Josh Wyant Josh Wyant - 4 months ago 23
C# Question

Throttling asynchronous tasks

I would like to run a bunch of async tasks, with a limit on how many tasks may be pending completion at any given time.

Say you have 1000 URLs, and you only want to have 50 requests open at a time; but as soon as one request completes, you open up a connection to the next URL in the list. That way, there are always exactly 50 connections open at a time, until the URL list is exhausted.

I also want to utilize a given number of threads if possible.

I came up with an extension method,

that does what I want. Is there a simpler solution already out there? I would assume that this is a common scenario.


class Program
static void Main(string[] args)
Enumerable.Range(1, 10).ThrottleTasksAsync(5, 2, async i => { Console.WriteLine(i); return i; }).Wait();

Console.WriteLine("Press a key to exit...");

Here is the code:

static class IEnumerableExtensions
public static async Task<Result_T[]> ThrottleTasksAsync<Enumerable_T, Result_T>(this IEnumerable<Enumerable_T> enumerable, int maxConcurrentTasks, int maxDegreeOfParallelism, Func<Enumerable_T, Task<Result_T>> taskToRun)
var blockingQueue = new BlockingCollection<Enumerable_T>(new ConcurrentBag<Enumerable_T>());

var semaphore = new SemaphoreSlim(maxConcurrentTasks);

// Run the throttler on a separate thread.
var t = Task.Run(() =>
foreach (var item in enumerable)
// Wait for the semaphore


var taskList = new List<Task<Result_T>>();

Parallel.ForEach(IterateUntilTrue(() => blockingQueue.IsCompleted), new ParallelOptions { MaxDegreeOfParallelism = maxDegreeOfParallelism },
_ =>
Enumerable_T item;

if (blockingQueue.TryTake(out item, 100))
// Run the task
.ContinueWith(tsk =>
// For effect

// Release the semaphore

return tsk.Result;

// Await all the tasks.
return await Task.WhenAll(taskList);

static IEnumerable<bool> IterateUntilTrue(Func<bool> condition)
while (!condition()) yield return true;

The method utilizes
to make it work. The throttler is run on one thread, and all the async tasks are run on the other thread. To achieve parallelism, I added a maxDegreeOfParallelism parameter that's passed to a
loop re-purposed as a

The old version was:

foreach (var master = ...)
var details = ...;
Parallel.ForEach(details, detail => {
// Process each detail record here
}, new ParallelOptions { MaxDegreeOfParallelism = 15 });
// Perform the final batch updates here

But, the thread pool gets exhausted fast, and you can't do

To get around the problem in
where an exception is thrown in
is called, I'm using the
overload with a timeout. If I didn't use the timeout in
, it would defeat the purpose of using a
won't block. Is there a better way? Ideally, there would be a


As requested, here's the code I ended up going with.

The work is set up in a master-detail configuration, and each master is processed as a batch. Each unit of work is queued up in this fashion:

var success = true;

// Start processing all the master records.
Master master;
while (null != (master = await StoredProcedures.ClaimRecordsAsync(...)))
    await masterBuffer.SendAsync(master);

// Finished sending master records

// Now, wait for all the batches to complete.
await batchAction.Completion;

return success;

Masters are buffered one at a time to save work for other outside processes. The details for each master are dispatched for work via the masterTransform TransformManyBlock. A BatchedJoinBlock is also created to collect the details in one batch.

The actual work is done in the detailTransform TransformBlock, asynchronously, 150 at a time. BoundedCapacity is set to 300 to ensure that too many Masters don't get buffered at the beginning of the chain, while also leaving room for enough detail records to be queued to allow 150 records to be processed at one time. The block outputs an object to its targets, because it's filtered across the links depending on whether it's a Detail or Exception.

The batchAction ActionBlock collects the output from all the batches, and performs bulk database updates, error logging, etc. for each batch.

There will be several BatchedJoinBlocks, one for each master. Since each ISourceBlock is output sequentially and each batch only accepts the number of detail records associated with one master, the batches will be processed in order. Each block only outputs one group, and is unlinked on completion. Only the last batch block propagates its completion to the final ActionBlock.

The dataflow network:

// The dataflow network
BufferBlock<Master> masterBuffer = null;
TransformManyBlock<Master, Detail> masterTransform = null;
TransformBlock<Detail, object> detailTransform = null;
ActionBlock<Tuple<IList<object>, IList<object>>> batchAction = null;

// Buffer master records to enable efficient throttling.
masterBuffer = new BufferBlock<Master>(new DataflowBlockOptions { BoundedCapacity = 1 });

// Sequentially transform master records into a stream of detail records.
masterTransform = new TransformManyBlock<Master, Detail>(async masterRecord =>
    var records = await StoredProcedures.GetObjectsAsync(masterRecord);

    // Filter the master records based on some criteria here
    var filteredRecords = records;

    // Only propagate completion to the last batch
    var propagateCompletion = masterBuffer.Completion.IsCompleted && masterTransform.InputCount == 0;

    // Create a batch join block to encapsulate the results of the master record.
    var batchjoinblock = new BatchedJoinBlock<object, object>(records.Count(), new GroupingDataflowBlockOptions { MaxNumberOfGroups = 1 });

    // Add the batch block to the detail transform pipeline's link queue, and link the batch block to the the batch action block.
    var detailLink1 = detailTransform.LinkTo(batchjoinblock.Target1, detailResult => detailResult is Detail);
    var detailLink2 = detailTransform.LinkTo(batchjoinblock.Target2, detailResult => detailResult is Exception);
    var batchLink = batchjoinblock.LinkTo(batchAction, new DataflowLinkOptions { PropagateCompletion = propagateCompletion });

    // Unlink batchjoinblock upon completion.
    // (the returned task does not need to be awaited, despite the warning.)
    batchjoinblock.Completion.ContinueWith(task =>

    return filteredRecords;
}, new ExecutionDataflowBlockOptions { BoundedCapacity = 1 });

// Process each detail record asynchronously, 150 at a time.
detailTransform = new TransformBlock<Detail, object>(async detail => {
        // Perform the action for each detail here asynchronously
        await DoSomethingAsync();

        return detail;
    catch (Exception e)
        success = false;
        return e;

}, new ExecutionDataflowBlockOptions { MaxDegreeOfParallelism = 150, BoundedCapacity = 300 });

// Perform the proper action for each batch
batchAction = new ActionBlock<Tuple<IList<object>, IList<object>>>(async batch =>
    var details = batch.Item1.Cast<Detail>();
    var errors = batch.Item2.Cast<Exception>();

    // Do something with the batch here
}, new ExecutionDataflowBlockOptions { MaxDegreeOfParallelism = 4 });

masterBuffer.LinkTo(masterTransform, new DataflowLinkOptions { PropagateCompletion = true });
masterTransform.LinkTo(detailTransform, new DataflowLinkOptions { PropagateCompletion = true });