The document discusses the JavaScript event loop, which is how JavaScript handles concurrency. It explains that JavaScript is single-threaded but uses an event loop model to simulate parallelism. Key points are: - JavaScript uses a single thread of execution but handles I/O asynchronously by placing callbacks into a queue to be executed later. - This allows I/O-heavy operations like networking to occur "in parallel" without blocking the main thread. - The event loop continuously runs through the call stack and queue, executing functions and callbacks. - While efficient for I/O, CPU-intensive tasks would block the single thread, so JavaScript is not ideal for those types of applications.

1. JavaScript Event Loop
Not yo mama’s multithreaded approach.
slidesha.re/ZPC2nD

2. Who is Thomas Hunter?
• Web developer for 8+ years
• Dow, Ford, Quicken, Startup, Barracuda
• Started development with PHP & MySQL
• Became a Node.js fanboy
• Writing a book on Backbone.js for Packt
• @tlhunter | me@thomashunter.name

3. What does MultiThreaded mean?
• Makes use of separate CPU Cores as “Threads”
• Uses a single process within the Operating System
• True concurrency
• Can have Race Conditions (simultaneous memory use)
• “Hard” to get it right
• Ran out of Ghz, hardware adds more cores

4. JavaScript is SingleThreaded
• Makes use of a single CPU core
• CPU intensive work is never “concurrent”
• Easier to pull off, as in less technical difficulties

5. Technical Implementation
• Stack:
• Functions to run and available variables
• More added as code is run
• Stuff guaranteed to run in order
• Heap:
• “Chaotic” listing of objects
• Queue:
• Gets added to stack when stack empty
• setTimeout and setInterval added here
Credit: Mozilla Developer Network
http://mzl.la/Y5Dh2x

6. Example Code-run
console.log("Adding code to the queue");
setTimeout(function() { // Added somewhere in Heap
console.log("Running next code from queue");
}, 0);
function a(x) { // Added somewhere in Heap
console.log("a() frame added to stack");
b(x);
console.log("a() frame removed from stack");
}
function b(y) { // Added somewhere in Heap
console.log("b() frame added to stack");
console.log("Value passed in is " + y);
console.log("b() frame removed from stack");
}
console.log("Starting work for this stack");
a(42);
console.log("Ending work for this stack");
Adding code to the queue
Starting work for this stacka() frame added to stackb()frame added to stackValue passed in is 42b() frameremoved from stacka() frame removed fromstackEnding work for this stackRunning next codefrom queue

7. Your App is Mostly Asleep
• Node.js:All I/O is non-blocking
• E.g. it gets thrown into the Queue
• Browser:Wait for a click to happen
• PHP:Wait for a MySQL query to run
• These show how slow I/O can be →
L1-Cache 3 cyclesL2-Cache
14 cyclesRAM 250
cyclesDisk 41,000,000
Network 240,000,000

8. Sequential vs Parallel
• Traditional web apps perform each I/O Sequentially
• With an Event Loop, they can be run in Parallel
• Since most time is wasted doing I/O, very inefficient

9. Non JS Event Loops
• Event Loops can be implemented in other languages
• However, JavaScript was built this way, feels “natural”
• Examples:
• Ruby: EventMachine
• Python:Twisted & Tornado
• PHP: ReactPHP

10. !JS Event Loop Examples
require 'eventmachine'
module Echo
def post_init
puts "Someone Connected"
end
def receive_data data
send_data "#{data}"
close_connection if data =~ /quit/if
end
end
EventMachine.run {
EventMachine.start_server "127.0.0.1", 8081, Echo
}
var net = require('net');
var server = net.createServer(function (socket) {
console.log('Someone Connected');
socket.pipe(socket);
});
server.listen(1337, '127.0.0.1');
EventMachine in Ruby Node.js

11. Event Loops are Awesome!
• No race conditions
• Typical web apps spend their time waiting on I/O
• No funky syntax; it just works
• Perform I/O operations “in parallel” easily
• Stateful web applications are easy compared to PHP
• Long run apps, don’t need web servers, shared data...

12. Event Loops aren’t Awesome!
• CPU intensive work will block your process
• You can offload work to different processes (Node.js)
• It isn’t making use of those 8 cores you’ve got
• You can use the Multi-node module though (Node.js)
• Memory leaks are possible, not so with PHP
• You can program better and prevent it though ;-)

13. Web Workers
• You can use MultiThreaded JavaScript in your browser
• IE10, Firefox 3.5, Chrome 4, Safari 4, Opera 10.6
• var worker = new Worker('task.js');
• Use Message Passing to share data
• You share simple JSON data, not complex objects
• Prevents deadlocks/race conditions because of this

14. Conclusion
• Great for I/O bound applications (most web apps)
• Horrible for CPU bound applications (do it in C ;)
• Appears to be a single multi-threaded process
• “Fakes” concurrency
• thomashunter.name @tlhunter slidesha.re/ZPC2nD