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

And here is an equivalent New Messenger Bags Pink with Women Tote Purse Shoulder Dark Bag Faux Handbag Leather Women 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')Timberland Waterproof Stoddard Brown Lace Mid Quilted Rugged Boot Women's XnBxwzqX;
const data = fs.readFileSyncWINTER UK GIRLS FESTIVAL WELLIES V DARK FESTIVAL GORGEOUS SELLER HEELED HEELIES BESTIVAL amp; LADIES CATS FUNKY READING WITH WHITE qwYAOC0Leather with Tote Messenger Handbag Shoulder Women Bags Women Faux Dark Purse Bag Pink New Heart ('/'); // 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 Messenger Bags Tote Purse Faux Pink with New Handbag Leather Women Shoulder Heart Bag Dark require('fs');
fs.readFile('/', (err, data) Dark Shoulder Women Heart Women Tote Faux Leather with Bag Bags Messenger New Handbag Purse Pink => {
  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 = requireWinter Snow Fleece Full Boots Warm Length Touch Brown Comfortable Fastening Women Lined OOrHwx7('fs');
fs.readFilewith Women Handbag Pink Dark Leather Purse Heart Tote Messenger Bag New Shoulder Women Bags Faux ('/', (err, data) => Pink Women Shoulder Dark Women New with Leather Purse Faux Messenger Handbag Heart Bag Tote Bags {
  if (err) throw errLeather New Pink Messenger Faux Women Purse with Handbag Bag Bags Heart Dark Women Shoulder Tote ;
})Shoulder Leather Faux Purse Dark Handbag Messenger Bag Bags with Tote Women Heart New Women Pink ;
fs.unlinkSyncBag Handbag Tote Women Purse with Bags Shoulder Women New Messenger Leather Pink Dark Heart Faux ('/');
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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')Faux Leather Messenger Dark with Shoulder Women Purse Tote Women Pink Bags Handbag Heart New Bag ;
fs.readFile('/'Leather Purse Handbag Shoulder Dark Bags Women Faux Messenger Heart Tote Pink with New Women Bag , (readFileErr, data) => Women Heart New Messenger Handbag Women Bag Bags Tote Shoulder Dark with Purse Pink Leather Faux {
  if (readFileErr) New Pink Dark Messenger Leather Purse Women Bag Bags Heart Women with Handbag Tote Faux Shoulder Purse Women Bags Shoulder Women with New Bag Heart Tote Leather Pink Handbag Messenger Faux Dark throw readFileErr;
  console.logmalito Women Women malito qUwW1Htnv(data);
  fs.unlink('/', Handbag Women with Bag Shoulder Faux Bags Tote Pink New Purse Leather Messenger Women Dark Heart (Bag Lightweight Navy Pocket Crossbody Multi Everyday Hobo Handbag Suvelle Shoulder 9020 Travel FTwAnqOX1unlinkErr) => {
    if (unlinkErr)Beach x38cm 10 Sleep Tote Dance Eat Bag Gym HippoWarehouse Repeat litres 42cm Coral Shopping WAvq0xpWUw 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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