Event groups

For some applications, it is useful to know which actor is processing an event derived from some user request. Coyote offers the notion of an EventGroup that can be tracked automatically. The following IActorRuntime APIs take an optional EventGroup parameter.

ActorId CreateActor(Type type, Event e = null, EventGroup group = null);
void SendEvent(ActorId target, Event e, EventGroup group = null);

When you pass a non-null group the runtime takes care of propagating it to any subsequent actors that might be created and across any SendEvent calls to any other actors. So all the work performed as a result of these actors and events can be grouped into a logical group.

The EventGroup argument on SendEvent is optional, the value null means pick up the CurrentEventGroup of the sender and pass it along to the target actor (if there is one).

The Actor class has a field that returns the current EventGroup. The EventGroup is stored with the event and CurrentEventGroup is set when the event is dequeued.

EventGroup CurrentEventGroup { get; set; }

Additionally you may use the following IActorRuntime API to get the current group of any actor.

EventGroup GetCurrentEventGroup(ActorId actorId);

The CurrentEventGroup is automatically passed along whenever you use CreateActor or SendEvent as shown below:

CreateActor CreateActor CreateActor SendEvent A B C CreateActor("A", g) EventGroup == g EventGroup == g EventGroup == g EventGroup == g when event is dequeued D E EventGroup == g


The base EventGroup class contains the following:

public Guid Id { get; internal set; }
public string Name { get; internal set; }

The Guid is automatically assigned to Guid.Empty. The Name defaults to null but you can provide any friendly name you want there.


As a convenience the following typed EventGroup is also provided that can also be used to wait for some result to be returned from the actors that share this group:

public class AwaitableEventGroup<T> : EventGroup
    public bool IsCompleted { get; }
    public Task<T> Task { get; }
    public virtual void SetResult(T result);
    public TaskAwaiter<T> GetAwaiter();

You can pass this group to your actors so that one of those actors can decide at some point to call SetResult. Then you can await on this object for the result. In this way you can asynchronously return a result from a collection of actors that are performing some job. SetResult also sets IsCompleted to true.

Clearing the CurrentEventGroup

You might need to clear the current event group at some point in your Actor. To do this you can simply set the property to null:

this.CurrentEventGroup = null;

However, this could be overridden by any subsequent event that is dequeued from the event queue. If you do not want the CurrentEventGroup to be passed along to the target actor you can pass a special NullEventGroup like this:

this.SendEvent(target, e, EventGroup.NullEventGroup);

This will put a null in the event queue of the target actor so that when this event is dequeued the CurrentEventGroup of the target actor will be set to null. In this way then you can propagate a null EventGroup to all your actors.

Custom EventGroups

The EventGroup class is unsealed so you can create any custom class that you need. The following is an example that counts a certain number of steps before completing the boolean AwaitableEventGroup:

public class EventGroupCounter : AwaitableEventGroup<bool>
    public int ExpectedCount;

    public EventGroupCounter(int expected)
        this.ExpectedCount = expected;

    public void Complete()
        var count = Interlocked.Decrement(ref this.ExpectedCount);
        if (count == 0)

This way you can have multiple actors calling Complete and the outer code that is waiting is not released until the expected count is reached.

Similarly you can create an EventGroup that gathers multiple results from various actors like this:

public class EventGroupList : AwaitableEventGroup<List<string>>
    public List<string> Items = new List<string>();

    public void AddItem(string msg)

    public void Complete()

Then when the final actor calls Complete() the list of gathered items is then made available to the waiting caller via SetResult.