Short running background task in .NET Core

Question

I just discovered IHostedService and .NET Core 2.1 BackgroundService class. I think idea is awesome. Documentation.

All examples I found are used for long running tasks (until application die). But I need it for short time. Which is the correct way of doing it?

For example:
I want to execute a few queries (they will take approx. 10 seconds) after application starts. And only if in development mode. I do not want to delay application startup so IHostedService seems good approach. I can not use Task.Factory.StartNew, because I need dependency injection.

Currently I am doing like this:

public class UpdateTranslatesBackgroundService: BackgroundService
{
    private readonly MyService _service;

    public UpdateTranslatesBackgroundService(MyService service)
    {
        //MService injects DbContext, IConfiguration, IMemoryCache, ...
        this._service = service;
    }

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        await ...
    }
}

startup:

public static IServiceProvider Build(IServiceCollection services, ...)
{
    //.....
    if (hostingEnvironment.IsDevelopment())
        services.AddSingleton<IHostedService, UpdateTranslatesBackgroundService>();
    //.....
}

But this seems overkill. Is it? Register singleton (that means class exists while application lives). I don't need this. Just create class, run method, dispose class. All in background task.

Answer 1

Well I think there is more then one question here. First let me point out something you are probably aware of async != multithreaded. So BackgroundService will not make you app "multithreaded" it can run inside a single thread without no problem. And if you are doing blocking operations on that thread it will still block startup. Lets say in the class you implement all the sql queries in a not real async way something similar to

public class StopStartupService : BackgroundService
    {
        protected override Task ExecuteAsync(CancellationToken stoppingToken)
        {
            System.Threading.Thread.Sleep(1000);
            return Task.CompletedTask;
        }
    }

This will still block startup.

So there is another question.

How should you run background jobs?

For this in simple cases Task.Run(Try to avoid Task.Factory.StartNew if you are not sure how to configure it) should do the job, but that is not to say this is the best or a good way to do it. There are a bunch of open source libraries that will do this for you and it might be good to have a look at what they provide. There are a lot of problems you might not be aware of , that can create frustrating bugs if you just use Task.Run The second question I can see is.

Should I do fire and forget in c#?

For me this is a definite NO(but XAML people might not agree). No matter what you do, you need to keep track of when the thing you are doing is done. In your case you might want to do a rollback in the database if someone stops the app before the queries are done. But more than that you would want to know when you can start using the data that the queries provided. So BackgroundService helps you to simplify the execution but is difficult to keep track of completion.

Should you use a singleton?

As you already mentioned using singletons can be a dangerous thing especially if you don't clean things properly, but more than that the context of the service you are using will be the same for the life time of the object. So with this all depends on your implementation of the service if there will be problems.

I do something like this to do what you want.

   public interface IStartupJob
    {
        Task ExecuteAsync(CancellationToken stoppingToken);
    }

    public class DBJob : IStartupJob
    {
        public Task ExecuteAsync(CancellationToken stoppingToken)
        {
            return Task.Run(() => System.Threading.Thread.Sleep(10000));
        }
    }

    public class StartupJobService<TJob> : IHostedService, IDisposable where TJob: class,IStartupJob
    {
        //This ensures a single start of the task this is important on a singletone
        private readonly Lazy<Task> _executingTask;

        private readonly CancellationTokenSource _stoppingCts = new CancellationTokenSource();

        public StartupJobService(Func<TJob> factory)
        {
            //In order for the transient item to be in memory as long as it is needed not to be in memory for the lifetime of the singleton I use a simple factory
            _executingTask = new Lazy<Task>(() => factory().ExecuteAsync(_stoppingCts.Token));
        }

        //You can use this to tell if the job is done
        public virtual Task Done => _executingTask.IsValueCreated ? _executingTask.Value : throw new Exception("BackgroundService not started");

        public virtual Task StartAsync(CancellationToken cancellationToken)
        {

            if (_executingTask.Value.IsCompleted)
            {
                return _executingTask.Value;
            }

            return Task.CompletedTask;
        }

        public virtual async Task StopAsync(CancellationToken cancellationToken)
        {
            if (_executingTask == null)
            {
                return;
            }

            try
            {
                _stoppingCts.Cancel();
            }
            finally
            {
                await Task.WhenAny(_executingTask.Value, Task.Delay(Timeout.Infinite,
                                                              cancellationToken));
            }

        }

        public virtual void Dispose()
        {
            _stoppingCts.Cancel();
        }

        public static void AddService(IServiceCollection services)
        {
            //Helper to register the job
            services.AddTransient<TJob, TJob>();

            services.AddSingleton<Func<TJob>>(cont => 
            {
                return () => cont.GetService<TJob>();
            });

            services.AddSingleton<IHostedService, StartupJobService<TJob>>();
        }
    }