How NOT to write in Node.js

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How NOT to write in
Node.js
Piotr Pelczar
me@athlan.pl

PHPers, 6 Feb 2014

About me

Piotr Pelczar
me@athlan.pl

About me

getfokus.com

useselly.com

How software lives in hardware?
• Operating systems are process based
• Each process has assigned
processor, registers, me...

How software lives in hardware?
• Process paralelism using
threads (thread pools)
• Switching processor over
processes/thr...

How software lives in hardware?

1. Avoid context switching = wasting time

How software lives in hardware?
In trivial, sequential approach
• Each operation is executed
sequentially:

O(t) > O(t+1)
...

How software lives in hardware?

This is cool, software flow is predictible
But not in high throughput I/O

http://blog.mixu.net/2011/02/01/understanding-the-node-js-event-loop/

How software lives in hardware?
High throughput I/O doesn’t mean:
• Memory operations
• Fast single-thread computing

How software lives in hardware?
High throughput I/O means:
• HTTP requests
• Database connections
• Queue system dispatchi...

Single-threaded, event loop model
Problem:
Imagine a man, who has a task:
1. Walk around
2. When bucket is full of water,
...

Single-threaded, event loop model
What is nonblocking I/O?
Imagine a man, who has a task:
1. Walk around
2. When bucket is...

Single-threaded, event loop model
Problem:

Imagine a man, who has a task:
1. Walk around
2. When bucket is full of water,...

Single-threaded, event loop model
How it is realised in low-level operating system?
• select()
• /dev/pool descriptors
• k...

Single-threaded, event loop model
How it is realised in low-level operating system?
#include <sys/types.h>
#include <sys/s...

Single-threaded, event loop model

2. Avoid I/O blocking

Node.js architecture

http://nodejs.org/logos/

Node.js architecture
JavaScript
C/C++

Node std library

node bindings (socket, http, …)

Google V8

Thread Pool
(libeio)
...

Node.js architecture
• Single-threaded
no context switching

• Event loop
no waits

• Javascript (Google V8)
• ECMA-262 ...

Node.js architecture

http://www.tocadoelfo.com.br/2012/04/por-que-estou-aprendendo-nodejs.html

ECMA-262 support
var arr = []
for(var i = 0, j = arr.length; i < j; ++i) {
var row = arr[i] // ufff…
}
var arr = []
arr.fo...

„Parallelism”
is a myth

Node.js architecture
• Everything runs in parallel except your code
• When currently code is running,
(not waiting for I/O...

„Parallelism”
• Let’s compute Fibonacci
function fib(n) {
return (n < 2) ? 1 : (fib(n-2)+fib(n-1));
}

This will simply bl...

„Parallelism”
• How about process.nextTick() ?

„Parallelism”
• … but next iteration over function is delayed into next
event loop iteration
• That means each descriptor ...

„Parallelism”

process.nextTick()
to speed up computations is a myth
Node.js is not good solution to make
single-process c...

„Parallelism”
You can still fork another process

• threads_a_gogo

pool = require('threads_a_gogo').createPool(5)
pool.an...

Callback hell
is a myth
we can deal with it

Callback hell
• Language construction – callback function
• When operation is ready, call function passed as an
argument
s...

Callback hell
• When many operations are ready…

var amqp = require('amqp');
var connection = amqp.createConnection();
connection.on('ready', function() {
connection.excha...

Callback hell
Deal with callback hell in many ways:
• Define callbacks as local variables
• Use async module
• Use PROMISE...

Callback hell
Define callbacks as local variables, like this:
var whenSthElseDone = function() {
// done...
}
var whenSthD...

Callback hell
Use async module:
async.series([
function(callback) {

],
function() {

operation1(callback)
},

})

functio...

Callback hell
PROMISE design pattern:
var Promise = require('promise');

var promise = new Promise(function (resolve, reje...

Callback hell
PROMISE design pattern:
promise.then(function(res) {
// done...

})

Events (event bus)
Triggering event does’t block whole event loop is a myth

Events
var eventbus = require('events').EventEmitter

myFunction() {
operation1(function() {
eventbus.emit('myFunctionIsDo...

Events
var eventbus = require('events').EventEmitter

eventbus.on('myFunctionIsDone', function(data) {
console.log('cool! ...

Scaling EventEmitter
Triggering events in event bus via EventEmitter does not scale!

Scaling EventEmitter
• Triggering events in event bus via EventEmitter
does not scale!
• Events are visible locally in mai...

Scaling EventEmitter

We need to use external queue system
to touch distributed nodes
(3rd party for our application writt...

Scaling EventEmitter
RabbitMQ is cool because:
• based on AMQP protocol
(so integrates 3rd party software – an abstraction...

Scaling EventEmitter
Emit:
1. Trigger event in your application via EventEmitter
2. Catch it locally
3. Re-trigger via Rab...

Scaling EventEmitter
Receive:
1. Subscribe on RabbitMQ exchange
2. Trigger local event in application

Scaling EventEmitter

This is only to separate responsibilities
and good design of application

Not only Node.js
Node.js is not a religion! It is only an implementation of async programming model

Not only Node.js
Node.js is not a religion. Last time - hype.
It only utilizes event-loop model, known from a long time…
•...

Event loop, single-threaded
model is not solution for all
your problems…
Especially for long-running computings in single ...

Thank you.
Q&A?

Piotr Pelczar
me@athlan.pl

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