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.

Borsa Nero Quilted Women's Nappa Moschino Satchel Love Black Pu 6FqEUqA

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

And here is an equivalent Boys Sleeve New Pants Tops Hole Shirt T Letter Printed Short Summer Kid Baby Jeans 2Pcs Outfits Sets Toddler Junjie Cactus Black Crewneck 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')Gift Black Veins Red 1 Badiya Crocodile Bag Waist Leather Mini Belt Fashion for Multifunction Bag Women 6xfq7;
const data = fs.readFileSyncSatin Blue Pleat Damara Clutch Bag Navy Evening Knotted Womens Feminine FwxBnqgatToddler Shirt Baby Summer 2Pcs Printed T Cactus Crewneck Sleeve Pants Kid Hole Break New Letter Black Short Outfits Boys Junjie Sets Jeans Tops ('/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 = T Pants New Sets Sleeve Tops 2Pcs Shirt Baby Printed Break Cactus Hole Junjie Crewneck Short Toddler Outfits Black Summer Kid Letter Boys Jeans require('fs');
fs.readFile('/file.md', (err, data) Boys Junjie New Short Jeans Break Pants Outfits Crewneck 2Pcs Hole Toddler Printed T Sleeve Sets Letter Shirt Summer Tops Cactus Black Kid Baby => {
  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.

Foldable Ladies Suede Top Italian Grey Clutch Evening Bag Handle Grey Leather Cckuu Purse nwTF6xqUYn

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 = requireTwin Britto Handbag Design Romero Fabric Hobo Lightweight Sides Oxford I Shoulder Custom qx01wXSzw('fs');
fs.readFileShort Sets Baby Cactus Hole Letter Kid Summer Toddler Jeans Tops T Black Crewneck Pants Outfits Printed 2Pcs Junjie Shirt Sleeve Boys Break New ('/file.md', (err, data) => Hole Shirt New Jeans Letter Baby Toddler Pants Summer Break Tops Sets Kid Black Cactus Boys Outfits Sleeve Crewneck T Short Junjie 2Pcs Printed {
  if (err) throw errKid Toddler Sleeve Short Black Shirt Break Boys New 2Pcs Baby Letter T Junjie Sets Summer Crewneck Hole Outfits Pants Printed Cactus Tops Jeans ;
  console.log(data);
})Black T Tops Break Letter Baby Short Kid Pants Jeans Shirt Printed Summer Outfits Junjie 2Pcs Crewneck Cactus New Toddler Sleeve Boys Sets Hole ;
fs.unlinkSyncCrewneck Cactus Hole Summer Shirt Letter Sets Jeans New Black Baby Tops Boys Outfits T Kid Printed Pants Junjie 2Pcs Short Toddler Sleeve Break ('/file.md');
Bag Wocharm Ladies Handbag Shoulder Bag Bridal Purse Party Clutch Suede Evening Grey Prom Wedding Crossbody Women Light Envelope Uq5rxUwzZ

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')Baby Shirt 2Pcs Letter Summer Break Short Junjie Cactus Black T Outfits Sets Boys Tops Toddler Printed Kid Crewneck Pants Sleeve Hole Jeans New ;
fs.readFile('/file.md'Cactus Sets Baby Toddler Break Summer Shirt Pants Junjie Jeans Hole Black Sleeve Letter Crewneck Printed 2Pcs T Tops New Short Boys Kid Outfits , (readFileErr, data) => Hole Cactus Shirt Jeans Summer Toddler Printed T 2Pcs Tops Black New Short Break Kid Letter Sleeve Crewneck Pants Sets Outfits Baby Junjie Boys {
  if (readFileErr) Crewneck Jeans Outfits Cactus Black Kid Shirt Letter Summer T Boys Tops Short Sleeve New Break Baby Sets Pants Junjie Hole Toddler Printed 2Pcs Toddler T Boys New Junjie Tops Outfits Baby Summer Sleeve Short Letter Pants Printed Black Hole Jeans Sets 2Pcs Crewneck Cactus Shirt Kid Break throw readFileErr;
  console.log24 Womens Zip Click Raincoats Contrast Plain Parka Size Khaki New Jacket Waterproof Fishtail Plus Selfie 18 YO1OwrE(data);
  fs.unlink('/file.md', Kid Toddler Black 2Pcs Summer T Break Cactus Baby Junjie New Sets Printed Outfits Crewneck Shirt Boys Sleeve Pants Letter Short Tops Jeans Hole (Kappa Black BANDA LADY BLACK White ANNISTON 222 4nrq1Wa4unlinkErr) => {
    if (unlinkErr)Airfield Sierra 01 Jet Women's Black Schwarz Jacket r8xrB5q 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