libuv

TL;DR

libuv is a multi-platform C library that provides support for asynchronous I/O based on event loops. It is the core engine behind Node.js’s non-blocking architecture, handling everything from reading files to establishing network connections.

Mental Model

How It Works

When Node.js encounters an asynchronous operation, it hands it off to libuv.

1. Network Operations: libuv delegates these directly to the OS’s native async interfaces (like epoll on Linux or kqueue on macOS).
2. File Systems & Heavy Computation: Because OS interfaces for file systems are inconsistent, libuv uses a Worker Thread Pool to execute file operations, DNS lookups, and crypto hashing (like pbkdf2) without blocking the main event loop.

Example

const crypto = require('crypto');

const start = Date.now();

// This is offloaded to libuv's Thread Pool
crypto.pbkdf2('password', 'salt', 100000, 512, 'sha512', () => {
  console.log(`1: ${Date.now() - start}ms`);
});

// If we do this 5 times, the 5th will take longer 
// because the default thread pool only has 4 threads!

Common Interview Questions

How can you change the size of the libuv thread pool?

You can change the number of threads by setting the UV_THREADPOOL_SIZE environment variable before running your Node app (e.g., UV_THREADPOOL_SIZE=8 node app.js). The max size is 1024.

Does libuv use the thread pool for HTTP requests?

No. Network I/O is handled by the operating system’s native asynchronous polling mechanisms (epoll, kqueue), not the thread pool. The thread pool is mainly for fs, crypto, zlib, and dns.lookup.