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.
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 adding this package as a dependency in your *project.json* file:
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":
The following code shows how to add the ``OwinHello`` middleware (shown above) to the ASP.NET pipeline with the :dn:method:`~Microsoft.AspNetCore.Builder.OwinExtensions.UseOwin` extension method.
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.
``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.
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/aspnet/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>`_.
