The `Main` method invokes `WebHost.CreateDefaultBuilder`, which follows the builder pattern to create a web application host. The builder has methods that define the web server (for example, `UseKestrel`) and the startup class (`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.
`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 `UseContentRoot` for specifying the root content directory. The `Build` and `Run` methods build the `IWebHost` object that hosts the app and begins listening for HTTP requests.
The `Main` method uses `WebHostBuilder`, which follows the builder pattern to create a web application host. The builder has methods that define the web server (for example, `UseKestrel`) and the startup class (`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 `UseIISIntegration` for hosting in IIS and IIS Express and `UseContentRoot` for specifying the root content directory. The `Build` and `Run` methods build the `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:
`ConfigureServices` defines the [Services](#dependency-injection-services) used by your app (for example, ASP.NET Core MVC, Entity Framework Core, Identity). `Configure` defines the [middleware](xref:fundamentals/middleware/index) for the request pipeline.
The content root is the base path to any content used by the app, such as views, [Razor Pages](xref:razor-pages/index), and static assets. By default, the content root is the same as application base path for the executable hosting the app.
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 **I**nversion **o**f **C**ontrol (IoC) container that supports [constructor injection](xref:mvc/controllers/dependency-injection#constructor-injection) by default. You can replace the default native container if you wish. In addition to its loose coupling benefit, DI makes services available throughout your app (for example, [logging](xref:fundamentals/logging/index)).
In ASP.NET Core, you compose your request pipeline using [middleware](xref:fundamentals/middleware/index). ASP.NET Core middleware performs asynchronous logic on an `HttpContext` and then either invokes the next middleware in the sequence or terminates the request directly. A middleware component called "XYZ" is added by invoking an `UseXYZ` extension method in the `Configure` method.
For information about using `IHttpClientFactory` to access `HttpClient` instances to make HTTP requests, see [Initiate HTTP requests](xref:fundamentals/http-requests).
ASP.NET Core uses a configuration model based on name-value pairs. The configuration model isn't based on `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) and environment variables to enable environment-based configuration. 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 features for handling errors in apps, including a developer exception page, custom error pages, static status code pages, and startup exception handling.
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 often run behind a production web server, such as [IIS](https://www.iis.net/) or [Nginx](http://nginx.org). Kestrel can be run as an edge server.
Creating a multilingual website with ASP.NET Core allows your site to reach a wider audience. ASP.NET Core provides services and middleware for localizing into different languages and cultures.
Web server implementation details related to HTTP requests and responses are defined in interfaces. These interfaces are used by server implementations and middleware to create and modify the app's hosting pipeline.
Background tasks are implemented as *hosted services*. A hosted service is a class with background task logic that implements the [IHostedService](/dotnet/api/microsoft.extensions.hosting.ihostedservice) interface.
For more information, see [Open Web Interface for .NET (OWIN)](xref:fundamentals/owin).
## WebSockets
[WebSocket](https://wikipedia.org/wiki/WebSocket) is a protocol that enables two-way persistent communication channels over TCP connections. It's used for apps such as chat, stock tickers, games, and anywhere you desire real-time functionality in a web app. ASP.NET Core supports web socket features.
For more information, see [WebSockets](xref:fundamentals/websockets).
The [Microsoft.AspNetCore.App](https://www.nuget.org/packages/Microsoft.AspNetCore.App/) metapackage simplifies package management. For more information, see [Microsoft.AspNetCore.App metapackage](xref:fundamentals/metapackage-app).
For more information on choosing between ASP.NET Core and ASP.NET, see [Choose between ASP.NET Core and ASP.NET](xref:fundamentals/choose-between-aspnet-and-aspnetcore).
