Sadek Drobi sadache

Is socket.push(bytes) all you need to program Realtime Web apps?

One of the goals of Play2 architecture is to provide a programming model for what is called Realtime Web Applications.

Realtime Web Applications

Realtime Web Applications are applications that make use of Websockets, Server Sent Events, Comet or other protocols offering/simulating an open socket between the browser and the server for continuous communication. Basically, these applications let users work with information as it is published - without having to periodically ping the service.

There are quite a few web frameworks that target the development of this type of application: but usually the solution is to simply provide an API that allows developers to push/receive messages from/to an open channel, something like:

Bits'bout Play2 Architecture

Play2 Simple HTTP API

Essential blueprint of Play2 architecture is pretty simple and it should be easy to explain in a fairly short blog post. The framework can be understood progressively at different levels; each time having better exposure to some aspects of its design.

The core of Play2 is really small, surrounded by a fair amount of useful APIs, services and structure to make Web Programming tasks easier.

Basically, Play2 is an API that abstractly have the folllowing type

If your csv doesn't contain escaped newlines then it is pretty easy to do a progressive parsing without putting the whole file into memory. The iteratee library comes with a method search inside play.api.libs.iteratee.Parsing :

def search (needle: Array[Byte]): Enumeratee[Array[Byte], MatchInfo[Array[Byte]]]

which will partition your stream into Matched[Array[Byte]] and Unmatched[Array[Byte]]

Then you can combine a first iteratee that takes a header and another that will fold into the umatched results. This should look like the following code:

// break at each match and concat unmatches and drop the last received element (the match)

	Enumeratee.map[Array[Byte]]{ e =>
	val length = e.size
	val CRLF = "\r\n".getBytes("UTF-8")
	(length.toHexString.getBytes ++ CRLF ++ e ++ CRLF):Array[Byte]
	} ><>
	Enumeratee.trailing(Seq((0.toHexString.getBytes) ++ (CRLF ++ CRLF)))

	package controllers

	import play.api._
	import play.api.mvc._


	object Application extends Controller {

	def index = Action {

	package controllers

	import play.api._
	import play.api.mvc._
	import play.api.libs.ws._
	import play.api.libs.iteratee._
	import play.api.libs.concurrent._

	object Application extends Controller {

	/**
	* Handles the comet event stream.
	*/
	def cometStream = Action {
	AsyncResult {
	implicit val timeout = Timeout(5.seconds)
	val actor=Akka.system.actorOf(Props[EventListener])
	// Actor is listening for event on the eventStream
	Akka.system.eventStream.subscribe(actor,classOf[ChangeEvent])
	// For each event, stream the data to client

	// What is a Promise sequence function ?
	// A function which transform a List[Promise[A]] into a Promise[List[A]]

	// First naive implementation. It's a synchronous implementation (blocking).
	def sequencePromises[A](list: List[Promise[A]]): Promise[List[A]] =
	list.foldLeft(Promise.pure(Promise[List[A]]()))((s,p) => s.flatMap( s => p.map(a => s :+ a)))

	//should be in the lines of this, didn't try to typecheck it though.

	@(contacts:List[(Long,String,String)])

	<html>
	<head>
	<title>ZenContact - Play! 2 Sample application</title>
	<link rel="shortcut icon" type="image/png" href="http://www.playframework.org/public/images/favicon.png">
	<link rel="stylesheet" type="text/css" media="screen" href="@routes.Assets.at("stylesheets/main.css")">
	<link rel="stylesheet" type="text/css" href="http://fonts.googleapis.com/css?family=Varela+Round\|Droid+Sans">
	</head>
	<body>

	def prevNext = {
	SQL(
	"""
	(
	select *, 'next' as pos from post
	where postedAt < {date} order by postedAt desc limit 1
	)
	union
	(
	select *, 'prev' as pos from post