Question: ActivitySource behavior with multiple subscribed ActivityListeners

[alanwest]

Description

When an Activity is started with the new ActivitySource API if any subscribed ActivityListener responds with anything higher than ActivityDataRequest.None then all subscribed ActivityListener.ActivityStarted/Stopped events are fired.

For example:

public static void Main(string[] args)
        {
            var listenerA = new ActivityListener()
            {
                ActivityStarted = (a) => System.Console.WriteLine("listenerA.ActivityStarted"),
                ActivityStopped = (a) => System.Console.WriteLine("listenerA.ActivityStopped"),
                ShouldListenTo = _ => true,
                GetRequestedDataUsingParentId = (ref ActivityCreationOptions<string> options) => ActivityDataRequest.None,
                GetRequestedDataUsingContext = (ref ActivityCreationOptions<ActivityContext> options) => ActivityDataRequest.None,
            };

            var listenerB = new ActivityListener()
            {
                ActivityStarted = (a) => System.Console.WriteLine("listenerB.ActivityStarted"),
                ActivityStopped = (a) => System.Console.WriteLine("listenerB.ActivityStopped"),
                ShouldListenTo = _ => true,
                GetRequestedDataUsingParentId = (ref ActivityCreationOptions<string> options) => ActivityDataRequest.AllData,
                GetRequestedDataUsingContext = (ref ActivityCreationOptions<ActivityContext> options) => ActivityDataRequest.AllData,
            };

            ActivitySource.AddActivityListener(listenerA);
            ActivitySource.AddActivityListener(listenerB);

            var activitySource = new ActivitySource("MyActivitySource");

            var activity = activitySource.StartActivity("MyActivity");
            activity?.Stop();
        }

This code produces this output:

listenerA.ActivityStarted
listenerB.ActivityStarted
listenerA.ActivityStopped
listenerB.ActivityStopped

Even though listenerA responds ActivityDataRequest.None it's ActivityStarted/Stopped events are fired. Is this the correct behavior? It does not seem intuitive.

@cijothomas

[ghost]

Tagging subscribers to this area: @tarekgh, @tommcdon, @pjanotti
See info in area-owners.md if you want to be subscribed.

[tarekgh]

This is by design, the responding value of ActivityDataRequest is not intended for getting the notification or not. it is intended to know if need to create the Activity object or not. If any listener responded with any value different than ActivityDataRequest.None then we'll create the Activity object and all listeners will get the notifications. The listener is free to set ActivityStarted/ActivityStopped to null if needed to stop getting any notifications.

[tarekgh]

CC @noahfalk

[cijothomas]

related: #31862 (comment)