2

I'm trying to execute logic on start up of my web API and this code will also be async.

I've thought of IStartupFilter but this is sync, which is a problem. StartUp.Configure() is also not an option, sync.

I've thought of IHostedService but this runs before the application is done loading and up for requests.

Any other ways of doing this ?

Camilo Terevinto
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Francis Ducharme
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2 Answers2

4

Something you probably didn't realize is that running the application has 2 parts, that are hidden by default when you call IHost.Run/IHost.RunAsync.

Basically, this:

var host = CreateHostBuilder(args).Build();
await host.RunAsync();

Is equivalent to this:

var host = CreateHostBuilder(args).Build();

await host.StartAsync();
await host.WaitForShutdownAsync();

When StartAsync returns, the application has already started and is ready to consume requests, so you probably want to do this:

var host = CreateHostBuilder(args).Build();

await host.StartAsync();

await PerformSomeWorkAfterStartupAsync();

await host.WaitForShutdownAsync();
Camilo Terevinto
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  • Very useful. Is there a way to register a service/middleware/etc so it runs right where `PerformSomeWorkAfterStartupAsync()` is without having a dev. edit that specific part ? – Francis Ducharme Oct 24 '21 at 19:10
  • @FrancisDucharme Not that I know of, this was the best I could find after quite an extensive search, as I needed something similar some months ago – Camilo Terevinto Oct 24 '21 at 19:11
0

Implement IStartupTask, register StartupTaskRunner and YourStartupTask in DI:

services    
    .AddStartupTasksRunner()
    .AddStartupTask<YourStartupTask>();

And here the code based on Andrew's Lock post:

public interface IAsyncStartupTask
{
    Task OnStartupAsync(CancellationToken cancellationToken);
}

internal class StartupTasksRunner : IHostedService
{
    private readonly IEnumerable<IAsyncStartupTask> _startupTasks;

    private readonly IHostApplicationLifetime _applicationLifetime;

    private readonly SemaphoreSlim _semaphore;

    public StartupTasksRunner(IEnumerable<IAsyncStartupTask> startupTasks, IHostApplicationLifetime applicationLifetime)
    {
        _startupTasks = startupTasks;
        _applicationLifetime = applicationLifetime;

        _semaphore = new SemaphoreSlim(0);
        applicationLifetime.ApplicationStarted.Register(() => _semaphore.Release());
    }

    public async Task StartAsync(CancellationToken cancellationToken)
    {
        // wait for ApplicationStarted event to execute when app is listening to web requests
        // await _semaphore.WaitAsync(cancellationToken);

        foreach (var task in _startupTasks)
        {
            try
            {
                await task.OnStartupAsync(cancellationToken).ConfigureAwait(false);
            }
            catch
            {
                // stop the application if failing startup task if fatal
                //_applicationLifetime.StopApplication();
                throw;
            }
        }
    }

    public Task StopAsync(CancellationToken cancellationToken)
    {
        return Task.CompletedTask;
    }
}

public static class ServiceCollectionExtensions
{
    public static IServiceCollection AddStartupTasksRunner(this IServiceCollection services)
    {
        return services.AddHostedService<StartupTasksRunner>();
    }

    public static IServiceCollection AddStartupTask<TStartupTask>(this IServiceCollection services)
        where TStartupTask : class, IAsyncStartupTask
    {
        return services.AddSingleton<IAsyncStartupTask, TStartupTask>();
    }
}

Notes
The app startup order is:

  1. Start HostedService's
  2. Start Kesterl Server
  3. Fire ApplicationStarted event

If you want to run your startup task before HostedServices started you need to make sure StartupTasksRunner registered before any other HostedService (they are started in registration order).

If you want to run your startup task after all HostedServices started but before application starts to receive web requests make sure you are registering StartupTasksRunner after any other HostedService.

If you want to run your startup task after Kestrel Server is started uncomment // await _semaphore.WaitAsync(cancellationToken); line. Be aware that it may run concurrently with web requests handling.

Also be aware that exceptions thrown by IHostedService.StartAsync method will be swallowed by framework. So if successful execution of startup tasks is necessary for your application uncomment // _applicationLifetime.StopApplication();

Artur
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