Reverse Ajax and Prototype with Mongrel handlers

The most common use of reverse Ajax or Comet calls is having some piece of information in your webpage that you want to update as soon as some remote event triggers, always without polling. Let's make a little experiment to see how this works:

Client side

So you start a request, the web server waits for the event to happen, and finally, a while later, the response is sent back to the client. When this process ends, it's very common that you want to start it again. You want to have a continuous ajax call when the event is also continuous, when it can happen many times.
To deal with this, as I'm using prototype, I made this javascript routine to start listening for the event:

function continuousAjaxCall(url, method, params,
                            successCallback){
  waitingForResponse = false;
  function iteration(){
    if(!waitingForResponse){
      waitingForResponse = true;
      new Ajax.Request(url, 
       {
         method: method,
         parameters: params,
         onSuccess: theSuccessCallback
       });
    }
  };

  function theSuccessCallback(transport){
   successCallback(transport.responseText);
   waitingForResponse = false;
  };
  
  return new PeriodicalExecuter(iteration, 1);
} 

So now if you do:

continuousAjaxCall("/WebFacade/SomeWebService", "post",
                   "taskid=" + taskNumber, callback);

you will start listening to the web service in /WebFacade/SomeWebService using method post and with taskid=taskNumber as the http body parameter.
The callback is a routine that will be executed each time the event is triggered. For example we could do this to show the result from the web service:

function callback(responseText)
{
  alert(responseText);
}

The routine returns a prototype PeriodicalExecuter that you can use to stop listening:

pe = continuousAjaxCall(...
pe.stop()

Server side

In the server side I have some Mongrel handlers serving the client pages. The key here is the line sleep 1 until @@submitted_value that waits until some new value is submitted and only then responds to the ajax request:

require 'rubygems'
require 'mongrel'
include Mongrel

@@submitted_value = nil

class ReverseAjaxHandler < HttpHandler
  def process(request, response)
    response.start(200) do |head,out|
      head['content-type'] = 'text/html'
      sleep 1 until @@submitted_value
      out << @@submitted_value
      @@submitted_value = nil
    end
  end
end

class SendHandler < HttpHandler
  def process(request, response)
    response.start(200) do |head,out|
      #Code that renders the "/send" page and sets the @@submitted_value
      #...  
      #...  
    end
  end
end

h = HttpServer.new("0.0.0.0", "5000")
h.register("/wait", ReverseAjaxHandler.new)
h.register("/send", SendHandler.new)
h.register("/files", DirHandler.new('.'))
h.run.join

It's nice to have this as a Mongrel handler because it's simple, but keep in mind that this implementation was made just for the purpose of this example and it is not thread safe. I don't control the access to the global variable @@submitted_value

Demo

I made a little demo about this using some mongrel handlers like the one in the previous post:

The code for this article and it's demo can be found here.

Standalone Mongrel handler Hello World

I think this is the simplest Mongrel handler you can create. It doesn't depend on Rails so this is not what you need to write if you want to install a Rails friendly Mongrel handler.

require 'rubygems'
require 'mongrel'

class HelloHandler < Mongrel::HttpHandler
  def process(request, response)
    response.start(200) do |head,out|
      out << 'Hello World!'
    end
  end
end

h = Mongrel::HttpServer.new("0.0.0.0", "5000")
h.register("/hello", HelloHandler.new)
h.run.join

Notes:

• If you want to output html remember to include head['content-type'] = 'text/html' inside the inner block


• To include static pages you can add h.register("/files", DirHandler.new('.')). After this you can access all files in the current directory through this url http://localhost:5000/files