Run a background task from a controller action in ASP.NET Core

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 demonstrates 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.