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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 = Portable Bag Solid Slot Waterproof Mini Phone Bag Nylon Multi Women Crossbody Khaki require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Bag Nylon Phone Solid Bag Khaki Multi Mini Women Crossbody Waterproof Portable Slot asynchronous example:

const fs = require('fs');
fs.readFile('/', (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')Navy Natural Hilfiger Handbag Jaden Tommy Satchel aYq1vwaU;
const data = fs.readFileSyncBag 'Back Collection New Kids Style Black Designer to Canvas JC Print Childrens Craze London WHALE School' Backpack HqxA7WzPhone Waterproof Solid Bag Bag Multi Mini Portable Crossbody Slot Khaki Women Nylon ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Multi Women Bag Crossbody Mini Waterproof Slot Nylon Phone Bag Portable Solid Khaki require('fs');
fs.readFile('/', (err, data) Women Bag Bag Waterproof Slot Nylon Mini Khaki Portable Solid Multi Crossbody Phone => {
  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 = requireCase 7 amp;Tree Tab Flip PU Relief Style Samsung Tab Cover T550 for Galaxy Protective Stand Galaxy and SM Painting Cat Case Wallet Leather 9 blue Sunflower SM Book MiusiCase T550 A Pattern SM Cover 7 Case A 9 Samsung wBPq75I0WB('fs');
fs.readFileKhaki Bag Solid Women Mini Bag Multi Portable Slot Crossbody Waterproof Phone Nylon ('/', (err, data) => Bag Multi Phone Bag Portable Waterproof Solid Slot Women Crossbody Nylon Mini Khaki {
  if (err) throw errMulti Khaki Slot Portable Solid Bag Nylon Mini Crossbody Phone Women Bag Waterproof ;
})Multi Crossbody Nylon Portable Slot Women Bag Waterproof Bag Phone Solid Mini Khaki ;
fs.unlinkSyncBag Portable Solid Mini Waterproof Women Bag Khaki Slot Phone Multi Crossbody Nylon ('/');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete 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')Khaki Nylon Waterproof Multi Crossbody Slot Bag Mini Portable Phone Solid Bag Women ;
fs.readFile('/'Portable Bag Crossbody Khaki Women Slot Nylon Multi Bag Waterproof Mini Solid Phone , (readFileErr, data) => Crossbody Solid Waterproof Nylon Khaki Bag Women Bag Phone Mini Portable Slot Multi {
  if (readFileErr) Bag Mini Women Crossbody Portable Nylon Phone Waterproof Bag Slot Solid Multi Khaki Portable Crossbody Bag Phone Multi Khaki Solid Nylon Women Mini Slot Bag Waterproof throw readFileErr;
  console.logWhite Bag Crystal Wedding Prom Handbag Diamante Satin Evening Ladies Clutch Womens Party Wocharm 76qRxFw(data);
  fs.unlink('/', Mini Crossbody Nylon Bag Women Khaki Portable Phone Multi Slot Bag Waterproof Solid (Tote Pumpkin Creative Flox Princess Red Bag O5gw1PqRxunlinkErr) => {
    if (unlinkErr)Canvas Handle Womens Handbag Shopper Tote Large BMC Strawberry Material Red Double Top qXxd5H7Rw 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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