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Lesson 3.1: Using Group routers to divide work among your actors

Welcome to Unit 3! As you know, this unit focuses on making our actor systems more scalable, resilient, and parallelized.

Over the course of Unit 3, you're going to build a sophisticated GitHub scraper that can simultaneously retrieve data from multiple GitHub repos at once. This system will also be able to fetch information about the GitHubbers who have participated in those repos (e.g. starred or forked). By the end, we'll have a nicely scalable system for retrieving data from the GitHub API, capable of coordinating a huge amount of data retrieval in parallel!

Heads up: This lesson is the most critical (and longest) of all the lessons in Unit 3. Grab some coffee and get comfortable!

The most important new concept we need to learn is Routers (docs). Let's get going.

Key Concepts / Background

Routers

What is a Router?

A Router is a special kind of actor that acts as a messaging hub to a group of other actors. The purpose of a Router is to divide and balance work (represented as message streams) across some group of other actors, which will actually do the work.

What's special about a Router?

A Router actually is an actor, but with one critical difference: it can process more than one message at a time.

Wait, what? I thought all actors only processed one message at a time?! That is true for 99.999% of cases. But remember: The purpose of a Router is to route messages onward, NOT to process them.

Since a Router does not need to actually process messages and take any significant action on the message—it just has to forward the message on to another actor—it can break the "one message at a time" rule and everything is fine. In fact, it's better that way. Why?

What does a Router give me?

Fundamentally, what a Router gives you is massive message throughput and scalability.

Routers are the critical component that breaks down large data streams into easily managed ones. Routers allow you to divide up a huge amount of work across a group of actors and scale up processing easily.

On the surface, Routers look like normal actors, but they are actually implemented differently. Routers are designed to be extremely efficient at one thing: receiving messages and quickly passing them on to routees.

A normal actor can be used for routing messages, but a normal actor's single-threaded processing can become a bottleneck. Routers achieve much higher throughput by changing the usual message-processing pipeline to allow concurrent message routing. This is achieved by embedding the routing logic of a Router directly in their ActorCell itself (the mesh between the actor's mailbox and the actor,) rather than in the receive loop / message-handling code of the router actor. This way, messages sent to a router's IActorRef are immediately routed to the routee, bypassing the single-threaded message-handling code of the router's actor entirely.

Fortunately, all of this complexity is invisible to consumers of the routing API.

Okay, what kinds of Routers are there?

There are two types of routers: Pool routers, and Group routers.

Pool Routers

A "Pool" router is a Router that creates and manages its worker actors ("routees"). You provide a NrOfInstances to the router and the router will handle routee creation (and supervision) by itself.

We cover Pool routers in depth in lesson 2.

Group Routers

A group router is a router that does not create/manage its routees. It only forwards them messages. You specify the routees when creating the group router by passing the router the ActorPaths for each routee.

In this lesson, we'll be working with Group routers.

How do I configure a Router?

You can configure a router directly in your code (dubbed "procedural" or "programatic" configuration), or you can configure the router using HOCON and App.config.

We'll use procedural configuration in this lesson and the next, but go in depth on using HOCON to configure routers in Lesson 3.

When a routee gets a message, is the Sender the Router, or the actor that sent the message to the Router?

Great question. Glad you asked!

The Sender of a message delivered to a routee is the actor who sent the message to the router. The router just forwards the message. A router is effectively transparent, and only works in one direction: to its routees.

So, if the routee replies to the message, that response will go to the original actor, bypassing the router altogether. When replying this way, the routee can also specify the router as the Sender for its response, so that any follow-up reply from the original actor flows back through the router.

How does a Router know where to forward a message?

A Router decides how to distribute messages to its routees based on the RoutingStrategy you assign it.

Routing Strategies

A RoutingStrategy encapsulates the logic used by a router to distribute messages to its routees.

There are quite a few types of RoutingStrategy available to you out of the box. But, we divide them into two general categories: special-case strategies, and load balancing strategies.

You can also define your own RoutingStrategy, but that's quite complex and beyond the scope of this course.

Let's go over these message routing strategies.

Special-case RoutingStrategys

Each of these is a RoutingStrategy with a specialized purpose.

  1. Broadcast
  2. Random
  3. ConsistentHash

Broadcast

Under this RoutingStrategy, the router will forward any message it receives to ALL of its routees.

Here's what the Broadcast RoutingStrategy looks like:

Broadcast RoutingStrategy

Random

Under the Random RoutingStrategy, each time the Router receives a message, it will pick a routee at random to forward the message to.

The router will repeat this process each time it receives a message, so messages 1, 2, 3...N will each be sent to a randomly selected actor.

ConsistentHash

With the ConsistentHash RoutingStrategy, the Router uses consistent hashing to select the routee based on the data of the message that was sent.

This has a specific use case where you want the routing of the message to depend on the message itself. For example, imagine that you had a system that needed to process events on a per-user level. To do this, your analytics events could flow to a ConsistentHashRouter that then forwards the event message on to a per-user UserEventTrackingActor, which will track all the events for that given user. The hash key would probably be the user UUID contained inside each of the user-generated events.

Here's what the ConsistentHash RoutingStrategy looks like:

ConsistentHash RoutingStrategy

Load balancing RoutingStrategys

These routing strategies are all different approaches to the same goal: attempting to evenly distribute work across the group.

  1. RoundRobin
  2. SmallestMailbox
  3. TailChopping
  4. Resizable
  5. ScatterGatherFirstCompleted

Let's review them quickly.

RoundRobin

Under this RoutingStrategy, the router will use round-robin to select a routee. For concurrent calls, RoundRobin is just a best effort—meaning that the router makes no complex effort to determine which routee is best suited for a message, it just sends it to the next in the round robin pattern and advances.

Here's what the RoundRobin RoutingStrategy looks like:

RoundRobin RoutingStrategy

In practice, a RoundRobin strategy is going to be fine for most situations and should be your go-to load balancing RoutingStrategy.

However, if you find yourself with a router under massive load (e.g. 10s of millions of messages per second) or needing a unique pattern, then you may want to look at the more esoteric load balancing strategies that follow.

TailChopping

The TailChoppingRouter will first send the message to one, randomly picked, routee and then after a small delay to to a second routee (picked randomly from the remaining routees) and so on. It waits for first reply it gets back and forwards it back to original sender. Other replies are discarded.

The goal of this router is to decrease latency by performing redundant queries to multiple routees, assuming that one of the other actors may still be faster to respond than the initial one.

ScatterGatherFirstCompleted

The ScatterGatherFirstCompletedRouter will send the message on to all its routees. It then waits for first reply it gets back. This result will be sent back to original sender. Other replies are discarded.

ScatterGatherFirstCompletedRouter expects at least one reply within a configured duration, otherwise it will reply to its Sender with an exception.

Here's what ScatterGatherFirstCompleted looks like:

ScatterGatherFirstCompletedRouter

SmallestMailbox

This RoutingStrategy only works with Pool routers, so we'll cover it in the next lesson.

ResizableRouter

This RoutingStrategy only works with Pool routers, so we'll cover it in the next lesson.

Special Router messages

Regardless of its RoutingStrategy, there are a few special messages that you can send to a Router to cause certain behaviors.

Broadcast

Sending a Broadcast message to a non-Broadcast router makes the router act like a BroadcastRouter for that single message. After the message is processed, the router will return to its normal RoutingStrategy.

When would you use this? It doesn't come up very often, but one use case we can think of is if a group of routees all needed to take some action in response to a global-level event.

For example, perhaps you have a group of actors that all must be alerted if a critical system goes down. In this case, you could send their router a Broadcast message and all the routees would be alerted.

// tell a router to broadcast message to all its routees
// regardless of what type of router it actually is
router.Tell(new Broadcast("Shields failing, Captain!"));

GetRoutees

The GetRoutees message type tells a router to return a list of its routees. This is most commonly used for debugging, but in our example we'll also use it to track how many jobs are open.

// get a list of a routers routees
router.Tell(new GetRoutees());

PoisonPill

A PoisonPill message has special handling for all actors, including for routers. When any actor receives a PoisonPill message, that actor will be stopped immediately.

For a group router, this only stops the router and does not stop its routees. For a pool router, it will also have the effect of shutting down its child routees as they are supervised children of the router.

Sending a PoisonPill will terminate any actor immediately:

// kill router actor
router.Tell(PoisonPill.Instance);

Great! Now that you know what the different kinds of routers are, and how to use them, let's wrap up by covering how group routers and their routees recover from failures.

How does supervision work with Group routers?

A group router does not supervise its routees.

Recall that group routers do not create their routees, but instead are passed the ActorPaths of their routees. This means that those routees exist somewhere else in the hierarchy, and are managed by whatever other parent actors created them.

Practically, this means that a group router usually won't know that its routees have died. A group router will attempt to DeathWatch its routees, but it doesn't always succeed in subscribing. Much of this is due to the fact that ActorPaths can have wildcards.

Isn't it bad that group routers usually don't know their routees have died?

Yes, it is bad.

This is one of the key reasons that, in general, we recommend using pool routers instead of group routers.

In this lesson, we're only using group routers, but going forward we'll be using pool routers almost exclusively.

Phew! That was a LOT of new information. Now let's put it to use and make something!

Exercise

If you build and run GithubActors.sln, you'll notice that we can only process one GitHub repository at a time right now:

GithubActors without parallelism

NOTE: If you're following along using the eBook / .ePub, you won't see the animation. Click here to see it.

The current state of our actor hierarchy for processing GitHub repositories currently looks like this:

Initial state of GithubActors hierarchy

We're going to modify the GithubCommanderActor to use a BroadcastGroup router so we can run multiple jobs in parallel by the end of this lesson!

Phase 1 - Add IWithUnboundedStash to the GithubCommanderActor

Open Actors\GithubCommanderActor.cs and make the following changes to the actor declaration:

// Actors\GithubCommanderActor.cs
public class GithubCommanderActor : ReceiveActor, IWithUnboundedStash

And then add the Stash property to GithubCommanderActor somewhere

// inside GithubCommanderActor class definition
public IStash Stash { get; set; }

Phase 2 - Add switchable behaviors to the GithubCommanderActor

Replace the GithubCommanderActor's constructor and current Receive<T> handlers with the following:

/* replace the current constructor and Receive<T> handlers
 * in Actors/GithubCommanderActor.cs with the following
 */
private int pendingJobReplies;

public GithubCommanderActor()
{
   Ready();
}

private void Ready()
{
    Receive<CanAcceptJob>(job =>
    {
        _coordinator.Tell(job);

        BecomeAsking();
    });
}

private void BecomeAsking()
{
    _canAcceptJobSender = Sender;
    pendingJobReplies = 3; //the number of routees
    Become(Asking);
}

private void Asking()
{
    // stash any subsequent requests
    Receive<CanAcceptJob>(job => Stash.Stash());

    Receive<UnableToAcceptJob>(job =>
    {
        pendingJobReplies--;
        if (pendingJobReplies == 0)
        {
            _canAcceptJobSender.Tell(job);
            BecomeReady();
        }
    });

    Receive<AbleToAcceptJob>(job =>
    {
        _canAcceptJobSender.Tell(job);

        // start processing messages
        Sender.Tell(new GithubCoordinatorActor.BeginJob(job.Repo));

        // launch the new window to view results of the processing
        Context.ActorSelection(ActorPaths.MainFormActor.Path).Tell(
			new MainFormActor.LaunchRepoResultsWindow(job.Repo, Sender));

        BecomeReady();
    });
}

private void BecomeReady()
{
    Become(Ready);
    Stash.UnstashAll();
}

Phase 3 - Modify GithubCommanderActor.PreStart to use a group router for _coordinator

The GithubCommanderActor is responsible for creating one GithubCoordinatorActor, who manages and coordinates all of the actors responsible for downloading data from GitHub via Octokit.

We're going to change the GithubCommanderActor to actually create 3 different GithubCoordinatorActor instances, and we're going to use a BroadcastGroup to communicate with them!

Replace the GithubCommanderActor.PreStart method with the following:

protected override void PreStart()
{
    // create three GithubCoordinatorActor instances
    var c1 = Context.ActorOf(Props.Create(() => new GithubCoordinatorActor()),
        ActorPaths.GithubCoordinatorActor.Name + "1");
    var c2 = Context.ActorOf(Props.Create(() => new GithubCoordinatorActor()),
        ActorPaths.GithubCoordinatorActor.Name + "2");
    var c3 = Context.ActorOf(Props.Create(() => new GithubCoordinatorActor()),
        ActorPaths.GithubCoordinatorActor.Name + "3");

    // create a broadcast router who will ask all of them 
    // if they're available for work
    _coordinator =
        Context.ActorOf(Props.Empty.WithRouter(
            new BroadcastGroup(ActorPaths.GithubCoordinatorActor.Path + "1",
            ActorPaths.GithubCoordinatorActor.Path + "2",
            ActorPaths.GithubCoordinatorActor.Path + "3")));
    base.PreStart();
}

And with that, you're all set!

Once you're done

You should be able to run GithubActors.sln now and see that you can launch up to three jobs in parallel - a big improvement that didn't take very much code!

GithubActors without parallelism

NOTE: If you're following along using the eBook / .ePub, you won't see the animation. Click here to see it.

As a result of the changes you made, the actor hierarchy for GithubActors now looks like this:

Final state of GithubActors hierarchy after lesson 1

Now we have 3 separate GithubCoordinatorActor instances who are all available for GitHub repository analysis jobs.

Great job!

Awesome job! You've successfully used Akka.NET routers to achieve the first layer of parallelism we're going to add to our GitHub scraper!

Let's move onto Lesson 2 - Using Pool routers to automatically create and manage pools of actors.

Any questions?

Come ask any questions you have, big or small, in this ongoing Bootcamp chat with the Petabridge & Akka.NET teams.

Problems with the code?

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.

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