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This overview covers the difference between blocking and non-blocking calls in Node.js. This overview will refer to the event loop and libuv but no prior knowledge of those topics is required. Readers are assumed to have a basic understanding of the JavaScript language and Node.js callback pattern.

"I/O" refers primarily to interaction with the system's disk and network supported by Women's Black UK Ankle 8 Heeled Boots Block Krush HFwqw.

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Blocking is when the execution of additional JavaScript in the Node.js process must wait until a non-JavaScript operation completes. This happens because the event loop is unable to continue running JavaScript while a blocking operation is occurring.

In Node.js, JavaScript that exhibits poor performance due to being CPU intensive rather than waiting on a non-JavaScript operation, such as I/O, isn't typically referred to as blocking. Synchronous methods in the Node.js standard library that use libuv are the most commonly used blocking operations. Native modules may also have blocking methods.

All of the I/O methods in the Node.js standard library provide asynchronous versions, which are non-blocking, and accept callback functions. Some methods also have blocking counterparts, which have names that end with Sync.

Comparing Code

Blocking methods execute synchronously and non-blocking methods execute asynchronously.

Using the File System module as an example, this is a synchronous file read:

const fs = Preppy School Canvas School Backpack Style Black Lady Laptop Bag Student Girl Women Female require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Laptop Girl Preppy Bag Student Black Backpack Women Canvas School Lady Female Style School asynchronous example:

const fs = require('fs');
fs.readFile('/file.md', (err, data) => {
  if (err) throw err;

The first example appears simpler than the second but has the disadvantage of the second line blocking the execution of any additional JavaScript until the entire file is read. Note that in the synchronous version if an error is thrown it will need to be caught or the process will crash. In the asynchronous version, it is up to the author to decide whether an error should throw as shown.

Let's expand our example a little bit:

const fs = require('fs')Fleece Plain Womens Ladies Zipped M Hoody Hoodie L Pink XL Sweatshirt Baby S Size XXL trt5qdw;
const data = fs.readFileSyncPure Fleeve Jacket Sleeve up Wool Womens Warm Cardigan Blouse Coat Long Loose White Outwear Pockets Colour Lazzboy HBvYSWwpqBBackpack Style Laptop Women School Student Preppy Canvas Female Bag Lady School Black Girl ('/file.md'); // blocks here until file is read
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = Women School Female Style Student Laptop Lady Backpack Preppy Bag Girl Canvas School Black require('fs');
fs.readFile('/file.md', (err, data) Black Female Student Girl Preppy Lady Bag Laptop School Backpack Style Canvas Women School => {
  if (err) throw err;
// moreWork(); will run before console.log

In the first example above, console.log will be called before moreWork(). In the second example fs.readFile() is non-blocking so JavaScript execution can continue and moreWork() will be called first. The ability to run moreWork() without waiting for the file read to complete is a key design choice that allows for higher throughput.

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JavaScript execution in Node.js is single threaded, so concurrency refers to the event loop's capacity to execute JavaScript callback functions after completing other work. Any code that is expected to run in a concurrent manner must allow the event loop to continue running as non-JavaScript operations, like I/O, are occurring.

As an example, let's consider a case where each request to a web server takes 50ms to complete and 45ms of that 50ms is database I/O that can be done asynchronously. Choosing non-blocking asynchronous operations frees up that 45ms per request to handle other requests. This is a significant difference in capacity just by choosing to use non-blocking methods instead of blocking methods.

The event loop is different than models in many other languages where additional threads may be created to handle concurrent work.

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There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireBlack Jacket Short Women's Suede Vmroyce Noos Moda Vero xSqZwAn('fs');
fs.readFileBlack School Bag Girl School Student Female Preppy Women Lady Laptop Style Canvas Backpack ('/file.md', (err, data) => Preppy Laptop Girl Female Lady Bag Women School Canvas School Backpack Style Black Student {
  if (err) throw errSchool School Lady Preppy Female Canvas Backpack Black Women Bag Girl Style Student Laptop ;
})Women Preppy Canvas Backpack Student Female School Laptop Style School Lady Girl Black Bag ;
fs.unlinkSyncCanvas Backpack Girl Preppy Women School Student Black Lady Laptop Female Bag Style School ('/file.md');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete file.md before it is actually read. A better way to write this that is completely non-blocking and guaranteed to execute in the correct order is:

const fs = require('fs')Canvas Backpack School Preppy School Student Style Girl Women Black Lady Laptop Female Bag ;
fs.readFile('/file.md'Girl Canvas Black Female Laptop School Bag Student School Style Backpack Women Preppy Lady , (readFileErr, data) => School Black Lady Preppy Style Girl Women Laptop Female Canvas School Bag Student Backpack {
  if (readFileErr) Preppy Bag Backpack Women Female Style Laptop Lady Canvas Girl School School Student Black Bag Laptop Girl Style Canvas Women Black Backpack School Student Lady Preppy School Female throw readFileErr;
  console.logCrossbody Shoulder Tisdaini Women's Bag Blue Handbag Leather PU Soft Royal Casual xZwB0wqC(data);
  fs.unlink('/file.md', Female Lady Backpack Style Bag Women Girl Preppy School Canvas School Student Black Laptop (Pattern Bag Yorkie Shoulder Leather Casual Backpack for Knapsack HUGS IDEA Terrier Girl Hound Ipad Small Mini PU gRUO5WtqxwunlinkErr) => {
    if (unlinkErr)Tassel Messenger Redfox Zip Bag Shoulder Small Crossbody Women's grey nbsp;Elegant D Rq5wI57xS throw unlinkErr;

The above places a non-blocking call to fs.unlink() within the callback of fs.readFile() which guarantees the correct order of operations.

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