How to implement parallel long running background tasks in .NET Core 3?

Question

I have .NET Core console application containing two independent tasks that should be running in parallel for the entire life-time of the application. I was thinking to use BackgroundService:

class BackgroundTaskOne : BackgroundService
{
    protected override Task ExecuteAsync(CancellationToken stoppingToken)
    {
        while (!stoppingToken.IsCancellationRequested)
        {
            try
            {
                // do long running task for the entire life-time of application
                while(true)
                {
                    // do work one
                }
            }
            catch (Exception e)
            {
                // log
            }
        }

        return Task.CompletedTask;
    }
}

class BackgroundTaskTwo : BackgroundService
{
    protected override Task ExecuteAsync(CancellationToken stoppingToken)
    {
        while (!stoppingToken.IsCancellationRequested)
        {
            try
            {
                // do long running task for the entire life-time of application
                while(true)
                {
                    // do work two
                }
            }
            catch (Exception e)
            {
                // log
            }
        }

        return Task.CompletedTask;
    }
}

And register them like this:

services.AddHostedService<BackgroundTaskOne>();
services.AddHostedService<BackgroundTaskTwo>();

But these are going to run in order. So I have two questions:

Is there a way to make these two run in parallel?
Are there any other alternatives to run two long-running background processes in .NET Core in parallel?

Have you look at MSDN's Task-based asynchronous programming? Lots of good ideas in there. You can create Tasks, you can create whole Threads... — Sean Skelly
– Sean Skelly, Commented Apr 21, 2020 at 19:20
add await Task.Yield() as the first call in the ExecuteAsync method — baltermia
– baltermia, Commented Nov 7, 2024 at 10:46

Sir Rufo · Accepted Answer · 2020-04-21 19:41:33Z

1

The docs of BackgroundService.ExecuteAsync say

The implementation should return a task that represents the lifetime of the long running operation(s) being performed.

Your implementation returns a completed task when the whole work is done. In fact you implemented it to run sync and not async and that is the reason for not running parallel.

Here is a sample implementation with some fake async work:

class BackgroundTaskOne : BackgroundService
{
    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        while (!stoppingToken.IsCancellationRequested)
        {
            try
            {
                // do work one
                await Task.Delay( 100 );
            }
            catch (Exception e)
            {
                // log
            }
        }
    }
}

answered Apr 21, 2020 at 19:41

Sir Rufo

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2 Comments

havij Over a year ago

Ah I see. Actually the work I am performing in the try block is the actual long-running task that needs to be running for the entire lifetime of the application (just updated the question), so awaing that wouldn't help. I want to fire and forget these two tasks.

Sir Rufo Over a year ago

It does not matter how long the task is running, you have to have a real task and not a sync operation and returning a completed task at the end

Yury Schkatula · Accepted Answer · 2020-04-21 19:41:43Z

1

As documentation says: https://github.com/dotnet/AspNetCore.Docs/blob/master/aspnetcore/fundamentals/host/hosted-services.md

No further services are started until ExecuteAsync becomes asynchronous, such as by calling await.

As long as you do not have async clause mentioned at the ExecuteAsync method above, I suspect your method is synchronous at whole. This is why two services are called sequentially, not in parallel. Give them a break, introduce good amount of awaitable code.

answered Apr 21, 2020 at 19:41

Yury Schkatula

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