How can I know when Kestrel has started listening?

Question

I need to notify systemd that my service has started up successfully, and a task it needs to run after startup requires that the server is already listening on the target Unix domain socket.

I am using IWebHost::Run to start the server, and that is a blocking call. Additionally, I am unable to find any obvious way to set a delegate or callback event for successful initialization.

Anyone?

Answer 1

• On .Net Core 1.x it is safe to just run IWebHost.Start() and assume that the server is initialized afterwards (instead of Run() that blocks the thread). Check the source.

  var host = new WebHostBuilder()
      .UseKestrel()
      (...)
      .Build();
  
  host.Start();
  

• If you are using .NET Core 2.0 Preview 1 (or later), the source is different, the synchronous method is not available anymore so you should await IWebHost.StartAsync() and assume everything is ready afterwards.

Answer 2

You may use Microsoft.AspNetCore.Hosting.IApplicationLifetime:

/// <summary>
/// Triggered when the application host has fully started and is about to wait
/// for a graceful shutdown.
/// </summary>
CancellationToken ApplicationStarted { get; }

Look into this SO post for the configuration example.

Answer 3

This is what I ended up going with, for now. Seems to be working fine:

host.Start();

Log.Information("Press Ctrl+C to shut down...");
Console.CancelKeyPress += OnConsoleCancelKeyPress;

var waitHandles = new WaitHandle[] {
    CancelTokenSource.Token.WaitHandle
};

WaitHandle.WaitAll(waitHandles);
Log.Information("Shutting down...");

Then, in the Ctrl+C event handler:

private static void OnConsoleCancelKeyPress(object sender, ConsoleCancelEventArgs e)
{
    Log.Debug("Got Ctrl+C from console.");
    CancelTokenSource.Cancel();
}