Multiple Akka ask getting same actor reference name.

[Love Hasija]

Hi,

I am observing a strange behavior but I am sure, I did something wrong.

I have a service class providing a DB Wrapper, which has async methods for providing services. Internally there is an actor workflow. but the service class is not part of actor system and simply invokes ask to the actor system.

Here's an example.

Async Service => Supervisor => Workers.
(ask)  => (results from the supervisor).

When I am testing the same using Junit by invoking multiple parallel requests., what I am observing is that the supervisors and workers behave properly, but only the first response comes back to ask actor, others are lost to the dead letter queue.

While debugging, I saw all the ask actor references are same: Actor[akka://MyActorSystem/temp/$a]

Is it something, I am doing wrong.

[Justin du coeur]

Hmm -- I think you may be misunderstanding ask.  Ask sends *one* message, and gets *one* response.  It sounds like you're trying to use it to get a stream of responses, and it just doesn't do that.

For that matter, ask gives you back a Future, and Future only returns a single response.  It's not designed to represent a stream.

To get a stream of responses, which sounds like what you're looking for, you're going to have to build something more sophisticated, I'm afraid.  My usual recommendation would be to add another Actor in the workflow, which receives the initial ask, sends out the request, collates the results from the workers and returns them as a *single* response to the non-Actor code...

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[Love Hasija]

Yeah you are right. Actually that's what is happening. I may not have explained it right.

The Actor system has a hierarchy of master and multiple workers. The master is responsible for correlating the stream of result from the workers, correlating, aggregating and returning a single response to the non-actor instance.

The non-actor instance here is using ask to get a single response. Where I am coming for is, when i use Junit and invoke multiple requests to the non-actor instance. I am invoking the actor system individually and expecting multiple ask returns. It is something like this.

for(i=0;i<10;i++) {
 non_actor_instance.request();
}

function request() {
 ask(supervisor, msg);

[Justin du coeur]

Hmm.  Nothing apparently wrong with that.  You say that messages are dropping into dead letters -- is that the requests or the responses that are being dropped?  If it's the responses, it might be helpful to see the code of the master, which is likely to be the source of the problem...

[Love Hasija]

Thanks for your help Justin.

I explored a little bit more on the Master worker and you were right. The problem was that for each ask i was keeping an actor instance level reference to the caller, and that's the reason for each ask message, the response was directing to a single actor reference, leading to dead letter references.

I updated it to have the caller references part of the message envelope.

[Justin du coeur]

Okay, cool -- glad to hear you found the issue...