Introduction to Node js

A K S H A Y M A T H U R
@AKSHAYMATHU
Getting Started with

Ground Rules
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 Post on FB and Tweet now
 Disturb Everyone during the
session
 Not by phone rings
 Not by local talks
 By more information and
questions

Let’s Know Each Other
@akshaymathu
3
 Do you code?
 OS?
 Node?
 JavaScript, JSON?
 Web Development?
 CoffeeScript
 Other Programing Language?
 Why are you attending?

Akshay Mathur
@akshaymathu
4
 Founding Team Member of
 ShopSocially (Enabling “social” for retailers)
 AirTight Neworks (Global leader of WIPS)
 15+ years in IT industry
 Currently Principal Architect at ShopSocially
 Mostly worked with Startups
 From Conceptualization to Stabilization
 At different functions i.e. development, testing, release
 With multiple technologies

JavaScript
@akshaymathu
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 Born in 1995 at Netscape
 Not at all related to Java
 Syntax influenced by C
 Interpreted ECMA scripting language
 Dynamically typed
 Object Oriented as well as Functional
 Prototype based

Typical Usage
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 Web programing
 Client side
 Web pages
 Browser plugins
 Server side
 SSJS (not in use)
 NodeJS
 PDF documents
 Desktop Widgets
 MongoDB

NodeJS
 JavaScript Runtime at command line
 Allows us to write JS programs outside browser
 Built on V8 JS engine
 V8 is open source JS engine developed by Google
 It also powers Google Chrome
 Written in C++
 Compiles JS code to native before execution
@akshaymathu
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Good for
 For IO heavy apps
 Web socket (chat) server
 Real time collaborative editor
 Fast file uploads
 Ad server
 Any other real time data app (e.g. streaming server)
 Crawler
 Asynchronous chaining of tasks
 NOT good for CPU heavy apps
 Weather prediction
 May not be good for big projects
@akshaymathu
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NodeJS is NOT
 A web framework
 Provides tools to create a web server
 Multi threaded
 Is Single threaded, event driven, asynchronous, non-blocking
 For beginners
 Needs programing at very low level
 You start with writing a web server
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Possible Issues
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 Can not utilize multicore processer because of single
thread
 Managing multiple processes from outside may be even bigger
problem
 Any CPU intensive task delays all the requests
 Requires constant attention (non-traditional thinking)
for not having blocking code
 In case of big products the code/logic gets distributed in
multiple callback functions
 If data needs to collected from different places and
correlated, synchronizing all callbacks becomes tough
 Garbage collection may also be an issue

S I N G L E T H R E A D E D
E V E N T D R I V E N
A S Y N C H R O N O U S
N O N - B L O C K I N G
What Jargons Mean

Single Threaded
 New Node process does not start for every request
 Only one code executes at a time
 Everything else remains in the queue
 No worry about different portions of code accessing the same
data structures at the same time
 Delay at one place delays everything after that
 Can not take advantage of multi-core CPU
 Then how NodeJS is fast?
@akshaymathu
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Non-blocking
 There is only one process that is executing
 If the process waits for something to complete, everything else
gets delayed
 So the code has to be structured in a way that wait
for IO happens outside the main execution
@akshaymathu
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Blocking Vs. Non-blocking
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var a = db.query('SELECT * from huge_table’);
console.log('result a:’, a);
console.log(‘Doing something else’);
 Blocking I/O
 Non-Blocking I/O
db.query('SELECT * from huge_table’, function(res) {
console.log('result a:’, res);
});
console.log(‘Doing something else’);

Asynchronous
 Not all code is executed in the same order it is
written
 Node (and you) divide the code into small pieces and
fire them in parallel
 Typically the code announces its state of execution
(or completion) via events
@akshaymathu
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Event Driven
 A code block gets into the queue of execution when
an event happens
 It gets executed on its turn
 Anyone can raise (or listen to) an event
 System
 Some library (module)
 Your code
 You have the choice (and ways) to attach a code
block to an event
 Also known as event callbacks
@akshaymathu
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The Event Queue
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‘King and Servants’ Analogy… by Felix Geisendörfer
everything runs in parallel, except your code. To understand that,
imagine your code is the king, and node is his army of servants.
The day starts by one servant waking up the king and asking him if he
needs anything. The king gives the servant a list of tasks and goes back
to sleep a little longer. The servant now distributes those tasks among
his colleagues and they get to work.
Once a servant finishes a task, he lines up outside the kings quarter to
report. The king lets one servant in at a time, and listens to things he
reports. Sometimes the king will give the servant more tasks on the
way out.
Life is good, for the king's servants carry out all of his tasks in parallel,
but only report with one result at a time, so the king can focus.
@akshaymathu
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D O T H E Y O C C U R O N S E R V E R A S W E L L ?
Events

Events we know…
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 The event familiar to us are
 Click
 Focus
 Blur
 Hover
 These events are raised when user interacts with
DOM elements
 But the DOM is not being rendered in NodeJS
 Then what events are we talking about in NodeJS?

Node Events
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 The most common event that a web server uses is
request
 Raised when a web request (url) hits the server
 Other events:
 When chunk of multipart data (file) is received
 When all chunks of file are received
 When system finishes reading a file
 When data set is returned by a database query
 …
 You can define your own events using event emitter

Let’s Revise: JS Function Facts
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 Function is a block of a code that gets executed when
called
 JS function may not have a name
 Anonymous functions can be used
 JS function accept any data type as argument
 Function is a valid data type in JS
 A function can be assigned to a variable
 A function can be passed as an argument to another
function
 A function can be defined inside another function

Event Callback
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 Because the system is single threaded
 And we do not want to block it for I/O
 We use asynchronous functions for getting work done
 We depend on the events to tell when some work is
finished
 And we want some code to execute when the event occurs
 Asynchronous functions take a function as an
additional argument and call the function when the
event occurs
 This function is known as callback function

Callback in Action
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callback = function(res) {
};
db.query('SELECT * from huge_table’, callback);
Or
db.query('SELECT * from huge_table’, function(res) {
});

N O T H I N G B U T L I B R A R I E S
Node Modules

Available Modules
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 Modules are nothing but collections of useful
functions
 Otherwise we call them libraries
 Built-in modules come with NodeJS installation
 http, tcp, url, dns, buffer, udp etc.
 People create more modules, package and publish
them for others to use
 less, coffee, express etc.
 1000+ modules are available via npm
 You can write your own custom module for
organizing your code better

Creating Custom Module
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 Write some useful code in a file
 Some function(s) achieving a goal
 Decide what should be available outside for others to
use
 Public API of your module
 Make the APIs available outside using exports object
my_api = function(){…};
exports.myApi = my_api;

Using Modules
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 ‘require’ functions loads a module
 Whatever has been exported becomes available with
‘require’ and can be assigned to a variable for later
use
var http = require(‘http’);
var custom = require(‘./my_api’);
custom.myApi();

A S I M P L E W E B S E R V E R
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Let’s Program

Hello World
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 Just one line is needed to write to STDOUT
console.log(‘Hello World’)
 Running the file with Node just works
node hello_world.js

Minimalistic HTTP Server
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var http = require("http");
on_request = function(request, response) {
response.writeHead(200,
{"Content-Type":
"text/plain"});
response.write("Hello World");
response.end();
};
http.createServer(on_request).listen(8888);
Console.log(‘Server Started’);

Improving the Server
@akshaymathu
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 Running this server with Node starts the server
node server.js
 The server always returns same string
 Try any url, browser always says “Hello World”
 Actually the server should respond content based on
URL
 It should route the request to proper handler
 Handler should return proper content

Organizing Code
@akshaymathu
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 Rather than writing everything in single file, it is a
good idea to divide the code into logical modules
 Main startup file: index.js
 Web server: server.js
 URL Router: routes.js
 Request Handler: requestHandlers.js
 …
 More files and directories will come as the code
grows

Initial Server Module
@akshaymathu
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function start() {
function onRequest(request, response){
console.log("Request received.");
{"Content-Type": "text/plain"});
response.end();
}
http.createServer(onRequest).listen(8888);
console.log("Server has started.");
}
exports.start = start;

Initial Startup File
@akshaymathu
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 As server became a module and exposes a start
function, we need to
 load server module
 Call start function to start the server
var server = require("./server");
server.start();
 Running the main file with now starts the server
node index.js
 But it still returns ‘Hello world’ for all URLs

Let’s Revise: Parts of URL
https://sub.domain.com:8086/a/folder/file.html?key
=val&key=val2#some_place
 Protocol
 Sub-domain, Domain and TLD
 Port
 Path
 File
 Query string
 Fragment
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Server – Router Interaction
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 For routing the requests based on URL, the router
must know pathname of the requested URL
 The URL can be read from the ‘request’ object available in
server module
 So what should we pass to the router?
 Request object
 URL (Extract URL from request in server)
 Pathname (Extract and parse URL in server)
 ??
 If server need to call router, how router becomes
available to server?

Initial Router
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function route(pathname) {
console.log("About to route a
request for " + pathname);
}
exports.route = route;

Router Aware Server
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var url = require("url");
function start(route) {
function onRequest(request, response){
var pathname =
url.parse(request.url).pathname;
route(pathname);
response.writeHead(200, "Content-Type": "text/plain"});
response.end();
}
}

Making Router Available to Server
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 Router is made available as an argument (dependency) to
server’s ‘start’ function
 This is also known as dependency injection
var router = require("./router");
server.start(router.route);
 Router module is loaded in startup file and the route
function is passed at the time of starting server
 Server then calls route function of the router with the
pathname

Adding Request Handlers
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 The actual work of creating response for a request
will be done by Request Handlers
 We need to add these handlers to the server
 The requestHandlers module will consist of a
function corresponding to each expected URL
 At some place, we also need mapping between URL
and the request handler function

Initial Request Handlers
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function start() {
console.log("Request for 'start’.");
return "Hello Start";
}
function upload() {
console.log("Request for 'upload.");
return "Hello Upload";
}
exports.upload = upload;

Including Handlers
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var router = require("./router");
var requestHandlers =
require("./requestHandlers");
var handle = {}
handle["/"] = requestHandlers.start;
handle["/start"] = requestHandlers.start;
handle["/upload"] = requestHandlers.upload;
server.start(router.route, handle);

Change in Server
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function start(route, handle) {
function onRequest(request, response) {
var pathname =
content = route(handle, pathname);
response.writeHead(200, {"Content-Type":
"text/plain"});
response.write(content);
response.end();
}
}

Real Routing
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function route(handle, pathname) {
console.log(”Routing request for " + pathname);
if (typeof handle[pathname] === 'function') {
return handle[pathname]();
} else {
console.log("No request handler found
for " + pathname);
}
}

 Done 
@akshaymathu
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Did we do everything Correct?
 Nop 

What is wrong?
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var pathname =
content = route(handle, pathname);
response.writeHead(200, {"Content-Type": "text/plain"});
response.write(content);
response.end();
}
}
Blocking
Code

What are the problems?
@akshaymathu
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 The way ‘content’ is being collected and being
written to response, it forces to write blocking
synchronous code in handlers
 Because handler has to return content when called
 If you write asynchronous code in handler, the server
will always return response with no content
 Because handler will return nothing and when callback will
return with the content, there will be no one to collect the
output

The Right Way
@akshaymathu
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 The content should be written into response object
when the content becomes available
 So the response object should be made available to
request handlers
 Response object is available in server
 Server is not directly calling request handlers
 So first, Response object will be passed to router
 And then, Router will pass it to request handlers

Corrected Server
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var url = require("url");
var pathname =
console.log("Request for " + pathname);
route(handle, pathname, response);
}
}

Corrected Router
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function route(handle, pathname, response) {
console.log(”Routing request for " + pathname);
if (typeof handle[pathname] === 'function') {
handle[pathname](response);
} else {
console.log("No handler found for " +
pathname);
response.writeHead(404, {"Content-Type":
"text/plain"});
response.write("404 Not found");
response.end();
}
}

Corrected Handlers
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function start(response) {
db.query(”select * from huge_table”,
function (error, stdout, stderr) {
{"Content-Type": "text/plain"});
response.write(stdout);
response.end();
});
}

 Done 
@akshaymathu
57
Did we do everything Correct?
 Yep 

Summary
 Node will require extra work and different thought
process
 But it will pay off for it
 Choose Node carefully only for the type of app it is
best suited
 You may not need to write code at the lowest level we
discussed here
 You may want to choose framework
@akshaymathu
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MVC in Node
 Express (Controller)
 Mongoose (Model)
 Jade (View)
 More …
@akshaymathu
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Thanks
@akshaymathu
61
@akshaymathu

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