CSC/ECE 517 Fall 2014/ch1a 1 sj

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Play Framework is a high velocity web development framework<ref>http://typesafe.com/blog/announcing-play-framework-21-the-high-velocit</ref> for Java and Scala. It uses the Model-view-controller (MVC) architectural pattern for implementing user interfaces. Play is based on a lightweight, stateless and web-friendly architecture written in Scala. The primary goal of Play framework is to provide a developer friendly development platform for web development.

The major advantage of Play over other Java EE frameworks is it's ability to build highly scalable applications. It's reactive model (Reactive Manifesto), thanks to Iteratee IO provides nominal consumption of resources (CPU cores, memory), a major requirement for ascendable and asynchronous applications.<ref>http://www.reactivemanifesto.org/</ref>

Some of the major companies that use Play framework are LinkedIn, Klout, TheGuardian, ZapTravel and GILT

Play framework logo

History

The early versions of Play framework can be traced back to early 2007 while it was being developed at Zenexity by software developer Guillaume Bort.<ref>http://en.wikipedia.org/wiki/Play_framework</ref> The first full version of Play was in October 2009. Later in 2011, Sadek Drobi, co-creator of Play, joined the Play development wagon to release Play 2.0 in conjunction with Typesafe Stack 2.0.

Release History

Below is the changes introduced in the releases: <ref>https://www.playframework.com/changelog</ref>

Release Version Date of Release Features
Play 1.x November 2010 - April 2011
  • Native Scala Support
  • Migration from Apache MINA to JBoss Netty
  • OAuth and HTTPS support
Play 2.0 March 2012
  • Native Support for Java and Scala
  • Asynchronous programming support by inclusion of WebSockets and Comet
  • Built-in relational database access libraries such as Ebean, JPA and Anorm
Play 2.1 Februrary 2013
  • Migration to Scala 2.10 API
  • RequireJS support
  • New Scala JSON API
Play 2.2 September 2013
  • Improved Java Promise API
  • SBT 0.13 support
  • Built in Gzip support
Play 2.3 May 2014
  • Introduced Activator command
  • Support for Scala 2.11
  • Custom SSL Engine for HTTPS
  • Upgrade to Netty 3.9.3

Motivation of Play framework

The classical web development frameworks assume a single-box architecture. Hence the organization of their design is around static, unique, change-intolerant data models which try to centralize the entire app's business logic. This traditional approach falls short in a number of areas such as complex data representations, data distributions, composability<ref>http://engineering.linkedin.com/play/play-framework-democratizing-functional-programming-modern-web-programmers.</ref> Functional programming is adept at handling these modern coding challenges. Although an increasing number of programmers are following this programming paradigm, it is time before main stream developers become fluent with this model. This has inspired one more essential motivation of Play design: a progressive learning curve. The release history shows the gradual introduction of functional programming concepts and the transition from Java to Scala(a functional programming language) Some of the major factors that motivated the development of Play framework are:

Traditional web frameworks are stateful and because of this scalability becomes an issue after a while. The creators of the Play framework found that stateless frameworks are more stable when there is higher traffic since no overhead is incurred in tracking and maintaining user state information.

  • Having an ‘evented’ web server architecture as opposed to a ‘threaded’ web server architecture

Since the Play framework is essentially stateless, an event driven web server architecture was more appropriate to base Play on than modelling it as threaded. Therefore, it was decided to base play on Netty – an event-driven client-server framework.

  • Provision of a full-stack web framework

The creators of the framework wanted to provide developers with a single complete framework that was capable of model-persistence and which had template (view), controller and testing mechanisms. In Play, model and controller can be realized with Java and Scala, views are realized with a template syntax and Scala and model persistence is done with Ebean by default with the option to use any other persistence mechanism. JUnit, Selenium WebDriver and FluentLenium are mainly used as the testing tools for Play.

One of the primary motivations of the Play framework was to make development as easy as possible for the developer. To do this features such as hot reload (mimicking interpreted languages), integration to JSON and integration of compilers for CoffeeScript, LESS etc. were provided. Development on Ruby on Rails and Django is fairly simple and the creators of Play wanted to mimic this and hence modeled Play on these frameworks. Play is also fully compiled, typesafe, RESTful by default and provides support for long living connections(WebSocket, Comet etc).

Features of Play Framework

  • Play is a stateless, light weight framework which is very flexible. The MVC architecture inherently provides flexibility but along with this there are many elements in Play which are pluggable, configurable or customizable.
  • It makes use of Akka which supports multiple programming models for message based asynchronous concurrency. As a result of this, even large scale applications built on Play consume minimum resources in terms of CPU, memory and threads.
  • Another advantage that Akka provides to Play is predictable scalability. The asynchronous model of Akka coupled with the stateless, non-blocking I/O nature of Play ensures that there are no hiccups while scaling up.
  • Development on Play is simple and straight forward. Owing to features such as hot reload, powerful console & build tools, type safety, built in testing tools and IDE support for Eclipse & IntelliJ IDEA all that is required for development would be a browser and a text editor. This enhances the productivity of developers dramatically.
  • Play provides many features for modern web and mobile app development. Being RESTful by default, integration for JSON and compilers for CoffeeScript, Less etc., support for protocol technologies such as WebSocket & Comet and extensive NoSQL & Big Data support are just some of these features.
  • The reactive nature of Play is another important feature. Apps developed based on a Service oriented architecture would require multiple parallel asynchronous requests to be sent out. Netty supports non - blocking I/O and hence it is not expensive to make such calls.
  • Play is open source which allows for evaluation of the underlying code and also facilitates monitoring of the framework code. There is also a large user community who contribute libraries and help clarify the doubts of fellow users. But, even though play is open source, there is an option to receive commercial support.
  • Play provides a developer friendly error handling mechanism where the error is directly mentioned in the browser itself with the file path, line number as well as the relevant code. This saves a lot of developer time and effort since he/she would not have to dig through and analyze large log files.

Components of Play Framework

Components of Play framework.
Components of Play framework

Java

The compiler and runtime (JVM)<ref>http://www.java.com/en/</ref> do the heavy lifting so that your apps run super-fast and keeps running under load.

Scala

After the core was rewritten in Scala after release 2.0, Play provides an extensive Scala support.

Templates

Play 2.x comes with a really powerful Scala-based template engine. This new template engine’s design was inspired by ASP.NET Razor. The Play template engine can be used to generate any text based format such as HTML, CSV or XML.

SBT Build System

Play includes a build system which is based on sbt, a non intrusive build tool for Java and Scala projects.<ref>http://www.scala-sbt.org/</ref> There are certain default build settings which can be altered to suit individual project needs.

Akka

Akka uses the Actor Model to raise the abstraction level and provide a better platform to build correct concurrent and scalable applications.

Web Sockets

Play comes with an in-built support for Web Sockets. This combined with the Akka system can be used to easily develop web sockets which can handle large amounts of data.

Architecture

MVC application model

Play follows a typical MVC architectural design pattern of web application development. This divides the application into two broad layers:<ref>https://www.playframework.com/documentation/1.0/main</ref>

  • The Model Layer: It handles the business logic of the actual domain problem.
  • The Presentation Layer: This can be categorized into two sub parts.
    • The View: It handles the actual GUI of the application
    • The Controller: It maps the actions on the user interface to the concerned models.

In a Play application, these three components are placed in the app directory, each in a separate package.

app/controllers

These are the Java classes that receive the HTTP requests. They are extended form the play.mvc.Controller class. Each HTTP request is attached to a action method. This is a public and static method having the following signature: 

public static void action_name(params...);

Various parameters can be defined as inputs to the action methods. Play automatically maps them from the corresponding HTTP request parameters. There is generally no limit on the number of parameters to be passed and they can also be sent as query strings.

app/models

These are the Java classes exhibiting typical Java behavior. Unlike the controllers, they follow the Java object oriented paradigm. This package contains the classes that represent the various actors in the use cases. All the corresponding operations pertaining to these actors are contained in the Model classes. The model classes access the data storage mediums to persist and update data.

app/views

This package contains special HTML, XML template files which are used to render the GUI on the user browser screen. The Play templating engine renders these views using special directives contained in these files. The Controller gets the data from the models and uses a template to decorate the views.

Standard Application Layout

The app directory

This directory contains a sub folder for each of the layer in the MVC pattern. This is the directory under which the developers can add their code as well as HTML pages. Apart from the above mentioned packages, developers can create their own packages.

The public directory

This directory contains the static resources which are not modified by the framework. their state remains the same after deployment. These contain the image files, CSS files, JavaScript files.

The conf directory

The conf directory contains all the configuration files required by the framework and the application. Two mandatory configuration files are:

application.conf file: This file contains the standard configurations at time of application start up. Play provides a sophisticated API to read values from this file 

Play.application().configuration().getString("key");

routes file: This file contains the configurable URLs. The routes defined here are mapped to controller functions which accept HTTP requests, render views etc. 

GET        /home                         Controllers.Application.index(Int ?= 2)
POST       /verifyuser                   Controllers.Login.verifyUser(String username)

Using Play

Online resources

Books

IDE Support

Since Play supports hot reloading, code can be typed on the fly and output can be received on the browser screen. Since there is no specific compilation phase involved, coding on Play framework does not require a Integrated Development Environment (IDE) Although code writing would not require the maximum usage of the IDE, activities like debugging and assisted refractoring in Java and Scala require the use of these sophisticated tools.<ref>https://www.playframework.com/documentation/2.4.x/IDE</ref> Play framework supports a host of famous IDE's like:

  • Eclipse

A simple command eclipse generates the required Eclipse configurations to convert the existing application to an Eclipse-ready one. 

[my-first-play-app] $ eclipse
  • IntelliJ

This IDE does not require a command prompt to setup the project configurations. It provides a direct Play2.x project selection while creating a new project. Only perquisite is that the Scala and Play 2 plugins are enabled in the IDE.

Miscellaneous

Asynchronous and Scalable programming

The feature that makes Play a preferred framework for asynchronous and scalable applications is the inclusion of Akka toolkit. Instead of doing it the Java way of concurrency using threads, semaphores, and locks; Akka emphasizes upon actor-based concurrency. It implements the Actor model. The actors, which are built on a fixed number of threads which is configurable, converse by the means of messages generated due to events. This helps Play achieve it's scalability without having to handle it programatically. Each actor consists of a mailbox and a queue. The mailbox registers the events and are passed as inputs to the actor. These actors, belonging to an actor system then process the messages and perform the assigned tasks in parallel.

Below is the sample creation of code: <ref>http://doc.akka.io/docs/akka/snapshot/java/untyped-actors.html</ref> 

import akka.actor.UntypedActor;
import akka.event.Logging;
import akka.event.LoggingAdapter;
 
public class MyActor extends UntypedActor {
  LoggingAdapter log = Logging.getLogger(getContext().system(), this);
 
  public void onReceive(Object message) throws Exception {
    if (message instanceof String) {
      log.info("Received String message: {}", message);
      getSender().tell(message, getSelf());
    } else
      unhandled(message);
  }
}

The actor MyActor can be created by the following line of code below: 

ActorRef myActor = Akka.system().actorOf(new Props(MyActor.class));

Also the number of actors initialized at application start up, by specifying a property in the application.conf file. This actor can start processing messages and the number of actors can be incremented dynamically by configuration changes.

Backward compatibility

As shown in the History of Releases tab, Play framework got a complete overhaul after Play 2.0 The major difference turns out to be that Play 1.x was in Java while Play 2.x is in Scala. This renders it as backward incompatible. This caused a lot of furor at the time of the release of 2.0 version as there was no portability script provided for migration from 1.x to 2.0. However, it was clearly seen that the advantages of this Scala version far outweighed the problems faced in application porting.

The following is a summary of the pros and cons of the 2 Play versions:<ref>http://stackoverflow.com/questions/8189893/what-are-the-major-differences-between-play-framework-1-0-and-2-0</ref>

Wins of Play 2.0

  • Suited for asynchronous programming
  • Improved and flexible cache support
  • Convenient Data Access Layer for database requests
  • Inclusion of Akka: Actor based, event driven framework

Downsides of Play 1.x

  • Involves Bytecode manipulations
  • Does not follow Java conventions
  • Poor Inversion of Control

Play vs Java EE frameworks

  • Stateless

The major difference between Play and other Java EE frameworks is that Play is stateless. There is no Java EE session per connection.

  • Independent of Servlet

Like other major Java frameworks such as Spring, JSF and Struts, Play does not depend on Servlets.

  • Asynchronous I/O

Because Play depends on JBoss Netty API, it can service long requests in a asynchronous manner rather than tying up HTTP request with business logic threads<ref>http://www.doublespring.com/play-framework/</ref>

  • Modular Architecture

Since Play is inspired from the Rails and Django frameworks, it adapts the concept of Modules.

References

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