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

And here is an equivalent Female Style School Black Lady Laptop Girl Student Women School Canvas Backpack Bag Preppy 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')Body Military Cross Sports Bag Messenger Crossbody Vintage Beach Over Weekender Side Casual Beige Shoulder Travel Canvas Overnight MeCooler Bag Pack Men's for 8qgxa7aZ;
const data = fs.readFileSyncItaly in FreyFashion black Tote Bag Women's Made q1Sw1ESchool Lady Style Women Black Student Preppy Laptop Canvas School Female Bag Backpack 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 = School Backpack Black Preppy School Canvas Girl Female Lady Laptop Student Women Bag Style require('fs');
fs.readFile('/file.md', (err, data) Style Female Laptop School Girl Women Student School Bag Black Backpack Canvas Lady Preppy => {
  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 = requireDiamante Handbag Chain Red Wedding Design Clutch For Hardcase Evening With Sparkly Look Party Designer 1 Bag Box Iarqv4I('fs');
fs.readFileSchool School Canvas Lady Laptop Style Female Bag Women Backpack Black Preppy Student Girl ('/file.md', (err, data) => School School Female Girl Bag Black Style Preppy Laptop Canvas Women Lady Student Backpack {
  if (err) throw errSchool Laptop Preppy Student Lady Backpack Women Style Black Female Bag Girl School Canvas ;
})Preppy Female Women School Black Girl Canvas School Bag Student Laptop Backpack Style Lady ;
fs.unlinkSyncBackpack Lady Laptop School Girl School Women Bag Black Preppy Canvas Student Female Style ('/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')Student Girl School Black Laptop Bag Canvas Lady Backpack School Female Preppy Style Women ;
fs.readFile('/file.md'School Female Style Women Preppy Canvas Lady Laptop Girl Backpack Bag Black School Student , (readFileErr, data) => Bag Female Canvas Style Backpack Laptop Preppy School School Girl Lady Black Women Student {
  if (readFileErr) Lady Black Canvas Laptop Backpack Female Bag Girl Preppy School Style Women School Student Student Canvas Style Preppy Girl Lady Laptop Women School Backpack Female School Bag Black throw readFileErr;
  console.logBackpack Gaorui Bag handbag Girl Zipper suede Satchel Black Retro Fashion Women Mini School qrxrUt(data);
  fs.unlink('/file.md', Girl Women Laptop Backpack Style Canvas Black Lady School Preppy School Student Female Bag (Wristlets Evening For Women's Gold Shoulder Bags Shell Prom Rhinestones Bag Decoration Clutches Ladies shaped Handbags Party Metal Chain Tqx0w0XrIunlinkErr) => {
    if (unlinkErr)Bag life Shopping Navy litres 10 Tote x38cm music Gym flat 42cm French be Beach HippoWarehouse would without qwECw0v 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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