Blocking and Non-Blocking in NodeJS

Last Updated : 20 Feb, 2025

In NodeJS, blocking and non-blocking are two ways of writing code. Blocking code stops everything else until it's finished, while non-blocking code lets other things happen while it's waiting. This difference is key to understanding how NodeJS works and writing fast, efficient applications.

What is Blocking?

Blocking in NodeJS means that when your code runs a task, it stops and waits for that task to completely finish before moving on to the next thing. It's like reading a book one word at a time, never skipping ahead.

The program waits patiently for the current operation to finish.
No other code can run until that operation is done.

JavaScript

function myFunction() {
    console.log("Starting a task...");
    // Simulate a long-running task (blocking)
    let sum = 0;
    for (let i = 0; i < 1000000000; i++) { // A big loop!
        sum += i;
    }
    console.log("Task finished!");
    return sum;
}

console.log("Before the function call");
let result = myFunction(); 
console.log("After the function call");
console.log("Result:", result);

The for loop acts like a long-running task. The program waits for it to complete.
The "After the function call" message doesn't print until the loop is totally done.
This makes the code run step-by-step, one thing at a time.

Output

What is Non-Blocking?

Non-blocking means your program can keep doing other things while a task is running in the background. It doesn't have to stop and wait for that task to finish before moving on.

The program doesn't wait for the current task to complete.
Other code can run while the task is working in the background.

JavaScript

function myFunction() {
    console.log("Starting a task...");
    // Simulate a long-running task (non-blocking) - using setTimeout
    setTimeout(() => {
        let sum = 0;
        for (let i = 0; i < 1000000000; i++) { // A big loop!
            sum += i;
        }
        console.log("Task finished!");
        console.log("Result:", sum);
    }, 0); // The 0 delay makes it asynchronous
}

console.log("Before the function call");
myFunction(); // The program doesn't wait here
console.log("After the function call");

The setTimeout makes the big loop run in the background. The program doesn't stop.
"After the function call" prints before "Task finished!" because the loop runs later.
This lets the program do multiple things "at the same time."

Output

Difference Between Blocking and Non-Blocking

Here's a table which shows the key differences between blocking and non-blocking operations

Feature	Blocking Operations	Non-Blocking Operations
Execution	Waits until the operation completes.	Continues immediately; operation completes later.
Thread Behavior	Current thread waits (is blocked).	Current thread continues; operation handled separately.
Resource Usage	Easier to implement but may waste resources while waiting.	More complex but uses resources more efficiently.
Responsiveness	Can cause delays if operations are slow.	Keeps application responsive during long operations.
Use Cases	Simple tasks where waiting is acceptable.	Tasks requiring high responsiveness, like user interfaces.
Complexity	Easier to implement.	Requires managing callbacks or async mechanisms.
Examples	Reading a file entirely before proceeding.	Starting a file read and continuing without waiting.
Error Handling	Errors handled immediately after the operation.	Errors handled later, often in callbacks or error handlers.

Real-World Examples

Let's see how blocking and non-blocking I/O work with web servers. This will show why non-blocking is so much better, especially in NodeJS.

Blocking HTTP Server

A blocking server handles one request at a time. If it's busy reading a file for one user, other users have to wait.

const http = require('http');
const fs = require('fs');
http.createServer((req, res) => {
    const data = fs.readFileSync('largeFile.txt', 'utf8'); // Blocking!
    res.end(data);
}).listen(3000);

console.log("Server running at http://localhost:3000/");

fs.readFileSync is blocking. The server stops and waits until the entire largeFile.txt is read.
If another user tries to access the server while it's reading the file, they have to wait.

Non-Blocking HTTP Server

A non-blocking server can handle many requests at the same time. If it needs to read a file, it starts the read and then goes back to handling other requests. When the file is ready, the server is told about it and then it can send the data to the user.

const http = require('http');
const fs = require('fs');

http.createServer((req, res) => {
    fs.readFile('largeFile.txt', 'utf8', (err, data) => { // Non-blocking!
        if (err) {
            res.writeHead(500);
            res.end("Error reading file");
            return;
        }
        res.end(data);
    });
}).listen(3000);

console.log("Server running at http://localhost:3000/");

fs.readFile is non-blocking, allowing the server to handle other requests while the file is being read.
This approach is much more efficient for concurrent requests, as the server remains responsive and doesn't wait for I/O operations to complete.

How NodeJS Handles Non-Blocking I/O

NodeJS handles non-blocking I/O using

The Event Loop: A single thread (the event loop) manages asynchronous operations. When a task like file I/O or a network request is initiated, NodeJS delegates it and doesn't wait. The event loop continues processing other events.
Callbacks: When the I/O operation completes, NodeJS is notified and executes a callback function associated with that task. This callback handles the result of the operation.
Thread Pool: Certain tasks (like some file operations or computations) are handled by a thread pool. This prevents these tasks from blocking the event loop. The thread pool also uses callbacks to notify the event loop when a task is done.

How Concurrency and throughput is handled?

NodeJS is single-threaded, but it achieves concurrency through the event loop, which efficiently handles non-JavaScript operations, such as I/O tasks. Callback functions are sent to the event loop and executed only after other operations are completed.

Concurrency: The event loop manages multiple operations, ensuring non-blocking asynchronous tasks are handled efficiently.
Throughput: Non-blocking asynchronous operations are significantly faster than blocking synchronous ones, which helps in maximizing throughput.

Drawback of mixing Blocking and Non-Blocking Code

Combining blocking and non-blocking code can cause issues, such as operations executing before their dependencies are complete. For example, trying to print file content before it has been fully read.

Problem: Mixing can lead to unexpected behavior and incomplete operations.
Solution: Always use non-blocking code for related operations to ensure proper execution order.

Best Practices for Blocking and Non-Blocking in NodeJS

Prefer Non-Blocking Methods: Utilize asynchronous APIs to prevent the event loop from stalling, ensuring efficient handling of multiple operations.
Use Promises and Async/Await: Implement these for clearer, more manageable asynchronous code, reducing callback complexity.
Monitor Event Loop Lag: Regularly check for delays in the event loop to identify and address potential performance bottlenecks.

Difference between Asynchronous and Non-blocking

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