Node.js is an asynchronous and event-driven JavaScript runtime built on Chrome's V8 JavaScript engine. It uses an event loop to handle asynchronous I/O in a non-blocking way without threads. The event loop listens for events like file system operations or network connections and queues microtasks or callbacks for execution when the events occur. A thread pool handles I/O-bound tasks like file system operations to improve performance. This single-threaded asynchronous model improves concurrency and scalability compared to traditional threaded models.

1. Node.js concurrency
Giacomo Fornari
University of Padua,
Sistemi concorrenti e distribuiti,
2016 - 2017

2. What is Node.js?
2

3. Two theses
1. waiting an I/O operation is a waste of time
2. thread-per-connection is memory-expensive
“Non-blocking”
L1-cache
L2-cache
RAM
Disk
Network
The cost of I/O
3 cycles
14 cycles
250 cycles
41 000 000 cycles
240 000 000 cycles
“Blocking”
3
Ryan Dahl: Introduction to Node.js, https://www.youtube.com/watch?v=M-sc73Y-zQA

4. Features
● Single-thread
● Highly concurrent
● No parallelism
● Event-driven
Programmer’s point of view
4
“everything runs in parallel except your code” 1
1. http://blog.mixu.net/2011/02/01/understanding-the-node-js-event-loop/

5. Architecture
Node Bindings
Node Standard Library
V8
Thread
Pool
Event
Loop
JavaScript
C
5

6. Event Loop
● Has a sequential queue of functions linked with events
● Runs on single-thread
○ it has only one stack
● The functions are defined by callbacks
○ short execution time
○ they can yield events and asynchronous operations
● Implicit usage
6

7. Event Loop diagram
Node.js
Application
Node.js
Event Loop
Thread 1 Thread 2 Thread n
…
(function,
callback)
Task 1
Task 2
Task 3
Return 1
Task 4
Callback 1
7

8. Thread Pool
● The strategy to achieve asynchronous I/O is not always a thread pool
○ Yes file-system operation
○ No network operations
● It has a fixed number of 4 thread
○ It is possible to set the number of thread before it is requested and created
● It has an internal queue of
○ function to execute
○ data for the function
○ function to call with the result
8

9. Thread delegation
9
Call function with callback Submit a work request
Return to JS codeContinue processing
Process the work request
Finish processing and
return result
Call the callback
Execute the callback of the
function
JavaScript C++
Main thread Worker thread

10. Event Loop and Thread Pool diagram
10https://strongloop.com/strongblog/node-js-is-faster-than-java/

11. return vs. callback
function add(a, b) {
return a + b;
}
function add(a, b, cb) {
cb(a + b);
}
11

12. sync op vs. async op
const dinner = ['pasta', 'meat', 'tiramisù'];
dinner.forEach((food) => console.log(food));
const fs = require('fs');
dinner.forEach((food) => {
fs.readdir('.', () => {
console.log(food);
});
});
console.log('prosecco');
12
pasta
meat
tiramisù
prosecco
meat
tiramisù
pasta

13. ● Concurrency reasoning easy
● No concurrent access of the state
● No locks
● No deadlock
● Still possible
○ starvation
○ race condition
Conclusions
13

14. 14
References
● http://blog.mixu.net/2011/02/01/understanding-the-node-js-event-loop/
● http://www.journaldev.com/7462/node-js-architecture-single-threaded-event-loop
● http://neilk.net/blog/2013/04/30/why-you-should-use-nodejs-for-CPU-bound-tasks/
● https://oparu.uni-ulm.de/xmlui/bitstream/handle/123456789/2450/vts_8082_11772.pdf, ch. 5.5
● https://strongloop.com/strongblog/node-js-is-faster-than-java/
● http://www.slideshare.net/RonPerlmuter/nodejs-meetup-at-palo-alto-networks-tel-aviv
● http://www.slideshare.net/ganeshiyer7/nodejs-event-driven-concurrency-for-web-applications
● http://stackoverflow.com/questions/10680601/nodejs-event-loop
● http://stackoverflow.com/questions/22644328/when-is-the-thread-pool-used
● https://www.youtube.com/watch?v=M-sc73Y-zQA