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The ASP.NET Framework has been around for over ten years, and the platform has enabled the development of countless Web sites and services. As Web application development strategies have evolved, the framework has been able to evolve in step with technologies like ASP.NET MVC and ASP.NET Web API. As Web application development takes its next evolutionary step into the world of cloud computing, project  provides the underlying set of components to ASP.NET applications, enabling them to be flexible, portable, lightweight, and provide better performance – put another way, project  cloud optimizes your ASP.NET applications.

Why Katana – Why Now?

The second group of target customers for ASP.NET was Windows business application developers. Unlike classic ASP developers, who were accustomed to writing HTML markup and the code to generate more HTML markup, WinForms developers (like the VB6 developers before them) were accustomed to a design time experience that included a canvas and a rich set of user interface controls. The first version of ASP.NET – also known as “Web Forms” provided a similar design time experience along with a server-side event model for user interface components and a set of infrastructure features (such as ViewState) to create a seamless developer experience between client and server side programming. Web Forms effectively hid the Web’s stateless nature under a stateful event model that was familiar to WinForms developers.

Challenges Raised by the Historical Model

The net result was a mature, feature-rich runtime and developer programming model. However, with that feature-richness came a couple notable challenges. Firstly, the framework was monolithic, with logically disparate units of functionality being tightly coupled in the same System.Web.dll assembly (for example, the core HTTP objects with the Web forms framework). Secondly, ASP.NET was included as a part of the larger .NET Framework, which meant that the time between releases was on the order of years. This made it difficult for ASP.NET to keep pace with all of the changes happening in rapidly evolving Web development. Finally, System.Web.dll itself was coupled in a few different ways to a specific Web hosting option: Internet Information Services (IIS).

Evolutionary steps: ASP.NET MVC and ASP.NET Web API

And lots of change was happening in Web development! Web applications were increasingly being developed as a series of small, focused components rather than large frameworks. The number of components as well as the frequency with which they were released was increasing at an ever faster rate. It was clear that keeping pace with the Web would require frameworks to get smaller, decoupled and more focused rather than larger and more feature-rich, therefore the ASP.NET team took several evolutionary steps to enable ASP.NET as a family of pluggable Web components rather than a single framework.

One of the early changes was the rise in popularity of the well-known model-view-controller (MVC) design pattern thanks to Web development frameworks like Ruby on Rails. This style of building Web applications gave the developer greater control over her application’s markup while still preserving the separation of markup and business logic, which was one of the initial selling points for ASP.NET. To meet the demand for this style of Web application development, Microsoft took the opportunity to position itself better for the future by developing ASP.NET MVC out of band (and not including it in the .NET Framework).  ASP.NET MVC was released as an independent download. This gave the engineering team the flexibility to deliver updates much more frequently than had been previously possible.

Another major shift in Web application development was the shift from dynamic, server-generated Web pages to static initial markup with dynamic sections of the page generated from client-side script communicating with backend Web APIs through AJAX requests. This architectural shift helped propel the rise of Web APIs, and the development of the ASP.NET Web API framework. As in the case of ASP.NET MVC, the release of ASP.NET Web API provided another opportunity to evolve ASP.NET further as a more modular framework. The engineering team took advantage of the opportunity and built ASP.NET Web API such that it had no dependencies on any of the core framework types found in System.Web.dll. This enabled two things: first, it meant that ASP.NET Web API could evolve in a completely self-contained manner (and it could continue to iterate quickly because it is delivered via NuGet). Second, because there were no external dependencies to System.Web.dll, and therefore, no dependencies to IIS, ASP.NET Web API included the capability to run in a custom host (for example, a console application, Windows service, etc.)

The Future: A Nimble Framework

By decoupling framework components from one another and then releasing them on NuGet, frameworks could nowiterate more independently and more quickly. Additionally, the power and flexibility of Web API’s self-hosting capability proved very attractive to developers who wanted a small, lightweight host for their services. It proved so attractive, in fact, that other frameworks also wanted this capability, and this surfaced a new challenge in that each framework ran in its own host process on its own base address and needed to be managed (started, stopped, etc.) independently. A modern Web application generally supports static file serving, dynamic page generation, Web API, and more recently real-time/push notifications. Expecting that each of these services should be run and managed independently was simply not realistic.

What was needed was a single hosting abstraction that would enable a developer to compose an application from a variety of different components and frameworks, and then run that application on a supporting host.

The Open Web Interface for .NET (OWIN)

Inspired by the benefits achieved by   in the Ruby community, several members of the .NET community set out to create an abstraction between Web servers and framework components. Two design goals for the OWIN abstraction were that it was simple and that it took the fewest possible dependencies on other framework types. These two goals help ensure:

•  New components could be more easily developed and consumed.
• Applications could be more easily ported between hosts and potentially entire platforms/operating systems.

The resulting abstraction consists of two core elements. The first is the environment dictionary. This data structure is responsible for storing all of the state necessary for processing an HTTP request and response, as well as any relevant server state. The environment dictionary is defined as follows:

An OWIN-compatible Web server is responsible for populating the environment dictionary with data such as the body streams and header collections for an HTTP request and response. It is then the responsibility of the application or framework components to populate or update the dictionary with additional values and write to the response body stream.

In addition to specifying the type for the environment dictionary, the OWIN specification defines a list of core dictionary key value pairs. For example, the following table shows the required dictionary keys for an HTTP request:

The second key element of OWIN is the application delegate. This is a function signature which serves as the primary interface between all components in an OWIN application. The definition for the application delegate is as follows:

Func<IDictionary<string, object>, Task>;

The application delegate then is simply an implementation of the Func delegate type where the function accepts the environment dictionary as input and returns a Task. This design has several implications for developers:

• There are a very small number of type dependencies required in order to write OWIN components. This greatly increases the accessibility of OWIN to developers.
• The asynchronous design enables the abstraction to be efficient with its handling of computing resources, particularly in more I/O intensive operations.
• Because the application delegate is an atomic unit of execution and because the environment dictionary is carried as a parameter on the delegate, OWIN components can be easily chained together to create complex HTTP processing pipelines.

From an implementation perspective, OWIN is a specification (). Its goal is not to be the next Web framework, but rather a specification for how Web frameworks and Web servers interact.

If you’ve investigated  or , you may also have noticed the  and Owin.dll. This library contains a single interface, , which formalizes and codifies the startup sequence described in  of the OWIN specification. While not required in order to build OWIN servers, the  interface provides a concrete reference point, and it is used by the Katana project components.

Project Katana

Whereas both the  specification and Owin.dll are community owned and community run open source efforts, the  project represents the set of OWIN components that, while still open source, are built and released by Microsoft. These components include both infrastructure components, such as hosts and servers, as well as functional components, such as authentication components and bindings to frameworks such as  and. The project has the following three high level goals:

• Portable – Components should be able to be easily substituted for new components as they become available. This includes all types of components, from the framework to the server and host. The implication of this goal is that third party frameworks can seamlessly run on Microsoft servers while Microsoft frameworks can potentially run on third party servers and hosts.
• Modular/flexible – Unlike many frameworks which include a myriad of features that are turned on by default, Katana project components should be small and focused, giving control over to the application developer in determining which components to use in her application.
• Lightweight/performant/scalable – By breaking the traditional notion of a framework into a set of small, focused components which are added explicitly by the application developer, a resulting Katana application can consume fewer computing resources, and as a result, handle more load, than with other types of servers and frameworks. As the requirements of the application demand more features from the underlying infrastructure, those can be added to the OWIN pipeline, but that should be an explicit decision on the part of the application developer. Additionally, the substitutability of lower level components means that as they become available, new high performance servers can seamlessly be introduced to improve the performance of OWIN applications without breaking those applications. 

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