Queue of async tasks with throttling which supports muti-threading

Question

I need to implement a library to request vk.com API. The problem is that API supports only 3 requests per second. I would like to have API asynchronous.

Answer 1

I solved a similar problem using a wrapper around SemaphoreSlim. In my scenario, I had some other throttling mechanisms as well, and I needed to make sure that requests didn't hit the external API too often even if request number 1 took longer to reach the API than request number 3. My solution was to use a wrapper around SemaphoreSlim that had to be released by the caller, but the actual SemaphoreSlim would not be released until a set time had passed.

public class TimeGatedSemaphore
{
    private readonly SemaphoreSlim semaphore;
    public TimeGatedSemaphore(int maxRequest, TimeSpan minimumHoldTime)
    {
        semaphore = new SemaphoreSlim(maxRequest);
        MinimumHoldTime = minimumHoldTime;
    }
    public TimeSpan MinimumHoldTime { get; }

    public async Task WaitAsync()
    {
        await semaphore.WaitAsync();
        return new InternalReleaser(semaphore, Task.Delay(MinimumHoldTime));
    }

    private class InternalReleaser : IDisposable
    {
        private readonly SemaphoreSlim semaphoreToRelease;
        private readonly Task notBeforeTask;
        public InternalReleaser(SemaphoreSlim semaphoreSlim, Task dependantTask)
        {
            semaphoreToRelease = semaphoreSlim;
            notBeforeTask = dependantTask;
        }
        public void Dispose()
        {
            notBeforeTask.ContinueWith(_ => semaphoreToRelease.Release());
        }
    }
}

Example usage:

private TimeGatedSemaphore requestThrottler = new TimeGatedSemaphore(3, TimeSpan.FromSeconds(1));
public async Task MyRequestSenderHelper(string endpoint)
{
    using (await requestThrottler.WaitAsync())
        return await SendRequestToAPI(endpoint);        
}