ASP .Net Core Queued background tasks parallel processing

Question

I have an ASP .NET core Web API which uses Queued background tasks like described here .

I've used the code sample provided and added the IBackgroundTaskQueue , BackgroundTaskQueue and QueuedHostedService exactly as described in the article.

In my Startup.cs , I'm registering only one QueuedHostedService instance as follows: services.AddHostedService<QueuedHostedService>();

Tasks coming from the WebApi's controller are enqueued and then dequeued and executed one by one by the QueuedHostedService .

I'll would like to allow more than one background processing thread that will dequeue and execute the incoming Tasks. The most straight forward solution i can come up with is to register more than one instance of the QueuedHostedService in my Startup.cs . ie, something like this:

 int maxNumOfParallelOperations;
 var isValid = int.TryParse(Configuration["App:MaxNumOfParallelOperations"], out maxNumOfParallelOperations);

 maxNumOfParallelOperations = isValid && maxNumOfParallelOperations > 0 ? maxNumOfParallelOperations : 2;

 for (int index = 0; index < maxNumOfParallelOperations; index++) 
 {
    services.AddHostedService<QueuedHostedService>();
 }

I've also noticed that thanks to the singal Semaphore in BackgroundTaskQueue , the QueuedHostedService instances are not really working all the time, but only awaken when a new Task is available in the queue.

This solution seems to works just fine in my tests.

But, In this particular use case - is it really a valid, recommended solution for parallel processing?

Answer 1

You can use an IHostedService with a number of threads to consume the IBackgroundTaskQueue .

Here is a basic implementation. I assume you're using the same IBackgroundTaskQueue and BackgroundTaskQueue described here .

public class QueuedHostedService : IHostedService
{
    private readonly ILogger _logger;

    private readonly Task[] _executors;
    private readonly int _executorsCount = 2; //--default value: 2
    private CancellationTokenSource _tokenSource;
    public IBackgroundTaskQueue TaskQueue { get; }

    public QueuedHostedService(IBackgroundTaskQueue taskQueue,
        ILoggerFactory loggerFactory,
        IConfiguration configuration)
    {
        TaskQueue = taskQueue;
        _logger = loggerFactory.CreateLogger<QueuedHostedService>();

        if (ushort.TryParse(configuration["App:MaxNumOfParallelOperations"], out var ct))
        {
            _executorsCount = ct;
        }
        _executors = new Task[_executorsCount];
    }

    public Task StartAsync(CancellationToken cancellationToken)
    {
        _logger.LogInformation("Queued Hosted Service is starting.");

        _tokenSource = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);

        for (var i = 0; i < _executorsCount; i++)
        {
            var executorTask = new Task(
                async () =>
                {
                    while (!cancellationToken.IsCancellationRequested)
                    {
#if DEBUG
                    _logger.LogInformation("Waiting background task...");
#endif
                    var workItem = await TaskQueue.DequeueAsync(cancellationToken);

                        try
                        {
#if DEBUG
                        _logger.LogInformation("Got background task, executing...");
#endif
                        await workItem(cancellationToken);
                        }
                        catch (Exception ex)
                        {
                            _logger.LogError(ex,
                                "Error occurred executing {WorkItem}.", nameof(workItem)
                            );
                        }
                    }
                }, _tokenSource.Token);

            _executors[i] = executorTask;
            executorTask.Start();
        }

        return Task.CompletedTask;
    }

    public Task StopAsync(CancellationToken cancellationToken)
    {
        _logger.LogInformation("Queued Hosted Service is stopping.");
        _tokenSource.Cancel(); // send the cancellation signal

        if (_executors != null)
        {
            // wait for _executors completion
            Task.WaitAll(_executors, cancellationToken);
        }

        return Task.CompletedTask;
    }
}

You need to register the services in ConfigureServices on Startup class.

...
services.AddSingleton<IBackgroundTaskQueue, BackgroundTaskQueue>();
services.AddHostedService<QueuedHostedService>();
...

Aditionally, you can set the number of threads in configuration ( appsettings.json )

...
"App": {
    "MaxNumOfParallelOperations": 4
}
...