KestrelServer詳解[3]: 自定義一個迷你版的KestrelServer
- 2022 年 3 月 30 日
- 筆記
- .NET 6, .NET Core, [01] 技術剖析, Asp.Net, Asp.Net Core, ASP.NET Core 6框架揭秘
和所有的服務器一樣，KestrelServer最終需要解決的是網絡傳輸的問題。在《網絡連接的創建》，我們介紹了KestrelServer如何利用連接接聽器的建立網絡連接，並再次基礎上演示了如何直接利用建立的連接接收請求和回復響應。本篇更進一步，我們根據其總體設計，定義了迷你版的KestrelServer讓讀者看看這個重要的服務器大體是如何實現的。本文提供的示例演示已經同步到《ASP.NET Core 6框架揭秘-實例演示版》）
一、ConnectionDelegate
二、IConnectionBuilder
三、HTTP 1.x/HTTP 2.x V.S. HTTP 3
四、MiniKestrelServer
一、ConnectionDelegate
ASP.NET CORE在「應用」層將針對請求的處理抽象成由中間件構建的管道，實際上KestrelServer面向「傳輸」層的連接也採用了這樣的設計。當代表連接的ConnectionContext上下文創建出來之後，後續的處理將交給由連接中間件構建的管道進行處理。我們可以根據需要註冊任意的中間件來處理連接，比如可以將並發連結的控制實現在專門的連接中間件中。ASP.NET CORE管道利用RequestDelegate委託來表示請求處理器，連接管道同樣定義了如下這個ConnectionDelegate委託。
public delegate Task ConnectionDelegate(ConnectionContext connection);
二、IConnectionBuilder
ASP.NET CORE管道中的中間件體現為一個Func<RequestDelegate, RequestDelegate>委託，連接管道的中間件同樣可以利用Func<ConnectionDelegate, ConnectionDelegate>委託來表示。ASP.NET CORE管道中的中間件註冊到IApplicationBuilder對象上並利用它將管道構建出來。連接管道依然具有如下這個IConnectionBuilder接口，ConnectionBuilder實現了該接口。
public interface IConnectionBuilder { IServiceProvider ApplicationServices { get; } IConnectionBuilder Use(Func<ConnectionDelegate, ConnectionDelegate> middleware); ConnectionDelegate Build(); } public class ConnectionBuilder : IConnectionBuilder { public IServiceProvider ApplicationServices { get; } public ConnectionDelegate Build(); public IConnectionBuilder Use(Func<ConnectionDelegate, ConnectionDelegate> middleware); }
IConnectionBuilder接口還定義了如下三個擴展方法來註冊連接中間件。第一個Use方法使用Func<ConnectionContext, Func<Task>, Task>委託來表示中間件。其餘兩個方法用來註冊管道末端的中間件，這樣的中間件本質上就是一個ConnectionDelegate委託，我們可以將其定義成一個派生於ConnectionHandler的類型。
public static class ConnectionBuilderExtensions { public static IConnectionBuilder Use(this IConnectionBuilder connectionBuilder,Func<ConnectionContext, Func<Task>, Task> middleware); public static IConnectionBuilder Run(this IConnectionBuilder connectionBuilder,Func<ConnectionContext, Task> middleware); public static IConnectionBuilder UseConnectionHandler<TConnectionHandler>(this IConnectionBuilder connectionBuilder) where TConnectionHandler : ConnectionHandler; } public abstract class ConnectionHandler { public abstract Task OnConnectedAsync(ConnectionContext connection); }
三、HTTP 1.x/HTTP 2.x V.S. HTTP 3
KestrelServer針對HTTP 1.X/2和HTTP 3的設計和實現基本上獨立的，這一點從監聽器的定義就可以看出來。就連接管道來說，基於HTTP 3的多路復用連接通過MultiplexedConnectionContext表示，它也具有「配套」的MultiplexedConnectionDelegate委託和IMultiplexedConnectionBuilder接口。ListenOptions類型同時實現了IConnectionBuilder和IMultiplexedConnectionBuilder接口，意味着我們在註冊終結點的時候還可以註冊任意中間件。
public delegate Task MultiplexedConnectionDelegate(MultiplexedConnectionContext connection); public interface IMultiplexedConnectionBuilder { IServiceProvider ApplicationServices { get; } IMultiplexedConnectionBuilder Use(Func<MultiplexedConnectionDelegate, MultiplexedConnectionDelegate> middleware); MultiplexedConnectionDelegate Build(); } public class MultiplexedConnectionBuilder : IMultiplexedConnectionBuilder { public IServiceProvider ApplicationServices { get; } public IMultiplexedConnectionBuilder Use(Func<MultiplexedConnectionDelegate, MultiplexedConnectionDelegate> middleware); public MultiplexedConnectionDelegate Build(); } public class ListenOptions : IConnectionBuilder, IMultiplexedConnectionBuilder
四、MiniKestrelServer
在了解了KestrelServer的連接管道後，我們來簡單模擬一下這種服務器類型的實現，為此我們定義了一個名為MiniKestrelServer的服務器類型。簡單起見，MiniKestrelServer只提供針對HTTP 1.1的支持。對於任何一個服務來說，它需要將請求交付給一個IHttpApplication<TContext>對象進行處理，MiniKestrelServer將這項工作實現在如下這個HostedApplication<TContext>類型中。
public class HostedApplication<TContext> : ConnectionHandler where TContext : notnull { private readonly IHttpApplication<TContext> _application; public HostedApplication(IHttpApplication<TContext> application) => _application = application; public override 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<IHttpRequestFeature>(handler.Request); features.Set<IHttpResponseFeature>(responseFeature); features.Set<IHttpResponseBodyFeature>(bodyFeature); return (features, handler.Request, responseFeature); } static async Task ApplyResponseAsync(ConnectionContext connection, IHttpResponseFeature response, Stream body) { var builder = new StringBuilder(); builder.AppendLine($"HTTP/1.1 {response.StatusCode} {response.ReasonPhrase}"); foreach (var kv in response.Headers) { builder.AppendLine($"{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<TContext>是對一個IHttpApplication<TContext>對象的封裝。它派生於抽象類ConnectionHandler，重寫的OnConnectedAsync方法將針對請求的讀取和處理置於一個無限循環中。為了將讀取的請求轉交給IHostedApplication<TContext>對象進行處理，它需要根據特性集合將TContext上下文創建出來。這裡提供的特性集合只包含三種核心的特性，一個是描述請求的HttpRequestFeature特性，它是利用HttpParser解析請求荷載內容得到的。另一個是描述響應的HttpResponseFeature特性，至於提供響應主體的特性由如下所示的StreamBodyFeature對象來表示。這三個特性的創建實現在CreateFeatures方法中。
public class 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; public void DisableBuffering() { } public Task SendFileAsync(string path, long offset, long? count, CancellationToken cancellationToken = default)=> throw new NotImplementedException(); public Task StartAsync(CancellationToken cancellationToken = default) => Task.CompletedTask; }
包含三大特性的集合隨後作為參數調用了IHostedApplication<TContext>對象的CreateContext方法將TContext上下文創建出來，此上下文作為參數傳入了同一對象的ProcessRequestAsync方法，此時中間件管道接管請求。待中間件管道完成處理後， ApplyResponseAsync方法被調用以完成最終的響應工作。ApplyResponseAsync方法將響應狀態從HttpResponseFeature特性中提取並生成首行響應內容（「HTTP/1.1 {StatusCode} {ReasonPhrase}」），然後再從這個特性中將響應報頭提取出來並生成相應的文本。響應報文的首行內容和報頭文本按照UTF-8編碼生成二進制數組後利用ConnectionContext上下文的Transport屬性返回的IDuplexPipe對象發送出去後，它再將StreamBodyFeature特性收集到的響應主體輸出流「拷貝」到這個IDuplexPipe對象中，進而完成了針對響應主體內容的輸出。
如下所示的是MiniKestrelServer類型的完整定義。該類型的構造函數中注入了用於提供配置選項的IOptions<KestrelServerOptions>特性和IConnectionListenerFactory工廠，並且創建了一個ServerAddressesFeature對象並註冊到Features屬性返回的特性集合中。
public class MiniKestrelServer : IServer { private readonly KestrelServerOptions _options; private readonly IConnectionListenerFactory _factory; private readonly List<IConnectionListener> _listeners = new(); public IFeatureCollection Features { get; } = new FeatureCollection(); public MiniKestrelServer(IOptions<KestrelServerOptions> optionsAccessor, IConnectionListenerFactory factory) { _factory = factory; _options = optionsAccessor.Value; Features.Set<IServerAddressesFeature>(new ServerAddressesFeature()); } public void Dispose() => StopAsync(CancellationToken.None).GetAwaiter().GetResult(); public Task StartAsync<TContext>(IHttpApplication<TContext> application, CancellationToken cancellationToken) where TContext : notnull { var feature = Features.Get<IServerAddressesFeature>()!; IEnumerable<ListenOptions> listenOptions; if (feature.PreferHostingUrls) { listenOptions = BuildListenOptions(feature); } else { listenOptions = _options.GetListenOptions(); if (!listenOptions.Any()) { listenOptions = BuildListenOptions(feature); } } foreach (var options in 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<TContext>(application); var pipeline = litenOptions.Use(next => context => hostedApplication.OnConnectedAsync(context)).Build(); while (true) { var connection = await listener.AcceptAsync(); if (connection != null) { _ = pipeline(connection); } } } IEnumerable<ListenOptions> BuildListenOptions(IServerAddressesFeature feature) { var options = new KestrelServerOptions(); foreach (var address in 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())); }
如上所示的演示程序將替換了針對IServer的服務註冊，意味着默認的KestrelServer將被替換成自定義的MiniKestrelServer。啟動該程序後，由瀏覽器發送的HTTP請求（不支持HTTPS）同樣會被正常處理，並得到如圖18-6所示的響應內容。需要強調一下，MiniKestrelServer僅僅用來模擬KestrelServer的實現原理，不要覺得真實的實現會如此簡單。
圖1 由MiniKestrelServer回復的響應內容
