Synchronous, event-based and asynchronous HTTP proxy

// NOTE: This snippet uses System.Net extensions from: http://fssnip.net/6d
// (such as HttpListener.Start and WebClient.AsyncDownloadData)

// Location where the proxy copies content from
let root = "http://msdn.microsoft.com"

// Maps requests from local URL to target URL
let getProxyUrl (ctx:HttpListenerContext) = 
  Uri(root + ctx.Request.Url.PathAndQuery)

// Handle exception - generate page with message
let handleError (ctx:HttpListenerContext) (e:exn) =
  use wr = new StreamWriter(ctx.Response.OutputStream)
  wr.Write("<h1>Request Failed</h1>")
  wr.Write("<p>" + e.Message + "</p>")
  ctx.Response.Close()

// Handle exception asynchronously - generate page with message
let asyncHandleError (ctx:HttpListenerContext) (e:exn) = async {
  use wr = new StreamWriter(ctx.Response.OutputStream)
  wr.Write("<h1>Request Failed</h1>")
  wr.Write("<p>" + e.Message + "</p>")
  ctx.Response.Close() }

  /// Handle request on a dedicated thread - This is not 
  /// scalable, because thread may be blocked for long time
  let handleRequest (ctx:HttpListenerContext) =
    let wc = new WebClient()
    try
      let data = wc.DownloadData(getProxyUrl(ctx))
      ctx.Response.OutputStream.Write(data, 0, data.Length)
      ctx.Response.Close()
    with e ->
      handleError ctx e
  
  // Start synchronous HTTP proxy 
  let token = HttpListener.StartSynchronous("http://localhost:8080/", handleRequest)
  token.Cancel()

  module Wrappers =
    Wrappers that provide event-based network API

  open Wrappers

  /// Handles request using callbacks - This scales well, but the
  /// code is difficult to write, because we need to use callbacks
  /// (This is similar to the style used by Node.js)
  let handleRequestCallback (ctx:HttpListenerContext) =
    let wc = new WebClient()
    wc.DownloadData
      ( getProxyUrl(ctx.Context), 
        success = (fun data ->
          ctx.Response.OutputStream.Write
            ( data, 0, data.Length,
              success = (fun _ -> ctx.Response.Close()),
              error = handleError ctx.Context )),
        error = handleError ctx.Context )

  // Start HTTP proxy that handles requests using callbacks 
  let token = HttpListener.StartSynchronous("http://localhost:8080/", fun ctx ->
    handleRequestCallback(HttpListenerContext(ctx)))
  token.Cancel()

  /// Handles request using asynchronous workflows - We get efficient & scalable
  /// code by wrapping synchronous solution in 'async' block and changing
  /// synchronous primitives to asynchronous (e.g. AsyncWrite)  
  let asyncHandleRequest (ctx:HttpListenerContext) = async {
    let wc = new WebClient()
    try
      let! data = wc.AsyncDownloadData(getProxyUrl(ctx))
      do! ctx.Response.OutputStream.AsyncWrite(data) 
    with e ->
      do! asyncHandleError ctx e }

  // Start HTTP proxy that handles requests asynchronously
  let token = HttpListener.Start("http://localhost:8080/", asyncHandleRequest)
  token.Cancel()