An API rate limiter

Question

I wrote an API rate limiter to use with Last.fm's API.

Last.fm's Tos states that I cannot make more than 5 requests per originating IP address per second, averaged over a 5 minute period.

Here is the class I wrote:

public class RateLimiter
{
    private static readonly List<DateTime> _requests = new List<DateTime>();

    private const double _perMillisecond = 1000.1;
    private const int _rateLimit = 5;
    private const int _rateLimitCooldownMs = 500;

    public static void CheckLimiter()
    {
        _requests.Add(DateTime.Now);

        var requestsDuringRateLimit = _requests.Where(w => (DateTime.Now - w).TotalMilliseconds < _perMillisecond).ToArray();

        if (requestsDuringRateLimit.Count() >= _rateLimit)
        {
            Thread.Sleep(_rateLimitCooldownMs);
            _requests.Clear();
            Console.Clear();
        }
    }
}

The CheckLimiter method is called before the HttpWebRequest is initiated, is this a good way to limit API requests?

Answer 1

This is quite fine in my opinion. Except that, there is a bug in this code. This is because what if each request is done more than a second after one another? It will never go inside that if block. Thus, some kind of memory leak because the _requests will grow larger over time and possibly never be cleared if my scenario above always happens.

Example:

for (int i = 0; i < 100; i++)
{
   RateLimiter.CheckLimiter();
   Thread.Sleep(2000);
}

What you can do is to remove entries in your _requests that are exceeding the 1 second rule like adding this line at the end of your method.

if (_requests.Count != 0)
{
    //remove irrelevant/expired entries
    _requests.RemoveAll(date => (DateTime.Now - date).TotalMilliseconds >= _perMillisecond);
}