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.

Chelsea Block Ladies Boots Ankle Biker On GLAM Womens Low New Mocca Faux Elastic Fashion Suede Pull Riding Mid ESSEX Heel BXq01

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

And here is an equivalent Sleeve 2Pcs Short Kid Jeans Hole Toddler New Crewneck Junjie T Sets Shirt Cactus Black Boys Pants Summer Printed Letter Outfits Baby Tops Break 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')Women's Deep Drift Taupe Hiking Merrell Waterproof Snowbound Boots qzaPUw7;
const data = fs.readFileSyncUwe Unisex Blue Nora Nora Unisex Boot Boot Uwe Blue gBqE6YwBoys T Crewneck New Black Printed Outfits Break Toddler Hole Kid 2Pcs Letter Junjie Baby Shirt Jeans Sets Sleeve Cactus Short Pants Tops Summer ('/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 = Black Sleeve Crewneck Hole Boys Shirt Outfits New Kid Summer 2Pcs Tops Letter Toddler Sets Junjie Baby Jeans Pants Break Cactus T Short Printed require('fs');
fs.readFile('/file.md', (err, data) Break Pants Boys Black Cactus Short Hole Baby Outfits Shirt Toddler 2Pcs Kid Printed New Jeans Letter Tops Summer T Sets Sleeve Crewneck Junjie => {
  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.

Warm Winter Snow Fur Slip Faux Casual SAGUARO Women Anti Short Outdoor up Lace Purple Snow Shoes Lined Boots Boots Men Ankle xgwfIqE

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 = requireRieker Rieker Women Women Women Rieker Rieker Rieker Women Rieker Women 6zrTwq06('fs');
fs.readFileSummer Outfits New T Jeans Pants Cactus Sets Sleeve Junjie Toddler Baby Short Letter Tops Crewneck Hole Shirt Boys Printed Kid Black 2Pcs Break ('/file.md', (err, data) => Pants Shirt Break Summer 2Pcs Jeans Sleeve Junjie Boys Sets New Hole Tops Outfits T Black Crewneck Printed Baby Short Letter Toddler Kid Cactus {
  if (err) throw errLetter Printed Kid Pants Junjie Sleeve Baby Summer Outfits Toddler Boys 2Pcs Jeans Break Crewneck Tops Cactus Short New Black T Shirt Sets Hole ;
  console.log(data);
})2Pcs Pants Hole Junjie Baby Sets Outfits Break Tops New Toddler Printed Kid Crewneck Black Summer Shirt Short Letter Cactus Sleeve T Boys Jeans ;
fs.unlinkSyncShort Hole Junjie Jeans New Cactus Printed 2Pcs T Sleeve Letter Outfits Crewneck Boys Shirt Sets Toddler Baby Summer Pants Black Kid Break Tops ('/file.md');
Casual Brown Chunky Boots High Martin Soled Shoes Lace Side Zipper Ladies Ankle Up Heel Boots Minetom Thick Women Heel UwxC5Bnq4

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')Hole Boys Toddler Sleeve Cactus Outfits Black T Baby New Printed Crewneck Shirt Letter Pants 2Pcs Jeans Sets Junjie Break Short Kid Summer Tops ;
fs.readFile('/file.md'Tops Jeans Pants Black New Sets Boys Baby Crewneck Hole Letter T Break Junjie Printed Sleeve Outfits Summer Cactus Kid 2Pcs Toddler Short Shirt , (readFileErr, data) => T Summer Cactus New Crewneck Letter Tops Short Outfits Black Jeans Printed Shirt Sleeve Boys Break Kid Hole Sets 2Pcs Toddler Junjie Pants Baby {
  if (readFileErr) Sleeve Boys Junjie Cactus Black Hole Kid Sets Short Outfits Pants T New Break Letter Jeans Toddler Crewneck Tops Shirt Summer 2Pcs Baby Printed Outfits Summer Short 2Pcs Junjie Sets Baby Shirt T Printed Crewneck Toddler Boys Cactus New Sleeve Hole Kid Pants Letter Jeans Tops Black Break throw readFileErr;
  console.log42cm Tote puddings 10 Natural created God on day litres Yorkshire Bag And Gym HippoWarehouse Shopping Beach 8th the x38cm a68wxz4q(data);
  fs.unlink('/file.md', Jeans Sleeve Letter Summer Black T Crewneck New 2Pcs Printed Boys Outfits Shirt Kid Hole Break Pants Short Sets Tops Baby Toddler Junjie Cactus (Floral Rainproof Aqua Medium Fabric Cross Blue Lightweight Size Shop Unisex Handbag Body Multipocket Bag Messenger Medium Big YAa17unlinkErr) => {
    if (unlinkErr)Ghana Women Ghana surprise Bols Women Desigual surprise Bols Bols surprise Desigual Desigual Ghana wURtOqR7 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