ASP.NET Core 5-Kestrel源码解读
上节讲到了kestrel服务器的配置及使用，相信很多同学已经对kestrel服务器有了初步的了解，那么有的同学可能会想更加深入的了解一下Kestrel服务器的是怎么实现监听和接收http请求的，今天我们来看下Kestrel服务器的源码，相信看完这些，你一定会对Kestrel服务器的运行机制有更深入的了解。
首先，让我们从程序启动类Program.cs开始分析。
public class Program{ public static void Main(string[] args) { CreateHostBuilder(args).Build().Run(); } public static IHostBuilder CreateHostBuilder(string[] args) => Host.CreateDefaultBuilder(args) .ConfigureWebHostDefaults(webBuilder => { webBuilder.UseStartup<Startup>(); });}
其中，Host类链式调用了两个方法：
- CreateDefaultBuilder
- ConfigureWebHostDefaults
首先我们来看下CreateDefaultBuidler方法：
public static IHostBuilder CreateDefaultBuilder(string[] args){ HostBuilder hostBuilder = new HostBuilder(); hostBuilder.UseContentRoot(Directory.GetCurrentDirectory()); hostBuilder.ConfigureHostConfiguration((Action<IConfigurationBuilder>) (config => { ... })); hostBuilder.ConfigureAppConfiguration((Action<HostBuilderContext, IConfigurationBuilder>) ((hostingContext, config) => { ... })).ConfigureLogging((Action<HostBuilderContext, ILoggingBuilder>) ((hostingContext, logging) => { ... })).UseDefaultServiceProvider((Action<HostBuilderContext, ServiceProviderOptions>) ((context, options) => { ... })); return (IHostBuilder) hostBuilder; } }
从上述代码可以看出，CreateDefaultBuilder并未涉及Kestrel服务器相关代码，仅仅是进行一些应用的初始化配置，例如，设置应用程序目录，设置配置文件等操作。
我们再来看下ConfigureWebHostDefaults方法：
public static IHostBuilder ConfigureWebHostDefaults( this IHostBuilder builder, Action<IWebHostBuilder> configure){ if (configure == null) throw new ArgumentNullException(nameof (configure)); return builder.ConfigureWebHost((Action<IWebHostBuilder>) (webHostBuilder => { Microsoft.AspNetCore.WebHost.ConfigureWebDefaults(webHostBuilder); configure(webHostBuilder); }));}
通过阅读源码可以发现： ConfigureWebHostDefaults方法中的Microsoft.AspNetCore.WebHost.ConfigureWebDefaults(IWebHostBuilder)为实际执行初始化Kestrel服务器的代码。
internal static void ConfigureWebDefaults(IWebHostBuilder builder){ ... builder.UseKestrel((Action<WebHostBuilderContext, KestrelServerOptions>) ((builderContext, options) => options.Configure((IConfiguration) builderContext.Configuration.GetSection("Kestrel"), true))).ConfigureServices((Action<WebHostBuilderContext, IServiceCollection>) ((hostingContext, services) => { services.PostConfigure<HostFilteringOptions>((Action<HostFilteringOptions>) (options => { ... } })).UseIIS().UseIISIntegration();}
看到这里，可能有的同学已经的迫不及待的想要看下Kestrel初始化流程相关的代码了。别着急，我们一步一步来。
首先我们查看一下上面的UseKestrel扩展方法：
public static IWebHostBuilder UseKestrel( this IWebHostBuilder hostBuilder, Action<WebHostBuilderContext, KestrelServerOptions> configureOptions) { return hostBuilder.UseKestrel().ConfigureKestrel(configureOptions); }
发现该方法只是对传入的配置项KestrelServerOptions做了封装，最终是调用了IWebHostBuilder的扩展方法UseKestrel和ConfigureKestrel(Action<WebHostBuilderContext, KestrelServerOptions> configureOptions)扩展方法来初始化Kestrel服务器配置，同样是链式调用。
现在我们来看下UseKestrel()这个扩展方法：
public static IWebHostBuilder UseKestrel(this IWebHostBuilder hostBuilder){ return hostBuilder.ConfigureServices((Action<IServiceCollection>) (services => { services.TryAddSingleton<IConnectionListenerFactory, SocketTransportFactory>(); services.AddTransient<IConfigureOptions<KestrelServerOptions>, KestrelServerOptionsSetup>(); services.AddSingleton<IServer, KestrelServerImpl>(); }));}
细心的同学可能会发现，配置一个Kestrel服务器居然只需要仅仅三行代码？是不是感觉有些不可思议？Kestrel服务器这么简单？是的，Kestrel服务器就是这么简单。那么，Kestrel服务器是如何实现监听和接收请求的呢？
首先看下IConnectionListenerFactory接口类：
public interface IConnectionListenerFactory{ ValueTask<IConnectionListener> BindAsync( EndPoint endpoint, CancellationToken cancellationToken = default (CancellationToken));}
这个接口职责只有一个，就是执行Sokcert的绑定EndPoint操作，然后返回一个IConnectionListener对象。EndPoint可以有三种实现：
- FileHandleEndPoint
- UnixDomainSocketEndPoint
- IPEndPoint
我们再来看下实现类SocketTransportFactory：
public sealed class SocketTransportFactory : IConnectionListenerFactory{ private readonly SocketTransportOptions _options; private readonly SocketsTrace _trace; public SocketTransportFactory( IOptions<SocketTransportOptions> options, ILoggerFactory loggerFactory) { if (options == null) throw new ArgumentNullException(nameof (options)); if (loggerFactory == null) throw new ArgumentNullException(nameof (loggerFactory)); this._options = options.Value; this._trace = new SocketsTrace(loggerFactory.CreateLogger("Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets")); } public ValueTask<IConnectionListener> BindAsync( EndPoint endpoint, CancellationToken cancellationToken = default (CancellationToken)) { SocketConnectionListener connectionListener = new SocketConnectionListener(endpoint, this._options, (ISocketsTrace) this._trace); connectionListener.Bind(); return new ValueTask<IConnectionListener>((IConnectionListener) connectionListener); }}
代码非常简单，先实例化SocketConnectionListener对象，然后调用SocketConnectionListener的Bind方法并根据传入的EndPoint类型来创建Socket对象，来实现对EndPoint的监听和绑定操作。
internal void Bind(){ if (this._listenSocket != null) throw new InvalidOperationException(SocketsStrings.TransportAlreadyBound); Socket listenSocket; switch (this.EndPoint) { case FileHandleEndPoint fileHandleEndPoint: this._socketHandle = new SafeSocketHandle((IntPtr) (long) fileHandleEndPoint.FileHandle, true); listenSocket = new Socket(this._socketHandle); break; case UnixDomainSocketEndPoint domainSocketEndPoint: listenSocket = new Socket(domainSocketEndPoint.AddressFamily, SocketType.Stream, ProtocolType.IP); BindSocket(); break; case IPEndPoint ipEndPoint: listenSocket = new Socket(ipEndPoint.AddressFamily, SocketType.Stream, ProtocolType.Tcp); if (ipEndPoint.Address == IPAddress.IPv6Any) listenSocket.DualMode = true; BindSocket(); break; default: listenSocket = new Socket(this.EndPoint.AddressFamily, SocketType.Stream, ProtocolType.Tcp); BindSocket(); break; } this.EndPoint = listenSocket.LocalEndPoint; listenSocket.Listen(this._options.Backlog); this._listenSocket = listenSocket; void BindSocket() { try { listenSocket.Bind(this.EndPoint); } catch (SocketException ex) when (ex.SocketErrorCode == SocketError.AddressAlreadyInUse) { throw new AddressInUseException(ex.Message, (Exception) ex); } }}
现在我们已经知道了Kestrel服务器内部是如何进行绑定和监听操作。那么Kestrel服务器是如何对http请求进行接收处理的呢？
接下来我们来看IServer接口：
public interface IServer : IDisposable{ IFeatureCollection Features { get; } Task StartAsync<TContext>(IHttpApplication<TContext> application, CancellationToken cancellationToken) where TContext : notnull; Task StopAsync(CancellationToken cancellationToken);}
IServer接口也非常简单，定义了一个Server最基本的有两个功能：启动和停止。那么Kestrel服务器是怎么实现的这个接口呢？
下面我们来看下微软官方为IServer注入的实现类KestrelServerImpl:
internal class KestrelServerImpl : IServer{ ... public IFeatureCollection Features { get; } public KestrelServerOptions Options => ServiceContext.ServerOptions; private ServiceContext ServiceContext { get; } private IKestrelTrace Trace => ServiceContext.Log; private AddressBindContext AddressBindContext { get; set; } public async Task StartAsync<TContext>(IHttpApplication<TContext> application, CancellationToken cancellationToken) { ... async Task OnBind(ListenOptions options) { if (!BitConverter.IsLittleEndian) { throw new PlatformNotSupportedException(CoreStrings.BigEndianNotSupported); } ValidateOptions(); if (_hasStarted) { // The server has already started and/or has not been cleaned up yet throw new InvalidOperationException(CoreStrings.ServerAlreadyStarted); } _hasStarted = true; ServiceContext.Heartbeat?.Start(); if ((options.Protocols & HttpProtocols.Http3) == HttpProtocols.Http3) { if (_multiplexedTransportFactory is null) { throw new InvalidOperationException($"Cannot start HTTP/3 server if no {nameof(IMultiplexedConnectionListenerFactory)} is registered."); } options.UseHttp3Server(ServiceContext, application, options.Protocols); var multiplexedConnectionDelegate = ((IMultiplexedConnectionBuilder)options).Build(); multiplexedConnectionDelegate = EnforceConnectionLimit(multiplexedConnectionDelegate, Options.Limits.MaxConcurrentConnections, Trace); options.EndPoint = await _transportManager.BindAsync(options.EndPoint, multiplexedConnectionDelegate, options.EndpointConfig).ConfigureAwait(false); } if ((options.Protocols & HttpProtocols.Http1) == HttpProtocols.Http1 || (options.Protocols & HttpProtocols.Http2) == HttpProtocols.Http2 || options.Protocols == HttpProtocols.None) // TODO a test fails because it doesn't throw an exception in the right place // when there is no HttpProtocols in KestrelServer, can we remove/change the test? { if (_transportFactory is null) { throw new InvalidOperationException($"Cannot start HTTP/1.x or HTTP/2 server if no {nameof(IConnectionListenerFactory)} is registered."); } options.UseHttpServer(ServiceContext, application, options.Protocols); var connectionDelegate = options.Build(); connectionDelegate = EnforceConnectionLimit(connectionDelegate, Options.Limits.MaxConcurrentConnections, Trace); options.EndPoint = await _transportManager.BindAsync(options.EndPoint, connectionDelegate, options.EndpointConfig).ConfigureAwait(false); } } AddressBindContext = new AddressBindContext { ServerAddressesFeature = _serverAddresses, ServerOptions = Options, Logger = Trace, CreateBinding = OnBind, }; await BindAsync(cancellationToken).ConfigureAwait(false); ... } public async Task StopAsync(CancellationToken cancellationToken) { ... } ... private async Task BindAsync(CancellationToken cancellationToken) { ... await AddressBinder.BindAsync(Options.ListenOptions, AddressBindContext).ConfigureAwait(false); ... } ...}
我们来整理一下StartAsync方法的流程：
- 字节序校验：不支持BigEndian
- 请求参数长度校验，最大8kb
- 判断服务器是否已经启动过
- 启动心跳检测
- 实例化AddressBindContext用于BindAsync方法使用
- 执行BindAsync方法来绑定地址操作
BindAsync调用了AddressBindContext的OnBind方法。OnBind方法会根据使用的http协议类型创建不同的HttpConnectionMiddleware中间件并加入到connection管道中，用于处理Http请求。
具体规则如下：
- 当协议是HttpProtocols.Http1/2时，创建HttpConnectionMiddleware中间件
- 当协议是HttpProtocols.Http3时，创建Http3ConnectionMiddleware中间件
目前常用的是HttpConnectionMiddleware：
IConnectionBuilder UseHttpServer<TContext>( this IConnectionBuilder builder, ServiceContext serviceContext, IHttpApplication<TContext> application, HttpProtocols protocols) { HttpConnectionMiddleware<TContext> middleware = new HttpConnectionMiddleware<TContext>(serviceContext, application, protocols); return builder.Use((Func<ConnectionDelegate, ConnectionDelegate>) (next => new ConnectionDelegate(middleware.OnConnectionAsync))); }
UseHttpServer方法为connection管道(注意不是IApplicationBuilder中的请求管道)添加了一个HttpConnectionmiddleware中间件，当请求到达时，会执行OnConnectionAsync方法来创建HttpConnection对象，然后通过该对象处理http请求：
public Task OnConnectionAsync(ConnectionContext connectionContext){ IMemoryPoolFeature memoryPoolFeature = connectionContext.Features.Get<IMemoryPoolFeature>(); HttpConnectionContext context = new HttpConnectionContext(); context.ConnectionId = connectionContext.ConnectionId; context.ConnectionContext = connectionContext; HttpProtocolsFeature protocolsFeature = connectionContext.Features.Get<HttpProtocolsFeature>(); context.Protocols = protocolsFeature != null ? protocolsFeature.HttpProtocols : this._endpointDefaultProtocols; context.ServiceContext = this._serviceContext; context.ConnectionFeatures = connectionContext.Features; context.MemoryPool = memoryPoolFeature?.MemoryPool ?? MemoryPool<byte>.Shared; context.Transport = connectionContext.Transport; context.LocalEndPoint = connectionContext.LocalEndPoint as IPEndPoint; context.RemoteEndPoint = connectionContext.RemoteEndPoint as IPEndPoint; return new HttpConnection(context).ProcessRequestsAsync<TContext>(this._application);}
ProcessRequestsAsync为具体的处理请求的方法，此方法会根据使用的http协议版本来创建Http1Connection还是Http2Connection，然后使用此httpConnection来创建context对象(注意不是HttpContext对象)。
Kestrel服务器对请求的接收是通过OnBind里面的TransportManager.BindAsync来实现的。
public async Task<EndPoint> BindAsync( EndPoint endPoint, ConnectionDelegate connectionDelegate, EndpointConfig? endpointConfig){ if (this._transportFactory == null) throw new InvalidOperationException("Cannot bind with ConnectionDelegate no IConnectionListenerFactory is registered."); IConnectionListener connectionListener = await this._transportFactory.BindAsync(endPoint).ConfigureAwait(false); this.StartAcceptLoop<ConnectionContext>((IConnectionListener<ConnectionContext>) new TransportManager.GenericConnectionListener(connectionListener), (Func<ConnectionContext, Task>) (c => connectionDelegate(c)), endpointConfig); return connectionListener.EndPoint;}
其中StartAcceptLoop方法为实际接收数据的方法，通过方法名“开始循环接收”，我们猜测，是不是Kestrel服务器是通过对Socket的Accept方法进行循环监听来接收数据的？那么到底是不是呢？让我们来继续跟踪一下connectionDispatcher.StartAcceptingConnections方法：
public Task StartAcceptingConnections(IConnectionListener<T> listener){ ThreadPool.UnsafeQueueUserWorkItem<IConnectionListener<T>>(new Action<IConnectionListener<T>>(this.StartAcceptingConnectionsCore), listener, false); return this._acceptLoopTcs.Task;}private void StartAcceptingConnectionsCore(IConnectionListener<T> listener){ AcceptConnectionsAsync(); async Task AcceptConnectionsAsync() { try { while (true) { T connectionContext = await listener.AcceptAsync(new CancellationToken()); if ((object) connectionContext != null) { long id = Interlocked.Increment(ref ConnectionDispatcher<T>._lastConnectionId); KestrelConnection<T> kestrelConnection = new KestrelConnection<T>(id, this._serviceContext, this._transportConnectionManager, this._connectionDelegate, connectionContext, this.Log); this._transportConnectionManager.AddConnection(id, (KestrelConnection) kestrelConnection); this.Log.ConnectionAccepted(connectionContext.ConnectionId); KestrelEventSource.Log.ConnectionQueuedStart((BaseConnectionContext) connectionContext); ThreadPool.UnsafeQueueUserWorkItem((IThreadPoolWorkItem) kestrelConnection, false); } else break; } } catch (Exception ex) { this.Log.LogCritical((EventId) 0, ex, "The connection listener failed to accept any new connections."); } finally { this._acceptLoopTcs.TrySetResult(); } }}
相信现在大家已经了解是怎么回事了吧？原来Kestrel服务器是通过while(true)循环接收的方式接收用户请求数据，然后通过线程池的ThreadPool.UnsafeQueueUserWorkItem方法将请求分发到CLR线程池来处理的。换句话说，在请求到来时，TransportManager将OnConnectionAsync方法加入线程池并待CLR线程池调度。
那么回到开始的时候，Kestrel服务器是如何启动的呢？
让我们再回顾一下Program.cs中的方法
public static void Main(string[] args){ CreateHostBuilder(args).Build().Run();}
相信聪明的同学已经猜到了，是通过Run()方法来执行的，Run()方法做了些什么呢？
Run方法实际上是执行了Host类中的StartAsync方法，此方法通过获取预先注入的GenericeWebHostService类中注入的IServer类来最终调用到IServer实现类的StartAsnyc方法的。
internal class GenericWebHostService : IHostedService{ ... public IServer Server { get; } ... public async Task StartAsync(CancellationToken cancellationToken) { ... var httpApplication = new HostingApplication(application, Logger, DiagnosticListener, HttpContextFactory); await Server.StartAsync(httpApplication, cancellationToken); ... }}
至此，Kestrel成功启动并开始监听用户请求。
一句话总结：其实ASP.NET Core 5中的Kestrel服务器只是对Socket的简单封装，简单到直接用socket通过while(true)的方式来循环接收socket请求，并直接放入clr线程池中来等待线程池调度处理。
原来，Kestrel服务器这么简单~
相信通过本文的介绍，大家已经对ASP.NET Core 5中的Kestrel服务器有了解了吧？
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