The Kestrel server is the newer HTTP server built by Microsoft. The Freya.Polyfills.Kestrel polyfill should be used whenever you are taking one of these hosting approaches. The polyfill should be used by inserting it in front of your Freya application in a pipeline composition:

open Freya.Core
open Freya.Core.Operators
open Freya.Polyfills.Kestrel

// Some pre-existing Freya pipeline (could be a router, function, etc.)
let myApp =
    freya {
        return Next }

// Compose a polyfill with the pre-existing pipeline
let composedApp =
    >?= myApp

// Use as normal...
let owinApp =
    OwinAppFunc.ofFreya composedApp


The polyfill currently adds additional data to the OWIN environment which enables routing to work more accurately (giving routers access to the raw, encoded form of the path and query).