So, have you been amazed by thin's async.callback? If not, go check it out. Come back here when you start missing your middleware.

So what is the issue with Rack and async.callback? Currently there are two ways of triggering a async responds. The first is to throw :async, the latter to return a status code of -1 (even though thin and ebb do disagree on that). Opposed to what others say, I would recommend using throw, as it simply skips middleware not able to handle :async. Also, it works on all servers supporting async.callback – thin, ebb, rainbows! and zbatery – about the same and copes better with middleware that is unable to handle an async respond.

That's the issue with async.callback: Most middleware is not aware of it. Let's say you got an app somewhat like that:

class Farnsworth
  def when_there_is_good_news
    Thread.new do # Well, actually, you want to hook into your event loop instead, I guess.
      wait_for_good_news
      yield
    end
  end
  
  def call(env)
    when_there_is_good_news do
      env["async.callback"].call [200, {'Content-Type' => 'text/plain'}, ['Good news, everyone!']]
    end
    throw :async
  end
end

Ok, now, since this app could end up on Reddit, you better prepare yourself for some heavy traffic. Say, you want to use the Rack::Deflate middleware, so you set it up in your config.ru and add the link to reddit yourself. The next day you get a call from your server admin. Why don't you at least compress your http response? Well what happened? The problem is, that by sending your response via env["async.callback"].call you talk directly to your web server (i.e. thin), bypassing all potential middleware.

Well, how do you avoid that? Simple: By just using middleware that plays well with async.callback. However, most middleware does not play well with it. In fact, most middleware that ships with rack does not play well with it. That's what I wrote this little library for. If you load async-rack it modifies all middleware that ships with rack, so it will work just fine with you throwing around your :async.

How does that work? Simple, whenever necessary, async-rack will replace async.callback with an appropriate proc object, so it has the chance to do it's response modifications whenever you feel like answering the http request.

Note: This library only 'fixes' the middleware that ships with rack, not other rack middleware. However, you can use the included helper classes to easily make other libraries handle async.callback.

There are three types of middleware:

Example: Rack:::MethodOverride

Such middleware already works fine with async.callback. Also, from our perspective, middleware either creating a own response and not calling your app at all, or calling your app without modifying neither request nor response falls into this category, too.

Such middleware can easily be identified by having @app.call(env) or something similar as last line or always prefixed with a return inside the call method.

Example: Rack:::ETag

Here it is a bit tricky. Essentially what you want is running #call again on an async.callback but replace @app.call(env) with the parameter passed to async.callback. Well, apparently this is the most common case inside rack, so I created a mixin for that:

# Ok, Rack::FancyStuff does currently not work with async responses
require 'rack/fancy_stuff'

class FixedFancyStuff < AsyncRack::AsyncCallback(:FancyStuff)
  include AsyncRack::AsyncCallback::SimpleWrapper
end

See below to get an idea what actually happens here.

Example: Rack::Runtime

# Let's assume there is some not so async middleware.
module Rack
  class FancyStuff
    def initialize(app)
      @app = app
    end
    
    def call(env)
      prepare_fancy_stuff env
      result = @app.call env
      perform_fancy_stuff result
    end
    
    def prepare_fancy_stuff(env)
      # ...
    end
    
    def perform_fancy_stuff(result)
      # ...
    end
  end
end

# What happens here is the following: We will subclass Rack::FancyStuff
# and then set Rack::FancyStuff = FixedFancyStuff. AsyncRack::AsyncCallback
# makes sure we don't screw that up.
class FixedFancyStuff < AsyncRack::AsyncCallback(:FancyStuff)
  # this method will handle async.callback
  def async_callback(result)
    # pass it on to thin / ebb / other middleware
    super perform_fancy_stuff(result)
  end
end

Rack::FancyStuff == FixedFancyStuff # => true

In general: Place a require 'async-rack' before setting up any middleware or you will end up with the synchronous version!

Please keep in mind that it only "fixes" middleware that ships with rack. Read: It works very well with Sinatra. With Rails and Merb, not so much!

In your config.ru:

require 'async-rack'
require 'your-app'

use Rack::SomeMiddleware
run YourApp

In your application file:

require 'async-rack'
require 'sinatra'

get '/' do
  # do some async stuff here
end

In your config/environment.rb, add inside the Rails::Initializer.run block:

config.gem 'async-rack'

In your Gemfile, add:

gem 'async-rack'