【译】ASP.NET Web API ：Web宿主
上一节描述了API的整个运行框架，即分为三层hosting、message handler pipeline 和 controller handling。此节讲其中一个宿主，WebHost 寄宿在asp.net 传统管道上。
Routing（路由）
在asp.net平台，路由是一般由RouteTables.Routes静态属性添加的，类型是RouteCollection，例如下面的MVC模板自带的添加路由的代码。
protected void Application_Start()
{
RegisterRoutes(RouteTable.Routes);
}
public static void RegisterRoutes(RouteCollection routes)
{
routes.IgnoreRoute("{resource}.axd/{*pathInfo}");
routes.MapRoute(
"Default", // Route name
"{controller}/{action}/{id}", // URL with parameters
new { controller = "Home", action = "Index", id = UrlParameter.Optional }
);
}
大部分路由逻辑都是在UrlRoutingModule里，属于PostResolveRequestCache asp.net管道事件。每一次请求，这个module都是重新匹配一次这个路由集合，并获取一个RouteData实例，如果匹配，则：
1、实例化RouteData同时获取一个route handler（HttpControllerRouteHandler）。
2、从routehandler获取一个http handle 继承自IHttphandler接口。IRouteHandler接口方法：IHttpHandler GetHttpHandler(RequestContext requestContext)
3、最后，当前的请求上下文(RequestContext）被映射到上面的这个httphandler。
因此，最后结果是asp.net管道请求被此handler处理。
Web API 集成
当寄宿在asp.net上，Web API特定的配置被定义在一个单例模式的HttpConfiguration对象里，通过静态属性GlobalCnfiguration.Configuration访问。
Web API 也定义了一对新的RouteCollection扩展方法MapHttpRoute，来注册Web API特殊的路由，下面是配置的例子代码：
HttpConfiguration config = GlobalConfiguration.Configuration;
config.Routes.MapHttpRoute("default", "{controller}/{id}", new {id = UrlParameter.Optional});
// other configuration settings
注意：
- 静态属性GlobalConfiguration.Configuration是用于获取配置的引用，内部这个对象指向的是全局的RouteTables.Routes这个集合。
- 当新增路由时使用的是MapHttpRoute这个扩展方法。
当一个route通过MapHttpRoute增加后匹配到一个Request，，HttpControllerRouteHandler会创建一个新的HttpControllerHandler，其继承自IAsyncHttpHandler，此handler通过RouteData(包含了路由的信息）初始化。
当被调用，HttpControllerhandler 在他的BeginReocessRequest方法里有以下行为：
- 为当前上下文创建HttpRequestMessage实例
- 使用GlobalConfiguration.Configuration获取配置并创建一个HttpServer，并且把HttpRequestMessage送入服务管道。
当这个请求被HttpServer接受后，进入宿主的独立处理阶段（Web API的新管道）
下面的类图是路由解决过程的摘要，并且分配给HttpServer（消息处理管道）
原文地址：
ASP.NET Web API: web hosting
看文字描述比较晦涩难懂，需要根据源码理解，上图容易理解。
HttpApplication -> UrlRoutingModule -> RouteCollection -> RouteData -> HttpControllerRouteHandler -> HttpControllerHandler -> HttpRequestMessage -> HttpServer
附加一些源码：
1、UrlRoutingModule获取RouteData
public virtual void PostResolveRequestCache(HttpContextBase context) { RouteData routeData = this.RouteCollection.GetRouteData(context); if (routeData == null) { return; } IRouteHandler routeHandler = routeData.RouteHandler; if (routeHandler == null) { throw new InvalidOperationException(string.Format(CultureInfo.CurrentCulture, SR.GetString("UrlRoutingModule_NoRouteHandler"), new object[0])); } if (routeHandler is StopRoutingHandler) { return; } RequestContext requestContext = new RequestContext(context, routeData); context.Request.RequestContext = requestContext; IHttpHandler httpHandler = routeHandler.GetHttpHandler(requestContext); if (httpHandler == null) { throw new InvalidOperationException(string.Format(CultureInfo.CurrentUICulture, SR.GetString("UrlRoutingModule_NoHttpHandler"), new object[] { routeHandler.GetType() })); } if (!(httpHandler is UrlAuthFailureHandler)) { context.RemapHandler(httpHandler); return; } if (FormsAuthenticationModule.FormsAuthRequired) { UrlAuthorizationModule.ReportUrlAuthorizationFailure(HttpContext.Current, this); return; } throw new HttpException(401, SR.GetString("Assess_Denied_Description3")); }
2、MapHttpRoute（RouteCollectionExtension）
public static Route MapHttpRoute(this RouteCollection routes, string name, string routeTemplate, object defaults, object constraints, HttpMessageHandler handler) { if (routes == null) { throw Error.ArgumentNull("routes"); } HttpRouteValueDictionary defaultsDictionary = new HttpRouteValueDictionary(defaults); HttpRouteValueDictionary constraintsDictionary = new HttpRouteValueDictionary(constraints); HostedHttpRoute httpRoute = (HostedHttpRoute)GlobalConfiguration.Configuration.Routes.CreateRoute(routeTemplate, defaultsDictionary, constraintsDictionary, dataTokens: null, handler: handler); Route route = httpRoute.OriginalRoute; routes.Add(name, route); return route; }
3、HttpControllerRouteHandler.GetHandler
protected virtual IHttpHandler GetHttpHandler(RequestContext requestContext) { return new HttpControllerHandler(requestContext.RouteData); }
4、HttpControllerHandler.ProcessRequest
internal async Task ProcessRequestAsyncCore(HttpContextBase contextBase) { HttpRequestMessage request = contextBase.GetHttpRequestMessage() ?? ConvertRequest(contextBase); // Add route data request.SetRouteData(_routeData); CancellationToken cancellationToken = contextBase.Response.GetClientDisconnectedTokenWhenFixed(); HttpResponseMessage response = null; try { response = await _server.SendAsync(request, cancellationToken); await CopyResponseAsync(contextBase, request, response, _exceptionLogger.Value, _exceptionHandler.Value, cancellationToken); } finally { // The other HttpTaskAsyncHandler is HttpRouteExceptionHandler; it has similar cleanup logic. request.DisposeRequestResources(); request.Dispose(); if (response != null) { response.Dispose(); } } }
5、HttpApplication -> HttpControllerHandler
public HttpControllerHandler(RouteData routeData) : this(routeData, GlobalConfiguration.DefaultServer) { } public HttpControllerHandler(RouteData routeData, HttpMessageHandler handler) { if (routeData == null) { throw Error.ArgumentNull("routeData"); } if (handler == null) { throw Error.ArgumentNull("handler"); } _routeData = new HostedHttpRouteData(routeData); _server = new HttpMessageInvoker(handler); }
HttpServer : DelegatingHandler : HttpMessageHandler