Node.js Patterns for Discerning Developers
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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
Employee.prototype = { marital_status: 'single', salute: function() { return 'My name is ' + this.firstname; } }
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
Employee.prototype = { marital_status: 'single', salute: function() { return 'My name is ' + this.firstname; } }
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
Anti-pattern: Synchronous Code
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()
Anti-pattern: Synchronous Code
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
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]); }); } });});
Get recent posts from web service API
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]); }); } });});
Open connection to DB
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]); }); } });});
Get user from DB for each post
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]); }); } });});
Return users
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]); }); } });});
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
Serial/Parallel Functions
async.parallel([ function(){ ... }, function(){ ... }], callback); async.series([ function(){ ... }, function(){ ... }]);
Serial/Parallel Functions
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); } } }
Based on examples from: https://github.com/nodebits/distilled-patterns/
// 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); } } }
Based on examples from: https://github.com/nodebits/distilled-patterns/
Is this file already being read?
// 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); } } }
Based on examples from: https://github.com/nodebits/distilled-patterns/
If not, start a new file read operation
// 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); } } }
Based on examples from: https://github.com/nodebits/distilled-patterns/
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);
}
Based on examples from: https://github.com/nodebits/distilled-patterns/
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); } }
Based on examples from: https://github.com/nodebits/distilled-patterns/
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); }
Based on examples from: https://github.com/nodebits/distilled-patterns/
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); } }}
Based on examples from: https://github.com/nodebits/distilled-patterns/
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