Background tasks with hosted services in ASP.NET Core

By Luke Latham

In ASP.NET Core, background tasks can be implemented as hosted services. A hosted service is a class with background task logic that implements the IHostedService interface. This topic provides three hosted service examples:

  • Background task that runs on a timer.
  • Hosted service that activates a scoped service. The scoped service can use dependency injection.
  • Queued background tasks that run sequentially.

View or download sample code (how to download)

The sample app is provided in two versions:

  • Web Host – Web Host is useful for hosting web apps. The example code shown in this topic is from Web Host version of the sample. For more information, see the Web Host topic.
  • Generic Host – Generic Host is new in ASP.NET Core 2.1. For more information, see the Generic Host topic.

Worker Service template

The ASP.NET Core Worker Service template provides a starting point for writing long running service apps. To use the template as a basis for a hosted services app:

  1. Create a new project.
  2. Select ASP.NET Core Web Application. Select Next.
  3. Provide a project name in the Project name field or accept the default project name. Select Create.
  4. In the Create a new ASP.NET Core Web Application dialog, confirm that .NET Core and ASP.NET Core 3.0 are selected.
  5. Select the Worker Service template. Select Create.

Package

Reference the Microsoft.AspNetCore.App metapackage or add a package reference to the Microsoft.Extensions.Hosting package.

IHostedService interface

Hosted services implement the IHostedService interface. The interface defines two methods for objects that are managed by the host:

  • StartAsync(CancellationToken)StartAsync contains the logic to start the background task. When using the Web Host, StartAsync is called after the server has started and IApplicationLifetime.ApplicationStarted is triggered. When using the Generic Host, StartAsync is called before ApplicationStarted is triggered.

  • StopAsync(CancellationToken) – Triggered when the host is performing a graceful shutdown. StopAsync contains the logic to end the background task. Implement IDisposable and finalizers (destructors) to dispose of any unmanaged resources.

    The cancellation token has a default five second timeout to indicate that the shutdown process should no longer be graceful. When cancellation is requested on the token:

    • Any remaining background operations that the app is performing should be aborted.
    • Any methods called in StopAsync should return promptly.

    However, tasks aren't abandoned after cancellation is requested—the caller awaits all tasks to complete.

    If the app shuts down unexpectedly (for example, the app's process fails), StopAsync might not be called. Therefore, any methods called or operations conducted in StopAsync might not occur.

    To extend the default five second shutdown timeout, set:

The hosted service is activated once at app startup and gracefully shut down at app shutdown. If an error is thrown during background task execution, Dispose should be called even if StopAsync isn't called.

Timed background tasks

A timed background task makes use of the System.Threading.Timer class. The timer triggers the task's DoWork method. The timer is disabled on StopAsync and disposed when the service container is disposed on Dispose:

internal class TimedHostedService : IHostedService, IDisposable
{
    private readonly ILogger _logger;
    private Timer _timer;

    public TimedHostedService(ILogger<TimedHostedService> logger)
    {
        _logger = logger;
    }

    public Task StartAsync(CancellationToken cancellationToken)
    {
        _logger.LogInformation("Timed Background Service is starting.");

        _timer = new Timer(DoWork, null, TimeSpan.Zero, 
            TimeSpan.FromSeconds(5));

        return Task.CompletedTask;
    }

    private void DoWork(object state)
    {
        _logger.LogInformation("Timed Background Service is working.");
    }

    public Task StopAsync(CancellationToken cancellationToken)
    {
        _logger.LogInformation("Timed Background Service is stopping.");

        _timer?.Change(Timeout.Infinite, 0);

        return Task.CompletedTask;
    }

    public void Dispose()
    {
        _timer?.Dispose();
    }
}

The service is registered in Startup.ConfigureServices with the AddHostedService extension method:

services.AddHostedService<TimedHostedService>();

Consuming a scoped service in a background task

To use scoped services within an IHostedService, create a scope. No scope is created for a hosted service by default.

The scoped background task service contains the background task's logic. In the following example, an ILogger is injected into the service:

internal interface IScopedProcessingService
{
    void DoWork();
}

internal class ScopedProcessingService : IScopedProcessingService
{
    private readonly ILogger _logger;
    
    public ScopedProcessingService(ILogger<ScopedProcessingService> logger)
    {
        _logger = logger;
    }

    public void DoWork()
    {
        _logger.LogInformation("Scoped Processing Service is working.");
    }
}

The hosted service creates a scope to resolve the scoped background task service to call its DoWork method:

internal class ConsumeScopedServiceHostedService : IHostedService
{
    private readonly ILogger _logger;

    public ConsumeScopedServiceHostedService(IServiceProvider services, 
        ILogger<ConsumeScopedServiceHostedService> logger)
    {
        Services = services;
        _logger = logger;
    }

    public IServiceProvider Services { get; }

    public Task StartAsync(CancellationToken cancellationToken)
    {
        _logger.LogInformation(
            "Consume Scoped Service Hosted Service is starting.");

        DoWork();

        return Task.CompletedTask;
    }

    private void DoWork()
    {
        _logger.LogInformation(
            "Consume Scoped Service Hosted Service is working.");

        using (var scope = Services.CreateScope())
        {
            var scopedProcessingService = 
                scope.ServiceProvider
                    .GetRequiredService<IScopedProcessingService>();

            scopedProcessingService.DoWork();
        }
    }

    public Task StopAsync(CancellationToken cancellationToken)
    {
        _logger.LogInformation(
            "Consume Scoped Service Hosted Service is stopping.");

        return Task.CompletedTask;
    }
}

The services are registered in Startup.ConfigureServices. The IHostedService implementation is registered with the AddHostedService extension method:

services.AddHostedService<ConsumeScopedServiceHostedService>();
services.AddScoped<IScopedProcessingService, ScopedProcessingService>();

Queued background tasks

A background task queue is based on the .NET 4.x QueueBackgroundWorkItem (tentatively scheduled to be built-in for ASP.NET Core 3.0):

public interface IBackgroundTaskQueue
{
    void QueueBackgroundWorkItem(Func<CancellationToken, Task> workItem);

    Task<Func<CancellationToken, Task>> DequeueAsync(
        CancellationToken cancellationToken);
}

public class BackgroundTaskQueue : IBackgroundTaskQueue
{
    private ConcurrentQueue<Func<CancellationToken, Task>> _workItems = 
        new ConcurrentQueue<Func<CancellationToken, Task>>();
    private SemaphoreSlim _signal = new SemaphoreSlim(0);

    public void QueueBackgroundWorkItem(
        Func<CancellationToken, Task> workItem)
    {
        if (workItem == null)
        {
            throw new ArgumentNullException(nameof(workItem));
        }

        _workItems.Enqueue(workItem);
        _signal.Release();
    }

    public async Task<Func<CancellationToken, Task>> DequeueAsync(
        CancellationToken cancellationToken)
    {
        await _signal.WaitAsync(cancellationToken);
        _workItems.TryDequeue(out var workItem);

        return workItem;
    }
}

In QueueHostedService, background tasks in the queue are dequeued and executed as a BackgroundService, which is a base class for implementing a long running IHostedService:

public class QueuedHostedService : BackgroundService
{
    private readonly ILogger _logger;

    public QueuedHostedService(IBackgroundTaskQueue taskQueue, 
        ILoggerFactory loggerFactory)
    {
        TaskQueue = taskQueue;
        _logger = loggerFactory.CreateLogger<QueuedHostedService>();
    }

    public IBackgroundTaskQueue TaskQueue { get; }

    protected async override Task ExecuteAsync(
        CancellationToken cancellationToken)
    {
        _logger.LogInformation("Queued Hosted Service is starting.");

        while (!cancellationToken.IsCancellationRequested)
        {
            var workItem = await TaskQueue.DequeueAsync(cancellationToken);

            try
            {
                await workItem(cancellationToken);
            }
            catch (Exception ex)
            {
                _logger.LogError(ex, 
                   $"Error occurred executing {nameof(workItem)}.");
            }
        }

        _logger.LogInformation("Queued Hosted Service is stopping.");
    }
}

The services are registered in Startup.ConfigureServices. The IHostedService implementation is registered with the AddHostedService extension method:

services.AddHostedService<QueuedHostedService>();
services.AddSingleton<IBackgroundTaskQueue, BackgroundTaskQueue>();

In the Index page model class:

  • The IBackgroundTaskQueue is injected into the constructor and assigned to Queue.
  • An IServiceScopeFactory is injected and assigned to _serviceScopeFactory. The factory is used to create instances of IServiceScope, which is used to create services within a scope. A scope is created in order to use the app's AppDbContext (a scoped service) to write database records in the IBackgroundTaskQueue (a singleton service).
public class IndexModel : PageModel
{
    private readonly AppDbContext _db;
    private readonly ILogger _logger;
    private readonly IServiceScopeFactory _serviceScopeFactory;

    public IndexModel(AppDbContext db, IBackgroundTaskQueue queue, 
        ILogger<IndexModel> logger, IServiceScopeFactory serviceScopeFactory)
    {
        _db = db;
        _logger = logger;
        Queue = queue;
        _serviceScopeFactory = serviceScopeFactory;
    }

    public IBackgroundTaskQueue Queue { get; }

When the Add Task button is selected on the Index page, the OnPostAddTask method is executed. QueueBackgroundWorkItem is called to enqueue the work item:

public IActionResult OnPostAddTaskAsync()
{
    Queue.QueueBackgroundWorkItem(async token =>
    {
        var guid = Guid.NewGuid().ToString();

        using (var scope = _serviceScopeFactory.CreateScope())
        {
            var scopedServices = scope.ServiceProvider;
            var db = scopedServices.GetRequiredService<AppDbContext>();

            for (int delayLoop = 1; delayLoop < 4; delayLoop++)
            {
                try
                {
                    db.Messages.Add(
                        new Message() 
                        { 
                            Text = $"Queued Background Task {guid} has " +
                                $"written a step. {delayLoop}/3"
                        });
                    await db.SaveChangesAsync();
                }
                catch (Exception ex)
                {
                    _logger.LogError(ex, 
                        "An error occurred writing to the " +
                        $"database. Error: {ex.Message}");
                }

                await Task.Delay(TimeSpan.FromSeconds(5), token);
            }
        }

        _logger.LogInformation(
            $"Queued Background Task {guid} is complete. 3/3");
    });

    return RedirectToPage();
}

Additional resources