OWIN 中 K Commands（OwinHost.exe）与 Microsoft.AspNet.Hosting 的角色问题

问题详情：K Commands（OwinHost.exe）是不是 OWIN 中的 Host 角色？如果是，那 Microsoft.AspNet.Hosting 对应的是 OWIN 中的哪个角色？

OWIN 中，除了 Host 和 Server 的概念容易混淆，K Commands（OwinHost.exe）与 Microsoft.AspNet.Hosting 也是很容易混淆的一点，先看一下它们的概念：

• 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.

上面是从网上各个地方搜刮的概念，再来结合 Microsoft.AspNet.Hosting/Program.cs 的源码：

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）的作用就是启动并加载 OWIN 组件，使你的应用程序处于运行状态，看上面 Program.cs 代码，就会发现 Microsoft.AspNet.Hosting 其实就是一个控制台项目，当然除此之外还会包含 Builder、Server、Startup 等一些操作，这些构成了基本的 OWIN Host，它是一个进程，负责启动并加载 OWIN 组件（在之前的博文中有说明），而 K Commands（OwinHost.exe）只不过是一个命令，用来去开启它，就像一个车钥匙，用来发动汽车一样。结合 IIS 的一些东西，K Commands（OwinHost.exe）就像我们点击“启动”、“停止” 的后台处理命令，当然还有一些 URL 绑定等，这些都通过命令去加载，过程大概是这样：K Commands -> project.json -> Microsoft.AspNet.Hosting -> Started。

在之前曾说过，如果采用 IIS 的部署，那 ASP.NET 5 的 project.json 配置，就是下面这么简单：

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

你会发现，没有了 Microsoft.AspNet.Hosting、Microsoft.AspNet.Server.WebListener，也没有了 Commands，为什么呢？因为 IIS 既是 Host，又是 Server，IIS 与 ASP.NET 5 的 OWIN 管道处理，是通过 Microsoft.AspNet.Server.IIS（AspNet.Loader.dll）打通的，所以 OWIN 的处理组件都是通过 IIS，这时候的 ASP.NET 5 其实就不是纯粹的 OWIN。

回答上面的问题，K Commands（OwinHost.exe）不是 OWIN 中的 Host 角色，Microsoft.AspNet.Hosting 才是，应该准确的说，K Commands（OwinHost.exe）和 OWIN 中的 Host 不存在概念问题，它只是一个命令，用来开启 Microsoft.AspNet.Hosting，Microsoft.AspNet.Hosting 是 OWIN 中 Host 概念的具体体现。