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Question

Eg of functionality There is 20 users and they clicked send button almost in one time, so methods stacking in queue and first user message is sent and response received, after second third and so on. Users wont chat with other people but with device which response is pretty fast
So I am trying to queue Task which sends Message.
I found code samples that uses Task queuing as shown in Example 1 and Example 2 .

Example 1

public class SerialQueue
{
    readonly object _locker = new object();
    WeakReference<Task> _lastTask;

    public Task Enqueue(Action action)
    {
        return Enqueue<object>(() => {
            action();
            return null;
        });
    }

    public Task<T> Enqueue<T>(Func<T> function)
    {
        lock (_locker)
        {
            Task lastTask = null;
            Task<T> resultTask = null;

            if (_lastTask != null && _lastTask.TryGetTarget(out lastTask))
            {
                resultTask = lastTask.ContinueWith(_ => function());
            }
            else
            {
                resultTask = Task.Run(function);
            }

            _lastTask = new WeakReference<Task>(resultTask);
            return resultTask;
        }
    }
}

Example 2

 public class TaskQueue
{
    private readonly SemaphoreSlim _semaphoreSlim;

    public TaskQueue()
    {
        _semaphoreSlim = new SemaphoreSlim(1);
    }

    public async Task<T> Enqueue<T>(Func<Task<T>> taskGenerator)
    {
        await _semaphoreSlim.WaitAsync();

        try
        {
            return await taskGenerator();
        }
        finally
        {
            _semaphoreSlim.Release();
        }
    }

    public async Task Enqueue(Func<Task> taskGenerator)
    {
        await _semaphoreSlim.WaitAsync();
        try
        {
            await taskGenerator();
        }
        finally
        {
            _semaphoreSlim.Release();
        }
    }
}

Problem is that when I'm passing task which I want to queue ( Example 3 ) each time I pressing button, tasks still are executed at the same time and interrupting each other.

Example 3

 [HttpPost(Name = "add-message")]
        public async Task<IActionResult> PostMessage([FromBody] MessengerViewModel messengerViewModel)
        {
            TaskQueue taskQueue = new TaskQueue();
            SerialQueue serialQueue = new SerialQueue();

            await taskQueue.Enqueue(() => SendMessage(messengerViewModel.PhoneNr, messengerViewModel.MessageBody,
                messengerViewModel.ContactId, messengerViewModel.State));
//I'm not running tasks at same time, using one or other at time
            await serialQueue.Enqueue(() => SendMessage(messengerViewModel.PhoneNr, messengerViewModel.MessageBody,
                messengerViewModel.ContactId, messengerViewModel.State));

            return Ok();
        }

How could I solve problem and stack task to queue by each click?

Answer 1

Your problem is that you create a new TaskQueue and SerialQueue everytime. Thus each time a user clicks/invokes PostMessage a new queue is created, and the task is the first task in the queue and executed directly.

You should use a static/singleton queue so each click/invoke works on the same queue object.

But that would deliver problems when you scale your webapp across multiple servers. To that end you should use things like (for example) Azure Queue Storage in combination with Azure Functions.

---

Startup.cs

public void ConfigureServices(IServiceCollection services)
{
    services.AddSingleton<TaskQueue>();
    services.AddSingleton<SerialQueue>();
    // the rest
}

SomeController.cs

[HttpPost(Name = "add-message")]
public async Task<IActionResult> PostMessage(
    [FromBody] MessengerViewModel messengerViewModel,
    [FromServices] TaskQueue taskQueue,
    [FromServices] SerialQueue serialQueue)
{
    await taskQueue.Enqueue(
        () => SendMessage(
                  messengerViewModel.PhoneNr, 
                  messengerViewModel.MessageBody,
                  messengerViewModel.ContactId, 
                  messengerViewModel.State));
    //I'm not running tasks at same time, using one or other at time
    await serialQueue.Enqueue(
        () => SendMessage(
                  messengerViewModel.PhoneNr, 
                  messengerViewModel.MessageBody,
                  messengerViewModel.ContactId, 
                  messengerViewModel.State));

    return Ok();
}