Owin 101 a progressive walkthrough

TLDR: Play with my repository if you want to get to grips with OWIN

Last week I did a presentation at work on a .NET website we put into production last december completely built on OSX just because … hipsterism is suppose.

hipster cat

On a more serious note: this was a great exercise in evaluating what it means to completely develop in C# outside off the microsoft realm.

I'll save all of the challenges, lessons learned for another blog post and focus on the glue, OWIN.

Precursor to what us .NET web dev's have had to deal with

Historically .NET web development happened inside the ASP.NET pipeline that was hosted inside IIS, end of story. Now whats particularly bad is that this pipeline was designed to accommodate for the ASP.NET WebForms paradigm, where clicks on the client would result in posts that would run server code. This to unify WinForms and WebForms sort off. This caused the entire pipeline to be way to complex, feast your eyes on this.

asp.net web-forms pipeline

This was back in 2002, somewhere around 2009 ASP.NET MVC was released. This did away with the notion of postbacks and let us program the web as it was meant to: stateless. However MVC was still shoehorned into the existing pipeline even if it does a great job of hiding it, you still need a global.asax Application_Startup Application_OnError still are the ways to initialise and catch errors for instance. Granted it did do away with 90% of the cruft in the previous diagram.

If you're interested read about the whole pipeline here

To add fault to injury historically aspx pages worked much like php or other interpreted languages as they were compiled as they were hit so they came with a first hist performance problem. Precompilation of these pages has been added as far back as 2005 but this never mitigated the problem of the first request being particularly hard to tame. I blame this on FirstRequestInit() still trying to scan for aspx pages, theres a mysterious call to System.Web.Compilation.CompilationLock, files get copied to %\Framework\[FRAMEWORK]\Temporary ASP.NET Files ARGH do I need to go on? So much cruft! Get rid of it!

I didn't even get into the split that is now introduced with ASP.NET MVC/ ASP.NET Web Api/ ASP.NET WebPages all introducing slightly different pipelines that makes it impossible to take a request filter written for mvc and reuse it in web api. I feel like screaming.

rage quit

Owin to the rescue!

OWIN is a specification that decouples the pipeline from frameworks and servers. Inspired by Rack in ruby, Connect in node, WSGI in python, the list goes on and now .NET has finally followed suit.

It defines a minimal interface for middleware to implement without needing to take ANY dependencies and promises to finally bring a lightweight composable pipeline to us .NET web developers.

The spec pretty much states, if your lib has a Startup class with a public Configuration method that returns a Func<Dictionary<string, env>, Task> you are writing a web application.

public class Startup
    public Func<IDictionary<string, object>, Task> Configuration()
        return async env =>
            object responseStream;
            if (!env.TryGetValue("owin.ResponseBody", out responseStream))
                throw new Exception("Expecting a valid owin dictionary");

            using (var s = (Stream) responseStream)
            using (var sr = new StreamWriter(s))
               await sr.WriteAsync("Hello world");

The OWIN specification states that whatever server runs this should make the response stream available under the key owin.ResponseBody.

Now much to Microsoft credit they have an awesome team working hard to make the OWIN ecosystem happening and much of it is under the codename Katana but with all the examples online being implemented with the Microsoft.Owin.* packages it's hard to distinguish whats OWIN and whats Katana. In fact even the Owin.dll inside the Owin package on nuget is NOT part of OWIN.

For last tuesday's talk I created a demo which progressively goes from vanilla OWIN into the world of KATANA in 6 steps

01 - Barebones

No dlls, just a simple owin handler. Owin101.BareBones's Readme

02 - Barebones Middleware

Still no dlls, combining owin middleware by ourselves. Owin101.BareBonesMiddleware's Readme

03 - Introducing IAppBuilder

We take a dependency on owin.dll and see what that gives us in return. Owin101.IntroducingIAppBuilder's Readme

04 - Helper assemblies

We take on additional dependencies on Owin.Extensions and Owin.Types and again look into how these help us write terser/better typed owin middleware. Owin101.HelperAssemblies's Readme

05 - Branching the builder

We take a dependency on Microsoft.Owin.Mapping to see how we can get a very simple routing going on. Up until now our handlers were completely sequential. This shows how to branch off and have different endpoints doing different things. Owin101.BranchingBuilder's Readme

06 - Hosting

This example shows how katana handles the hosting be it self host in a console/service or in IIS. Owin101.Hosting's Readme

Checkout the repos here the root readme also goes into how to run the examples.

Hopefully this is of some use to someone out there!


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