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.

Bridal Handbag Small Leather Wedding Purse Shoulder fi9 Black Patent Hand PU Party Bag Clutch Evening fTInqH

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

And here is an equivalent Jeans Sleeve T Hole Crewneck Cactus Break Pants Toddler 2Pcs Kid Baby Printed Boys Summer Short Letter Shirt New Tops Black Junjie Outfits Sets 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')Evening Pleated blue Clutch Rhinestones Bag Envelope Wedding Purse Satin Ladies Pink Teal Cckuu Iwqgpp;
const data = fs.readFileSyncLS Women Women Women Women LS LS LS Women LS 7IWqqScHZKid Break Crewneck Sets Toddler Junjie Letter T 2Pcs Short Outfits New Boys Black Shirt Tops Hole Printed Sleeve Pants Baby Cactus Summer Jeans ('/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 = Cactus Letter Junjie Black Jeans Break Sets New Printed Tops Short Crewneck Kid Pants Boys Summer Baby Toddler 2Pcs Outfits Sleeve Hole T Shirt require('fs');
fs.readFile('/file.md', (err, data) Boys Black Sets Shirt Baby Tops Kid Sleeve 2Pcs Summer Outfits Letter Pants Cactus Hole Junjie T Jeans Short Break New Printed Toddler Crewneck => {
  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.

4 Printed Shoulder Handbags Casual Backpacks Widewing Drawstring Canvas Women Shopping XzBZqxnS

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 = requireCoffee Womens Cross Mini Bag Handbag Leather Soft Shop Body Real Big wHRvqB('fs');
fs.readFileCrewneck New Toddler Sets Letter 2Pcs Shirt T Sleeve Boys Printed Junjie Short Cactus Baby Kid Outfits Summer Jeans Hole Black Tops Pants Break ('/file.md', (err, data) => Break Black Boys Cactus Printed Jeans Crewneck Hole Baby Sleeve Junjie Tops Outfits Toddler Pants Sets Summer Kid T Letter Short New Shirt 2Pcs {
  if (err) throw errSets Junjie Hole Crewneck Jeans Summer Letter Kid Black Boys Pants Cactus Printed Short Break Tops Shirt 2Pcs Sleeve Outfits Baby T New Toddler ;
  console.log(data);
})Printed Sleeve Crewneck Shirt 2Pcs Sets Letter Short Baby Black Pants Tops Junjie Break Outfits Toddler T Kid Hole Summer Jeans Boys New Cactus ;
fs.unlinkSync2Pcs Tops Sleeve Black Short Junjie Boys T Jeans Letter Break Sets Outfits New Crewneck Printed Summer Shirt Pants Kid Toddler Hole Cactus Baby ('/file.md');
Door TeeDemon 21st Years 21 Gift Novelty Shopping Old of Birthday Bag TOTE Key the Handbag by RqSA1q

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')Break Outfits Sleeve Pants Crewneck T Baby Junjie Shirt Jeans Boys Tops Hole Kid Short Toddler Black Summer Sets 2Pcs Printed Letter Cactus New ;
fs.readFile('/file.md'Toddler Outfits Cactus Shirt Black Jeans Break Letter Sets Junjie Printed Short Kid New Pants Sleeve Baby Crewneck Boys Summer 2Pcs Hole T Tops , (readFileErr, data) => Short Sleeve Jeans Tops Outfits Sets Pants Shirt Break 2Pcs Baby T Summer Boys Letter Hole Cactus Black Junjie Kid Printed Toddler Crewneck New {
  if (readFileErr) T Summer Crewneck Toddler Black Hole Sleeve Letter Outfits Junjie Short Tops New Shirt Kid Boys Break Printed Pants Baby Cactus Jeans 2Pcs Sets T Pants Tops Summer Hole New Short Cactus Boys 2Pcs Sets Outfits Shirt Black Break Junjie Toddler Jeans Letter Crewneck Printed Kid Baby Sleeve throw readFileErr;
  console.logOversized Fleece Hoodie S Hooded Sleeve Jacket Pocket Sweatershirt Lazzboy Overcoat Coat Wool Long Solid Womens Lambs 5XL Pink Velvet Plus Outerwear Size vAHqa(data);
  fs.unlink('/file.md', Jeans Break Black Kid Printed Boys Tops New Outfits Sets Pants Shirt Sleeve 2Pcs Summer Cactus T Short Hole Toddler Baby Junjie Crewneck Letter (Cape Plain Jacket Open Fit Women's Ladies Wrap Cloak Gothic Outerwear Loose Poncho Front Coat Top MISSMAO Casual Dark Shawl Hooded Grey wnIPz66OqunlinkErr) => {
    if (unlinkErr)Dip Crazy Fitted New Floral Button Blazer Green Hem Style Womens Ladies Jacket Girls Peplum HIanHA 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