Why Node.js Is Fast on a Single Thread: Understanding Non-Blocking I/O

Overview

Node.js runs your code on a single thread, yet it can handle thousands of requests at once. This video untangles that apparent contradiction with a restaurant analogy: the restaurant is the server, the waiter is a thread, and each group of customers is a request.

One waiter can look after several tables because customers do not need the waiter the whole time.

Key Points

• A request splits into CPU work (time spent thinking) and I/O work (time spent waiting for files, external services or timers). Traditional servers such as Apache create a thread per request, which gets heavy at scale because of per-thread memory and context switches.
• With non-blocking I/O, the thread moves on to another request while one is waiting. The event loop hands off slow work and picks up whatever has finished, and an event demultiplexer built on epoll, kqueue or IOCP watches many I/O operations at once.
• libuv, a library written in C, does the actual I/O: the OS handles network I/O asynchronously, while file access and cryptography run in a thread pool of four threads by default.
• Node.js suits I/O-heavy work such as web servers, chat and APIs, not CPU-heavy tasks such as video conversion or machine learning. Heavy computation can be moved to worker threads.
• The video also clears up three common questions: await is not blocking, the real cost of blocking is context switching, and multithreading means concurrency but does not guarantee parallelism.

Chapters

• 0:00 Introduction
• 0:55 Chapter 1: Servers and threads
• 1:27 Chapter 2: A server is a restaurant
• 2:39 Chapter 3: CPU work and I/O work
• 3:54 Chapter 4: Processes and threads
• 4:37 Chapter 5: The call stack
• 5:14 Chapter 6: One heavy task
• 6:14 Chapter 7: Adding more threads
• 7:04 Chapter 8: Non-blocking I/O
• 8:55 Chapter 9: The event loop
• 9:26 Chapter 10: The demultiplexer
• 10:35 Chapter 11: libuv
• 12:17 Chapter 12: Four kinds of I/O
• 13:23 Chapter 13: Checking it in code
• 14:43 Chapter 14: What it is bad at
• 15:59 Chapter 15: Common questions
• 18:09 Summary