Moving towardsReactive Programming

LSPE MeetUpMarch 14, 2015

Hello !!I am Deepak Shevani

- Work at Flipkart for Supply Chain team- Recently got interested in @reactive @functional @programming- Contact : shevanideepak@gmail.com

Agenda for talk

▧ Awareness, what’s up ??▧ Reactive, what’s this ??▧ Reactive Programming▧ Functional Reactive Programming▧ Demo(s)

Disclaimer : While I make every effort to tell correct information, I am still a learner :)

Noob Alert !!

“Be aware of technology advancements.

Java 8 design is heavily influenced by core principles of functional programming !!

Brian Goetz

#1 JAVA is functional

We should start thinking events as signals emitted from some asynchronous data stream

Eric Meijer

#2 Streams everywhere

Think about this !!

Suppose, we have to design a button with click counter that tracks different clicks and perform appropriate actions.

Say, we’ll print separate messages for each click observed.

Pure JS Implementation

var timer, timeout = 200; // timer reset in ms

button.addEventListener("dblclick", function (evt) {

timer = setTimeout(function () {

timer = null;

}, timeout);

});

button.addEventListener("click", function (evt) {

if (timer) {

console.log("triple");

label.textContent = "success!";

clearTimeout(timer);

timer = null;

}

});

1.Reactive, what ??

Let’s understand - what reactive means ?

What reactive means ??

Merriam Webster

adjective re·ac·tive

\rē-ˈak-tiv\

Readily responsive to a stimulus

Wikipedia

A reactive system is a system that responds (reacts) to external events.

Typically, computer systems are reactive, when they react to external as well as internal events.

Reactive Manifesto !!

Excerpts from Manifesto

#Changing Needs

…. These changes are happening because modern application requirements have changed dramatically in recent years.

#New Architectures

…. Today's demands are simply not met by yesterday’s software architectures.

…. A new architecture has evolved to let developers build applications to satisfy these needs.

#Reactive Applications

…. We want systems that are responsive, resilient, elastic and message Driven.

We call these Reactive Systems.

Changing requirements

Few years ago Now

Server Nodes 10’s 1000’s

Response Times seconds milliseconds

Maintenance downtimes hours none

Data volume GBs TBs -> PBs

Traits of Reactive Applications

Event DrivenTraditionally, systems are composed of threads which communicate with shared mutable state

Systems are better composed of loosely coupled event handlers + asynchronous IO

ResilientSystem should quickly recover from failures (hardware, software, network)

How ? Loose coupling, thought out right from beginning, handle exceptions, fbs

ScalableSystems should be able to adjust itself based on usage

- scale up : make use of parallelism- scale out : multiple server nodes

Responsive (GOAL)Application is ‘responsive’ if it provides rich, real-time interaction with its users even under load and in presence of failures

Event-Driven

Handling events is not new. Its often done using callbacks.

Heard of - EventHandlers?

Problems :

- Shared mutable state- Call-back hell

We do this already. No ??

Scalable

Distributed systems generally allow scaling-out. What about vertical scaling ? Is our code easy to parallelize ?

Problems :

- Asynchronous programming is hard, but we need this.

Is this revolutionary ??or evolutionary ??

2.Reactive Programming :)

Having understood what is reactive systemLet’s summarize our learnings& start reactive programming

1. Never Block- unless you really have to- use non-blocking IO- use lock-free concurrency

2. Go Async- use asynchronous events/messages- nothing to be shared (mutable state)- design workflows with events flowing- strive for loosely coupled message handlers

3. Go lazy- efficiency != doing tasks faster- avoid tasks that shouldn’t be done in the

first place.- function composition and lazy evaluation

are pillars of reactive programming

Reactive programming is programming withasynchronous data streams

Think - everything is a stream (not just clicks and hover events)

Anything can be stream - variables, user inputs, data structures

?

Functional Reactive Programming (FRP) is a variant of Reactive Programming thats seeks

to be purely functional.

?

Functional => Lambdas, Closures, (Mostly) Pure, Composition

Reactive => Asynchronous, Events, Push based

Finally !! Tool set

RxThe Reactive Extensions is a library for composing asynchronous event-based programs. Developed by Microsoft Open Technologies.

Bacon.jsA small functional reactive programming library for JavaScript. Turns your event spaghetti using functional programming paradigms.

RACReactive Cocoa (RAC) is an Objective-C framework inspired by Functional Reactive Programming.

ElmA functional reactive language for interactive applications.

PlayWritten in Scala and Java, play makes iterative, Reactive application development very simple.

AkkaAkka is a tool kit and runtime for building highly concurrent distributed, and resilient message-driven applications on the JVM

4.Demo Time

Show me some code

Just a moment !!

Streams

A stream is a sequence of ongoing events ordered in time. Emits three things

- value- error- completed

Observables

If you have heard of Observer Pattern, this is a logical extension where we deal with streams of data that

- signals end

- handles failures

- does lazy evaluation

- uses push instead of pull interaction

Subscriber

Captures emitted events synchronously

Defines separate functions for

- emitted values- handle errors- completion

Demo 1 : Click Counter

In this demo, we will consider click events arising from a button, and performs actions like

- track double clicks- track multiple (2+) clicks

as double click events- subscribe to events

Demo 1 - Code

var clickStream = Rx.Observable.fromEvent(button, 'click');

var multiClickStream =

clickStream

.buffer (

function() {

return clickStream.throttle(250);

})

.map (

function(list) {

return list.length;

})

.filter (

function(x) {

return x >= 2;

});

Demo 1 - Code

multiClickStream.subscribe (

function (numclicks) {

document.querySelector('h4').textContent = 'This was '+numclicks+'x click';

}

);

Rx.Observable.merge (singleClickStream, multiClickStream)

.throttle (5000)

.subscribe (

function (suggestion) {

document.querySelector('h4').textContent = 'Idle period for me ...';

}

);

Demo 2 : Rx-Java

In this demo, we will create a reactive stock server application using Rx Java

We will

- learn working with streams

- learn handling errors- see laziness live

- We will use Rx Java to create streams out of server responses and let subscribers to

- subscribe- unsubscribe- filter etc

Demo 2 - Lazy Code

public static void main(String[] args) {

List<Integer> values = Arrays.asList(1, 2, 3, 4, 5, 6);

System.out.println(

values.stream()

.filter(LazyStreamDemo::isGreaterThan3)

.filter(LazyStreamDemo::isEven)

.map(LazyStreamDemo::doubleIt)

.findFirst()

) ; } }

isGreaterThan3 - 3

isGreaterThan3 - 4

isEven - 4

doubleIt - 4

Thanks !!You were a wonderful audience

Any questions?

You can find me at@deepak_shevani

shevanideepak@gmail.com