上節講到了kestrel服務器的配置及使用，相信很多同學已經對kestrel服務器有了初步的了解，那么有的同學可能會想更加深入的了解一下Kestrel服務器的是怎么實現監聽和接收http請求的，今天我們來看下Kestrel服務器的源碼，相信看完這些，你一定會對Kestrel服務器的運行機制有更深入的了解。
首先，讓我們從程序啟動類Program.cs開始分析。
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類鏈式調用了兩個方法：
- CreateDefaultBuilder
- ConfigureWebHostDefaults
首先我們來看下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; } }
從上述代碼可以看出，CreateDefaultBuilder並未涉及Kestrel服務器相關代碼，僅僅是進行一些應用的初始化配置，例如，設置應用程序目錄，設置配置文件等操作。
我們再來看下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); })); }
通過閱讀源碼可以發現： ConfigureWebHostDefaults方法中的Microsoft.AspNetCore.WebHost.ConfigureWebDefaults(IWebHostBuilder)為實際執行初始化Kestrel服務器的代碼。
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(); }
看到這里，可能有的同學已經的迫不及待的想要看下Kestrel初始化流程相關的代碼了。別着急，我們一步一步來。
首先我們查看一下上面的UseKestrel擴展方法：
public static IWebHostBuilder UseKestrel( this IWebHostBuilder hostBuilder, Action<WebHostBuilderContext, KestrelServerOptions> configureOptions) { return hostBuilder.UseKestrel().ConfigureKestrel(configureOptions); }
發現該方法只是對傳入的配置項KestrelServerOptions做了封裝，最終是調用了IWebHostBuilder的擴展方法UseKestrel和ConfigureKestrel(Action<WebHostBuilderContext, KestrelServerOptions> configureOptions)擴展方法來初始化Kestrel服務器配置，同樣是鏈式調用。
現在我們來看下UseKestrel()這個擴展方法：
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>(); })); }
細心的同學可能會發現，配置一個Kestrel服務器居然只需要僅僅三行代碼？是不是感覺有些不可思議？Kestrel服務器這么簡單？是的，Kestrel服務器就是這么簡單。那么，Kestrel服務器是如何實現監聽和接收請求的呢？
首先看下IConnectionListenerFactory接口類：
public interface IConnectionListenerFactory { ValueTask<IConnectionListener> BindAsync( EndPoint endpoint, CancellationToken cancellationToken = default (CancellationToken)); }
這個接口職責只有一個，就是執行Sokcert的綁定EndPoint操作，然后返回一個IConnectionListener對象。EndPoint可以有三種實現：
- FileHandleEndPoint
- UnixDomainSocketEndPoint
- IPEndPoint
我們再來看下實現類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); } }
代碼非常簡單，先實例化SocketConnectionListener對象，然后調用SocketConnectionListener的Bind方法並根據傳入的EndPoint類型來創建Socket對象，來實現對EndPoint的監聽和綁定操作。
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); } } }
現在我們已經知道了Kestrel服務器內部是如何進行綁定和監聽操作。那么Kestrel服務器是如何對http請求進行接收處理的呢？
接下來我們來看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接口也非常簡單，定義了一個Server最基本的有兩個功能：啟動和停止。那么Kestrel服務器是怎么實現的這個接口呢？
下面我們來看下微軟官方為IServer注入的實現類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); ... } ... }
我們來整理一下StartAsync方法的流程：
- 字節序校驗：不支持BigEndian
- 請求參數長度校驗，最大8kb
- 判斷服務器是否已經啟動過
- 啟動心跳檢測
- 實例化AddressBindContext用於BindAsync方法使用
- 執行BindAsync方法來綁定地址操作
BindAsync調用了AddressBindContext的OnBind方法。OnBind方法會根據使用的http協議類型創建不同的HttpConnectionMiddleware中間件並加入到connection管道中，用於處理Http請求。
具體規則如下：
- 當協議是HttpProtocols.Http1/2時，創建HttpConnectionMiddleware中間件
- 當協議是HttpProtocols.Http3時，創建Http3ConnectionMiddleware中間件
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方法為connection管道(注意不是IApplicationBuilder中的請求管道)添加了一個HttpConnectionmiddleware中間件，當請求到達時，會執行OnConnectionAsync方法來創建HttpConnection對象，然后通過該對象處理http請求：
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為具體的處理請求的方法，此方法會根據使用的http協議版本來創建Http1Connection還是Http2Connection，然后使用此httpConnection來創建context對象(注意不是HttpContext對象)。
Kestrel服務器對請求的接收是通過OnBind里面的TransportManager.BindAsync來實現的。
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方法為實際接收數據的方法，通過方法名“開始循環接收”，我們猜測，是不是Kestrel服務器是通過對Socket的Accept方法進行循環監聽來接收數據的？那么到底是不是呢？讓我們來繼續跟蹤一下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(); } } }
相信現在大家已經了解是怎么回事了吧？原來Kestrel服務器是通過while(true)循環接收的方式接收用戶請求數據，然后通過線程池的ThreadPool.UnsafeQueueUserWorkItem方法將請求分發到CLR線程池來處理的。換句話說，在請求到來時，TransportManager將OnConnectionAsync方法加入線程池並待CLR線程池調度。
那么回到開始的時候，Kestrel服務器是如何啟動的呢？
讓我們再回顧一下Program.cs中的方法
public static void Main(string[] args) { CreateHostBuilder(args).Build().Run(); }
相信聰明的同學已經猜到了，是通過Run()方法來執行的，Run()方法做了些什么呢？
Run方法實際上是執行了Host類中的StartAsync方法，此方法通過獲取預先注入的GenericeWebHostService類中注入的IServer類來最終調用到IServer實現類的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成功啟動並開始監聽用戶請求。
一句話總結：其實ASP.NET Core 5中的Kestrel服務器只是對Socket的簡單封裝，簡單到直接用socket通過while(true)的方式來循環接收socket請求，並直接放入clr線程池中來等待線程池調度處理。
原來，Kestrel服務器這么簡單~
相信通過本文的介紹，大家已經對ASP.NET Core 5中的Kestrel服務器有了解了吧？