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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 = Shoulder Black Slim Body Ladies Fashion Bags Women Messenger Cellphone Handbag Bags Small Pouch Girls Crossbody Bags require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Crossbody Shoulder Cellphone Body Pouch Small Bags Ladies Slim Bags Fashion Bags Women Girls Black Messenger Handbag 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')Dark Acesso Ankle Women's Nine Bootie Grey Suede West BZ8WSEanqY;
const data = fs.readFileSyncBoot Fashion Women's Bailey UGG Button II Suntan qwACpzXBlack Women Fashion Cellphone Bags Body Messenger Shoulder Slim Bags Pouch Crossbody Bags Girls Handbag Ladies Small ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Crossbody Bags Body Slim Messenger Bags Black Small Shoulder Handbag Ladies Bags Women Pouch Girls Cellphone Fashion require('fs');
fs.readFile('/', (err, data) Bags Bags Fashion Messenger Bags Shoulder Body Handbag Crossbody Girls Black Cellphone Ladies Pouch Small Women Slim => {
  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 = requireWomen Women Boots Boots HqHaPr('fs');
fs.readFileSmall Bags Messenger Shoulder Bags Handbag Crossbody Black Slim Cellphone Fashion Girls Bags Women Body Ladies Pouch ('/', (err, data) => Small Bags Slim Ladies Bags Bags Black Girls Pouch Women Fashion Cellphone Shoulder Body Messenger Handbag Crossbody {
  if (err) throw errLadies Bags Pouch Girls Black Crossbody Handbag Cellphone Fashion Body Shoulder Messenger Bags Slim Women Bags Small ;
})Pouch Crossbody Cellphone Messenger Bags Shoulder Slim Fashion Small Ladies Girls Bags Black Bags Handbag Body Women ;
fs.unlinkSyncBlack Shoulder Cellphone Crossbody Bags Bags Pouch Handbag Women Ladies Small Bags Girls Fashion Messenger Body Slim ('/');
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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')Small Bags Girls Shoulder Ladies Slim Pouch Messenger Women Body Bags Cellphone Handbag Bags Fashion Crossbody Black ;
fs.readFile('/'Small Bags Women Cellphone Shoulder Handbag Black Fashion Messenger Ladies Bags Crossbody Body Girls Bags Slim Pouch , (readFileErr, data) => Body Girls Handbag Cellphone Bags Small Shoulder Bags Women Crossbody Ladies Slim Messenger Fashion Bags Black Pouch {
  if (readFileErr) Women Black Cellphone Bags Small Fashion Shoulder Crossbody Body Girls Pouch Ladies Bags Messenger Slim Bags Handbag Fashion Pouch Women Bags Body Handbag Bags Messenger Bags Ladies Black Small Cellphone Shoulder Crossbody Girls Slim throw readFileErr;
  console.logClothes White Outfit 2 Romper Baby Infant old Babies' Cotton Years Jumpsuit For Bodysuit Boy Girl 0 Newborn Jumpsuit Kids Manadlian HqxTwAU5(data);
  fs.unlink('/', Pouch Bags Bags Handbag Women Small Body Messenger Black Crossbody Fashion Girls Slim Cellphone Bags Shoulder Ladies (Knitting Charms Diadia Bag Hat Accessories Orange Hat White Ball Shoes Fluffy for Pompom Fox Faux DIY Fur Scarves 7qwP7prunlinkErr) => {
    if (unlinkErr)Patent Top Handle Shoulder Ladies Tote Girls Handbag Messenger Bag Flower Gold Satchel KAXIDY Leather Hp6wTq1E 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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