前言

限流是應對流量暴增或某些用戶惡意攻擊等場景的重要手段之一，然而微軟官方從未支持這一重要特性，AspNetCoreRateLimit這一第三方庫限流庫一般作為首選使用，然而其配置參數過于繁多，對使用者造成較大的學習成本。令人高興的是，在剛剛發布的.NET 7 Preview 4中開始支持限流中間件。

UseRateLimiter嘗鮮

安裝.NET 7.0 SDK（v7.0.100-preview.4）
通過nuget包安裝Microsoft.AspNetCore.RateLimiting
創建.Net7網站應用，注冊中間件

全局限流并發1個

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

根據不同資源不同限制并發數，/api前綴的資源租約數2，等待隊列長度為2，其他默認租約數1，隊列長度1。

app.UseRateLimiter(new?RateLimiterOptions()
{//?觸發限流的響應碼DefaultRejectionStatusCode?=?500,OnRejected?=?async?(ctx,?rateLimitLease)?=>{//?觸發限流回調處理},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));}})
});

本地測試

新建一個webapi項目，并注冊限流中間件如下

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();

啟動項目，使用jmeter測試100并發,請求接口/WeatherForecast所有請求處理成功，失敗0！

這個結果是不是有點失望，其實RateLimitPartition.CreateConcurrencyLimiter創建的限流器是ConcurrencyLimiter,后續可以實現各種策略的限流器進行替換之。看了ConcurrencyLimiter的實現，其實就是令牌桶的限流思想，上面配置的new ConcurrencyLimiterOptions(1, QueueProcessingOrder.NewestFirst, 1)),第一個1代表令牌的個數，第二個1代表可以當桶里的令牌為空時，進入等待隊列，而不是直接失敗，當前面的請求結束后，會歸還令牌，此時等待的請求就可以拿到令牌了，QueueProcessingOrder.NewestFirst代表最新的請求優先獲取令牌，也就是獲取令牌時非公平的，還有另一個枚舉值QueueProcessingOrder.OldestFirst老的優先，獲取令牌是公平的。只要我們獲取到令牌的人干活速度快，雖然我們令牌只有1，并發就很高。3. 測試觸發失敗場景只需要讓我們拿到令牌的人持有時間長點，就能輕易的觸發。調整jmater并發數為10

相應內容也是我們設置的內容。

ConcurrencyLimiter源碼

令牌桶限流思想

獲取令牌

protected?override?RateLimitLease?AcquireCore(int?permitCount){//?These?amounts?of?resources?can?never?be?acquiredif?(permitCount?>?_options.PermitLimit){throw?new?ArgumentOutOfRangeException(nameof(permitCount),?permitCount,?SR.Format(SR.PermitLimitExceeded,?permitCount,?_options.PermitLimit));}ThrowIfDisposed();//?Return?SuccessfulLease?or?FailedLease?to?indicate?limiter?stateif?(permitCount?==?0){return?_permitCount?>?0???SuccessfulLease?:?FailedLease;}//?Perf:?Check?SemaphoreSlim?implementation?instead?of?lockingif?(_permitCount?>=?permitCount){lock?(Lock){if?(TryLeaseUnsynchronized(permitCount,?out?RateLimitLease??lease)){return?lease;}}}return?FailedLease;}

嘗試獲取令牌核心邏輯

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?permitsif?(_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?availablelease?=?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?newestif?(_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;}

令牌獲取失敗后進入等待隊列

protected?override?ValueTask<RateLimitLease>?WaitAsyncCore(int?permitCount,?CancellationToken?cancellationToken?=?default){//?These?amounts?of?resources?can?never?be?acquiredif?(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?availableif?(permitCount?==?0?&&?_permitCount?>?0?&&?!_disposed){return?new?ValueTask<RateLimitLease>(SuccessfulLease);}//?Perf:?Check?SemaphoreSlim?implementation?instead?of?lockinglock?(Lock){if?(TryLeaseUnsynchronized(permitCount,?out?RateLimitLease??lease)){return?new?ValueTask<RateLimitLease>(lease);}//?Avoid?integer?overflow?by?using?subtraction?instead?of?additionDebug.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?requestdo{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?OldestFirstreturn?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);}}

歸還令牌

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?canceledif?(!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();}}}