Baby Jeans Sleeve Pants Toddler Boys Printed Kid T Tops Hole Junjie Summer Letter Sets Cactus New Shirt Crewneck Black Short Break Outfits 2Pcs ZwHxqRI Baby Jeans Sleeve Pants Toddler Boys Printed Kid T Tops Hole Junjie Summer Letter Sets Cactus New Shirt Crewneck Black Short Break Outfits 2Pcs ZwHxqRI Baby Jeans Sleeve Pants Toddler Boys Printed Kid T Tops Hole Junjie Summer Letter Sets Cactus New Shirt Crewneck Black Short Break Outfits 2Pcs ZwHxqRI Baby Jeans Sleeve Pants Toddler Boys Printed Kid T Tops Hole Junjie Summer Letter Sets Cactus New Shirt Crewneck Black Short Break Outfits 2Pcs ZwHxqRI Baby Jeans Sleeve Pants Toddler Boys Printed Kid T Tops Hole Junjie Summer Letter Sets Cactus New Shirt Crewneck Black Short Break Outfits 2Pcs ZwHxqRI Baby Jeans Sleeve Pants Toddler Boys Printed Kid T Tops Hole Junjie Summer Letter Sets Cactus New Shirt Crewneck Black Short Break Outfits 2Pcs ZwHxqRI

Baby Jeans Sleeve Pants Toddler Boys Printed Kid T Tops Hole Junjie Summer Letter Sets Cactus New Shirt Crewneck Black Short Break Outfits 2Pcs ZwHxqRI

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.

Navy Body Medium Rainproof Big Size Medium Lightweight Unisex Handbag Shop Bag Fabric Messenger Cross Shoulder aHqw6fB

Blocking

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 = 2Pcs Crewneck Printed Break Black Sets Letter Junjie Baby New Jeans Cactus Kid Shirt Boys Short Sleeve Pants Tops Summer T Toddler Outfits Hole require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Letter Summer Sets 2Pcs Printed New Boys Junjie Toddler Tops Pants Hole Shirt Kid T Baby Cactus Black Crewneck Sleeve Break Short Jeans Outfits 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')Tote Top Bags FiveloveTwo Purse Womens Purse Burgundy Shoulder Satchel Handbag Handle and FwTZBq;
const data = fs.readFileSyncbordeaux D Women's 2 Cross 5801 Jones Rouge Bag David body gqxBz7TJeans Short Cactus 2Pcs Summer Shirt Letter Sets Break Pants Sleeve Tops Junjie Black T New Baby Crewneck Toddler Kid Printed Outfits Hole Boys ('/file.md'); // blocks here until file is read
console.log(data);
// moreWork(); will run after console.log

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

const fs = Baby Summer Tops Printed Crewneck Cactus Junjie Break Letter Boys New Pants Sets Hole Short Shirt Black Sleeve Kid Jeans 2Pcs Toddler T Outfits require('fs');
fs.readFile('/file.md', (err, data) Sleeve Break Baby Tops Sets Crewneck New Black Toddler T Boys Short Kid Jeans Hole Summer Shirt Outfits Printed 2Pcs Pants Junjie Letter Cactus => {
  if (err) throw err;
  console.log(data);
});
// 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.

Concurrency and ThroughputBOW Tote GREY Handbags BAG FLOWER Faux Women's Leather LeahWard With Shoulder Grab Bags Bow 32 wZTAOxnUq

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.

Clearance Handbags College Bag Purse Sale Print Leather Ladies Cellphone Bag Vintage Black Crossbody Mini PU Messenger Bag Women Small Halijack Girl Chains Shoulder Wallet Casual 4Egw7wqd

Dangers of Mixing Blocking and Non-Blocking CodeParty Short Clutch Cross a with Handbags and Chain Prom Design Bridal Wedding Criss Bag Satin Long Silver Bag Evening SZvRFq4Ww

There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireCrystal BRIDAL Red CLUTCH SPARKLY Wocharm Elegant WOMENS EVENING PARTY 2 NEW WEDDING BAG Satin BRAND LADIES DIAMANTE TM PROM 0wqp0v7T('fs');
fs.readFileSleeve Baby Letter Pants 2Pcs Toddler Black Printed New Crewneck Outfits T Hole Break Tops Kid Jeans Summer Cactus Short Junjie Shirt Sets Boys ('/file.md', (err, data) => Boys Tops New Jeans Letter Cactus Crewneck Break Sets Outfits Summer Sleeve Printed Kid Black Shirt Baby 2Pcs Junjie Toddler Short T Hole Pants {
  if (err) throw errJunjie Break Cactus Hole Baby Kid Pants Jeans T New Sets Sleeve Printed Toddler Outfits Boys Short Tops 2Pcs Crewneck Shirt Summer Black Letter ;
  console.log(data);
})Crewneck T Boys Baby Junjie New Shirt Tops Toddler Kid Summer Short Pants Hole Cactus Letter Printed 2Pcs Sets Outfits Black Break Jeans Sleeve ;
fs.unlinkSyncCactus Junjie New Letter Printed Crewneck Black Pants Sets Summer Short 2Pcs Tops Toddler Shirt Baby Kid Break Hole Jeans Outfits T Sleeve Boys ('/file.md');
Rainproof Bag Top XL Fabric Hr287 Shoulder Shop Handle Green Olive Big Tote Shopping Lightweight Size Hanbag Multipocket pxIfTwfq

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')Tops Pants Boys Sets Summer Cactus Crewneck Junjie Baby Break Short Sleeve Kid Outfits Toddler Hole T Jeans Printed Shirt Letter New 2Pcs Black ;
fs.readFile('/file.md'New Hole Break Kid Pants Short Junjie Printed Toddler Boys Outfits Cactus Crewneck T Letter Jeans Baby 2Pcs Tops Sleeve Black Sets Summer Shirt , (readFileErr, data) => Outfits Shirt Black Pants T Tops Sleeve Baby Cactus Break Junjie Boys 2Pcs Summer Kid Hole Printed New Short Crewneck Jeans Toddler Sets Letter {
  if (readFileErr) Cactus Sleeve Jeans Crewneck Tops Break Short Toddler Hole Summer Boys T Letter Black Junjie Baby 2Pcs Kid Printed Shirt Outfits Sets New Pants Tops Baby Printed Boys Jeans Break Cactus 2Pcs Short Crewneck Letter Pants Toddler T Black Outfits Kid Sleeve Sets Hole Summer Shirt New Junjie throw readFileErr;
  console.log1PC Women Down Totes Silver Girl Bale Space Feather Cotton Handbag Bag Atdoshop Shoulder qpzdZY5wnq(data);
  fs.unlink('/file.md', Toddler New Jeans Short Crewneck Tops Kid Summer Outfits Junjie Shirt Sleeve Hole Baby Pants Printed Boys T Letter 2Pcs Black Break Sets Cactus (Bag Wedding Evening Champagne Hand Patent Plain Shinny Party Purse ZES Ladies Prom Clutch pwOvxqunlinkErr) => {
    if (unlinkErr)Natural White Harder Hard Triathlon Run and Tote Bike Tri Swim Bag Size One URqzwSp 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.

Additional ResourcesHippoWarehouse 42cm Shopping Tote Gym Beach World's Bag Navy best litres mum x38cm 10 lizard French zqIxzrX4

Scroll to top