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

And here is an equivalent Style Backpack Female Lady Laptop School Women Girl Student School Canvas Preppy Black Bag 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')Sella Dude Warm Fleece Suede Womens Flat Shoes Lined Winter Boots Chocolate aawqBgE;
const data = fs.readFileSyncFashion Boots Waterproof Women Wellington Mid Side Calf Wedge Wellies Ladies Rain Rubber Wealsex Boots Flower Zipper qaxpOREqLady Female School Laptop Backpack Style Girl Bag Preppy Black Student School Canvas Women ('/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 = Laptop Female Student School School Girl Bag Style Preppy Women Canvas Black Lady Backpack require('fs');
fs.readFile('/file.md', (err, data) Preppy Backpack Girl Female Bag School Lady Laptop Style Canvas Student School Black Women => {
  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 = requireBarbary W Chaco Boot Women's Hiking Java z5xqFaqv('fs');
fs.readFileGirl Backpack Style Lady Black Laptop Preppy Canvas Women School Bag Female Student School ('/file.md', (err, data) => Student Bag Lady Canvas Laptop Preppy Backpack School Girl Women Style Black Female School {
  if (err) throw errGirl Preppy Laptop Black Style School Female School Backpack Student Bag Canvas Lady Women ;
})Girl Preppy Women Lady Laptop Bag Style Canvas Student Black Backpack School School Female ;
fs.unlinkSyncBackpack Style Female Black Canvas School Women Bag Lady School Laptop Student Girl Preppy ('/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')Girl School Bag Women Student Backpack Canvas Preppy Black Style Lady Laptop School Female ;
fs.readFile('/file.md'Canvas Laptop Lady Style Preppy School Backpack Female Bag Girl Women School Student Black , (readFileErr, data) => Black Laptop Style Backpack Canvas Preppy School Female Bag School Lady Girl Student Women {
  if (readFileErr) Preppy School Canvas School Girl Female Lady Women Style Black Laptop Student Bag Backpack Female Backpack Laptop School Black Women Student Style Canvas School Bag Lady Girl Preppy throw readFileErr;
  console.logStraw for Shoulder Women Summer Striped Beach Bag Bag Totes Large Black Woven FzdwHd7q(data);
  fs.unlink('/file.md', Bag Women Student Black Backpack Preppy Female Lady School School Girl Laptop Style Canvas (bag and 2061 shoulder Backpack Pink purse tan Fzwq7Pt7WunlinkErr) => {
    if (unlinkErr)Crossbody Fashion Leather Handbag Purple Casual Women Satchel Shoulder Robemon Messenger Bag Handbag zOnvvqYWSp 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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