Run a background task from a controller action in asp.net core 2

Question

I am developing a web application with a REST Api using C# with asp.net core 2.0

What I want to achieve is when the client send a request to an endpoint I will run a background task separated from the client request context which will be ended if the task started successfully.

I know there is HostedService but the problem is that the HostedService starts when the server starts, and as far as I know there is no way to start the HostedService manually from a controller.

Here is a simple code that demonstrate the question.

[Authorize(AuthenticationSchemes = "UsersScheme")]
public class UsersController : Controller
{

    [HttpPost]
    public async Task<JsonResult> StartJob([FromForm] string UserId, [FromServices] IBackgroundJobService backgroundService) {

           //check user account
           (bool isStarted, string data) result = backgroundService.Start();

           return JsonResult(result);
    }
}

Answer 1

You still can use IHostedService as base for background tasks in combination with BlockingCollection.

Create wrapper for BlockingCollection so you can inject it as singleton.

public class TasksToRun
{
    private readonly BlockingCollection<TaskSettings> _tasks;

    public TasksToRun() => _tasks = new BlockingCollection<TaskSettings>();

    public void Enqueue(TaskSettings settings) => _tasks.Add(settings);

    public TaskSettings Dequeue(CancellationToken token) => _tasks.Take(token);
}

Then in implementation of IHostedService "listen" for tasks and when tasks "arrive" execute it.
BlockingCollection will stop execution if collection is empty - so your while loop will not consume processor time.
.Take method accept cancellationToken as argument. With token you can cancel "waiting" for next task when application stops.

public class BackgroundService : IHostedService
{
    private readonly TasksToRun _tasks;

    private CancellationTokenSource _tokenSource;

    private Task _currentTask;

    public BackgroundService(TasksToRun tasks) => _tasks = tasks;

    public async Task StartAsync(CancellationToken cancellationToken)
    {
        _tokenSource = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
        while (cancellationToken.IsCancellationRequested == false)
        {
            try
            {
                var taskToRun = _tasks.Dequeue(_tokenSource.Token);

                // We need to save executable task, 
                // so we can gratefully wait for it's completion in Stop method
                _currentTask = ExecuteTask(taskToRun);               
                await _currentTask;
            }
            catch (OperationCanceledException)
            {
                // execution cancelled
            }
        }
    }

    public async Task StopAsync(CancellationToken cancellationToken)
    {
        _tokenSource.Cancel(); // cancel "waiting" for task in blocking collection

        if (_currentTask == null) return;

        // wait when _currentTask is complete
        await Task.WhenAny(_currentTask, Task.Delay(-1, cancellationToken));
    }
}

And in the controller you simply add task you want to run to our collection

public class JobController : Controller
{
    private readonly TasksToRun _tasks;

    public JobController(TasksToRun tasks) => _tasks = tasks;

    public IActionResult PostJob()
    {
        var settings = CreateTaskSettings();

        _tasks.Enqueue(settings);

        return Ok();
    }
}

Wrapper for blocking collection should be registered for dependency injection as singleton

services.AddSingleton<TasksToRun, TasksToRun>();

Register background service

services.AddHostedService<BackgroundService>();

Answer 2

Microsoft has documented the same at https://docs.microsoft.com/en-us/aspnet/core/fundamentals/host/hosted-services?view=aspnetcore-2.1

It accomplishes using BackgroundTaskQueue, which gets work assigned from Controller and the work is performed by QueueHostedService which derives from BackgroundService.