node.js patterns for discerning developers

Node.js PatternsFor the Discerning Developer

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

Me

@aaroncois

www.codehenge.net

github.com/cacois

Disclaimer: Though I am an employee of the Software Engineering Institute at Carnegie Mellon University, this work was not funded by the SEI and does not reflect the work or opinions of the SEI or its customers.

Let’s talk about

Node.js Basics

• JavaScript

• Asynchronous

• Non-blocking I/O

• Event-driven

So, JavaScript?

The Basics

Prototype-based Programming

• JavaScript has no classes• Instead, functions define objects

function Person() {}

var p = new Person();

Image: http://tech2.in.com/features/gaming/five-wacky-gaming-hardware-to-look-forward-to/315742

Prototype

Classless Programming

What do classes do for us?

• Define local scope / namespace• Allow private attributes / methods• Encapsulate code• Organize applications in an object-

oriented way

Prototype-based Programming

function Person(firstname, lastname){ this.firstname = firstname; this.lastname = lastname; } var p = new Person(“Philip”, “Fry”);

What else can do that?

Prototype Inheritance

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

Employee.prototype = { marital_status: 'single', salute: function() { return 'My name is ' + this.firstname; } }

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

Watch out! function Person(firstname, lastname){ this.firstname = firstname; this.lastname = lastname; } // Create new class Employee = Person;//Inherit from superclass


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

The ‘new’ is very important!

If you forget, your new object will have global scope internally

Another option function Person(firstname, lastname){ this.firstname = firstname; this.lastname = lastname; } Employee = Person;//Inherit from superclass


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

Works, but you can’t initialize attributes in constructor

Anti-Pattern: JavaScript Imports

• Spread code around files• Link libraries

• No way to maintain private local scope/state/namespace

• Leads to:– Name collisions– Unnecessary access

Pattern: Modules

• An elegant way of encapsulating and reusing code

• Adapted from YUI, a few years before Node.js

• Takes advantage of the anonymous closure features of JavaScript

Image: http://wallpapersus.com/

Modules in the Wild

var http = require('http'), io = require('socket.io'), _ = require('underscore');

If you’ve programmed in Node, this looks familiar

Anatomy of a module

var privateVal = 'I am Private!'; module.exports = { answer: 42, add: function(x, y) { return x + y; } }

mymodule.js

Usage

mod = require('./mymodule'); console.log('The answer: '+ mod.answer); var sum = mod.add(4,5); console.log('Sum: ' + sum);

Modules are used everywhere // User model var mongoose = require('mongoose') , Schema = mongoose.Schema; var userSchema = new Schema({ name: {type: String, required: true}, email: {type: String, required: true}, githubid: String, twitterid: String, dateCreated: {type: Date, default: Date.now} }); userSchema.methods.validPassword = function validPass(pass) { // validate password… } module.exports = mongoose.model('User', userSchema);

My config files? Modules.

var config = require('config.js'); console.log('Configured user is: ' + config.user);

module.exports = { user: 'maurice.moss' }

config.js

app.js

Asynchronous

Asynchronous Programming

• Node is entirely asynchronous• You have to think a bit differently• Failure to understand the event loop

and I/O model can lead to anti-patterns

Event Loop

Node.js Event Loop

Node app

Event Loop

Node.js Event Loop

Node apps pass async tasks to the event loop, along with a callback

(function, callback)

Node app

Event Loop

Node.js Event Loop

The event loop efficiently manages a thread pool and executes tasks efficiently…

Thread 1

Thread 2

Thread n

…Task 1

Task 2

Task 3

Task 4

Return 1

Callback1()

…and executes each callback as tasks complete

Node app

Async I/O

The following tasks should be done asynchronously, using the event loop:

• I/O operations• Heavy computation• Anything requiring blocking

Your Node app is single-threaded

Anti-pattern: Synchronous Code

for (var i = 0; i < 100000; i++){ // Do anything }

Your app only has one thread, so:

…will bring your app to a grinding halt


But why would you do that? Good question.

But in other languages (Python), you may do this:

for file in files: f = open(file, ‘r’) print f.readline()


The Node.js equivalent is:

Based on examples from: https://github.com/nodebits/distilled-patterns/

var fs = require('fs'); for (var i = 0; i < files.length; i++){ data = fs.readFileSync(files[i]); console.log(data); }

…and it will cause severe performance problems

Pattern: Async I/O

fs = require('fs'); fs.readFile('f1.txt','utf8',function(err,data){ if (err) { // handle error } console.log(data); });

Async I/O

fs = require('fs'); fs.readFile('f1.txt','utf8',function(err,data){ if (err) { // handle error } console.log(data); });

Anonymous, inline callback

Async I/O

fs = require('fs'); fs.readFile('f1.txt','utf8', function(err,data){ if (err) { // handle error } console.log(data); } );

Equivalentsyntax

Callback Hell

When working with callbacks, nesting can get quite out of hand…

Callback Hell

var db = require('somedatabaseprovider');//get recent postshttp.get('/recentposts', function(req, res) { // open database connection db.openConnection('host', creds,function(err, conn){ res.param['posts'].forEach(post) { conn.query('select * from users where id='+post['user'],function(err,users){ conn.close(); res.send(users[0]); }); } });});

Callback Hell


Get recent posts from web service API

Callback Hell


Open connection to DB

Callback Hell


Get user from DB for each post

Callback Hell


Return users

Callback Hell


Anti-Pattern: Callback Hellfs.readdir(source, function(err, files) { if (err) { console.log('Error finding files: ' + err) } else { files.forEach(function(filename, fileIndex) { console.log(filename) gm(source + filename).size(function(err, values) { if (err) { console.log('Error identifying file size: ' + err) } else { console.log(filename + ' : ' + values) aspect = (values.width / values.height) widths.forEach(function(width, widthIndex) { height = Math.round(width / aspect) console.log('resizing ' + filename + 'to ' + height + 'x' + height) this.resize(width, height).write(destination+'w’+width+'_’+filename, function(err){ if (err) console.log('Error writing file: ' + err) }) }.bind(this)) } }) }) }})

http://callbackhell.com/

Solutions

• Separate anonymous callback functions (cosmetic)

• Async.js• Promises• Generators

Pattern: Separate Callbacks

fs = require('fs'); callback = function(err,data){ if (err) { // handle error } console.log(data); }

fs.readFile('f1.txt','utf8',callback);

Can Turn This var db = require('somedatabaseprovider');

http.get('/recentposts', function(req, res){ db.openConnection('host', creds, function(err,

conn){ res.param['posts'].forEach(post) { conn.query('select * from users where id=' +

post['user'],function(err,results){ conn.close(); res.send(results[0]); }); } });});

Into This var db = require('somedatabaseprovider'); http.get('/recentposts', afterRecentPosts); function afterRecentPosts(req, res) {

db.openConnection('host', creds, function(err, conn) { afterDBConnected(res, conn); }); } function afterDBConnected(err, conn) { res.param['posts'].forEach(post) { conn.query('select * from users where id='+post['user'],afterQuery); } } function afterQuery(err, results) { conn.close(); res.send(results[0]); }

This is really a Control Flow issue

Pattern: Async.js

Async.js provides common patterns for async code control flow

https://github.com/caolan/async

Also provides some common functional programming paradigms



Serial/Parallel Functions

• Sometimes you have linear serial/parallel computations to run, without branching callback growth

Function 1

Function 2

Function 3

Function 4

Function 1

Function 2

Function 3

Function 4


async.parallel([ function(){ ... }, function(){ ... }], callback); async.series([ function(){ ... }, function(){ ... }]);


async.parallel([ function(){ ... }, function(){ ... }], callback); async.series([ function(){ ... }, function(){ ... }], callback);

Single Callback

Waterfall

Async.waterfall([ function(callback){ ... }, function(input,callback){ ... }, function(input,callback){ ... },], callback);

Map

var arr = ['file1','file2','file3']; async.map(arr, fs.stat, function(err, results){ // results is an array of stats for each file console.log('File stats: ' + JSON.stringify(results)); });

Filter

var arr = ['file1','file2','file3']; async.filter(arr, fs.exists, function(results){ // results is a list of the existing files console.log('Existing files: ' + results); });

With great power…

Carefree

var fs = require('fs'); for (var i = 0; i < 10000; i++) { fs.readFileSync(filename); }

With synchronous code, you can loop as much as you want:

The file is opened once each iteration.

This works, but is slow and defeats the point of Node.

Synchronous Doesn’t Scale

What if we want to scale to 10,000+ concurrent users?

File I/O becomes the bottleneck

Users get in a long line

Async to the Rescue

var fs = require('fs'); function onRead(err, file) { if (err) throw err; } for (var i = 0; i < 10000; i++) { fs.readFile(filename, onRead); }

What happens if I do this asyncronously?

Ruh Roh

The event loop is fast

This will open the file 10,000 times at once

This is unnecessary…and on most systems, you will run out of file descriptors!

Pattern: The Request Batch

• One solution is to batch requests• Piggyback on existing requests for

the same file• Each file then only has one open

request at a time, regardless of requesting clients

// Batching wrapper for fs.readFile() 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 complete function onRead(err, file) { delete requestBatches[filename]; for(var i = 0;i < callbacks.length; i++) { // execute callback, passing arguments along callbacks[i](err, file); } } }




Is this file already being read?



If not, start a new file read operation



When read finished, return to all requests

Usage

//Request the resource 10,000 times at once for (var i = 0; i < 10000; i++) { batchedReadFile(file, onComplete); }

function onComplete(err, file) {if (err) throw err;else console.log('File contents: ' + file);

}


Pattern: The Request Batch

This pattern is effective on many read-type operations, not just file reads

Example: also good for web service API calls

Shortcomings

Batching requests is great for high request spikes

Often, you are more likely to see steady requests for the same resource

This begs for a caching solution

Pattern: Request Cache

Let’s try a simple cache

Persist the result forever and check for new requests for same resource

// Caching wrapper around fs.readFile() var requestCache = {}; function cachingReadFile(filename, callback) { //Do we have resource in cache? if (filename in requestCache) { var value = requestCache[filename]; // Async behavior: delay result till next tick process.nextTick(function () { callback(null, value); }); return; } // If not, start a new request fs.readFile(filename, onRead); // Cache the result if there is no error function onRead(err, contents) { if (!err) requestCache[filename] = contents; callback(err, contents); } }


Usage

// Request the file 10,000 times in series // Note: for serial requests we need to iterate // with callbacks, rather than within a loop var its = 10000; cachingReadFile(file, next); function next(err, contents) { console.log('File contents: ' + contents); if (!(its--)) return; cachingReadFile(file, next); }


Almost There!

You’ll notice two issues with the Request Cache as presented:• Concurrent requests are an issue

again• Cache invalidation not handled

Let’s combine cache and batch strategies:

// Wrapper for both caching and batching of requestsvar requestBatches = {}, requestCache = {};function readFile(filename, callback) { if (filename in requestCache) { // Do we have resource in cache? var value = requestCache[filename]; // Delay result till next tick to act async process.nextTick(function () { callback(null, value); }); return; } if (filename in requestBatches) {// Else, does file have a batch? requestBatches[filename].push(callback); return; } // If neither, create new batch and request var callbacks = requestBatches[filename] = [callback]; fs.readFile(filename, onRead); // Cache the result and flush batch function onRead(err, file) { if (!err) requestCache[filename] = file; delete requestBatches[filename]; for (var i=0;i<callbacks.length;i++) { callbacks[i](err, file); } }}


scale-fs

I wrote a module for scalable File I/O

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

Usage:

var fs = require(’scale-fs'); for (var i = 0; i < 10000; i++) { fs.readFile(filename); }



Final Thoughts

Most anti-patterns in Node.js come from:

• Sketchy JavaScript heritage• Inexperience with Asynchronous

Thinking

Remember, let the Event Loop do the heavy lifting!

Thanks

Code samples from this talk at:

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



Disclaimer

Though I am an employee of the Software Engineering Institute at Carnegie Mellon University, this wok was not funded by the SEI and does not reflect the work or opinions of the SEI or its customers.

Let’s chat

@aaroncois

www.codehenge.net

github.com/cacois

Node.js Event Loop

The event loop efficiently manages a thread pool and executes tasks efficiently…

Thread 1

Thread 2

Thread n

…Task 1

Task 2

Task 3

Task 4

Return 1

Callback1()

…and executes each callback as tasks complete

Node.js app

Node apps pass async tasks to the event loop, along with a callback

(function, callback)

1 2

3

node.js patterns for discerning developers

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