Understanding the Node.js event loop and non-blocking I/O

Anton Ioffe - November 4th 2023 - 10 minutes read

Welcome to a deep exploration into the beating heart of Node.js as we unravel the intricacies of its event loop and non-blocking I/O mechanisms. Throughout this article, we’ll sail beyond theoretical concepts to uncover how these fundamentals enhance performance, influence concurrency, and power modern web applications in Node.js. Expect a practical excursion filled with high quality, real-world code examples for crystal-clear visualization, and guidance on how to leverage these concepts in crafting efficient and scalable Node.js web applications. Ferrying you from a basic understanding to a level of mastery, this piece holds invaluable insights for every seasoned developer striving to exploit the fullest potential of Node.js. Hold onto your curiosity and let's delve in!

Unpacking Node.js Non-Blocking I/O

The solvent essence of Node.js is encapsulated in its remarkable capability to execute non-blocking Input/Output (I/O) operations. Typically, conventional I/O processes, encompassing actions like file reads/writes or network requests, hold the potential to inhibit code execution until the operation is successfully completed. As a result, an intensive I/O operation may pose a significant delay in your application by leaving the software immobile. Node.js, however, turns the tables by introducing a revolutionary model - non-blocking I/O.

Let's consider a real-world example:

const fs = require('fs');

fs.readFile('./file.txt', 'utf-8', (err, data) => {
    if (err) throw err;

console.log('Reading file...');

This Node.js snippet employs readFile, a non-blocking I/O operation, to read a file. The console.log('Reading file...') statement gets executed straightway, despite the I/O operation not having completed yet, showcasing asynchronous or non-blocking I/O.

A salient characteristic of Node.js, non-blocking or asynchronous I/O, operates via the passing of a callback function as an argument to an I/O operation. As the I/O operation is being executed in the background, Node.js continues executing other JavaScript code. Upon the completion of the I/O operation, the callback function gets executed.

One common mistake while using callbacks involves incorrect error handling. Novice programmers often forget to handle errors which could potentially crash the application. The correct approach involves checking the error object in the callback function:

fs.readFile('./non-existent-file.txt', 'utf-8', (err, data) => {
    if (err) {
        console.error('Could not read the file', err);

In this example, if the file doesn't exist or is inaccessible, the error is properly handled which prevents application crash.

Node.js's non-blocking I/O model drastically reshapes your app's performance dynamics. There are tangible benefits, notably scalability. Thanks to Node.js's effective handling of concurrent I/O operations, multiple requests can be served efficiently, positioning it as an enviable selection for building high-performance applications. Moreover, responsiveness is significantly enhanced due to the event-driven framework of Node.js which facilitates swift action on incoming events.

An important additional attribute of Node.js is resource efficiency. Non-blocking I/O operations enable Node.js applications to optimally utilize system resources, lower the aggregate memory footprint and boost throughput. This finest utilization of resources, coupled with the event-driven, non-blocking I/O model, predicates the burgeoning popularity of Node.js for modern web development. Thus, while conceptualizing your subsequent major application, consider the extraordinary benefits rendered by Node.js's non-blocking I/O model for scalability, responsiveness, and efficiency.

Deep Dive into the Node.js Event Loop

The Node.js event loop is the heart of every Node.js application, responsible for managing the sequence of operations. The event loop is an implementation of the libuv library, a multi-platform C library that provides support for asynchronous I/O based on event loops. It is the process that allows Node.js to handle operations sequentially with the callback queue handling events associated with these calls. Let's see an example with the setTimeout() function, one of the mechanisms to schedule functions for future execution.

function printImmediately() { 
    console.log('Hello from the printImmediately function');

function printWithSetTimeout() { 
    setTimeout(() =>  console.log('Hello from the printWithSetTimeout function'), 0); 


// Expected console output: 
// "Hello from the printImmediately function"
// "Hello from the printWithSetTimeout function"

In the above code, despite printWithSetTimeout() being called first, printImmediately() gets executed first. This occurs because when setTimeout() is invoked, Node.js starts a timer and continues with other tasks. When the timer expires, the callback function is added to the event queue. However, the callbacks in Node's event queue will not be processed until all callbacks in the main thread's call stack are completed; this is true even when the timer is set to 0 milliseconds.

Node.js employs a C++ core to manage the event loop. During a single loop cycle, it executes all micro-tasks queued up for a macro-task. These micro-tasks include promise resolutions and callback functions for asynchronous operations.

let start = Date.now();

// Create a promise that resolves after 2 seconds
let promise = new Promise((resolve) => { 
    setTimeout(() => { 
    console.log("Promise resolved"); 
    resolve("Promise resolved");
    }, 2000); 

// Log the start of the event loop 
console.log("Start of event loop"); 

// Resolving Promise 
promise.then((msg) => console.log("After event loop : ", msg));

console.log("End of event loop", Date.now() - start);

// Expected console output:
// "Start of event loop"
// "End of event loop"
// "Promise resolved"
// "After event loop : Promise resolved"

In this example, the event loop initially triggers the start and end logs, and then waits for the Promise to resolve. Once resolved after 2 seconds, it processes the then() method attached to the resolved Promise and triggers the respective logs. Modern JavaScript and Node.js environments have a "microtask queue". Promise handlers such as .then, catch, and finally are executed right after the current code is finished and a promise is pending. This ensures that asynchronous tasks are completed as soon as possible, mirroring the nature of the event loop and JavaScript’s single-threaded model.

A deeper appreciation of the event loop and its role allows developers to manage tasks, timers, and operations effectively under Node.js environment. Having a grasp on these mechanisms enables developers to control the sequence of operations and callbacks, promoting smooth application operation.

Phases of the Node.js Event Loop

The Node.js event loop is a semi-infinite loop comprising six major phases – Timers, I/O Callbacks, Idle/Preparation, Poll, Check, and Close Callbacks. Each of these phases fulfills specific functions and has a first-in-first-out queue of callbacks, holding the tasks scheduled for execution.

Timers phase executes timer callbacks. Scheduled using setTimeout() or setInterval(), these timers specify a delay after which a particular piece of code should run. Here's a little insight into how it works:

setTimeout(() => {
    console.log('Hello after 2 seconds');
}, 2000);

This piece of code simply logs a message after waiting for 2 seconds. Notice that the actual execution might slightly deviate from the intended delay, due to the nature of the timer mechanism.

I/O Callbacks phase is responsible for system operations such as networking, disk I/O. The callback functions associated with these operations get executed during this phase, allowing Node.js to handle any I/O operations thrown its way.

Idle/Preparation phase is primarily a preparation phase for upcoming I/O callback operations.

Poll phase is where Node.js retrieves new I/O events from the system and the respective callbacks are queued for the next phase. If there are no callbacks in queue or events waiting, Node.js would check for any setImmediate() callbacks and execute them in the next phase.

Check phase is dedicated to callback functions of setImmediate() calls. This phase allows developers to queue functions to be executed after the I/O callbacks, and before the event loop goes back to manage Timers.

setImmediate(() => {
    console.log('This comes after I/O callbacks');

This code block queues a function to be executed in the Check phase, after all I/O callbacks.

Close Callbacks phase handles close callbacks. For example, if a socket event such as socket.on('close', callback) is invoked, the callback would be executed in this phase.

Understanding of these six phases provides you with a visualization of sequence in which synchronous and asynchronous tasks are managed, giving you better control over the code execution in Node.js applications. Are there any efficiencies to be gained by ordering your operations across different phases? Is it feasible or beneficial to separate concerns across different phases of the event loop?

Event Loop and Concurrency in Node.js

Node.js operates on a single thread, employing an event-driven, non-blocking I/O model. At the heart of this model, lies the event loop which provides the mechanism to handle asynchronous operations. In essence, the event loop waits for tasks, executes them, and then sleeps until it receives more tasks. Internal to Node.js, a library called Libuv provides the event loop functionality. By default, Libuv creates a thread pool with four threads to handle such operations. When an I/O operation in the script is encountered, Node.js delegates it to the system kernel if possible due to most modern kernels being multi-threaded. Thus, the Node.js main thread can continue executing other code without waiting, simulating a form of multi-threading.

The event loop manages all the callbacks that result from these I/O operations. When a callback is ready to run, it gets queued in the event loop to be executed. However, it's crucial to note that the event loop can only address the callback queue when the call stack, where all the currently executing and queued operations lie, is empty. It means that if you block the thread with synchronous code or tight loops, the event loop stalls, delaying the processing of the I/O callbacks. Therefore, any heavy computation that doesn't yield to the event loop can starve I/O operations, causing noticeable performance problems.

Consider a hypothetical situation where each request to a web server takes 50 milliseconds, out of which 45 milliseconds is spent on asynchronous database I/O. If the remaining time is filled with blocking tasks, incoming requests queue up, causing a significant delay in response time. However, if the non-I/O time is kept trivial or allocated to non-blocking I/O, the event loop will execute the callbacks immediately after the I/O tasks complete, thereby improving the overall throughput.

Understanding this model allows developers to unlock the full potential of Node.js. Code can be structured effectively to run tasks concurrently, achieving high throughput even under a heavy load. By ensuring that the main thread is not blocked by time-consuming operations, and I/O tasks are handled asynchronously by offloading to system kernel whenever possible, the application can remain highly responsive. Be mindful, however, that incorrect synchronization amongst these callbacks can lead to dilemmas like callback hell or race conditions. Are you making sure I/O tasks don't starve because of blocked event loops? How could you improve the structure of your code to optimally manage tasks in Node.js? Is there a chance that you might be unknowingly causing a callback hell in your application? How could promises or async/await help you manage your asynchronous code better?

Building Web Applications with an Understanding of the Event Loop and Non-blocking I/O

In the context of web application development, our understanding of the event loop and non-blocking I/O can play an instrumental role in determining application robustness and scalability. As developers, we should consider leveraging these concepts for better task prioritization, efficient use of system resources and a more responsive application.

One key consideration is how the event loop handles incoming client requests. In typical web applications, where numerous requests are being made simultaneously, a traditional sequential execution flow can exhaust system resources and cause delays. Node.js addresses this issue by utilizing an event-driven architecture in combination with a non-blocking I/O model. Instead of waiting for a task to be completed (such as a file read/write or database operation) before proceeding to the next task, Node.js registers these tasks with associated callbacks and continues with the next code line, thereby participants in concurrent request handling.

Here's an example; suppose we have a Node.js-powered e-commerce application with a getUserOrders() function that fetches data from a database. Conventional blocking I/O would have the function wait for the database to return data before executing the next line of code. In contrast, the event-driven non-blocking I/O model in Node.js registers the database operation as a task, assigns a callback function to handle the result, and proceeds without delay. When the result is ready, the callback function is called with the data.

function getUserOrders(userId, callback) {
    // I/O operation is non-blocking. Callback will be executed once data is ready.
    database.fetch('SELECT orders FROM users WHERE id = ' + userId, callback);

Finally, it's important to understand that while the event-driven, non-blocking I/O model at the crux of Node.js is powerful, it's also a double-edged sword. For compute-heavy operations (like complex computations, image processing etc.), the single-threaded Node.js application can be blocked, causing other, less compute-intensive tasks to be delayed. Hence, when processing intensive tasks, a farmer-worker pattern should be employed via child processes or with worker threads in Node.js. But, with a clear understanding of these dynamics, developers can design applications that optimize the power of Node.js's event loop, delivering highly scalable and efficient web applications.

const {Worker} = require('worker_threads');

function executeHeavyCalculation(userInput) {
    // Create a new worker
    const worker = new Worker('./compute.js', {workerData: userInput});

    // Method to execute when the worker has a result
    worker.on('message', (result) => {
        console.log('Result: ', result);

In the code snippet above, a compute-intensive task such as executeHeavyCalculation(userInput) is offloaded to a worker thread, thereby freeing up the event loop to process additional incoming requests. The event-driven, non-blocking I/O model in Node.js, when appropriately managed, can power highly efficient web applications.


Summary: The article explores the concepts of the Node.js event loop and non-blocking I/O, highlighting their significance in modern web development. It emphasizes the benefits of non-blocking I/O in terms of scalability, responsiveness, and resource efficiency. The article also delves into the phases of the event loop and emphasizes the importance of understanding it for better control over code execution. Furthermore, it discusses how the event loop and non-blocking I/O enable concurrency in Node.js and provides insights into optimizing code structure for improved performance. The article concludes by emphasizing the role of understanding these concepts in building robust and scalable web applications.

Challenging Task: Consider a scenario where you have a Node.js application that needs to handle multiple concurrent database queries. Design a solution that leverages the event loop and non-blocking I/O to ensure efficient and responsive query execution. Consider factors such as managing the callback functions, prioritizing tasks, and optimizing the use of system resources.

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