Wow, look at you! Here we are on our last lesson of Bootcamp together. We want to say thank you for coming on this journey with us, and to give yourself a big pat on the back for your dedication to your craft.
In this lesson, we'll be going over how to handle timeouts within actors and prevent deadlocks, where one actor is waiting for another indefinitely.
This lesson will show you how to prevent deadlocks by using a ReceiveTimeout.
ReceiveTimeout lets you specify what an actor should do when it hasn't received a message for a certain period of time. Once this timeout has been hit, the actor will send itself the ReceiveTimeout singleton as a message.
Once set up, the ReceiveTimeout stays in effect and will continue firing repeatedly every time the specified interval passes without the actor receiving a message.
You can use ReceiveTimeout whenever you want to take some action after a period of inactivity.
Here are some common cases where you may want to use a ReceiveTimeout:
- To shut an actor down after it goes a certain amount of time without receiving a message
- To confirm that other actors are doing work and sending in their status messages
- To prevent deadlocks where one actor thinks another is doing work
You call Context.SetReceiveTimeout() and pass it a TimeSpan. If that amount of time passes and the actor hasn't received a message, the actor will send itself the ReceiveTimeout singleton as a message, e.g.
// send ourselves a ReceiveTimeout message if no message within 3 seconds
Context.SetReceiveTimeout(TimeSpan.FromSeconds(3));Then, you just need to handle the ReceiveTimeout message and take whatever action is appropriate.
Let's assume you wanted to shut an actor down after a period of inactivity, and inform its parent. Here's one basic way you could do that:
// have actor shut down after a long period of inactivity
Receive<ReceiveTimeout>(timeout =>
{
// inform parent that shutting down
Context.Parent.Tell(new ImShuttingDown());
// shut self down
Context.Stop(Self);
});You call Context.SetReceiveTimeout() and pass it null, e.g.
// cancel ReceiveTimeout
Context.SetReceiveTimeout(null);Yes, you can set the ReceiveTimeout after every message, or as often as you want. Setting a new timeout will cancel the previous timeout and schedule a new one.
1 millisecond is the minimum timeout interval.
Yes, you can use ReceiveTimeout with any actor type.
The only difference would be the syntax differences between how you match the message between a ReceiveActor and an UntypedActor.
ReceiveTimeout can create false positives. For example, it's possible for the timeout to occur and another message to arrive in the actors mailbox before the ReceiveTimeout message does. In this case, another message would get processed before the ReceiveTimeout message, making it invalid.
It is not guaranteed that upon reception of the ReceiveTimeout that there must have been an idle period beforehand as configured via this method.
This is an edge case, but there are ways to code around it.
We're going to use ReceiveTimeout to eliminate a potential deadlock that might occur inside the GithubCommanderActor - if one of the GithubCoordinatorActor it routes to suddenly dies before it has a chance to reply to a CanAcceptJob message, the GithubCommanderActor will be permanently stuck in its Asking state.
We can prevent this from happening using ReceiveTimeout!
We're going to hang onto the current job we're inquiring about as an instance variable inside the GithubCommanderActor, so open up Actors/GithubCommanderActor.cs and make the following changes:
// add this field anywhere inside the GithubCommanderActor
private RepoKey _repoJob;And modify the GithubCommanderActor.Ready method to look like this:
// modify the GithubCommanderActor.Ready method to look like this
private void Ready()
{
Receive<CanAcceptJob>(job =>
{
_coordinator.Tell(job);
_repoJob = job.Repo;
BecomeAsking();
});
}We need to set a few calls to Context.ReceiveTimeout in order to get it to work properly with our GithubCommanderActor when we're inside the Asking state.
First, modify the BecomeAsking method on the GithubCommanderActor to look like this:
// modify the `BecomeAsking` method on the `GithubCommanderActor` to look like this
private void BecomeAsking()
{
_canAcceptJobSender = Sender;
// block, but ask the router for the number of routees. Avoids magic numbers.
pendingJobReplies = _coordinator.Ask<Routees>(new GetRoutees())
.Result.Members.Count();
Become(Asking);
// send ourselves a ReceiveTimeout message if no message within 3 seconds
Context.SetReceiveTimeout(TimeSpan.FromSeconds(3));
}This means that once the GithubCommanderActor enters the Asking behavior, it will automatically send itself a ReceiveTimeout message if it hasn't received any other message for longer than three seconds.
Speaking of which, let's add a handler for the ReceiveTimeout message type inside the Asking method on GithubCommanderActor.
// add this inside the GithubCommanderActor.Asking method
// means at least one actor failed to respond
Receive<ReceiveTimeout>(timeout =>
{
_canAcceptJobSender.Tell(new UnableToAcceptJob(_repoJob));
BecomeReady();
});
We're going to treat every ReceiveTimeout as a "busy" signal from one of the GithubCoordinatorActor instances, so we'll send ourselves a UnableToAcceptJob message every time we receive a ReceiveTimeout.
Once the GithubCommanderActor has received all of the replies its expecting and it switches back to its Ready state, we need to cancel the ReceiveTimeout.
Modify the GithubCommanderActor's BecomeReady method to look like the following:
// modify the GithubCommanderActor.BecomeReady method to read like the following:
private void BecomeReady()
{
Become(Ready);
Stash.UnstashAll();
// cancel ReceiveTimeout
Context.SetReceiveTimeout(null);
}And that's it!
Build and run GithubActors.sln, and you should see the following output if you try querying the Akka.NET GitHub Repository (go give them a star while you're at it!)
NOTE: If you're following along using the eBook / .ePub, you won't see the animation. Click here to see it.
And here's what the final output looks like - sadly, for a different repo since hit the GitHub API rate limit with Akka.NET :(
Wow! You made it, awesome!
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Gratefully, The Petabridge Team.
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If there is a problem with the code running, or something else that needs to be fixed in this lesson, please create an issue and we'll get right on it. This will benefit everyone going through Bootcamp.