OWIN 中 K Commands（OwinHost.exe）與 Microsoft.aspNet.Hosting 的角色問題

問題詳情：K Commands（OwinHost.exe）是不是 OWIN 中的 Host 角色？如果是，那 Microsoft.AspNet.Hosting 對應的是 OWIN 中的哪個角色？

OWIN 中，除了 Host 和 Server 的概念容易混淆，K Commands（OwinHost.exe）與 Microsoft.AspNet.Hosting 也是很容易混淆的一點，先看一下它們的概念：

• OwinHost.exe: While some will want to write a custom PRocess to run Katana Web applications, many would prefer to simply launch a pre-built executable that can start a server and run their application. For this scenario, the Katana component suite includes OwinHost.exe. When run from within a project’s root directory, this executable will start a server (it uses the HttpListener server by default) and use conventions to find and run the user’s startup class. For more granular control, the executable provides a number of additional command line parameters.
• K Commands: Whenever you want to run your app in command line using K* commands, you will use k run. The K command is your entry point to the runtime. To run an application you can use K run to build you can use K build, and all other commands that are about taking your application and running it.
• Microsoft.AspNet.Hosting: The Hosting repo contains code required to host an asp.net vNext application, it is the entry point used when self-hosting an application.

上面是從網上各個地方搜刮的概念，再來結合 Microsoft.AspNet.Hosting/Program.cs 的源碼：

using System;using System.IO;using System.Threading;using System.Threading.Tasks;using Microsoft.Framework.ConfigurationModel;using Microsoft.Framework.DependencyInjection;using Microsoft.Framework.DependencyInjection.Fallback;using Microsoft.Framework.Logging;using Microsoft.Framework.Runtime;namespace Microsoft.AspNet.Hosting{    public class Program    {        private const string HostingIniFile = "Microsoft.AspNet.Hosting.ini";        private readonly IServiceProvider _serviceProvider;        public Program(IServiceProvider serviceProvider)        {            _serviceProvider = serviceProvider;        }        public void Main(string[] args)        {            var config = new Configuration();            if (File.Exists(HostingIniFile))            {                config.AddIniFile(HostingIniFile);            }            config.AddEnvironmentVariables();            config.AddCommandLine(args);            var services = HostingServices.Create(_serviceProvider, config)                .BuildServiceProvider();            var appEnv = services.GetRequiredService<IApplicationEnvironment>();            var hostingEnv = services.GetRequiredService<IHostingEnvironment>();            var context = new HostingContext()            {                Services = services,                Configuration = config,                ServerName = config.Get("server"), // TODO: Key names                ApplicationName = config.Get("app")  // TODO: Key names                    ?? appEnv.ApplicationName,                EnvironmentName = hostingEnv.EnvironmentName,            };            var engine = services.GetRequiredService<IHostingEngine>();            var loggerFactory = services.GetRequiredService<ILoggerFactory>();            var appShutdownService = _serviceProvider.GetRequiredService<IApplicationShutdown>();            var shutdownHandle = new ManualResetEvent(false);            var serverShutdown = engine.Start(context);            appShutdownService.ShutdownRequested.Register(() =>            {                try                {                    serverShutdown.Dispose();                }                catch (Exception ex)                {                    var logger = loggerFactory.Create<Program>();                    logger.WriteError("TODO: Dispose threw an exception", ex);                }                shutdownHandle.Set();            });            var ignored = Task.Run(() =>            {                Console.WriteLine("Started");                Console.ReadLine();                appShutdownService.RequestShutdown();            });            shutdownHandle.WaitOne();        }    }}

K Commands（OwinHost.exe）的作用就是啟動并加載 OWIN 組件，使你的應用程序處于運行狀態，看上面 Program.cs 代碼，就會發現 Microsoft.AspNet.Hosting 其實就是一個控制臺項目，當然除此之外還會包含 Builder、Server、Startup 等一些操作，這些構成了基本的 OWIN Host，它是一個進程，負責啟動并加載 OWIN 組件（在之前的博文中有說明），而 K Commands（OwinHost.exe）只不過是一個命令，用來去開啟它，就像一個車鑰匙，用來發動汽車一樣。結合 IIS 的一些東西，K Commands（OwinHost.exe）就像我們點擊“啟動”、“停止” 的后臺處理命令，當然還有一些 URL 綁定等，這些都通過命令去加載，過程大概是這樣：K Commands -> project.json -> Microsoft.AspNet.Hosting -> Started。

在之前曾說過，如果采用 IIS 的部署，那 ASP.NET 5 的 project.json 配置，就是下面這么簡單：

{    "webroot": "wwwroot",    "dependencies": {        "Microsoft.AspNet.Server.IIS": "1.0.0-beta1"    }}

你會發現，沒有了 Microsoft.AspNet.Hosting、Microsoft.AspNet.Server.WebListener，也沒有了 Commands，為什么呢？因為 IIS 既是 Host，又是 Server，IIS 與 ASP.NET 5 的 OWIN 管道處理，是通過 Microsoft.AspNet.Server.IIS（AspNet.Loader.dll）打通的，所以 OWIN 的處理組件都是通過 IIS，這時候的 ASP.NET 5 其實就不是純粹的 OWIN。

回答上面的問題，K Commands（OwinHost.exe）不是 OWIN 中的 Host 角色，Microsoft.AspNet.Hosting 才是，應該準確的說，K Commands（OwinHost.exe）和 OWIN 中的 Host 不存在概念問題，它只是一個命令，用來開啟 Microsoft.AspNet.Hosting，Microsoft.AspNet.Hosting 是 OWIN 中 Host 概念的具體體現。