The `Main` method invokes [WebHost.CreateDefaultBuilder](xref:Microsoft.AspNetCore.WebHost.CreateDefaultBuilder*), which follows the [builder pattern](https://wikipedia.org/wiki/Builder_pattern) to create a web host. The builder has methods that define the web server (for example, <xref:Microsoft.AspNetCore.Hosting.WebHostBuilderKestrelExtensions.UseKestrel*>) and the startup class (<xref:Microsoft.AspNetCore.Hosting.WebHostBuilderExtensions.UseStartup*>). In the preceding example, the [Kestrel](xref:fundamentals/servers/kestrel) web server is automatically allocated. ASP.NET Core's web host attempts to run on IIS, if available. Other web servers, such as [HTTP.sys](xref:fundamentals/servers/httpsys), can be used by invoking the appropriate extension method. `UseStartup` is explained further in the next section.
<xref:Microsoft.AspNetCore.Hosting.IWebHostBuilder>, the return type of the `WebHost.CreateDefaultBuilder` invocation, provides many optional methods. Some of these methods include `UseHttpSys` for hosting the app in HTTP.sys and <xref:Microsoft.Extensions.Hosting.HostingHostBuilderExtensions.UseContentRoot*> for specifying the root content directory. The <xref:Microsoft.AspNetCore.Hosting.IWebHostBuilder.Build*> and <xref:Microsoft.AspNetCore.Hosting.WebHostExtensions.Run*> methods build the <xref:Microsoft.AspNetCore.Hosting.IWebHost> object that hosts the app and begins listening for HTTP requests.
The `Main` method uses <xref:Microsoft.AspNetCore.Hosting.WebHostBuilder>, which follows the [builder pattern](https://wikipedia.org/wiki/Builder_pattern) to create a web app host. The builder has methods that define the web server (for example, <xref:Microsoft.AspNetCore.Hosting.WebHostBuilderKestrelExtensions.UseKestrel*>) and the startup class (<xref:Microsoft.AspNetCore.Hosting.WebHostBuilderExtensions.UseStartup*>). In the preceding example, the [Kestrel](xref:fundamentals/servers/kestrel) web server is used. Other web servers, such as [WebListener](xref:fundamentals/servers/weblistener), can be used by invoking the appropriate extension method. `UseStartup` is explained further in the next section.
`WebHostBuilder` provides many optional methods, including <xref:Microsoft.AspNetCore.Hosting.WebHostBuilderIISExtensions.UseIISIntegration*> for hosting in IIS and IIS Express and <xref:Microsoft.Extensions.Hosting.HostingHostBuilderExtensions.UseContentRoot*> for specifying the root content directory. The <xref:Microsoft.AspNetCore.Hosting.IWebHostBuilder.Build*> and <xref:Microsoft.AspNetCore.Hosting.WebHostExtensions.Run*> methods build the <xref:Microsoft.AspNetCore.Hosting.IWebHost> object that hosts the app and begins listening for HTTP requests.
The `Startup` class is where you define the request handling pipeline and where any services needed by the app are configured. The `Startup` class must be public and contain the following methods:
<xref:Microsoft.AspNetCore.Hosting.IStartup.ConfigureServices*> defines the [Services](#dependency-injection-services) used by your app (for example, ASP.NET Core MVC, Entity Framework Core, Identity). <xref:Microsoft.AspNetCore.Hosting.IStartup.Configure*> defines the [middleware](xref:fundamentals/middleware/index) called in the request pipeline.
The content root is the base path to any content used by the app, such as [Razor Pages](xref:razor-pages/index), MVC views, and static assets. By default, the content root is the same location as the app base path for the executable hosting the app.
The webroot of an app is the directory in the project containing public, static resources, such as CSS, JavaScript, and image files. By default, *wwwroot* is the webroot.
For Razor (*.cshtml*) files, the tilde-slash `~/` points to the webroot. Paths beginning with `~/` are referred to as virtual paths.
A *service* is a component that's intended for common consumption in an app. Services are made available through [dependency injection (DI)](xref:fundamentals/dependency-injection). ASP.NET Core includes a native Inversion of Control (IoC) container that supports [constructor injection](xref:mvc/controllers/dependency-injection#constructor-injection) by default. You can replace the default container if you wish. In addition to its [loose coupling benefit](/dotnet/standard/modern-web-apps-azure-architecture/architectural-principles#encapsulation), DI makes services, such as [logging](xref:fundamentals/logging/index), available throughout your app.
In ASP.NET Core, you compose your request pipeline using [middleware](xref:fundamentals/middleware/index). ASP.NET Core middleware performs asynchronous operations on an `HttpContext` and then either invokes the next middleware in the pipeline or terminates the request.
ASP.NET Core includes a rich set of built-in middleware, and you can write your own custom middleware. [Open Web Interface for .NET (OWIN)](xref:fundamentals/owin), which allows web apps to be decoupled from web servers, is supported in ASP.NET Core apps.
Environments, such as *Development* and *Production*, are a first-class notion in ASP.NET Core and can be set using an environment variable, settings file, and command-line argument.
The ASP.NET Core hosting model doesn't directly listen for requests. The hosting model relies on an HTTP server implementation to forward the request to the app. The forwarded request is wrapped as a set of feature objects that can be accessed through interfaces. ASP.NET Core includes a managed, cross-platform web server, called [Kestrel](xref:fundamentals/servers/kestrel). Kestrel is commonly run behind a production web server, such as [IIS](https://www.iis.net/) or [Nginx](http://nginx.org) in a reverse proxy configuration. Kestrel can also be run as a public-facing edge server exposed directly to the Internet in ASP.NET Core 2.0 or later.
ASP.NET Core uses a configuration model based on name-value pairs. The configuration model isn't based on <xref:System.Configuration> or *web.config*. Configuration obtains settings from an ordered set of configuration providers. The built-in configuration providers support a variety of file formats (XML, JSON, INI), environment variables, and command-line arguments. You can also write your own custom configuration providers.
ASP.NET Core supports a Logging API that works with a variety of logging providers. Built-in providers support sending logs to one or more destinations. Third-party logging frameworks can be used.
ASP.NET Core has built-in scenarios for handling errors in apps, including a developer exception page, custom error pages, static status code pages, and startup exception handling.
Background tasks are implemented as *hosted services*. A hosted service is a class with background task logic that implements the <xref:Microsoft.Extensions.Hosting.IHostedService> interface.
`HttpContext` is automatically available when processing requests with Razor Pages and MVC. In circumstances where `HttpContext` isn't readily available, you can access the `HttpContext` through the <xref:Microsoft.AspNetCore.Http.IHttpContextAccessor> interface and its default implementation, <xref:Microsoft.AspNetCore.Http.HttpContextAccessor>.
