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Question

I'd like to run a Task that has a " heartbeat " that keeps running at a specific time interval until the task completes.

I'm thinking an extension method like this would work well:

public static async Task WithHeartbeat(this Task primaryTask, TimeSpan heartbeatInterval, Action<CancellationToken> heartbeatAction, CancellationToken cancellationToken)

For example:

public class Program {
    public static void Main() {
        var cancelTokenSource = new CancellationTokenSource();
        var cancelToken = cancelTokenSource.Token;
        var longRunningTask = Task.Factory.StartNew(SomeLongRunningTask, cancelToken, TaskCreationOptions.LongRunning, TaskScheduler.Current);
        var withHeartbeatTask = longRunningTask.WithHeartbeat(TimeSpan.FromSeconds(1), PerformHeartbeat, cancelToken);
        withHeartbeatTask.Wait();
        Console.WriteLine("Long running task completed!");
        Console.ReadLine()
    }

    private static void SomeLongRunningTask() {
        Console.WriteLine("Starting long task");
        Thread.Sleep(TimeSpan.FromSeconds(9.5));
    }
    private static int _heartbeatCount = 0;
    private static void PerformHeartbeat(CancellationToken cancellationToken) {
        Console.WriteLine("Heartbeat {0}", ++_heartbeatCount);
    }
}

This program should output:

Starting long task
Heartbeat 1
Heartbeat 2
Heartbeat 3
Heartbeat 4
Heartbeat 5
Heartbeat 6
Heartbeat 7
Heartbeat 8
Heartbeat 9
Long running task completed!

Note that it should not (under normal circumstances) output "Heartbeat 10" since the heartbeat starts after the initial timeout (ie 1 second). Similarly, if the task takes less time than the heartbeat interval, the heartbeat should not occur at all.

What is a good way to implement this?

Background information: I have a service that's listening to an Azure Service Bus queue. I'd like to not Complete the message (which would permanently remove it from the queue) until I finish processing it, which could take longer than the maximum message LockDuration of 5 minutes. Thus, I need to use this heartbeat approach to call RenewLockAsync before the lock duration expires so that the message doesn't timeout while lengthy processing is occurring.

Answer 1

Here's my attempt:

public static class TaskExtensions {
    /// <summary>
    /// Issues the <paramref name="heartbeatAction"/> once every <paramref name="heartbeatInterval"/> while <paramref name="primaryTask"/> is running.
    /// </summary>
    public static async Task WithHeartbeat(this Task primaryTask, TimeSpan heartbeatInterval, Action<CancellationToken> heartbeatAction, CancellationToken cancellationToken) {
        if (cancellationToken.IsCancellationRequested) {
            return;
        }

        var stopHeartbeatSource = new CancellationTokenSource();
        cancellationToken.Register(stopHeartbeatSource.Cancel);

        await Task.WhenAny(primaryTask, PerformHeartbeats(heartbeatInterval, heartbeatAction, stopHeartbeatSource.Token));
        stopHeartbeatSource.Cancel();
    }

    private static async Task PerformHeartbeats(TimeSpan interval, Action<CancellationToken> heartbeatAction, CancellationToken cancellationToken) {
        while (!cancellationToken.IsCancellationRequested) {
            try {
                await Task.Delay(interval, cancellationToken);
                if (!cancellationToken.IsCancellationRequested) {
                    heartbeatAction(cancellationToken);
                }
            }
            catch (TaskCanceledException tce) {
                if (tce.CancellationToken == cancellationToken) {
                    // Totally expected
                    break;
                }
                throw;
            }
        }
    }
}

or with a slight tweak, you can even make the heartbeat async as in:

    /// <summary>
    /// Awaits a fresh Task created by the <paramref name="heartbeatTaskFactory"/> once every <paramref name="heartbeatInterval"/> while <paramref name="primaryTask"/> is running.
    /// </summary>
    public static async Task WithHeartbeat(this Task primaryTask, TimeSpan heartbeatInterval, Func<CancellationToken, Task> heartbeatTaskFactory, CancellationToken cancellationToken) {
        if (cancellationToken.IsCancellationRequested) {
            return;
        }

        var stopHeartbeatSource = new CancellationTokenSource();
        cancellationToken.Register(stopHeartbeatSource.Cancel);

        await Task.WhenAll(primaryTask, PerformHeartbeats(heartbeatInterval, heartbeatTaskFactory, stopHeartbeatSource.Token));

        if (!stopHeartbeatSource.IsCancellationRequested) {
            stopHeartbeatSource.Cancel();
        }
    }

    public static Task WithHeartbeat(this Task primaryTask, TimeSpan heartbeatInterval, Func<CancellationToken, Task> heartbeatTaskFactory) {
        return WithHeartbeat(primaryTask, heartbeatInterval, heartbeatTaskFactory, CancellationToken.None);
    }

    private static async Task PerformHeartbeats(TimeSpan interval, Func<CancellationToken, Task> heartbeatTaskFactory, CancellationToken cancellationToken) {
        while (!cancellationToken.IsCancellationRequested) {
            try {
                await Task.Delay(interval, cancellationToken);
                if (!cancellationToken.IsCancellationRequested) {
                    await heartbeatTaskFactory(cancellationToken);
                }
            }
            catch (TaskCanceledException tce) {
                if (tce.CancellationToken == cancellationToken) {
                    // Totally expected
                    break;
                }
                throw;
            }
        }
    }

which would allow you to change the sample code to something like this:

private static async Task PerformHeartbeat(CancellationToken cancellationToken) {
    Console.WriteLine("Starting heartbeat {0}", ++_heartbeatCount);
    await Task.Delay(1000, cancellationToken);
    Console.WriteLine("Finishing heartbeat {0}", _heartbeatCount);
}

The PerformHeartbeat could be replaced with an async call like RenewLockAsync so that you wouldn't have to waste thread time using a blocking call like RenewLock that the Action approach would require.

I'm answering my own question per SO guidelines , but I'm also open to more elegant approaches to this problem.

Answer 2

Here's my approach

using System;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApplication3
{
class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine("Start Main");
        StartTest().Wait();
        Console.ReadLine();
        Console.WriteLine("Complete Main");
    }

    static async Task StartTest()
    {
        var cts = new CancellationTokenSource();

        // ***Use ToArray to execute the query and start the download tasks. 
        Task<bool>[] tasks = new Task<bool>[2];
        tasks[0] = LongRunningTask("", 20, cts.Token);
        tasks[1] = Heartbeat("", 1, cts.Token);

        // ***Call WhenAny and then await the result. The task that finishes 
        // first is assigned to firstFinishedTask.
        Task<bool> firstFinishedTask = await Task.WhenAny(tasks);

        Console.WriteLine("first task Finished.");
        // ***Cancel the rest of the downloads. You just want the first one.
        cts.Cancel();

        // ***Await the first completed task and display the results. 
        // Run the program several times to demonstrate that different
        // websites can finish first.
        var isCompleted = await firstFinishedTask;
        Console.WriteLine("isCompleted:  {0}", isCompleted);
    }

    private static async Task<bool> LongRunningTask(string id, int sleep, CancellationToken ct)
    {
        Console.WriteLine("Starting long task");


        await Task.Delay(TimeSpan.FromSeconds(sleep));

        Console.WriteLine("Completed long task");
        return true;
    }

    private static async Task<bool> Heartbeat(string id, int sleep, CancellationToken ct)
    {
        while(!ct.IsCancellationRequested)
        {
            await Task.Delay(TimeSpan.FromSeconds(sleep));
            Console.WriteLine("Heartbeat Task Sleep: {0} Second", sleep);
        }

        return true;
    }

}

}