wait for actor to finish processing entire mailbox

[bobdebuildr]

wait for actor to finish processing entire mailbox

[bobdebuildr]

Ah I see, thank you very much for the help! The linked example makes it very clear.

[Restioson]

Ideally, I would structure such an example so that all the actors naturally finish when their work is done. With this, and the use of weak addresses, the final actor would have no strong references to it, so it would stop. Then, you can just await the actor future and return the relevant data from Stopped. Otherwise, it could stop itself when done, presuming it has some level of introspection as to when it is done. If it does not, I would try and design it so that when there is no more input, then there are no more strong addresses to the actor, so the actor will naturally stop. That is the ideal use of the xtra actor lifecycle, imo.

Let me know if this doesn't work for any reason (clarification would be appreciated!)

[bobdebuildr]

Ok, I think I understand the idea, and it sounds exactly like what I would want. Am I correct in assuming that by "await the actor future" you're referring to the join function?

I have something like this now:

let first_adr = xtra::spawn_tokio(MyActor::new(), None);
first_adr.send(DoSomething {}).await.unwrap();
// ...
// somewhere along the line there's a last future
last_adr = xtra::spawn_tokio(MyLastActor::new(), None);
let weakadr = last_adr.downgrade();
drop(last_adr);
// the following will wait for all copies of the strong address to be dropped
weakadr.join().await;

The only thing that's not working is to get something when it's stopped- it doesn't seem to be possible when spawning actors like this (it requires Stop = ()). I that correct or am I missing something?

Thanks for your help!

[Restioson]

Ok, I think I understand the idea, and it sounds exactly like what I would want. Am I correct in assuming that by "await the actor future" you're referring to the join function?

I am referring to the actor itself's future. See this example for more. join simply returns when the actor itself stops, so it will return at the same time but not with the value of Stop. Based on your example, I would try this:

let (first_adr, context) = Context::new(None);
let run_future = context.run(MessageCounter::default()); // `run_future` will resolve to `Actor::Stop`.
let handle = tokio::spawn(run_future);
let first_adr = xtra::spawn_tokio(MyActor::new(), None);
first_adr.send(DoSomething {}).await.unwrap();
// ...
// somewhere along the line there's a last future
let last_adr = xtra::spawn_tokio(MyLastActor::new(), None);
drop(last_adr);
// the following will wait for all copies of the strong address to be dropped [or the actor to call Context::stop]
let result = handle.await;

The only thing that's not working is to get something when it's stopped- it doesn't seem to be possible when spawning actors like this (it requires Stop = ()). I that correct or am I missing something?

This is an artificial limit to prevent users from throwing away the Stop value by spawning an actor. You can use context.run(actor) in this case instead.

[Restioson]

Glad I could be of assistance!

[bobdebuildr]

I have one more question about how actors stop:
When there are no more strong Addresses left, does the actor stop immediately, or does it first finish processing all messages in its mailbox? I wasn't able to find anything about this in the documentation.
Also, I assume that when the actor is busy processing the last message in its mailbox, address.len() will be zero, correct?

[Restioson]

When there are no more strong Addresses left, does the actor stop immediately, or does it first finish processing all messages in its mailbox? I wasn't able to find anything about this in the documentation.

It will finish processing all messages. This should probably be mentioned explicitly in the docs but it isn't, like you noted.

Also, I assume that when the actor is busy processing the last message in its mailbox, address.len() will be zero, correct?

Yes.

[bobdebuildr]

Ok great, thanks for clearing that up. I'm collecting some small suggestions for the documentation and will make a pull request when it's ready.

There's some unexpected behavior I came across when trying to make sure that I don't have any more strong addresses to an actor left.
In my main function, I pass around a strong address (by cloning) to various actors, which will be dropped when the actors finish. I then want to overwrite the remaining variable in the main function by a weak address, using: let adr = adr.downgrade();. The adr at the left is weak, on the right-hand side it is strong. In my understanding, this should drop the strong address.

However, that doesn't seem to be the case. The statement above is not equivalent to the following:

let adr = {
    let a = adr.downgrade();
    drop(adr);
    a
};

What am I missing here?

[Restioson]

This doesn't drop the strong address as Address::downgrade takes &self, so it is not consumed. You could also do

let weak = addr.downgrade();
drop(addr);

[bobdebuildr]

This doesn't drop the strong address as Address::downgrade takes &self, so it is not consumed. You could also do

Aah, I see.. should have taken a look at the signature. Thanks for clearing that up!