nodejs design patterns 130602183400 phpapp02

8/15/2019 Nodejs Design Patterns 130602183400 Phpapp02

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Node.js PatternsFor the Discerning Developer

C. Aaron Cois, Ph.D. :: Carnegie Mellon University, SE


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Me

!aaroncois

""".codehenge.net

gith#$.co%&cacois

Disclaimer: Though I am an employee of the Software Engineering Institute at Carnegie Mellon University,this work was not fune !y the SEI an oes not re"ect the work or opinions of the SEI or its customers#


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'et(s tal) a$o#t


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Node.js *asics

• +avaScript

• Asynchrono#s

• Non$loc)ing &-

• Eventdriven


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So, +avaScript


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/he *asics


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Prototype$ased Progra%%ing

• +avaScript has no classes

• nstead, 0#nctions de1ne o$jects

function Person() {}

var p = new Person();

%age: http:&&tech2.in.co%&0eat#res&ga%ing&1ve"ac)yga%inghard"aretoloo)0or"ardto&345672

$rototype


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Classless Progra%%ing

8hat do classes do 0or #s

• De1ne local scope & na%espace

• Allo" private attri$#tes & %ethods

• Encaps#late code

•

-rgani9e applications in an o$jectoriented "ay


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Prototype$ased Progra%%ing

function Person(firstname, lastname){ this.firstnam e = firstname; this.lastnam e = lastname;

}

var p = new Person(“Philip”, “Fry”);

8hat else can do that


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Prototype nheritance

function Person(firstname, lastname){ this.firstnam e = firstname; this.lastnam e = lastname; } %% Create new class

Employee = Person; %%Inherit from superclass

Employee.prototype = { marital_status: 'single',

salute: function() { return 'M y nam e is ' + this.firstname; } }

var p = new Employee (“Philip”, “Fry”);


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8atch o#t

function Person(firstname, lastname){ this.firstnam e = firstname; this.lastnam e = lastname; } %% Create new class Employee = Person; %%Inherit from superclass


salute: function() { return 'M y nam e is ' + this.firstname; } }

var p = new Employee (“Philip”, “Fry”);

/he ;ne"( is very i%portant

0 yo# 0orget, yo#r ne" o$ject"ill have glo$al scope internally


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Another option

function Person(firstname, lastname){ this.firstnam e = firstname; this.lastnam e = lastname; } Employee = Person; %%Inherit from superclass


salute: function() { return 'M y nam e is ' + this.firstname;

} }

var p = Object.create(Employee); p.firstname = 'Philip'; p.lastname = 'Fry';

8or)s, $#t yo# can(t initiali9eattri$#tes in constr#ctor


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AntiPattern: +avaScript %ports

• Spread code aro#nd 1les

• 'in) li$raries

• No "ay to %aintain private localscope&state&na%espace

•

'eads to: – Na%e collisions

– Unnecessary access


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Pattern: Mod#les

• An elegant "ay o0 encaps#lating andre#sing code

• Adapted 0ro%


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Mod#les in the 8ild

var http = require('http'),

io = require('socketio'),

_ = require('!n"erscore');

0 yo#(ve progra%%ed in Node, this

loo)s 0a%iliar


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Anato%y o0 a %od#le

var privateVal = '# am Pri$ate%';

module.e&ports = { answer: ,

add: function(x, y) {

return x + y;

}

}

%y%od#le.js


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Usage

mod = require('m ym o"!le');

console.log('*he ans er: '+ mod.ans er);

var sum = mod.a""(,);

console.log('-!m : ' + sum);


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Mod#les are #sed every"here

%% User moel var mongoose = require('m ongoose')

, Schema = mongoose.-chem a;

var userSchema = new Schema({

nam e: {type: String, required: true},

email: {type: String, required: true}, githubid: String,

twitterid: String,

dateCreated: {type: Date, default: Date.no }

});

userSchema.m etho"s.$ali"Passor" = function validPass(pass) {

%% valiate passwor&

}

module.e&ports = mongoose.m o"el('.ser', userSchema);


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My con1g 1les Mod#les.

var config = require('config/s');

console.log('0onfig!re" !ser is: ' + config.user);

module.e&ports = {

user: 'm a!ricem oss'

}

con1g.js

app.js


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Asynchrono#s


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Asynchrono#s Progra%%ing

• Node is entirely asynchrono#s

•


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Event 'oop

Node.jsEvent

'oop

Node app


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Event 'oop

Node.jsEvent

'oop

Node apps pass asynctas)s to the eventloop, along "ith a

call$ac)

>0#nction, call$ac)?

Node app


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Event 'oop

Node.jsEvent

'oop

/he event loop e@ciently%anages a thread pool andeec#tes tas)s e@cientlyB

/hread4

/hread2

/hreadn

B /as) 4

/as) 2

/as) 3

/as) 7

et#rn 4

Call$ac)4>?

Band eec#tes eachcall$ac) as tas)s co%plete

Node app


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Async &-

/he 0ollo"ing tas)s sho#ld $e doneasynchrono#sly, #sing the event loop:

• &- operations

• eavy co%p#tation

•

Anything re#iring $loc)ing


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Antipattern: Synchrono#s Code

for (var i = 1; i 2 311111; i+ + ){

4o anything

}


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*#t "hy "o#ld yo# do that

ood #estion.

*#t in other lang#ages >Python?, yo# %ay do this:

for file in files: f = open(file, 5r6) print frea"line()


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/he Node.js e#ivalent is:

*ased on ea%ples 0ro%: https:&&gith#$.co%&node$its&distilled

var fs = require('fs');

for (var i = 1;i 2 files.length;i+ + ){ data = fs.rea"File-ync(files7i8); console.log(data);

}

Band it "ill ca#se severe per0or%ancepro$le%s


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Pattern: Async &-

fs = require('fs');

fs.rea"File('f3t&t','!tf9',function(err,data){

if (err) {

// handle error

}

console.log(data);

});


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Async &-

fs = require('fs');

fs.rea"File('f3t&t','!tf9',function(err,data){

if (err) {

// handle error

}

console.log(data);

});

Anony%o#s, inline call$ac)


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Async &-

fs = require('fs');

fs.rea"File('f3t&t','!tf9',

function(err,data){

if (err) {

// handle error

}

console.log(data);

}

);

E#ivalent

synta


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Call$ac) ell

8hen "or)ing "ith call$ac)s, nestingcan get #ite o#t o0 handB


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Call$ac) ell

var db = require('som e"ataasepro$i"er');

%%get recent posts

http.get('recentposts', function(req, res) {

%% open ata!ase connection

db.open0onnection('host', creds,function(err, conn){ res.param 7'posts'8.forach(post) {

conn.

conn.close();

res.sen"(users718);

});

}

});

});


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Call$ac) ell


%%get recent posts




conn.

conn.close();

res.sen"(users718);

});

}

});

});

et recent posts 0ro% "e$service AP


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Call$ac) ell


%%get recent posts




conn.

conn.close();

res.sen"(users718);

});

}

});

});

-pen connection to D*


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Call$ac) ell


%%get recent posts




conn.

conn.close();

res.sen"(users718);

});

}

});

});

et #ser 0ro% D* 0or eachpost


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Call$ac) ell


%%get recent posts




conn.

conn.close();

res.sen"(users718);

});

}

});

});

et#rn #sers


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Call$ac) ell


%%get recent posts




conn.

conn.close();

res.sen"(users718);

});

}

});

});


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AntiPattern: Call$ac) ellfs.rea""ir(source, function(err, files) {

if (err) {

console.log('rror fin"ing files: ' + err)

} else {

files.forach(function(filename, fileIndex) {

console.log(filename)

gm(source + filename).si>e(function(err, values) {

if (err) {

console.log('rror i"entifying file si>e: ' + err)

} else { console.log(filename + ' : ' + values)

aspect = (values. i"th values.height)

widths.forach(function(width, widthIndex) {

height = Math.ro!n"(width aspect)

console.log('resi>ing ' + filename + 'to ' + height + '&' + height)

this.resi>e(width, height).write(destination+ ' 6+ width+ '?6+ filename, function(err){

if (err) console.log('rror riting file: ' + err)

}) }.in"(this))

}

})

})

}

})

http:&&call$ac)hell.co%&


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Sol#tions

• Separate anony%o#s call$ac)0#nctions >cos%etic?

• Async.js

• Pro%ises

• enerators


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Pattern:

Separate Call$ac)s

fs = require('fs');

callback = function(err,data){ if (err) {

// handle error

}

console.log(data); }

fs.rea"File('f3t&t','!tf9',callack);


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Can /#rn /his


http.get('recentposts', function(req, res){

db.open0onnection('host',creds,function(err, conn){

res.param 7'posts'8.forach(post) { conn.

post7'!ser'8,function(err,results){

conn.close();

res.sen"(results718);

});

}

});

});


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nto /his


http.get('recentposts', afterRecentPosts);

function afterRecentPosts(req, res) {

db.open0onnection('host', creds, function(err, conn) {

afterDBConnected(res, conn); });

}

function afterDBConnected(err, conn) {

res.param 7'posts'8.forach(post) {

conn.

} }

function afterQuery(err, results) {

conn.close();

res.sen"(results718);

}


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/his is really a Control 'low iss#e


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Pattern: Async.js

Async.js provides co%%on patterns 0orasync code control Go"

https:&&gith#$.co%&caolan&async

Also provides so%e co%%on 0#nctionalprogra%%ing paradig%s

https://github.com/caolan/asynchttps://github.com/caolan/asynchttps://github.com/caolan/async


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Serial&Parallel F#nctions

• So%eti%es yo# have linearserial&parallel co%p#tations to r#n,"itho#t $ranching call$ac) gro"th

F#nction4

F#nction2

F#nction3

F#nction7

F#nction4

F#nction2

F#nction3

F#nction7


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async.parallel(7

function(){ ... },

function(){ ... }

8, callback);

async.series(7

function(){ ... }, function(){ ... }

8);


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async.parallel(7

function(){ ... },

function(){ ... }

8, callback);

async.series(7

function(){ ... }, function(){ ... }

8, callback);

Single Call$ac)


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8ater0all

Async. aterfall(7

function(callack){ ... },

function(inp!t,callack){ ... },

function(inp!t,callack){ ... },

8, callback);


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Map

var arr = 7'file3','file','file@'8;

async.m ap(arr, fs.stat, function(err, results){

%% results is an array of stats for each (le

console.log('File stats: ' +

JSON.stringify(results));

});


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Filter

var arr = 7'file3','file','file@'8;

async.filter(arr, fs.e&ists, function(results){

%% results is a list of the e)isting (les

console.log('&isting files: ' + results);

});


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8ith great po"erB


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Care0ree


for (var i = 1; i 2 31111; i+ + ) {

fs.rea"File-ync(filename);

}

8ith synchrono#s code, yo# can loop as %#chas yo# "ant:

/he 1le is opened once each iteration.

/his "or)s, $#t is slo" and de0eats the point o0

Node.


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Synchrono#s Doesn(t Scale

8hat i0 "e "ant to scale to 4H,HHHIconc#rrent #sers

File &-$eco%es the$ottlenec)

Users get in along line


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Async to the esc#e


function onRead(err, file) { if (err) throw err;

}

for (var i = 1; i 2 31111; i+ + ) {

fs.rea"File(filename, onRead);

}

8hat happens i0 do this asyncrono#sly


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#h oh

/he event loop is fast

/his "ill open the 1le 4H,HHH ti%es atonce

/his is #nnecessaryBand on %ostsyste%s, yo# "ill r#n o#t o0 1ledescriptors

P tt


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Pattern:

/he e#est *atch

• -ne sol#tion is to $atch re#ests

• Piggy$ac) on eisting re#ests 0orthe sa%e 1le

• Each 1le then only has one openre#est at a ti%e, regardless o0

re#esting clients

// Batching wrapper for fs.readFile()


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var requestBatches = {};

function batchedReadFile(filename, callback) {

// Is there already a batch for this file?

if (filename in requestBatches) {

// if so, push callback into batch

requestBatches[filename].push(callback);

return;

}

// If not, start a new request

var callbacks = requestBatches[filename] = [callback];

fs.readFile(filename, onRead);

// Flush out the batch on coplete

function onRead(err, file) {

delete requestBatches[filename];

for(var i = 0;i


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return;

}







for(var i = 0;i


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return;

}







for(var i = 0;i


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return;

}







for(var i = 0;i


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Usage

%%*e+uest the resource -,--- times at once

for (var i = 1; i 2 31111; i+ + ) {

batchedReadFile(file, onComplete);

}

function onComplete(err, file) {

if (err

) throw

err;else console.log('File contents: ' + file);

}


Pattern


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Pattern:

/he e#est *atch

/his pattern is e=ective on %any readtype operations, not j#st 1le reads

Ea%ple: also good 0or "e$ service APcalls


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Shortco%ings

*atching re#ests is great 0or highre#est spi)es

-0ten, yo# are %ore li)ely to seesteady re#ests 0or the sa%e reso#rce

/his $egs 0or a caching sol#tion

Pattern:


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Pattern:

e#est Cache

'et(s try a si%ple cache

Persist the res#lt 0orever and chec) 0orne" re#ests 0or sa%e reso#rce

%% Caching wrapper aroun fs#rea'ile./


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var requestCache = {};

function cachingReadFile(filename, callback) {

%%Do we have resource in cache0

if (filename in requestCache) {

var value = requestCache7filename8;

%% 1sync !ehavior: elay result till ne)t tick

process.ne&t*ick(function () { callback(null, value); });

return;

}

%% If not, start a new re+uest


%% Cache the result if there is no error

function onRead(err, contents) {

if (%err) requestCache7filename8 = contents; callback(err, contents);

}

}



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Usage

%% *e+uest the (le -,--- times in series

%% 2ote: for serial re+uests we nee to iterate

%% with call!acks, rather than within a loop

var its = 31111; cachingReadFile(file, next);

function next(err, contents) {

console.log('File contents: ' + contents); if (%(itsAA)) return;

cachingReadFile(file, next);

}



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Al%ost /here


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var requestBatches = {}, requestCache = {};

function readFile(filename, callback) {

if (filename in requestCache) { %% Do we have resource in cache0

var value = requestCache7filename8;

%% Delay result till ne)t tick to act async process.ne&t*ick(function () { callback(null, value); });

return;

}

if (filename in requestBatches) { %% Else, oes (le have a !atch0

requestBatches7filename8.p!sh(callback);

return; }

%% If neither, create new !atch an re+uest

var callbacks = requestBatches7filename8 = 7callback8;


%% Cache the result an "ush !atch

function onRead(err, file) { if (%err) requestCache7filename8 = file;

delete requestBatches7filename8;

for (var i= 1;i2 callbacks.length;i+ + ) { callbacks7i8(err, file); }

}

}



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scale0s

"rote a %od#le 0or scala$le File &-

https:&&""".np%js.org&pac)age&scale0s

Usage:

var fs = require(6scaleAfs');

for (var i = 1; i 2 31111; i+ + ) {

fs.rea"File(filename);

}

https://www.npmjs.org/package/scale-fshttps://www.npmjs.org/package/scale-fshttps://www.npmjs.org/package/scale-fshttps://www.npmjs.org/package/scale-fs


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Final /ho#ghts

Most antipatterns in Node.js co%e 0ro%:

•

S)etchy +avaScript heritage• neperience "ith Asynchrono#s /hin)ing

e%e%$er, let the Event 'oop do theheavy li0ting


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/han)s

Code sa%ples 0ro% this tal) at:

https:&&gith#$.co%&cacois&nodepatternsdiscerning

https://github.com/cacois/node-patterns-discerninghttps://github.com/cacois/node-patterns-discerninghttps://github.com/cacois/node-patterns-discerninghttps://github.com/cacois/node-patterns-discerning


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Disclai%er

/ho#gh a% an e%ployee o0 theSo0t"are Engineering nstit#te atCarnegie Mellon University, this "o)

"as not 0#nded $y the SE and doesnot reGect the "or) or opinions o0 theSE or its c#sto%ers.


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'et(s chat

!aaroncois

""".codehenge.net

gith#$.co%&cacois

/he event loop e@ciently

Node apps pass asynctas)s to the event

1 2


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Node.jsEvent

'oop

%anages a thread pool andeec#tes tas)s e@cientlyB

/hread4

/hread2

/hreadn

B /as) 4

/as) 2

/as) 3

/as) 7

et#rn 4

Call$ac)4>?

d t h

Node.jsapp

tas)s to the eventloop, along "ith acall$ac)

>0#nction, call$ac)?

1 2

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