How to call TriggerBatch automagically after a timeout if the number of queued items is less than the BatchSize?

Question

Using Dataflow CTP (in the TPL)

Is there a way to call BatchBlock.TriggerBatch automatically if the number of currently queued or postponed items is less than the BatchSize, after a timeout ?

And better: this timeout should be reset to 0 each time the block receives a new item.

Answer 1

Yes, you can accomplish this rather elegantly by chaining together blocks. In this case you want to setup a TransformBlock which you link "before" the BatchBlock. That would look something like this:

Timer triggerBatchTimer = new Timer(() => yourBatchBlock.TriggerBatch());

TransformBlock<T, T> timeoutTransformBlock = new TransformBlock<T, T>((value) =>
{
    triggerBatchTimer.Change(5000, Timeout.Infinite);

    return value; 
});

timeoutTransformBlock.LinkTo(yourBatchBlock);

yourBufferBlock.LinkTo(timeoutTransformBlock);

Answer 2

Here is a policed version of the excellent Drew Marsh's solution. This one uses the DataflowBlock.Encapsulate method to create a dataflow block that encapsulates the timer+batch functionality. Beyond the new argument timeout, the CreateBatchBlock method also supports all options available to the normal BatchBlock constructor.

public static IPropagatorBlock<T, T[]> CreateBatchBlock<T>(int batchSize,
    int timeout, GroupingDataflowBlockOptions dataflowBlockOptions = null)
{
    dataflowBlockOptions = dataflowBlockOptions ?? new GroupingDataflowBlockOptions();
    var batchBlock = new BatchBlock<T>(batchSize, dataflowBlockOptions);
    var timer = new System.Threading.Timer(_ => batchBlock.TriggerBatch());
    var transformBlock = new TransformBlock<T, T>((T value) =>
    {
        timer.Change(timeout, Timeout.Infinite);
        return value;
    }, new ExecutionDataflowBlockOptions()
    {
        BoundedCapacity = dataflowBlockOptions.BoundedCapacity,
        CancellationToken = dataflowBlockOptions.CancellationToken,
        EnsureOrdered = dataflowBlockOptions.EnsureOrdered,
        MaxMessagesPerTask = dataflowBlockOptions.MaxMessagesPerTask,
        NameFormat = dataflowBlockOptions.NameFormat,
        TaskScheduler = dataflowBlockOptions.TaskScheduler
    });
    transformBlock.LinkTo(batchBlock, new DataflowLinkOptions()
    {
        PropagateCompletion = true
    });
    return DataflowBlock.Encapsulate(transformBlock, batchBlock);
}

---

Alternative: below is a BatchUntilInactiveBlock<T> class that offers the whole range of the BatchBlock<T> functionality. This implementation is a thin wrapper around a BatchBlock<T> instance. It has less overhead than the previous CreateBatchBlock implementation, while having a similar behavior.

/// <summary>
/// Provides a dataflow block that batches inputs into arrays.
/// A batch is produced when the number of currently queued items becomes equal
/// to BatchSize, or when a Timeout period has elapsed after receiving the last item.
/// </summary>
public class BatchUntilInactiveBlock<T> : IPropagatorBlock<T, T[]>,
    IReceivableSourceBlock<T[]>
{
    private readonly BatchBlock<T> _source;
    private readonly Timer _timer;
    private readonly TimeSpan _timeout;

    public BatchUntilInactiveBlock(int batchSize, TimeSpan timeout,
        GroupingDataflowBlockOptions dataflowBlockOptions)
    {
        _source = new BatchBlock<T>(batchSize, dataflowBlockOptions);
        _timer = new Timer(_ => _source.TriggerBatch());
        _timeout = timeout;
    }

    public BatchUntilInactiveBlock(int batchSize, TimeSpan timeout) : this(batchSize,
        timeout, new GroupingDataflowBlockOptions())
    { }

    public int BatchSize => _source.BatchSize;
    public TimeSpan Timeout => _timeout;
    public Task Completion => _source.Completion;
    public int OutputCount => _source.OutputCount;

    public void Complete() => _source.Complete();

    void IDataflowBlock.Fault(Exception exception)
        => ((IDataflowBlock)_source).Fault(exception);

    public IDisposable LinkTo(ITargetBlock<T[]> target,
        DataflowLinkOptions linkOptions)
            => _source.LinkTo(target, linkOptions);

    public void TriggerBatch() => _source.TriggerBatch();

    public bool TryReceive(Predicate<T[]> filter, out T[] item)
        => _source.TryReceive(filter, out item);

    public bool TryReceiveAll(out IList<T[]> items)
        => _source.TryReceiveAll(out items);

    DataflowMessageStatus ITargetBlock<T>.OfferMessage(
        DataflowMessageHeader messageHeader, T messageValue, ISourceBlock<T> source,
        bool consumeToAccept)
    {
        var offerResult = ((ITargetBlock<T>)_source).OfferMessage(messageHeader,
            messageValue, source, consumeToAccept);
        if (offerResult == DataflowMessageStatus.Accepted)
            _timer.Change(_timeout, System.Threading.Timeout.InfiniteTimeSpan);
        return offerResult;
    }

    T[] ISourceBlock<T[]>.ConsumeMessage(DataflowMessageHeader messageHeader,
        ITargetBlock<T[]> target, out bool messageConsumed)
            => ((ISourceBlock<T[]>)_source).ConsumeMessage(messageHeader,
                target, out messageConsumed);

    bool ISourceBlock<T[]>.ReserveMessage(DataflowMessageHeader messageHeader,
        ITargetBlock<T[]> target)
            => ((ISourceBlock<T[]>)_source).ReserveMessage(messageHeader, target);

    void ISourceBlock<T[]>.ReleaseReservation(DataflowMessageHeader messageHeader,
        ITargetBlock<T[]> target)
            => ((ISourceBlock<T[]>)_source).ReleaseReservation(messageHeader, target);
}

Answer 3

Thanks to Drew Marsh for the idea of using a TransformBlock which greatly helped me with a recent solution. However, I believe that the timer needs to be reset AFTER the batch block (i.e. after it has either been triggered by the batch size being reached OR the TriggerBatch method being explicitly called within the timer callback). If you reset the timer every time you get a single item then it can potentially keep resetting several times without actually triggering a batch at all (constantly pushing the "dueTime" on the Timer further away).

This would make the code snippet look like the following:

Timer triggerBatchTimer = new Timer(() => yourBatchBlock.TriggerBatch(), null, 5000, Timeout.Infinite);

TransformBlock<T[], T[]> timeoutTransformBlock = new TransformBlock<T[], T[]>((value) =>
{
    triggerBatchTimer.Change(5000, Timeout.Infinite);

    return value; 
});

yourBufferBlock.LinkTo(yourBatchBlock);
yourBatchBlock.LinkTo(timeoutTransformBlock)
timeoutTransformBlock.LinkTo(yourActionBlock);

// Start the producer which is populating the BufferBlock etc.