ASP.NET Core supports the Open Web Interface for .NET (OWIN). OWIN allows web apps to be decoupled from web servers. It defines a standard way for middleware to be used in a pipeline to handle requests and associated responses. ASP.NET Core applications and middleware can interoperate with OWIN-based applications, servers, and middleware.
OWIN provides a decoupling layer that allows two frameworks with disparate object models to be used together. The `Microsoft.AspNetCore.Owin` package provides two adapter implementations:
- ASP.NET Core to OWIN
- OWIN to ASP.NET Core
This allows ASP.NET Core to be hosted on top of an OWIN compatible server/host, or for other OWIN compatible components to be run on top of ASP.NET Core.
Note: Using these adapters comes with a performance cost. Applications using only ASP.NET Core components should not use the Owin package or adapters.
[View or download sample code](https://github.com/aspnet/Docs/tree/master/aspnetcore/fundamentals/owin/sample) ([how to download](xref:tutorials/index#how-to-download-a-sample))
ASP.NET Core's OWIN support is deployed as part of the `Microsoft.AspNetCore.Owin` package. You can import OWIN support into your project by installing this package.
OWIN middleware conforms to the [OWIN specification](http://owin.org/spec/spec/owin-1.0.0.html), which requires a `Func<IDictionary<string, object>, Task>` interface, and specific keys be set (such as `owin.ResponseBody`). The following simple OWIN middleware displays "Hello World":
OWIN-based servers can host ASP.NET applications. One such server is [Nowin](https://github.com/Bobris/Nowin), a .NET OWIN web server. In the sample for this article, I've included a project that references Nowin and uses it to create an `IServer` capable of self-hosting ASP.NET Core.
`IServer` is an interface that requires an `Features` property and a `Start` method.
`Start` is responsible for configuring and starting the server, which in this case is done through a series of fluent API calls that set addresses parsed from the IServerAddressesFeature. Note that the fluent configuration of the `_builder` variable specifies that requests will be handled by the `appFunc` defined earlier in the method. This `Func` is called on each request to process incoming requests.
We'll also add an `IWebHostBuilder` extension to make it easy to add and configure the Nowin server.
## Run ASP.NET Core on an OWIN-based server and use its WebSockets support
Another example of how OWIN-based servers' features can be leveraged by ASP.NET Core is access to features like WebSockets. The .NET OWIN web server used in the previous example has support for Web Sockets built in, which can be leveraged by an ASP.NET Core application. The example below shows a simple web app that supports Web Sockets and echoes back everything sent to the server through WebSockets.
This [sample](https://github.com/aspnet/Docs/tree/master/aspnetcore/fundamentals/owin/sample) is configured using the same `NowinServer` as the previous one - the only difference is in how the application is configured in its `Configure` method. A test using [a simple websocket client](https://chrome.google.com/webstore/detail/simple-websocket-client/pfdhoblngboilpfeibdedpjgfnlcodoo?hl=en) demonstrates the application:
OWIN depends on an `IDictionary<string,object>` object to communicate information throughout an HTTP Request/Response exchange. ASP.NET Core implements the keys listed below. See the [primary specification, extensions](http://owin.org/#spec), and [OWIN Key Guidelines and Common Keys](http://owin.org/spec/spec/CommonKeys.html).
