限流是应对流量暴增或某些用户恶意攻击等场景的重要手段之一，然而微软官方从未支持这一重要特性，AspNetCoreRateLimit这一第三方库限流库一般作为首选使用，然而其配置参数过于繁多，对使用者造成较大的学习成本。令人高兴的是，在刚刚发布的.NET 7 Preview 4中开始支持限流中间件。

UseRateLimiter尝鲜

1. 安装.NET 7.0 SDK（v7.0.100-preview.4）
2. 通过nuget包安装Microsoft.AspNetCore.RateLimiting
3. 创建.Net7网站应用，注册中间件

全局限流并发1个

app.UseRateLimiter(new RateLimiterOptions
{
    Limiter = PartitionedRateLimiter.Create<HttpContext, string>(resource =>
    {
        return RateLimitPartition.CreateConcurrencyLimiter("MyLimiter",
            _ => new ConcurrencyLimiterOptions(1, QueueProcessingOrder.NewestFirst, 1));
    })
});

根据不同资源不同限制并发数，/api前缀的资源租约数2，等待队列长度为2，其他默认租约数1，队列长度1。

app.UseRateLimiter(new RateLimiterOptions()
{
    // 触发限流的响应码
    DefaultRejectionStatusCode = 500,
    OnRejected = async (ctx, rateLimitLease) =>
    {
        // 触发限流回调处理
    },
    Limiter = PartitionedRateLimiter.Create<HttpContext, string>(resource =>
    {
        if (resource.Request.Path.StartsWithSegments("/api"))
        {
            return RateLimitPartition.CreateConcurrencyLimiter("WebApiLimiter",
                _ => new ConcurrencyLimiterOptions(2, QueueProcessingOrder.NewestFirst, 2));
        }
        else
        {
            return RateLimitPartition.CreateConcurrencyLimiter("DefaultLimiter",
                _ => new ConcurrencyLimiterOptions(1, QueueProcessingOrder.NewestFirst, 1));
        }
    })
});

1. 新建一个webapi项目，并注册限流中间件如下

using Microsoft.AspNetCore.RateLimiting;
using System.Threading.RateLimiting;

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.

builder.Services.AddControllers();
// Learn more about configuring Swagger/OpenAPI at https://aka.ms/aspnetcore/swashbuckle
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

var app = builder.Build();

// Configure the HTTP request pipeline.
if (app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI();
}

app.UseRateLimiter(new RateLimiterOptions
{
    DefaultRejectionStatusCode = 500,
    OnRejected = async (ctx, lease) =>
    {
        await Task.FromResult(ctx.Response.WriteAsync("ConcurrencyLimiter"));
    },
    Limiter = PartitionedRateLimiter.Create<HttpContext, string>(resource =>
    {
        return RateLimitPartition.CreateConcurrencyLimiter("MyLimiter",
            _ => new ConcurrencyLimiterOptions(1, QueueProcessingOrder.NewestFirst, 1));
    })
});

app.UseHttpsRedirection();

app.UseAuthorization();

app.MapControllers();

app.Run();

1. 启动项目，使用jmeter测试100并发,请求接口/WeatherForecast
   
   
   
   所有请求处理成功，失败0！

这个结果是不是有点失望，其实RateLimitPartition.CreateConcurrencyLimiter创建的限流器是
ConcurrencyLimiter,后续可以实现个各种策略的限流器进行替换之。
看了ConcurrencyLimiter的实现，其实就是令牌桶的限流思想，上面配置的new ConcurrencyLimiterOptions(1, QueueProcessingOrder.NewestFirst, 1)),第一个1代表令牌的个数，第二个1代表可以当桶里的令牌为空时，进入等待队列，而不是直接失败，当前面的请求结束后，会归还令牌，此时等待的请求就可以拿到令牌了，QueueProcessingOrder.NewestFirst代表最新的请求优先获取令牌，也就是获取令牌时非公平的，还有另一个枚举值QueueProcessingOrder.OldestFirst老的优先，获取令牌是公平的。只要我们获取到令牌的人干活速度快，虽然我们令牌只有1，并发就很高。
3. 测试触发失败场景
只需要让我们拿到令牌的人持有时间长点，就能轻易的触发。

调整jmater并发数为10

相应内容也是我们设置的内容。

ConcurrencyLimiter源码

令牌桶限流思想

        protected override RateLimitLease AcquireCore(int permitCount)
        {
            // These amounts of resources can never be acquired
            if (permitCount > _options.PermitLimit)
            {
                throw new ArgumentOutOfRangeException(nameof(permitCount), permitCount, SR.Format(SR.PermitLimitExceeded, permitCount, _options.PermitLimit));
            }

            ThrowIfDisposed();

            // Return SuccessfulLease or FailedLease to indicate limiter state
            if (permitCount == 0)
            {
                return _permitCount > 0 ? SuccessfulLease : FailedLease;
            }

            // Perf: Check SemaphoreSlim implementation instead of locking
            if (_permitCount >= permitCount)
            {
                lock (Lock)
                {
                    if (TryLeaseUnsynchronized(permitCount, out RateLimitLease? lease))
                    {
                        return lease;
                    }
                }
            }

            return FailedLease;
        }

尝试获取令牌核心逻辑

        private bool TryLeaseUnsynchronized(int permitCount, [NotNullWhen(true)] out RateLimitLease? lease)
        {
            ThrowIfDisposed();

            // if permitCount is 0 we want to queue it if there are no available permits
            if (_permitCount >= permitCount && _permitCount != 0)
            {
                if (permitCount == 0)
                {
                    // Edge case where the check before the lock showed 0 available permits but when we got the lock some permits were now available
                    lease = SuccessfulLease;
                    return true;
                }

                // a. if there are no items queued we can lease
                // b. if there are items queued but the processing order is newest first, then we can lease the incoming request since it is the newest
                if (_queueCount == 0 || (_queueCount > 0 && _options.QueueProcessingOrder == QueueProcessingOrder.NewestFirst))
                {
                    _idleSince = null;
                    _permitCount -= permitCount;
                    Debug.Assert(_permitCount >= 0);
                    lease = new ConcurrencyLease(true, this, permitCount);
                    return true;
                }
            }

            lease = null;
            return false;
        }

令牌获取失败后进入等待队列

 protected override ValueTask<RateLimitLease> WaitAsyncCore(int permitCount, CancellationToken cancellationToken = default)
        {
            // These amounts of resources can never be acquired
            if (permitCount > _options.PermitLimit)
            {
                throw new ArgumentOutOfRangeException(nameof(permitCount), permitCount, SR.Format(SR.PermitLimitExceeded, permitCount, _options.PermitLimit));
            }

            // Return SuccessfulLease if requestedCount is 0 and resources are available
            if (permitCount == 0 && _permitCount > 0 && !_disposed)
            {
                return new ValueTask<RateLimitLease>(SuccessfulLease);
            }

            // Perf: Check SemaphoreSlim implementation instead of locking
            lock (Lock)
            {
                if (TryLeaseUnsynchronized(permitCount, out RateLimitLease? lease))
                {
                    return new ValueTask<RateLimitLease>(lease);
                }

                // Avoid integer overflow by using subtraction instead of addition
                Debug.Assert(_options.QueueLimit >= _queueCount);
                if (_options.QueueLimit - _queueCount < permitCount)
                {
                    if (_options.QueueProcessingOrder == QueueProcessingOrder.NewestFirst && permitCount <= _options.QueueLimit)
                    {
                        // remove oldest items from queue until there is space for the newest request
                        do
                        {
                            RequestRegistration oldestRequest = _queue.DequeueHead();
                            _queueCount -= oldestRequest.Count;
                            Debug.Assert(_queueCount >= 0);
                            if (!oldestRequest.Tcs.TrySetResult(FailedLease))
                            {
                                // Updating queue count is handled by the cancellation code
                                _queueCount += oldestRequest.Count;
                            }
                        }
                        while (_options.QueueLimit - _queueCount < permitCount);
                    }
                    else
                    {
                        // Don't queue if queue limit reached and QueueProcessingOrder is OldestFirst
                        return new ValueTask<RateLimitLease>(QueueLimitLease);
                    }
                }

                CancelQueueState tcs = new CancelQueueState(permitCount, this, cancellationToken);
                CancellationTokenRegistration ctr = default;
                if (cancellationToken.CanBeCanceled)
                {
                    ctr = cancellationToken.Register(static obj =>
                    {
                        ((CancelQueueState)obj!).TrySetCanceled();
                    }, tcs);
                }

                RequestRegistration request = new RequestRegistration(permitCount, tcs, ctr);
                _queue.EnqueueTail(request);
                _queueCount += permitCount;
                Debug.Assert(_queueCount <= _options.QueueLimit);

                return new ValueTask<RateLimitLease>(request.Tcs.Task);
            }
        }

 private void Release(int releaseCount)
        {
            lock (Lock)
            {
                if (_disposed)
                {
                    return;
                }

                _permitCount += releaseCount;
                Debug.Assert(_permitCount <= _options.PermitLimit);

                while (_queue.Count > 0)
                {
                    RequestRegistration nextPendingRequest =
                        _options.QueueProcessingOrder == QueueProcessingOrder.OldestFirst
                        ? _queue.PeekHead()
                        : _queue.PeekTail();

                    if (_permitCount >= nextPendingRequest.Count)
                    {
                        nextPendingRequest =
                            _options.QueueProcessingOrder == QueueProcessingOrder.OldestFirst
                            ? _queue.DequeueHead()
                            : _queue.DequeueTail();

                        _permitCount -= nextPendingRequest.Count;
                        _queueCount -= nextPendingRequest.Count;
                        Debug.Assert(_permitCount >= 0);

                        ConcurrencyLease lease = nextPendingRequest.Count == 0 ? SuccessfulLease : new ConcurrencyLease(true, this, nextPendingRequest.Count);
                        // Check if request was canceled
                        if (!nextPendingRequest.Tcs.TrySetResult(lease))
                        {
                            // Queued item was canceled so add count back
                            _permitCount += nextPendingRequest.Count;
                            // Updating queue count is handled by the cancellation code
                            _queueCount += nextPendingRequest.Count;
                        }
                        nextPendingRequest.CancellationTokenRegistration.Dispose();
                        Debug.Assert(_queueCount >= 0);
                    }
                    else
                    {
                        break;
                    }
                }

                if (_permitCount == _options.PermitLimit)
                {
                    Debug.Assert(_idleSince is null);
                    Debug.Assert(_queueCount == 0);
                    _idleSince = Stopwatch.GetTimestamp();
                }
            }
        }

虽然这次官方对限流进行了支持，但貌似还不能支持对ip或client级别的限制支持，对于更高级的限流策略仍需要借助第三方库或自己实现，期待后续越来越完善。

__EOF__