How to write a linux daemon with .Net Core - linux

Question

I could just write a long-running CLI app and run it, but I'm assuming it wouldn't comply to all the expectations one would have of a standards-compliant linux daemon (responding to SIGTERM, Started by System V init process, Ignore terminal I/O signals, etc.)
Most ecosystems have some best-practice way of doing this, for example, in python, you can use https://pypi.python.org/pypi/python-daemon/
Is there some documentation about how to do this with .Net Core?

Answer 1

I toyed with an idea similar to how .net core web host waits for shutdown in console applications. I was reviewing it on GitHub and was able to extract the gist of how they performed the Run
https://github.com/aspnet/Hosting/blob/15008b0b7fcb54235a9de3ab844c066aaf42ea44/src/Microsoft.AspNetCore.Hosting/WebHostExtensions.cs#L86
public static class ConsoleHost {
/// <summary>
/// Block the calling thread until shutdown is triggered via Ctrl+C or SIGTERM.
/// </summary>
public static void WaitForShutdown() {
WaitForShutdownAsync().GetAwaiter().GetResult();
}
/// <summary>
/// Runs an application and block the calling thread until host shutdown.
/// </summary>
/// <param name="host">The <see cref="IWebHost"/> to run.</param>
public static void Wait() {
WaitAsync().GetAwaiter().GetResult();
}
/// <summary>
/// Runs an application and returns a Task that only completes when the token is triggered or shutdown is triggered.
/// </summary>
/// <param name="host">The <see cref="IConsoleHost"/> to run.</param>
/// <param name="token">The token to trigger shutdown.</param>
public static async Task WaitAsync(CancellationToken token = default(CancellationToken)) {
//Wait for the token shutdown if it can be cancelled
if (token.CanBeCanceled) {
await WaitAsync(token, shutdownMessage: null);
return;
}
//If token cannot be cancelled, attach Ctrl+C and SIGTERN shutdown
var done = new ManualResetEventSlim(false);
using (var cts = new CancellationTokenSource()) {
AttachCtrlcSigtermShutdown(cts, done, shutdownMessage: "Application is shutting down...");
await WaitAsync(cts.Token, "Application running. Press Ctrl+C to shut down.");
done.Set();
}
}
/// <summary>
/// Returns a Task that completes when shutdown is triggered via the given token, Ctrl+C or SIGTERM.
/// </summary>
/// <param name="token">The token to trigger shutdown.</param>
public static async Task WaitForShutdownAsync(CancellationToken token = default (CancellationToken)) {
var done = new ManualResetEventSlim(false);
using (var cts = CancellationTokenSource.CreateLinkedTokenSource(token)) {
AttachCtrlcSigtermShutdown(cts, done, shutdownMessage: string.Empty);
await WaitForTokenShutdownAsync(cts.Token);
done.Set();
}
}
private static async Task WaitAsync(CancellationToken token, string shutdownMessage) {
if (!string.IsNullOrEmpty(shutdownMessage)) {
Console.WriteLine(shutdownMessage);
}
await WaitForTokenShutdownAsync(token);
}
private static void AttachCtrlcSigtermShutdown(CancellationTokenSource cts, ManualResetEventSlim resetEvent, string shutdownMessage) {
Action ShutDown = () => {
if (!cts.IsCancellationRequested) {
if (!string.IsNullOrWhiteSpace(shutdownMessage)) {
Console.WriteLine(shutdownMessage);
}
try {
cts.Cancel();
} catch (ObjectDisposedException) { }
}
//Wait on the given reset event
resetEvent.Wait();
};
AppDomain.CurrentDomain.ProcessExit += delegate { ShutDown(); };
Console.CancelKeyPress += (sender, eventArgs) => {
ShutDown();
//Don't terminate the process immediately, wait for the Main thread to exit gracefully.
eventArgs.Cancel = true;
};
}
private static async Task WaitForTokenShutdownAsync(CancellationToken token) {
var waitForStop = new TaskCompletionSource<object>();
token.Register(obj => {
var tcs = (TaskCompletionSource<object>)obj;
tcs.TrySetResult(null);
}, waitForStop);
await waitForStop.Task;
}
}
I tried adapting something like a IConsoleHost but quickly realized I was over-engineering it. Extracted the main parts into something like await ConsoleUtil.WaitForShutdownAsync(); that operated like Console.ReadLine
This then allowed the utility to be used like this
public class Program {
public static async Task Main(string[] args) {
//relevant code goes here
//...
//wait for application shutdown
await ConsoleUtil.WaitForShutdownAsync();
}
}
from there creating a systemd as in the following link should get you the rest of the way
Writing a Linux daemon in C#

Answer 2

The best I could come up with is based on the answer to two other questions: Killing gracefully a .NET Core daemon running on Linux and Is it possible to await an event instead of another async method?
using System;
using System.Runtime.Loader;
using System.Threading.Tasks;
namespace ConsoleApp1
{
public class Program
{
private static TaskCompletionSource<object> taskToWait;
public static void Main(string[] args)
{
taskToWait = new TaskCompletionSource<object>();
AssemblyLoadContext.Default.Unloading += SigTermEventHandler;
Console.CancelKeyPress += new ConsoleCancelEventHandler(CancelHandler);
//eventSource.Subscribe(eventSink) or something...
taskToWait.Task.Wait();
AssemblyLoadContext.Default.Unloading -= SigTermEventHandler;
Console.CancelKeyPress -= new ConsoleCancelEventHandler(CancelHandler);
}
private static void SigTermEventHandler(AssemblyLoadContext obj)
{
System.Console.WriteLine("Unloading...");
taskToWait.TrySetResult(null);
}
private static void CancelHandler(object sender, ConsoleCancelEventArgs e)
{
System.Console.WriteLine("Exiting...");
taskToWait.TrySetResult(null);
}
}
}

Answer 3

Implementing Linux Daemon or service for windows quite easy with single codebase using Visual Studio 2019. Just create project using WorkerService template. In my case I have
Coraval library to schedule the tasks.
Program.cs class
public class Program
{
public static void Main(string[] args)
{
Log.Logger = new LoggerConfiguration()
.MinimumLevel.Debug()
.MinimumLevel.Override("Microsoft", LogEventLevel.Warning)
.Enrich.FromLogContext()
.WriteTo.File(#"C:\temp\Workerservice\logfile.txt").CreateLogger();
IHost host = CreateHostBuilder(args).Build();
host.Services.UseScheduler(scheduler =>
{
scheduler
.Schedule<ReprocessInvocable>()
.EveryThirtySeconds();
});
host.Run();
}
public static IHostBuilder CreateHostBuilder(string[] args) =>
Host.CreateDefaultBuilder(args).UseSystemd() //.UseWindowsService()
.ConfigureServices(services =>
{
services.AddScheduler();
services.AddTransient<ReprocessInvocable>();
});
}
ReprocessInvocable.cs class
public class ReprocessInvocable : IInvocable
{
private readonly ILogger<ReprocessInvocable> _logger;
public ReprocessInvocable(ILogger<ReprocessInvocable> logger)
{
_logger = logger;
}
public async Task Invoke()
{
//your code goes here
_logger.LogInformation("Information - Worker running at: {time}", DateTimeOffset.Now);
_logger.LogWarning("Warning - Worker running at: {time}", DateTimeOffset.Now);
_logger.LogCritical("Critical - Worker running at: {time}", DateTimeOffset.Now);
Log.Information("Invoke has called at: {time}", DateTimeOffset.Now);
}
}
For linux daemon use UseSystemd and for windows service use UseWindowsService as per the above code.

Answer 4

If you're trying to find something more robust, I found an implementation on Github that looks promising: .NET Core Application blocks for message-based communication. It uses Host, HostBuilder, ApplicationServices, ApplicationEnvironment, etc classes to implement a messaging service.
It doesn't quite look ready for black box reuse, but it seems like it could be a good starting point.
var host = new HostBuilder()
.ConfigureServices(services =>
{
var settings = new RabbitMQSettings { ServerName = "192.168.80.129", UserName = "admin", Password = "Pass#word1" };
})
.Build();
Console.WriteLine("Starting...");
await host.StartAsync();
var messenger = host.Services.GetRequiredService<IRabbitMQMessenger>();
Console.WriteLine("Running. Type text and press ENTER to send a message.");
Console.CancelKeyPress += async (sender, e) =>
{
Console.WriteLine("Shutting down...");
await host.StopAsync(new CancellationTokenSource(3000).Token);
Environment.Exit(0);
};
...

Answer 5

Have you tried Thread.Sleep (Timeout.Infinite) ?
using System;
using System.IO;
using System.Threading;
namespace Daemon {
class Program {
static int Main(string[] args) {
if (Environment.OSVersion.Platform == PlatformID.Win32NT) {
Log.Critical("Windows is not supported!");
return 1;
}
Agent.Init();
Agent.Start();
if (Agent.Settings.DaemonMode || args.FirstOrDefault() == "daemon") {
Log.Info("Daemon started.");
Thread.Sleep(Timeout.Infinite);
}
Agent.Stop();
}
}
}

Answer 6

I'm not sure it is production grade, but for a quick and dirty console app this works well:
await Task.Delay(-1); //-1 indicates infinite timeout