c# - Simple way to rate limit HttpClient requests

Question

I am using the HTTPClient in System.Net.Http to make requests against an API. The API is limited to 10 requests per second.

My code is roughly like so:

    List<Task> tasks = new List<Task>();
    items..Select(i => tasks.Add(ProcessItem(i));

    try
    {
        await Task.WhenAll(taskList.ToArray());
    }
    catch (Exception ex)
    {
    }

The ProcessItem method does a few things but always calls the API using the following: await SendRequestAsync(..blah). Which looks like:

private async Task<Response> SendRequestAsync(HttpRequestMessage request, CancellationToken token)
{    
    token.ThrowIfCancellationRequested();
    var response = await HttpClient
        .SendAsync(request: request, cancellationToken: token).ConfigureAwait(continueOnCapturedContext: false);

    token.ThrowIfCancellationRequested();
    return await Response.BuildResponse(response);
}

Originally the code worked fine but when I started using Task.WhenAll I started getting 'Rate Limit Exceeded' messages from the API. How can I limit the rate at which requests are made?

Its worth noting that ProcessItem can make between 1-4 API calls depending on the item.

Answer 1

The API is limited to 10 requests per second.

Then just have your code do a batch of 10 requests, ensuring they take at least one second:

Items[] items = ...;

int index = 0;
while (index < items.Length)
{
  var timer = Task.Delay(TimeSpan.FromSeconds(1.2)); // ".2" to make sure
  var tasks = items.Skip(index).Take(10).Select(i => ProcessItemsAsync(i));
  var tasksAndTimer = tasks.Concat(new[] { timer });
  await Task.WhenAll(tasksAndTimer);
  index += 10;
}

Update

My ProcessItems method makes 1-4 API calls depending on the item.

In this case, batching is not an appropriate solution. You need to limit an asynchronous method to a certain number, which implies a SemaphoreSlim. The tricky part is that you want to allow more calls over time.

I haven't tried this code, but the general idea I would go with is to have a periodic function that releases the semaphore up to 10 times. So, something like this:

private readonly SemaphoreSlim _semaphore = new SemaphoreSlim(10);

private async Task<Response> ThrottledSendRequestAsync(HttpRequestMessage request, CancellationToken token)
{
  await _semaphore.WaitAsync(token);
  return await SendRequestAsync(request, token);
}

private async Task PeriodicallyReleaseAsync(Task stop)
{
  while (true)
  {
    var timer = Task.Delay(TimeSpan.FromSeconds(1.2));

    if (await Task.WhenAny(timer, stop) == stop)
      return;

    // Release the semaphore at most 10 times.
    for (int i = 0; i != 10; ++i)
    {
      try
      {
        _semaphore.Release();
      }
      catch (SemaphoreFullException)
      {
        break;
      }
    }
  }
}

Usage:

// Start the periodic task, with a signal that we can use to stop it.
var stop = new TaskCompletionSource<object>();
var periodicTask = PeriodicallyReleaseAsync(stop.Task);

// Wait for all item processing.
await Task.WhenAll(taskList);

// Stop the periodic task.
stop.SetResult(null);
await periodicTask;