Are there any third-party tools for dynamic checks?
The purpose of ActorRef is to completely abstract the recipient. Sending a message to it provides absolutely no guarantees about a response or even suitability of the message being sent. The recipient could drop the message, route it, stash it or handle it. Any contract about handling and causing possible response messages to be emitted is entirely an informal agreement.
Now, that sounds like a lot to give up in a statically typed environment, but it provides a programming model that brings its own slew of advantages which by their design require that you are sending and receiving messages with the assumption that the messages will be handled but without any knowledge where or when they will be handled.
Regarding how this task is typically solved in akka applications is by configuration and/discovery. The contract of acceptable Messages is usually placed into a Protocol object, while the valid recipient for those Messages is either injected into the calling Actor at creation or queryable via some DiscoveryProtocol (itself hidden behind an ActorRef)
Let's say you have a UserRepository you want to query, you would create protocol like this:
case class User(id: Int, ... )
object UserRepositoryProtocol {
case class GetUser(userId: Int)
}
Further, let's assume that the ActorRef of UserRepository was not injected, but because it is just one of many services your actor might use has to be discovered via a general Discovery service:
object DiscoveryProtocol {
case class Discover(type: String)
case class Discovered(type: String, ref: ActorRef)
}
Now you can fetch a user like this:
(discoveryRef ? Discover("UserRepository")).flatMap {
case Discovered("UserRepository",repository) =>
(repository ? GetUser(id)).map {
case user:User => // do something with the user
}
}
The above condenses discovery and calls into a chain of ask operations. Chances are you would want to cache the discovered ref and or hand off the retrieved user to some other Actor that's doing the work, breaking each '?' into a ! and a matching receive in the same or different actor.
This last point illustrates the power of the actor model. In a traditional request => response model, the requestor and recipient of the response would have to be the same just by virtue of function signatures, but in the actor model, you can send from one actor, spawn a worker that will handle the response, etc.
Assume that an actor is not only encapsulated behind an actor ref, but that the physical location of an actor is also unknown. An actor can be running on another physical server or VM. You can't call instanceOf on an object in another VM - how can you expect to get the class of an actor then?
Now, when building, I would recommend you consider that all actors are remote via Akka's location transparency. (http://doc.akka.io/docs/akka/snapshot/general/remoting.html) If you assume all actors are remote, suddenly you'll think about your designs a little differently. Think of Akka as a Distribution Toolkit - that is its primary benefit!
If you're trying to reflect on actors during runtime, then there is probably something wrong with your design. Instead, you need to know what messages actors can accept and respond to.
If you really want to know what an actor can and can't do, then you could think of modelling some sort of "Accepts" method where an actor would reply with the current version of the described API that the actor implements for example - in this way your client and server can talk back and forth about what capabilities etc are supported dynamically during runtime.
I hope that contributes something to the discussion - just remember to always think of an actor as something that's running somewhere else and you'll design appropriately. The benefit of doing so is that you'll be able to scale out your applications with very minimal effort if your user base unexpectedly explodes!
Is there any standard way of formalizing my scala/akka actor api? Imho, situation where I need to look into implementation to know what to send is not really a good option. Also, if implementation has changed and my message is no longer valid(not invoking action I think it invokes) I don't get any warning or error.
This is a question that is discussed very much in the community. I heard that maybe Akka 3 will have better support for typesafe actors, but that is some time down the road.
In the mean time you could use TypedActors, though the general suggestion is to use them at the boundaries of your application.
A nice approach that does not give you any typesafety, but makes the contract of an actor more visible, is to define the messages an actor can receive in their companion object. This way each time you want to send a message to an actor you choose from the message its companion object defines. This of course works best if you have specific messages for each actor. If you change the implementation you could remove the old message and add a new one, so that everyone who wanted to use the old implementation would get compiler errors.
Lastly there was a nice pattern last week on the mailing list. He creates traits to define the contracts for the actors and their consumers, but you still need to take care that the consumer mix in the correct trait.
In my experience, the best way to make sure everything works is an extensive test suite which will test each actor for itself, but also the communication between specific actors.
The approach generally taken in Erlang is to avoid sending messages to a process directly, and to provide additional API in the same module which defines the behavior of the process. In Akka it would look like
class Foo extends Actor {
// handles messages Bar(x: Int) and Baz
}
object Foo {
def bar(foo: ActorRef, x: Int) = foo ! Bar(x)
def baz(foo: ActorRef) = (foo ? Baz).mapTo[TypeOfResponseToBaz]
}
One problem is handling return messages, since Erlang generally promotes more synchronous style than Akka does. This may be handled by a naming convention (e.g. BarResponse or FooBarResponse if different actors handle the same message with different responses).
I'm building up an actor hierarchy in which some actors reply to messages they get directly to a well-known actor (fixed name). I.e. these actors (far-removed in the hierarchy) obtain an actorRef via context.actorFor("akka://...").
I.e. for example, I have an "orchestrating" actor:
system.actorOf(Props[OrchestratingActor], name = "orchestrator")
which will then have an address of the kind akka://application/user/orchestrator
and someplace else, a random worker that received a message and wants to talk to the orchestrator:
class RandomWorker extends Actor {
def theOrchestrator = context.actorFor("akka://application/user/orchestrator")
def receive = {
case Foo =>
theOrchestrator ! "Bar"
}
}
Now, I'd like to test those actors and am wondering about how to deal with those addresses: when unit-testing an actor (e.g. using TestActorRef), how do I go about checking what's being sent to the remote address? One idea would be to (when possible) provide the address to the well-known actor via the constructor, and pass in the address of a TestActor to see what's being received. However I wonder if there isn't a way to "impersonate" a given address in the test, esp. in situation where the addresses aren't simple.
In other words I'd like to test the behavior of the actor (will it indeed have sent "Bar" to the orchestrator upon receiving a Foo
My recommendation would be to avoid using look-ups of actors for the purpose you show. It is rather a tool for the setup phase of your application, when wiring it all together. But even then most actors’ supervisors will know the dependencies of their children without using look-ups.
Within a local actor system all ActorRefs can be injected top to bottom (using constructor arguments or introduction messages). Look-ups are most useful when introducing remote systems with each other.
There is nothing wrong with injecting the address via the constructor. Let it me know if you need any details, because at the moment I don't know how to make this more clear, since you basically answered your own question. And btw, I don't know, which Akka version you are using, but actorFor has been recently deprecated in favor of ActorSelection, and there are good reasons for this.
So this question is related to an old one of mine: Do I need to re-use the same Akka ActorSystem or can I just create one every time I need one?
I asked a question about the lifecycle of actors, and I knew something was wrong in my mind, but couldn't phrase it correctly. Hopefully I can now :-).
Here's the situation. I want to test actors that have dependencies to other components and actors, so I went about composing my actors in bootstrap time (I'm using scalatra but however you bootstrap your app). I therefore have something like this:
trait DependencyComponent
{
val dependency : Dependency
}
trait ActorComponentA extends Actor with DependencyComponent {
val actorB : ActorRef
}
trait ActorComponentB extends Actor with DependencyComponent
Ok, so now I can test my actors by extending the traits and providing mock dependencies, all good. And I can bootstrap my app like so:
Bootstrap
val system = ActorSystem()
val actorA = system.actorOf(Props[DefaultActorA])
class DefaultActorB extends ActorComponentB {
val dependency = new RealDependency()
}
class DefaultActorA extends ActorComponentA {
val dependency = new RealDependency()
val actorB = context.actorOf(Props[DefaultActorB]).withRouter(RoundRobinRouter(nrOfInstances = 100)))
}
Cool, Im happy :-), now I can use the actorSystem and actorA within my app, and it has a 100 actorB routed to pass work to. So when actorA decideds that the work is done, it's my understanding that it should broadcast to the routed actors to shutdown. At this point when another request comes in actorA can no longer send messages to the router because all its actors are dead.
If I wasn't setting this up at boot time then actorA and its dependencies could be created when needed in my app. But that is very much like "newing up on object" in DI world. In order to test I would end up overriding the places where the actors were created.
Scalatra docs are suggesting creating my actors at boot time, so I feel that I am missing somehting here. Any help appreciated.
Cheers, Chris.
EDIT
I've +1 both #futurechimp and #cmbaxter as these both seem valid but slightly conflicting. So this is an open comment to both of you.
So #cmbaxter am I right in thinking that your suggesting never calling 'stop' on the routed actors and just maintaining a pool of them for use by ALL requests. And #futurechimp, your suggesting having the servlet instantiate the actors per request and killing them at the end of there lifecycle. Right?
It seems like per-request will spawn more actors (but dispose of them). Where the poll will have only a limited set for all requests in which case is there a potential bottle neck to this approach?
I guess basically, I'm asking if my assumptions are correct and if so what are the advantage and disadvantages to both approaches?
Instantiating an ActorSystem is expensive - however instantiating an Actor isn't. If you only want to instantiate your ActorSystem in ScalatraBootstrap, and your Actors elsewhere, that should work fine if that's what you need to do. I'll talk to some other people to confirm this, and then change the docs in Scalatra's Akka Guide to avoid confusion in future.
One of the questions you have to ask yourself here is: Are my actors going to be stateful or stateless. If stateless (and I would prefer this approach personally when possible), then they can be "long-lived" and you can start them when the server boots up and leave them running for the duration of the server's life. When you need to talk to them from elsewhere in the code, use system.actorFor(String) or system.actorSelection(String) (depending on what version of akka you are using) to look up the actor and send it a message. If the actors are going to be stateful, then they probably should be "short-lived" and started up in response to an individual request. In this case, you will not start them up when the server boots up; you will only start up the ActorSystem itself. Then, when the request comes in, you will instantiate via system.actorOf instead and make sure that when the work is done that you stop ActorA as it's the supervisor of all the ActorBs and stopping A will stop all of the Bs started by A.
I have a setup in which an actor(A) creates an actor(B) which it supervises and restarts when it crashes. From outside the actor system I want to get some value from the 'nested' actor B.
I can directly ask the actorSystem for the nested actor using:
val nestedActorB = myActorSystem.actorFor("/user/A/B")
Can I keep a reference to this actor or should I ask the actorSystem for the ref each time I want to ask the nested actor something. (Or should I ask the parent actor for the ref)?
What would be a best practice in this scenario?
(scala 2.9.1 and akka 2.0.4)
Thanks,
Albert
Yes you can keep a reference to the actor and not ask the system every time with actorFor.
There is a difference between local and remote actors and the documentation passage you are referring to is ambiguous and I've opened a ticket.
A local actor is only looked up once during the actorFor call so it needs to exist before you look it up, while sends to a remote actor are always by path and the remote actor is looked up on the remote node every time.
Even after the restart the reference stays the same. B will always be a child of A so the "/A/B" path is stable. If A is a top level actor, you will always find him under"/user/A". So yeah, "/user/A/B" will always point to the same actor.
As you can see in the akka doc, a reference is an object that, at runtime, traverses the actor tree every time you send a message or check the liveliness of the actor you are referring to.
This means you can use the reference you obtain with the actorFor method any time you want to communicate with that actor, even if the actor belonging to that path is restarted or even if it's already terminated.