How to stop writing spaghetti code

Ya ho o! In c

What is the event loop?

Multi-tasking, one thing at a time.

var http = require('http'); server = http.createServer(); server.on('request', function (req, res) { res.writeHead(200, {'Content-Type': 'text/plain'}); res.end('Hello Worldn'); }); server.listen(8124, "127.0.0.1"); console.log('Server running at http://127.0.0.1:8124/');

var http = require('http'); server = http.createServer(); server.on('request', function (req, res) { res.writeHead(200, {'Content-Type': 'text/plain'}); res.end('Hello Worldn'); }); server.listen(8124, "127.0.0.1"); console.log('Server running at http://127.0.0.1:8124/'); Step 1. Evaluate 'Main'

var http = require('http'); server = http.createServer(); server.on('request', function (req, res) { res.writeHead(200, {'Content-Type': 'text/plain'}); res.end('Hello Worldn'); }); server.listen(8124, "127.0.0.1"); console.log('Server running at http://127.0.0.1:8124/'); Step 1. variables: http -> http module server -> http server listeners: server.request -> function

var http = require('http'); server = http.createServer(); * server.on('request', function (req, res) { res.writeHead(200, {'Content-Type': 'text/plain'}); res.end('Hello Worldn'); }); server.listen(8124, "127.0.0.1"); console.log('Server running at http://127.0.0.1:8124/'); Step 2. Event Loop

var http = require('http'); server = http.createServer(); * server.on('request', function (req, res) { res.writeHead(200, {'Content-Type': 'text/plain'}); res.end('Hello Worldn'); }); server.listen(8124, "127.0.0.1"); console.log('Server running at http://127.0.0.1:8124/'); Step 2. Do we have active listeners? listeners: server.request -> function Yes! Wait for listeners.

var http = require('http'); server = http.createServer(); server.on('request', function (req, res) { res.writeHead(200, {'Content-Type': 'text/plain'}); res.end('Hello Worldn'); }); server.listen(8124, "127.0.0.1"); console.log('Server running at http://127.0.0.1:8124/'); Step 3. Event Calls

var http = require('http'); server = http.createServer(); server.on('request', function (req, res) { res.writeHead(200, {'Content-Type': 'text/plain'}); res.end('Hello Worldn'); }); server.listen(8124, "127.0.0.1"); console.log('Server running at http://127.0.0.1:8124/'); Step 3. 'request' is called. Since listeners: server.request -> function Call function

var http = require('http'); server = http.createServer(); server.on('request', function (req, res) { res.writeHead(200, {'Content-Type': 'text/plain'}); res.end('Hello Worldn'); }); server.listen(8124, "127.0.0.1"); console.log('Server running at http://127.0.0.1:8124/'); Step 3. Loop! (go to Step 2.)

Breaking the event loop

EE = require('events').EventEmitter; ee = new EE(); die = false; ee.on('die', function() { die = true; }); setTimeout(function() { ee.emit('die'); }, 100); while(!die) { } console.log('done');

Why non-blocking matters

var result = db.query("select * from T"); // use result

What are we waiting for?

"Blocking" is as bad as stopping

Event Loop vs. Threads

Apache vs NGINX concurrency × reqs/sec http://blog.webfaction.com/a-little-holiday-present

Apache vs NGINX concurrency × memory http://blog.webfaction.com/a-little-holiday-present

The less you block the more I/O Node can do

Programming Style

What kinds of applications do we build?

Applications • Command line applications • Web servers • Streaming servers • Peer-to-peer

Generalising • 'Main' scripts • Request/Response driven • Streaming

'Main' Scripts • Simple JavaScript • Doesn't use anything blocking • Doesn't use callbacks • Almost certainly a simple CLI script

Request/Response Driven • Discrete request/response cycle • Encapsulated requests/responses

Streaming • Arbitrary IN/OUT data • Stream data may not be encapsulated

var http = require('http'); http.createServer(function (req, res) { res.writeHead(200, {'Content-Type': 'text/plain'}); res.end('Hello Worldn'); }).listen(8124, "127.0.0.1"); console.log('Server running at http://127.0.0.1:8124/'

http.createServer(function (req, res) { })

//Server CommonJS Module Skeleton var sys = require('sys'), EE = require('events').EventEmitter; var Server = exports.Server = function() { //Server Setup }; sys.inherits(Server, EE); Server.prototype.Request = function() { //Request Setup }; sys.inherits(Sever.prototype.Request, EE); Server.prototype._createRequest = function(req, res) { //Request Factory return new this.Request(); }; exports.createServer = function() { //Server Factory return new Server(); };

var server = Server.createServer(); server.on('request', server._createReq);

Step 1. Create new Server

Step 2. Listen for requests

Step 3. On request create new Request

Step 4. Encapsulate request state in request object

Step 5. Request life-cycle

Closure Encapsulation DB File Query dbResponse Access Init ﬁleReady Server Request Init Request Respond Process Processed

Summary • Event Loop does 1 thing at a time • Until a loop loops events aren't ﬁred • Emitted events run immediately • Deﬁne a "server" structure • Use factory methods to encapsulate with named objects • Use events to maximize the event loop

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How to stop writing spaghetti code

by Tom Hughes-Croucher on Nov 10, 2010

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