深入ASP.NET Core源代码之 - Web Server Kestrel

时间：2023-05-03 03:27:02 | 来源：网站运营

时间：2023-05-03 03:27:02 来源：网站运营

深入ASP.NET Core源代码之 - Web Server Kestrel：

测试代码运行环境：
OS:Ubuntu 18.04 64bit
.NET Core SDK Version: 3.1.101
源代码版本：release/3.1

Kestrel的本质 - Web Server

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Kestrel是http://ASP.NET Core框架内置的默认Web Server 什么是Web Server? 根据维基百科的定义: Web Server是可以处理来自客户端的HTTP协议请求并返回网页的软件或硬件。 因此Kestrel的主要功能就是接收来自网络客户端的HTTP请求，并根据请求返回对应的网页（数据也是一种网页）。

定义 - IServer、IHttpApplication<TContext>

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http://ASP.NET Core定义了两个基本的接口IServer，及IHttpApplication<TContext>，IServer接口定义了Web Server的基本功能，IHttpApplication<TContext>则定义了处理HTTP协议的应用程序的基本功能，我们首先来看下这两个定义:

Web 服务器 - IServer

namespace Microsoft.AspNetCore.Hosting.Server{ /// <summary> /// Represents a server. /// </summary> public interface IServer : IDisposable { /// <summary> /// A collection of HTTP features of the server. /// </summary> IFeatureCollection Features { get; } /// <summary> /// Start the server with an application. /// </summary> /// <param name="application">An instance of <see cref="IHttpApplication{TContext}"/>.</param> /// <typeparam name="TContext">The context associated with the application.</typeparam> /// <param name="cancellationToken">Indicates if the server startup should be aborted.</param> Task StartAsync<TContext>(IHttpApplication<TContext> application, CancellationToken cancellationToken); /// <summary> /// Stop processing requests and shut down the server, gracefully if possible. /// </summary> /// <param name="cancellationToken">Indicates if the graceful shutdown should be aborted.</param> Task StopAsync(CancellationToken cancellationToken); }}Features 是一个功能集合，其中可以包含所有应用程序需要的，用以处理HTTP协议各个阶段和组成部分的功能集，以接口的形式注入到Features中。

StartAsync方法可以启动IServer对象，用来接受用户请求。包含两个参数：IHttpApplication<TContext>和CancellationToken。 IHttpApplicatoin<TContext>是最终处理HTTP请求的应用程序入口点，在ASP.NET Core应用程序中，默认的IHttpApplication<TContext>实现是：HostingApplication，我们会在稍后的部分进行详细的介绍。 而CancellationToken用来响应中断应用程序启动的请求。

StopAsync方法用来处理停止服务的请求，接受一个参数CancellationToken，用来响应中断停止应用程序的请求。

Http应用程序 - IHttpApplication<TContext>

namespace Microsoft.AspNetCore.Hosting.Server{ /// <summary> /// Represents an application. /// </summary> /// <typeparam name="TContext">The context associated with the application.</typeparam> public interface IHttpApplication<TContext> { /// <summary> /// Create a TContext given a collection of HTTP features. /// </summary> /// <param name="contextFeatures">A collection of HTTP features to be used for creating the TContext.</param> /// <returns>The created TContext.</returns> TContext CreateContext(IFeatureCollection contextFeatures); /// <summary> /// Asynchronously processes an TContext. /// </summary> /// <param name="context">The TContext that the operation will process.</param> Task ProcessRequestAsync(TContext context); /// <summary> /// Dispose a given TContext. /// </summary> /// <param name="context">The TContext to be disposed.</param> /// <param name="exception">The Exception thrown when processing did not complete successfully, otherwise null.</param> void DisposeContext(TContext context, Exception exception); }}IHttpApplication<TContext>接口的定义包含了三个方法： CreateContext方法用来创建处理请求的上下文中所需要的所有相关数据，组成Context对象，由接口的实现自己定义类型， ProcessRequestAsync方法使用CreateContext方法创建的Context对象处理本次请求。 DisposeContext方法在完成请求的处理后，负责释放Context对象。

实现 - KestrelServer

http://ASP.NET Core提供了默认的IServer：KestrelServer，下面我们就来看看KestrelServer具体都做了些什么。

KestrelServer 定义在dotnet/aspnetcore项目中（GITHUB REPO）。 项目名称为：Microsoft.AspNetCore.Server.Kestrel.Core 名称空间:Microsoft.AspNetCore.Server.Kestrel.Core 源代码

服务器启动：端口监听，协议解析及请求处理。

我们先看一下KestrelServer.StartAsync()方法的代码实现：

public async Task StartAsync<TContext>(IHttpApplication<TContext> application, CancellationToken cancellationToken) { try { 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(); async Task OnBind(ListenOptions options) { // Add the HTTP middleware as the terminal connection middleware options.UseHttpServer(ServiceContext, application, options.Protocols); var connectionDelegate = options.Build(); // Add the connection limit middleware if (Options.Limits.MaxConcurrentConnections.HasValue) { connectionDelegate = new ConnectionLimitMiddleware(connectionDelegate, Options.Limits.MaxConcurrentConnections.Value, Trace).OnConnectionAsync; } var connectionDispatcher = new ConnectionDispatcher(ServiceContext, connectionDelegate); var transport = await _transportFactory.BindAsync(options.EndPoint).ConfigureAwait(false); // Update the endpoint options.EndPoint = transport.EndPoint; var acceptLoopTask = connectionDispatcher.StartAcceptingConnections(transport); _transports.Add((transport, acceptLoopTask)); } await AddressBinder.BindAsync(_serverAddresses, Options, Trace, OnBind).ConfigureAwait(false); } catch (Exception ex) { Trace.LogCritical(0, ex, "Unable to start Kestrel."); Dispose(); throw; } }Kestrel首先会检查服务器的字节序，目前是不支持大端序的。 然后检查最大请求长度限制的设置项，以及服务器是否已经启动。

最后，通过AddressBinder对预先配置的IP地址或终结点(EndPoint)名称进行监听，开始接受客户端的请求。

当每有一个新的HTTP请求通过TCP协议或其他协议和服务器成功简历连接后，AddressBinder使用ThreadPool.UnsafeQueueUserWorkItem()方法将OnBind()方法添加到线程池中，等待线程池的调度。

如果此时进程有可用的线程，就会调用OnBind()方法，处理用户的HTTP请求。

OnBind()方法默认使用HttpConnectionMiddleware<ServiceContext>中间件，处理新接入的用户请求，当设置了MaxConcurrentConnections值为True时，则会默认使用ConnectionLimitMiddleware中间件，限制最大可用连接数，如果当前请求数已经达到最大可接受连接数，则拒绝用户的请求并断开连接，否则调用HttpConnectionMiddleware<ServiceContext>中间件，继续处理用户的请求。

处理HTTP请求 - HttpConnectionMiddleware<ServiceContext>、HttpConnection

HttpConnectionMiddleware<ServiceContext>中间件负责组装连接相关的上下文数据HttpConnectionContext，并使用HttpConnection类处理用户请求。

internal class HttpConnectionMiddleware<TContext> { private readonly ServiceContext _serviceContext; private readonly IHttpApplication<TContext> _application; private readonly HttpProtocols _protocols; public HttpConnectionMiddleware(ServiceContext serviceContext, IHttpApplication<TContext> application, HttpProtocols protocols) { _serviceContext = serviceContext; _application = application; _protocols = protocols; } public Task OnConnectionAsync(ConnectionContext connectionContext) { var memoryPoolFeature = connectionContext.Features.Get<IMemoryPoolFeature>(); var httpConnectionContext = new HttpConnectionContext { ConnectionId = connectionContext.ConnectionId, ConnectionContext = connectionContext, Protocols = _protocols, ServiceContext = _serviceContext, ConnectionFeatures = connectionContext.Features, MemoryPool = memoryPoolFeature.MemoryPool, Transport = connectionContext.Transport, LocalEndPoint = connectionContext.LocalEndPoint as IPEndPoint, RemoteEndPoint = connectionContext.RemoteEndPoint as IPEndPoint }; var connection = new HttpConnection(httpConnectionContext); return connection.ProcessRequestsAsync(_application); } }

HTTP版本控制 - HttpConnection

当用户创建HttpConnection类时，在初始化过程中，会根据用户请求声明的HTTP协议版本，分别创建对应版本的Connection类，并使用该类处理用户请求：

public async Task ProcessRequestsAsync<TContext>(IHttpApplication<TContext> httpApplication) { try { // Ensure TimeoutControl._lastTimestamp is initialized before anything that could set timeouts runs. _timeoutControl.Initialize(_systemClock.UtcNowTicks); IRequestProcessor requestProcessor = null; switch (SelectProtocol()) { case HttpProtocols.Http1: // _http1Connection must be initialized before adding the connection to the connection manager requestProcessor = _http1Connection = new Http1Connection<TContext>(_context); _protocolSelectionState = ProtocolSelectionState.Selected; break; case HttpProtocols.Http2: // _http2Connection must be initialized before yielding control to the transport thread, // to prevent a race condition where _http2Connection.Abort() is called just as // _http2Connection is about to be initialized. requestProcessor = new Http2Connection(_context); _protocolSelectionState = ProtocolSelectionState.Selected; break; case HttpProtocols.None: // An error was already logged in SelectProtocol(), but we should close the connection. break; default: // SelectProtocol() only returns Http1, Http2 or None. throw new NotSupportedException($"{nameof(SelectProtocol)} returned something other than Http1, Http2 or None."); } _requestProcessor = requestProcessor; if (requestProcessor != null) { var connectionHeartbeatFeature = _context.ConnectionFeatures.Get<IConnectionHeartbeatFeature>(); var connectionLifetimeNotificationFeature = _context.ConnectionFeatures.Get<IConnectionLifetimeNotificationFeature>(); // These features should never be null in Kestrel itself, if this middleware is ever refactored to run outside of kestrel, // we'll need to handle these missing. Debug.Assert(connectionHeartbeatFeature != null, nameof(IConnectionHeartbeatFeature) + " is missing!"); Debug.Assert(connectionLifetimeNotificationFeature != null, nameof(IConnectionLifetimeNotificationFeature) + " is missing!"); // Register the various callbacks once we're going to start processing requests // The heart beat for various timeouts connectionHeartbeatFeature?.OnHeartbeat(state => ((HttpConnection)state).Tick(), this); // Register for graceful shutdown of the server using var shutdownRegistration = connectionLifetimeNotificationFeature?.ConnectionClosedRequested.Register(state => ((HttpConnection)state).StopProcessingNextRequest(), this); // Register for connection close using var closedRegistration = _context.ConnectionContext.ConnectionClosed.Register(state => ((HttpConnection)state).OnConnectionClosed(), this); await requestProcessor.ProcessRequestsAsync(httpApplication); } } catch (Exception ex) { Log.LogCritical(0, ex, $"Unexpected exception in {nameof(HttpConnection)}.{nameof(ProcessRequestsAsync)}."); } finally { if (_http1Connection?.IsUpgraded == true) { _context.ServiceContext.ConnectionManager.UpgradedConnectionCount.ReleaseOne(); } } }HTTP1和HTTP2处理HTTP协议的方式有所不同，HTTP1协议解析完成后，会立即调用IHttpApplication<TContext>处理请求，HTTP2协议解析完成后，会再次调用ThreadPool.UnsafeQueueUserWorkItem()方法等待线程池可用线程。

结束语

Kestrel服务的代码量并不下，其中主要是辅助接受用户请求和解析HTTP协议的代码，在这里不做详细的介绍，各位读者有兴趣的，可以详细阅读源代码。

我们看到，Kestrel服务在接受和处理请求时，都用到了线程池，可以极大的提高服务器的吞吐量。

后面，我们还会详细介绍系统默认的IHttpApplication<TContext>实现，看看ASP.NET Core是如何将HTTP转发到Controller和Action，其中又有哪些精妙的代码呢。

敬请期待。