awaitable Task based queue

Question

I'm wondering if there exists an implementation/wrapper for ConcurrentQueue, similar to BlockingCollection where taking from the collection does not block, but is instead asynchronous and will cause an async await until an item is placed in the queue.

I've come up with my own implementation, but it does not seem to be performing as expected. I'm wondering if I'm reinventing something that already exists.

Here's my implementation:

public class MessageQueue<T>
{
    ConcurrentQueue<T> queue = new ConcurrentQueue<T>();

    ConcurrentQueue<TaskCompletionSource<T>> waitingQueue = 
        new ConcurrentQueue<TaskCompletionSource<T>>();

    object queueSyncLock = new object();

    public void Enqueue(T item)
    {
        queue.Enqueue(item);
        ProcessQueues();
    }

    public async Task<T> Dequeue()
    {
        TaskCompletionSource<T> tcs = new TaskCompletionSource<T>();
        waitingQueue.Enqueue(tcs);
        ProcessQueues();
        return tcs.Task.IsCompleted ? tcs.Task.Result : await tcs.Task;
    }

    private void ProcessQueues()
    {
        TaskCompletionSource<T> tcs=null;
        T firstItem=default(T);
        while (true)
        {
            bool ok;
            lock (queueSyncLock)
            {
                ok = waitingQueue.TryPeek(out tcs) && queue.TryPeek(out firstItem);
                if (ok)
                {
                    waitingQueue.TryDequeue(out tcs);
                    queue.TryDequeue(out firstItem);
                }
            }
            if (!ok) break;
            tcs.SetResult(firstItem);
        }
    }
}

Answer 1

I don't know of a lock-free solution, but you can take a look at the new Dataflow library, part of the Async CTP. A simple BufferBlock<T> should suffice, e.g.:

BufferBlock<int> buffer = new BufferBlock<int>();

Production and consumption are most easily done via extension methods on the dataflow block types.

Production is as simple as:

buffer.Post(13);

and consumption is async-ready:

int item = await buffer.ReceiveAsync();

I do recommend you use Dataflow if possible; making such a buffer both efficient and correct is more difficult than it first appears.

Answer 2

Simple approach with C# 8.0 IAsyncEnumerable and Dataflow library

// Instatiate an async queue
var queue = new AsyncQueue<int>();

// Then, loop through the elements of queue.
// This loop won't stop until it is canceled or broken out of
// (for that, use queue.WithCancellation(..) or break;)
await foreach(int i in queue) {
    // Writes a line as soon as some other Task calls queue.Enqueue(..)
    Console.WriteLine(i);
}

With an implementation of AsyncQueue as follows:

public class AsyncQueue<T> : IAsyncEnumerable<T>
{
    private readonly SemaphoreSlim _enumerationSemaphore = new SemaphoreSlim(1);
    private readonly BufferBlock<T> _bufferBlock = new BufferBlock<T>();

    public void Enqueue(T item) => _bufferBlock.Post(item);

    public async IAsyncEnumerator<T> GetAsyncEnumerator(CancellationToken token = default)
    {
        // We lock this so we only ever enumerate once at a time.
        // That way we ensure all items are returned in a continuous
        // fashion with no 'holes' in the data when two foreach compete.
        await _enumerationSemaphore.WaitAsync(token);

        try 
        {
            // Return new elements until cancellationToken is triggered.
            while (true) 
            {
                // Make sure to throw on cancellation so the Task will transfer into a canceled state
                token.ThrowIfCancellationRequested();
                yield return await _bufferBlock.ReceiveAsync(token);
            }
        } 
        finally 
        {
            _enumerationSemaphore.Release();
        }

    }
}

Answer 3

There is an official way to do this now: System.Threading.Channels. It's built into the core runtime on .NET Core 3.0 and higher (including .NET 5.0 and 6.0), but it's also available as a NuGet package on .NET Standard 2.0 and 2.1. You can read through the docs here.

var channel = System.Threading.Channels.Channel.CreateUnbounded<int>();

To enqueue work:

// This will succeed and finish synchronously if the channel is unbounded.
channel.Writer.TryWrite(42);

To complete the channel:

channel.Writer.TryComplete();

To read from the channel:

var i = await channel.Reader.ReadAsync();

Or, if you have .NET Core 3.0 or higher:

await foreach (int i in channel.Reader.ReadAllAsync())
{
    // whatever processing on i...
}

Answer 4

One simple and easy way to implement this is with a SemaphoreSlim:

public class AwaitableQueue<T>
{
    private SemaphoreSlim semaphore = new SemaphoreSlim(0);
    private readonly object queueLock = new object();
    private Queue<T> queue = new Queue<T>();

    public void Enqueue(T item)
    {
        lock (queueLock)
        {
            queue.Enqueue(item);
            semaphore.Release();
        }
    }

    public T WaitAndDequeue(TimeSpan timeSpan, CancellationToken cancellationToken)
    {
        semaphore.Wait(timeSpan, cancellationToken);
        lock (queueLock)
        {
            return queue.Dequeue();
        }
    }

    public async Task<T> WhenDequeue(TimeSpan timeSpan, CancellationToken cancellationToken)
    {
        await semaphore.WaitAsync(timeSpan, cancellationToken);
        lock (queueLock)
        {
            return queue.Dequeue();
        }
    }
}

The beauty of this is that the SemaphoreSlim handles all of the complexity of implementing the Wait() and WaitAsync() functionality. The downside is that queue length is tracked by both the semaphore and the queue itself, and they both magically stay in sync.

Answer 5

My atempt (it have an event raised when a "promise" is created, and it can be used by an external producer to know when to produce more items):

public class AsyncQueue<T>
{
    private ConcurrentQueue<T> _bufferQueue;
    private ConcurrentQueue<TaskCompletionSource<T>> _promisesQueue;
    private object _syncRoot = new object();

    public AsyncQueue()
    {
        _bufferQueue = new ConcurrentQueue<T>();
        _promisesQueue = new ConcurrentQueue<TaskCompletionSource<T>>();
    }

    /// <summary>
    /// Enqueues the specified item.
    /// </summary>
    /// <param name="item">The item.</param>
    public void Enqueue(T item)
    {
        TaskCompletionSource<T> promise;
        do
        {
            if (_promisesQueue.TryDequeue(out promise) &&
                !promise.Task.IsCanceled &&
                promise.TrySetResult(item))
            {
                return;                                       
            }
        }
        while (promise != null);

        lock (_syncRoot)
        {
            if (_promisesQueue.TryDequeue(out promise) &&
                !promise.Task.IsCanceled &&
                promise.TrySetResult(item))
            {
                return;
            }

            _bufferQueue.Enqueue(item);
        }            
    }

    /// <summary>
    /// Dequeues the asynchronous.
    /// </summary>
    /// <param name="cancellationToken">The cancellation token.</param>
    /// <returns></returns>
    public Task<T> DequeueAsync(CancellationToken cancellationToken)
    {
        T item;

        if (!_bufferQueue.TryDequeue(out item))
        {
            lock (_syncRoot)
            {
                if (!_bufferQueue.TryDequeue(out item))
                {
                    var promise = new TaskCompletionSource<T>();
                    cancellationToken.Register(() => promise.TrySetCanceled());

                    _promisesQueue.Enqueue(promise);
                    this.PromiseAdded.RaiseEvent(this, EventArgs.Empty);

                    return promise.Task;
                }
            }
        }

        return Task.FromResult(item);
    }

    /// <summary>
    /// Gets a value indicating whether this instance has promises.
    /// </summary>
    /// <value>
    /// <c>true</c> if this instance has promises; otherwise, <c>false</c>.
    /// </value>
    public bool HasPromises
    {
        get { return _promisesQueue.Where(p => !p.Task.IsCanceled).Count() > 0; }
    }

    /// <summary>
    /// Occurs when a new promise
    /// is generated by the queue
    /// </summary>
    public event EventHandler PromiseAdded;
}

Answer 6

It may be overkill for your use case (given the learning curve), but Reactive Extentions provides all the glue you could ever want for asynchronous composition.

You essentially subscribe to changes and they are pushed to you as they become available, and you can have the system push the changes on a separate thread.

Answer 7

Check out https://github.com/somdoron/AsyncCollection, you can both dequeue asynchronously and use C# 8.0 IAsyncEnumerable.

The API is very similar to BlockingCollection.

AsyncCollection<int> collection = new AsyncCollection<int>();

var t = Task.Run(async () =>
{
    while (!collection.IsCompleted)
    {
        var item = await collection.TakeAsync();

        // process
    }
});

for (int i = 0; i < 1000; i++)
{
    collection.Add(i);
}

collection.CompleteAdding();

t.Wait();

With IAsyncEnumeable:

AsyncCollection<int> collection = new AsyncCollection<int>();

var t = Task.Run(async () =>
{
    await foreach (var item in collection)
    {
        // process
    }
});

for (int i = 0; i < 1000; i++)
{
    collection.Add(i);
}

collection.CompleteAdding();

t.Wait();

Answer 8

Here's the implementation I'm currently using.

public class MessageQueue<T>
{
    ConcurrentQueue<T> queue = new ConcurrentQueue<T>();
    ConcurrentQueue<TaskCompletionSource<T>> waitingQueue = 
        new ConcurrentQueue<TaskCompletionSource<T>>();
    object queueSyncLock = new object();
    public void Enqueue(T item)
    {
        queue.Enqueue(item);
        ProcessQueues();
    }

    public async Task<T> DequeueAsync(CancellationToken ct)
    {
        TaskCompletionSource<T> tcs = new TaskCompletionSource<T>();
        ct.Register(() =>
        {
            lock (queueSyncLock)
            {
                tcs.TrySetCanceled();
            }
        });
        waitingQueue.Enqueue(tcs);
        ProcessQueues();
        return tcs.Task.IsCompleted ? tcs.Task.Result : await tcs.Task;
    }

    private void ProcessQueues()
    {
        TaskCompletionSource<T> tcs = null;
        T firstItem = default(T);
        lock (queueSyncLock)
        {
            while (true)
            {
                if (waitingQueue.TryPeek(out tcs) && queue.TryPeek(out firstItem))
                {
                    waitingQueue.TryDequeue(out tcs);
                    if (tcs.Task.IsCanceled)
                    {
                        continue;
                    }
                    queue.TryDequeue(out firstItem);
                }
                else
                {
                    break;
                }
                tcs.SetResult(firstItem);
            }
        }
    }
}

It works good enough, but there's quite a lot of contention on queueSyncLock, as I am making quite a lot of use of the CancellationToken to cancel some of the waiting tasks. Of course, this leads to considerably less blocking I would see with a BlockingCollection but...

I'm wondering if there is a smoother, lock free means of achieving the same end

Answer 9

Well 8 years later I hit this very question and was about to implement the MS AsyncQueue<T> class found in nuget package/namespace: Microsoft.VisualStudio.Threading

Thanks to @Theodor Zoulias for mentioning this api may be outdated and the DataFlow lib would be a good alternative.

So I edited my AsyncQueue<> implementation to use BufferBlock<>. Almost the same but works better.

I use this in an AspNet Core background thread and it runs fully async.

protected async Task MyRun()
{
    BufferBlock<MyObj> queue = new BufferBlock<MyObj>();
    Task enqueueTask = StartDataIteration(queue);

    while (await queue.OutputAvailableAsync())
    {
        var myObj = queue.Receive();
        // do something with myObj
    }

}

public async Task StartDataIteration(BufferBlock<MyObj> queue)
{
    var cursor = await RunQuery();
    while(await cursor.Next()) { 
        queue.Post(cursor.Current);
    }
    queue.Complete(); // <<< signals the consumer when queue.Count reaches 0
}

I found that using the queue.OutputAvailableAsync() fixed the issue that I had with AsyncQueue<> -- trying to determine when the queue was complete and not having to inspect the dequeue task.

Answer 10

You could just use a BlockingCollection ( using the default ConcurrentQueue ) and wrap the call to Take in a Task so you can await it:

var bc = new BlockingCollection<T>();

T element = await Task.Run( () => bc.Take() );