Dealing with throttling/rate limits (429 error) when using async/await

Question

I have the following async code that gets called from so many places in my project:

public async Task<HttpResponseMessage> MakeRequestAsync(HttpRequestMessage request)
{            
    var client = new HttpClient();
    return await client.SendAsync(request).ConfigureAwait(false);                
}

An example of how the above method gets called:

 var tasks = items.Select(async i =>
            {                    
                var response = await MakeRequestAsync(i.Url);           
                //do something with response    
            });

The ZenDesk API that I'm hitting allows about 200 requests per minute after which I'm getting a 429 error. I need to do some sort of a Thread.sleep if I encounter the 429 error, but with with async/await, there may be so many requests in parallel threads waiting to process, I am not sure how I can make all of them sleep for 5 seconds or so and then resume again.

What's the correct way to approach this problem? I'd like to hear quick solutions as well as good-design solutions.

Answer 1

I do not think that this is a duplicate, as marked recently. The other SO poster does not need a time-based sliding window (or time based throttling) and the answer there does not cover this situation. That works only when you want to set a hard limit on outgoing requests.

Anyway, a quasi-quick solution is to make the throttling in the MakeRequestAsync method. Something like this:

public async Task<HttpResponseMessage> MakeRequestAsync(HttpRequestMessage request)
{            
    //Wait while the limit has been reached. 
    while(!_throttlingHelper.RequestAllowed) 
    {
      await Task.Delay(1000);
    }

    var client = new HttpClient();

    _throttlingHelper.StartRequest();
    var result = await client.SendAsync(request).ConfigureAwait(false);
    _throttlingHelper.EndRequest();

    return result; 
}

The class ThrottlingHelper is just something I made now so you may need to debug it a bit (read - may not work out of the box). It tries to be a timestamp sliding window.

public class ThrottlingHelper : IDisposable
{
    //Holds time stamps for all started requests
    private readonly List<long> _requestsTx;
    private readonly ReaderWriterLockSlim _lock;

    private readonly int _maxLimit;
    private TimeSpan _interval;

    public ThrottlingHelper(int maxLimit, TimeSpan interval)
    {
        _requestsTx = new List<long>();
        _maxLimit = maxLimit;
        _interval = interval;
        _lock = new ReaderWriterLockSlim(LockRecursionPolicy.NoRecursion);
    }

    public bool RequestAllowed
    {
        get
        {
            _lock.EnterReadLock();
            try
            {
                var nowTx = DateTime.Now.Ticks;
                return _requestsTx.Count(tx => nowTx - tx < _interval.Ticks) < _maxLimit;
            }
            finally
            {
                _lock.ExitReadLock();
            }
        }
    }

    public void StartRequest()
    {
        _lock.EnterWriteLock();
        try
        {
            _requestsTx.Add(DateTime.Now.Ticks);
        }
        finally
        {
            _lock.ExitWriteLock();
        }
    }

    public void EndRequest()
    {
        _lock.EnterWriteLock();
        try
        {
            var nowTx = DateTime.Now.Ticks;
            _requestsTx.RemoveAll(tx => nowTx - tx >= _interval.Ticks);
        }
        finally
        {
            _lock.ExitWriteLock();
        }
    }

    public void Dispose()
    {
        _lock.Dispose();
    }
}

You would use it as a member in the class that makes the requests, and instantiate it like this:

_throttlingHelper = new ThrottlingHelper(200, TimeSpan.FromMinutes(1));

Don't forget to dispose it when you're done with it.

A bit of documentation about ThrottlingHelper:

1. Constructor params are the maximum requests you want to be able to do in a certain interval and the interval itself as a time span. So, 200 and 1 minute means that that you want no more than 200 requests/minute.
2. Property RequestAllowed lets you know if you are able to do a request with the current throttling settings.
3. Methods StartRequest & EndRequest register/unregister a request by using the current date/time.

EDIT/Pitfalls

As indicated by @PhilipABarnes, EndRequest can potentially remove requests that are still in progress. As far as I can see, this can happen in two situations:

1. The interval is small, such that requests do not get to complete in good time.
2. Requests actually take more than the interval to execute.

The proposed solution involves actually matching EndRequest calls to StartRequest calls by means of a GUID or something similar.

Answer 2

if there are multiple requests waiting in the while loop for RequestAllowed some of them might start at the same time. how about a simple StartRequestIfAllowed?

public class ThrottlingHelper : DisposeBase
{
    //Holds time stamps for all started requests
    private readonly List<long> _requestsTx;
    private readonly Mutex _mutex = new Mutex();

    private readonly int _maxLimit;
    private readonly TimeSpan _interval;

    public ThrottlingHelper(int maxLimit, TimeSpan interval)
    {
        _requestsTx = new List<long>();
        _maxLimit = maxLimit;
        _interval = interval;
    }


    public bool StartRequestIfAllowed
    {
        get
        {
            _mutex.WaitOne();
            try
            {
                var nowTx = DateTime.Now.Ticks;
                if (_requestsTx.Count(tx => nowTx - tx < _interval.Ticks) < _maxLimit)
                {
                    _requestsTx.Add(DateTime.Now.Ticks);
                    return true;
                }
                else
                {
                    return false;
                }
            }
            finally
            {
                _mutex.ReleaseMutex();
            }
        }
    }

    public void EndRequest()
    {
        _mutex.WaitOne();
        try
        {
            var nowTx = DateTime.Now.Ticks;
            _requestsTx.RemoveAll(tx => nowTx - tx >= _interval.Ticks);
        }
        finally
        {
            _mutex.ReleaseMutex();
        }
    }

    protected override void DisposeResources()
    {
        _mutex.Dispose();
    }
}