Bucks County Tech Meetup: node.js introduction

Good Times
with Node.js
Matt Kemmerer
Diana Shkolnikov
MeetMe, Inc.

Topics


Motivation



Introduction to Javascript



node.js Event Loop



npm



Event Emitters



Operation order & Code Flow



async module

Code can be found here: https://github.com/dshkolnikov/nodejs-intro

Motivation

Millions

CPU Cycles
300
250

200
150
100

50
0
L1

route
request

L2

RAM

Disk

process
results

Network

format
response
I/O

your
code
query db or
web service

write to
log file

Solution
•

callbacks and events

•

non-blocking I/O
single thread

•

Google V8 Engine

•
what happens under the

•
•

what the developer sees…

Javascript

libuv

•

Javascript core libraries:

hood…

assert, http, etc.

•

node binding in C++

let’s see some code…
git checkout step1

The Magic

event queue

event loop

thread pool
filesystem
network
process
other

Asynchronous Example

var http = require('http');
function onRequest(request, response) {
response.writeHead(200);
response.end('Hey World');
}
http.createServer(onRequest).listen(8888);

back to code…
git checkout step2

npm
• https://www.npmjs.org/ : stands for node.js package manager
• FREE and open to ANYONE

• catalogs and serves all modules ever published
• Total Packages: 61,008 (as of today)
• Notable packages
o
o
o
o
o
o
o
o
•

async
express / restify
mocha
assert
should
socket.io
commander
Winston

npm provides a CLI tool for defining, installing, and maintaining your module’s
dependencies

package.json
• all node.js projects should have package.json
•

npm init

: will create a package.json file

• required fields
•

name and version : do NOT put "js" or "node" in the name

• general fields
•

main : module ID relative to the root of the package

•

scripts : supports a set of commands, for example start,
•
•

npm start
npm run my-script

•

dependencies

•

devDependencies

•

bundledDependencies

•

optionalDependencies

: used during npm install

stop, restart, test

let’s see one…
run from project root:
npm init

Event Emitters

var redis = require("redis").createClient(null, null, {});
redis.on("error", function (err) {
console.log("Redis says: " + err);
});

redis.on("ready", function () {
console.log("Redis ready.");
});
redis.on("reconnecting", function (arg) {
console.log("Redis reconnecting: " + JSON.stringify(arg));
});
redis.on("connect", function () {
console.log("Redis connected.");
});

Event Emitters

•

deriving from EventEmitter class allows the subclass to emit events
and register listeners for emitted events
EventEmitter.emit( event, [arg1], [arg2], [...] )
EventEmitter.on( event, handlerCallback )
EventEmitter.once( event, handlerCallback )
EventEmitter.removeListener( event, handlerCallback )
EventEmitter.removeAllListeners( event )

let’s whip one up…
git checkout eventEmitter

EventEmitter error handling
•

if an ‘error’ event is emitted and there is no registered listener for it, the
event parameter will be thrown as an exception

var EventEmitter = require('events').EventEmitter;
var emitter = new EventEmitter();
emitter.on('error', function(err) {
console.log('error: ', err);
});

error:

the sky is falling!

emitter.emit('error',
new Error('the sky is falling!'));

var EventEmitter = require('events').EventEmitter;
var emitter = new EventEmitter();
emitter.emit('error',
new Error('the sky is falling!'));

throw er; // Unhandled 'error' event
^
Error: the sky is falling!
...stack trace...

process.nextTick
• defers execution of argument function by placing it at the end of the event
queue
• runs before any other I/O events fire

function foo() {
console.error('foo');
}
process.nextTick(foo);
console.error('bar');

bar
foo

code time again…
git checkout nextTick
look at nextTick.js

Starving I/O

code time again…
git checkout nextTick
look at compute.js

Callback Hell
function myAsyncFunction(callback) {
asyncFunction1(function (err) {
if (err) {
callback(err);
return;
}
asyncFunction2(function (err, data) {
callback(null, data);
});
});

function myAsyncFunction(callback) {
async.series(
[
function (callback) { asyncFunction1(callback); },
function (callback) { asyncFunction2(callback); }
],
function (err, data) {
if (err) {
callback(err);
} else {
callback(null, data[1]);
}
});

}

}

Memory management
• node.js has automatic garbage collection
• by default, V8 will perform garbage collections
• on failed allocations
• after every N allocations
• on a notification of idle condition by Node.js
• pointers for memory-friendly code
• global variables are never garbage collected for the life of the
application, so use sparingly
• be conscious of variable scope
• unbind event listeners where they are no longer required

Bucks County Tech Meetup: node.js introduction

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