ASP.NET限流器的简单实现

public interface IRateLimiter
{
    bool TryAcquire();
}
 
public sealed class SliddingWindowRateLimiter: IRateLimiter
{
    private readonly TimeSpan _window;
    private readonly ChannelReader _reader;
    private readonly ChannelWriter _writer;
    public SliddingWindowRateLimiter(TimeSpan window, int permit)
    {
        _window = window;
        var options = new BoundedChannelOptions (permit)
        {
            FullMode = BoundedChannelFullMode.Wait,
            SingleReader = false,
            SingleWriter = true
        };
        var channel = Channel.CreateBounded(options);
        _reader = channel.Reader;
        _writer = channel.Writer;
        Task.Factory.StartNew(Trim,TaskCreationOptions.LongRunning);
    }
 
    public bool TryAcquire() => _writer.TryWrite(DateTimeOffset.UtcNow);
    private void Trim()
    {
        if (!_reader.TryPeek(out var timestamp))
        {
            Task.Delay(_window).Wait();
            Trim();
        }
        else
        {
            var delay = _window - (DateTimeOffset.UtcNow - timestamp);
            if (delay > TimeSpan.Zero)
            {
                Task.Delay(delay).Wait();
                Trim();
            }
            else
            {
                var valueTask = _reader.ReadAsync();
                if (!valueTask.IsCompleted) _ = valueTask.Result;
                Trim();
            }
        }
    }
}

在实现的TryAcquire方法中，我们试着将当前时间戳写入这个Channel，并将写入的结果（成功或者失败）作为返回值。为了让Channel中只包含指定时间窗口的时间戳，我们利用一个LongRuning的Task执行Trim方法对过期的时间戳进行“裁剪”。Trim会调用ChannelReader的TRyPeek方法，如果返回False，意味着Channel为空，此时会等待一段窗口时间再进行“裁剪”。如果提取出来时间戳在Now-Window与当前时间之间，意味着Channel里面的时间戳均在设定的窗口内，此时同样需要等待，等待时间为Window - (Now - Timestamp)；只有在提取的时间超出窗口范围，我们才需要将其从Channel中移除。

public sealed class FixedWindowRateLimiter : IRateLimiter
{
    private readonly long _windowTicks;
    private readonly int _permit;
    private long _nextWindowStartTimeTicks;
    private volatile int _count = 0;
 
    public FixedWindowRateLimiter(TimeSpan window, int permit)
    {
        _windowTicks = window.Ticks;
        _permit = permit;
        _nextWindowStartTimeTicks = DateTimeOffset.UtcNow.Add(window).Ticks;
    }
 
    public bool TryAcquire()
    {
        // 超出时间窗口，重置计数器，并调整下一个时间窗口的开始时间
        var now = DateTimeOffset.UtcNow.Ticks;
        var nextWindowStartTimeTicks = nextWindowStartTimeTicks;
        if (now >= nextWindowStartTimeTicks && Interlocked.CompareExchange(ref _nextWindowStartTimeTicks, now + _windowTicks, nextWindowStartTimeTicks) == nextWindowStartTimeTicks)
        {
            Interlocked.Exchange(ref _count, 1);
            return true;
        }
        return _count < _permit && Interlocked.Increment(ref _count) <= _permit;
    }
}

在实现的TryAcquire方法中，我们先确定当前时间是否超过了设定的“下一个窗口开始时间”，如果是则调用Interlocked.CompareExchange方法修改__nextWindowStartTimeTicks字段。成功修改__nextWindowStartTimeTicks的线程会调整窗口开始时间，并重置计数器_count为1，并返回True。如果计数器大于等于设定阈值，方法返回False。否则我们让计数器+1，如果该值<=阈值，返回True，否则返回False。