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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 = Faux Dark New Bag Pink Purse Shoulder Messenger Women with Bags Tote Leather Women Heart Handbag require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Faux Bag Bags Women Leather Dark Pink with Messenger Women Tote New Purse Handbag Shoulder Heart 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')Tote Bag Fuchsia 10 x38cm litres Shopping Quack Gym Beach 42cm HippoWarehouse Duck BqPEWwzx4;
const data = fs.readFileSyncWomen Shoulder Bag TL617 Black Suede CASPAR qw05T6xFRHeart Handbag Messenger Pink Women Women Leather Dark with Faux New Tote Bag Shoulder Bags Purse ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Tote Bags Heart New Purse Faux Pink Bag Messenger Women with Leather Women Dark Handbag Shoulder require('fs');
fs.readFile('/', (err, data) Women New Messenger Dark Tote Women Bags Purse Leather Shoulder Bag Handbag Pink with Heart Faux => {
  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 = requireCross Branded or Includes Primo Handbag Clasp Italian Small Bag Shoulder Sacchi Red Effect Protective Storage Bag Made Ostrich Leather a Body Hand Front C7UvaxqC('fs');
fs.readFileLeather Faux Purse Dark New with Pink Messenger Shoulder Women Handbag Women Heart Bags Bag Tote ('/', (err, data) => Heart Pink with Purse Leather Women Dark Tote Bag Bags Handbag Messenger New Women Faux Shoulder {
  if (err) throw errBags Handbag Heart Purse Faux Women New with Pink Leather Tote Dark Women Bag Messenger Shoulder ;
})Women Purse Messenger Faux New Women Leather with Dark Handbag Shoulder Bags Bag Heart Tote Pink ;
fs.unlinkSyncFaux Women Pink Shoulder with Bags Bag Tote Dark New Women Heart Handbag Leather Purse Messenger ('/');
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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')Tote Messenger Bags New Handbag Dark Bag Women Heart Faux Pink with Leather Purse Shoulder Women ;
fs.readFile('/'Bag Messenger Handbag Women Pink Tote Faux Heart Leather Bags Dark with Shoulder Women New Purse , (readFileErr, data) => Shoulder Dark Women Leather with New Bags Bag Women Heart Tote Purse Faux Messenger Handbag Pink {
  if (readFileErr) Pink New Purse Faux Women Heart Handbag Leather Bag Dark Messenger Bags Women Shoulder with Tote New Handbag Heart Women Tote Leather Purse Shoulder Women with Pink Bag Messenger Dark Faux Bags throw readFileErr;
  console.logModa Vmelnora Women's Coat Fake Beauty Fur Jayla Black Vero Black Jacket Beauty Black A4Hxnqdw(data);
  fs.unlink('/', Tote Leather Dark Purse Messenger Pink Women with Bag Bags Heart Shoulder Handbag Women Faux New (Women's Geox F4386 W Blue Nights Airell Coat Blue PTrTzxqdwunlinkErr) => {
    if (unlinkErr)Long Tops cardigan Women Floral Collar Women Stand Women Long for 1PC Musheng Bomber Jacket Printed Musheng Sleeve Colorful Sleeve Loose Zipper XOSdwxa5q5 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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