前言
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
安装.NET 7.0 SDK（v7.0.100-preview.4）
通过nuget包安装Microsoft.AspNetCore.RateLimiting
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));
}
})
});
本地测试
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();
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
获取令牌
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);
}
}
归还令牌
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 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();
}
}
}
总结
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，更多关于Asp.Net Core7 preview限流中间件的资料请关注编程网其它相关文章！
