KestrelServer详解自定义一个迷你版的KestrelServer

和所有的服务器一样，KestrelServer最终需要解决的是网络传输的问题。在《网络连接的创建》，我们介绍了KestrelServer如何利用连接接听器的建立网络连接，并再次基础上演示了如何直接利用建立的连接接收请求和回复响应。本篇更进一步，我们根据其总体设计，定义了迷你版的KestrelServer让读者看看这个重要的服务器大体是如何实现的。本文提供的示例演示已经同步到《ASP.NET Core 6框架揭秘-实例演示版》）

一、ConnectionDelegate
二、IConnectionBuilder
三、HTTP 1.x/HTTP 2.x V.S. HTTP 3
四、MiniKestrelServer

一、ConnectionDelegate

ASP.NET CORE在“应用”层将针对请求的处理抽象成由中间件构建的管道，实际上KestrelServer面向“传输”层的连接也采用了这样的设计。当代表连接的ConnectionContext上下文创建出来之后，后续的处理将交给由连接中间件构建的管道进行处理。我们可以根据需要注册任意的中间件来处理连接，比如可以将并发连结的控制实现在专门的连接中间件中。ASP.NET CORE管道利用RequestDelegate委托来表示请求处理器，连接管道同样定义了如下这个ConnectionDelegate委托。

publicdelegate Task ConnectionDelegate(ConnectionContext connection);

二、IConnectionBuilder

ASP.NET CORE管道中的中间件体现为一个Func委托，连接管道的中间件同样可以利用Func委托来表示。ASP.NET CORE管道中的中间件注册到IApplicationBuilder对象上并利用它将管道构建出来。连接管道依然具有如下这个IConnectionBuilder接口，ConnectionBuilder实现了该接口。

publicinterface IConnectionBuilder {     IServiceProvider ApplicationServices {get; }     IConnectionBuilder Use(Func middleware);     ConnectionDelegate Build(); }publicclass ConnectionBuilder : IConnectionBuilder {public IServiceProvider ApplicationServices {get; }public ConnectionDelegate Build();public IConnectionBuilder Use(Func middleware); }

IConnectionBuilder接口还定义了如下三个扩展方法来注册连接中间件。第一个Use方法使用Func, Task>委托来表示中间件。其余两个方法用来注册管道末端的中间件，这样的中间件本质上就是一个ConnectionDelegate委托，我们可以将其定义成一个派生于ConnectionHandler的类型。

publicstaticclass ConnectionBuilderExtensions {publicstatic IConnectionBuilder Use(this IConnectionBuilder connectionBuilder,Func, Task> middleware);publicstatic IConnectionBuilder Run(this IConnectionBuilder connectionBuilder,Func middleware);publicstatic IConnectionBuilder UseConnectionHandler(this IConnectionBuilder connectionBuilder) where TConnectionHandler : ConnectionHandler; }publicabstractclass ConnectionHandler {publicabstract Task OnConnectedAsync(ConnectionContext connection); }

三、HTTP 1.x/HTTP 2.x V.S. HTTP 3

KestrelServer针对HTTP 1.X/2和HTTP 3的设计和实现基本上独立的，这一点从监听器的定义就可以看出来。就连接管道来说，基于HTTP 3的多路复用连接通过MultiplexedConnectionContext表示，它也具有“配套”的MultiplexedConnectionDelegate委托和IMultiplexedConnectionBuilder接口。ListenOptions类型同时实现了IConnectionBuilder和IMultiplexedConnectionBuilder接口，意味着我们在注册终结点的时候还可以注册任意中间件。

publicdelegate Task MultiplexedConnectionDelegate(MultiplexedConnectionContext connection);publicinterface IMultiplexedConnectionBuilder {     IServiceProvider ApplicationServices {get; }     IMultiplexedConnectionBuilder Use(Func middleware);     MultiplexedConnectionDelegate Build(); }publicclass MultiplexedConnectionBuilder : IMultiplexedConnectionBuilder {public IServiceProvider ApplicationServices {get; }public IMultiplexedConnectionBuilder Use(Func middleware);public MultiplexedConnectionDelegate Build(); }publicclass ListenOptions : IConnectionBuilder, IMultiplexedConnectionBuilder

四、MiniKestrelServer

在了解了KestrelServer的连接管道后，我们来简单模拟一下这种服务器类型的实现，为此我们定义了一个名为MiniKestrelServer的服务器类型。简单起见，MiniKestrelServer只提供针对HTTP 1.1的支持。对于任何一个服务来说，它需要将请求交付给一个IHttpApplication对象进行处理，MiniKestrelServer将这项工作实现在如下这个HostedApplication类型中。

publicclass HostedApplication : ConnectionHandler where TContext : notnull {privatereadonly IHttpApplication _application;public HostedApplication(IHttpApplication application) => _application = application;publicoverride async Task OnConnectedAsync(ConnectionContext connection)     {         var reader = connection!.Transport.Input;while (true)         {             var result = await reader.ReadAsync();using (var body =new MemoryStream())             {                 var (features, request, response) = CreateFeatures(result, body);                 var closeConnection = request.Headers.TryGetValue("Connection",out var vallue) && vallue == "Close";                 reader.AdvanceTo(result.Buffer.End);                  var context = _application.CreateContext(features);                 Exception? exception =null;try                 {                     await _application.ProcessRequestAsync(context);                     await ApplyResponseAsync(connection, response, body);                 }catch (Exception ex)                 {                     exception = ex;                 }finally                 {                     _application.DisposeContext(context, exception);                 }if (closeConnection)                 {                     await connection.DisposeAsync();return;                 }             }if (result.IsCompleted)             {break;             }         }static (IFeatureCollection, IHttpRequestFeature, IHttpResponseFeature) CreateFeatures(ReadResult result, Stream body)         {             var handler =new HttpParserHandler();             var parserHandler =new HttpParser(handler);             var length = (int)result.Buffer.Length;             var array = ArrayPool<byte>.Shared.Rent(length);try             {                 result.Buffer.CopyTo(array);                 parserHandler.Execute(new ArraySegment<byte>(array, 0, length));             }finally             {                 ArrayPool<byte>.Shared.Return(array);             }             var bodyFeature =new StreamBodyFeature(body);              var features =new FeatureCollection();             var responseFeature =new HttpResponseFeature();             features.Set(handler.Request);             features.Set(responseFeature);             features.Set(bodyFeature);return (features, handler.Request, responseFeature);         }static async Task ApplyResponseAsync(ConnectionContext connection, IHttpResponseFeature response, Stream body)         {             var builder =new StringBuilder();             builder.AppendLine($"HTTP/1.1 {response.StatusCode} {response.ReasonPhrase}");foreach (var kvin response.Headers)             {                 builder.AppendLine($"{kv.Key}: {kv.Value}");             }             builder.AppendLine($"Content-Length: {body.Length}");             builder.AppendLine();             var bytes = Encoding.UTF8.GetBytes(builder.ToString());              var writer = connection.Transport.Output;             await writer.WriteAsync(bytes);             body.Position = 0;             await body.CopyToAsync(writer);         }     } }

HostedApplication是对一个IHttpApplication对象的封装。它派生于抽象类ConnectionHandler，重写的OnConnectedAsync方法将针对请求的读取和处理置于一个无限循环中。为了将读取的请求转交给IHostedApplication对象进行处理，它需要根据特性集合将TContext上下文创建出来。这里提供的特性集合只包含三种核心的特性，一个是描述请求的HttpRequestFeature特性，它是利用HttpParser解析请求荷载内容得到的。另一个是描述响应的HttpResponseFeature特性，至于提供响应主体的特性由如下所示的StreamBodyFeature对象来表示。这三个特性的创建实现在CreateFeatures方法中。

publicclass StreamBodyFeature : IHttpResponseBodyFeature {public Stream 	Stream {get; }public PipeWriter 	Writer {get; }public StreamBodyFeature(Stream stream)     {         Stream = stream;         Writer = PipeWriter.Create(Stream);     }public Task CompleteAsync() => Task.CompletedTask;publicvoid DisableBuffering() { }public Task SendFileAsync(string path,long offset,long? count,     CancellationToken cancellationToken =default)=>thrownew NotImplementedException();public Task StartAsync(CancellationToken cancellationToken =default) => Task.CompletedTask; }

包含三大特性的集合随后作为参数调用了IHostedApplication对象的CreateContext方法将TContext上下文创建出来，此上下文作为参数传入了同一对象的ProcessRequestAsync方法，此时中间件管道接管请求。待中间件管道完成处理后， ApplyResponseAsync方法被调用以完成最终的响应工作。ApplyResponseAsync方法将响应状态从HttpResponseFeature特性中提取并生成首行响应内容（“HTTP/1.1 {StatusCode} {ReasonPhrase}”），然后再从这个特性中将响应报头提取出来并生成相应的文本。响应报文的首行内容和报头文本按照UTF-8编码生成二进制数组后利用ConnectionContext上下文的Transport属性返回的IDuplexPipe对象发送出去后，它再将StreamBodyFeature特性收集到的响应主体输出流“拷贝”到这个IDuplexPipe对象中，进而完成了针对响应主体内容的输出。

如下所示的是MiniKestrelServer类型的完整定义。该类型的构造函数中注入了用于提供配置选项的IOptions特性和IConnectionListenerFactory工厂，并且创建了一个ServerAddressesFeature对象并注册到Features属性返回的特性集合中。

publicclass MiniKestrelServer : IServer {privatereadonly KestrelServerOptions _options;privatereadonly IConnectionListenerFactory _factory;privatereadonly List _listeners =new();public IFeatureCollection Features {get; } =new FeatureCollection();public MiniKestrelServer(IOptions optionsAccessor, IConnectionListenerFactory factory)     {         _factory = factory;         _options = optionsAccessor.Value;         Features.Set(new ServerAddressesFeature());     }publicvoid Dispose() => StopAsync(CancellationToken.None).GetAwaiter().GetResult();public Task StartAsync(IHttpApplication application, CancellationToken cancellationToken) where TContext : notnull     {         var feature = Features.Get()!;         IEnumerable listenOptions;if (feature.PreferHostingUrls)         {             listenOptions = BuildListenOptions(feature);         }else         {             listenOptions = _options.GetListenOptions();if (!listenOptions.Any())             {                 listenOptions = BuildListenOptions(feature);             }         }foreach (var optionsin listenOptions)         {             _ = StartAsync(options);         }return Task.CompletedTask;          async Task StartAsync(ListenOptions litenOptions)         {             var listener = await _factory.BindAsync(litenOptions.EndPoint,cancellationToken);             _listeners.Add(listener!);
var hostedApplication = new HostedApplication(application);             var pipeline = litenOptions.Use(next => context => hostedApplication.OnConnectedAsync(context)).Build();
while (true)             {                 var connection = await listener.AcceptAsync();if (connection !=null)                 {                     _ = pipeline(connection);                 }             }         }          IEnumerable BuildListenOptions(IServerAddressesFeature feature)         {             var options =new KestrelServerOptions();foreach (var addressin feature.Addresses)             {                 var url =new Uri(address);if (string.Compare("localhost", url.Host,true) == 0)                 {                     options.ListenLocalhost(url.Port);                 }else                 {                     options.Listen(IPAddress.Parse(url.Host), url.Port);                 }              }return options.GetListenOptions();         }     }public Task StopAsync(CancellationToken cancellationToken) => Task.WhenAll(_listeners.Select(it => it.DisposeAsync().AsTask())); }

实现的StartAsync方法先将IServerAddressesFeature特性提取出来，并利用其PreferHostingUrls属性决定应该使用直接注册到KestrelOptions配置选项上的终结点还是使用注册在该特定上的监听地址。如果使用后者，注册的监听地址会利用BuildListenOptions方法转换成对应的ListenOptions列表，否则直接从KestrelOptions对象的ListenOptions属性提取所有的ListenOptions列表，由于这是一个内部属性，不得不利用如下这个扩展方法以反射的方式获取这个列表。

publicstaticclass KestrelServerOptionsExtensions {publicstatic IEnumerable GetListenOptions(this KestrelServerOptions options)     {         var property =typeof(KestrelServerOptions).GetProperty("ListenOptions",BindingFlags.NonPublic | BindingFlags.Instance);return (IEnumerable)property!.GetValue(options)!;     } }

对于每一个表示注册终结点的ListenOptions配置选项，StartAsync方法利用IConnectionListenerFactory工厂将对应的IConnectionListener监听器创建出来，并绑定到指定的终结点上监听连接请求。表示连接的ConnectionContext上下文一旦被创建出来后，该方法便会利用构建的连接管道对它进行处理。在调用ListenOptions配置选项的Build方法构建连接管道前，StartAsync方法将HostedApplication对象创建出来并作为中间件进行了注册。所以针对连接的处理将被这个HostedApplication对象接管。

using App;using Microsoft.AspNetCore.Hosting.Server;using Microsoft.Extensions.DependencyInjection.Extensions;  var builder = WebApplication.CreateBuilder(); builder.WebHost.UseKestrel(kestrel => kestrel.ListenLocalhost(5000));builder.Services.Replace(ServiceDescriptor.Singleton()); var app = builder.Build(); app.Run(context => context.Response.WriteAsync("Hello World!")); app.Run();

如上所示的演示程序将替换了针对IServer的服务注册，意味着默认的KestrelServer将被替换成自定义的MiniKestrelServer。启动该程序后，由浏览器发送的HTTP请求（不支持HTTPS）同样会被正常处理，并得到如图18-6所示的响应内容。需要强调一下，MiniKestrelServer仅仅用来模拟KestrelServer的实现原理，不要觉得真实的实现会如此简单。

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图1 由MiniKestrelServer回复的响应内容

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