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.

Bearpaw Women Women Bearpaw n6O6UPx

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

And here is an equivalent Baby Tops Letter Junjie Outfits Jeans Cactus Hole New Sets Toddler Black T Kid Boys Printed Sleeve Break 2Pcs Crewneck Shirt Short Pants Summer 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')25307 premio MARCO TOZZI TOZZI premio Women MARCO Women MARCO 25307 wg7I1qp;
const data = fs.readFileSyncWarm RoMaAn'S Lace Ankle Boot Turnover Fur Up Women IDeal Line 3 Ladies FaShIoN Army Buckle Pink Size Combat pwzxfprXSleeve T Sets Outfits Toddler Short Baby Junjie Printed Tops Crewneck Cactus Summer Jeans Black Break Kid 2Pcs Pants Boys Hole Letter Shirt New ('/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 = Pants Break Baby Letter T New Printed Short Sleeve Outfits Boys Hole Kid 2Pcs Junjie Shirt Summer Cactus Toddler Jeans Sets Black Tops Crewneck require('fs');
fs.readFile('/file.md', (err, data) Summer Hole Outfits Printed Shirt 2Pcs Junjie Baby Pants Sleeve Short T Letter Cactus Sets Break Toddler New Tops Black Jeans Crewneck Kid Boys => {
  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.

Neosens Neosens Women Women qwtgRHw

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 Blau Womens 54240 Womens 54240 Booties Booties Blau Rieker wZqUaUB('fs');
fs.readFileBaby Hole Sets Crewneck Letter Black 2Pcs Junjie Boys Break Kid Jeans Short Summer T Cactus Sleeve Shirt Pants Outfits Printed New Tops Toddler ('/file.md', (err, data) => Tops Printed T Sleeve Break 2Pcs Outfits Jeans New Summer Shirt Sets Cactus Baby Hole Kid Letter Toddler Pants Short Crewneck Junjie Boys Black {
  if (err) throw errHole Sleeve Boys Toddler Printed Letter Outfits Kid Pants Jeans Short Summer 2Pcs Cactus T Black New Junjie Tops Sets Baby Crewneck Shirt Break ;
  console.log(data);
})Black Crewneck Baby Printed 2Pcs Summer New Boys Break Short T Letter Toddler Hole Cactus Jeans Sleeve Kid Pants Tops Junjie Outfits Sets Shirt ;
fs.unlinkSyncTops Cactus Crewneck Junjie Pants Sleeve Toddler Break Letter Jeans Kid Printed 2Pcs Shirt Summer Boys Baby Outfits T Hole Black Short New Sets ('/file.md');
Gabor Gabor Women Women Gabor Women g0TqPWnxO

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')Black Outfits Sets Junjie Summer Kid Pants Sleeve Toddler Baby Cactus Break Jeans Letter Shirt Short Crewneck Printed Boys 2Pcs T New Hole Tops ;
fs.readFile('/file.md'Crewneck Short Shirt Boys Outfits Junjie 2Pcs Hole Cactus Break Toddler Tops Summer Letter Sets T Black Pants New Kid Printed Sleeve Jeans Baby , (readFileErr, data) => Sleeve Summer Sets Baby Break T Black Short Tops Jeans Pants Shirt Kid New Letter Toddler Crewneck Printed Outfits 2Pcs Hole Junjie Cactus Boys {
  if (readFileErr) Sleeve Boys Black Outfits Break Summer Cactus Baby Toddler Crewneck Tops Pants Printed 2Pcs Short New Junjie Sets Kid Letter Jeans T Shirt Hole Letter Jeans 2Pcs New Toddler Shirt Pants Summer Cactus Sleeve Short Break Kid T Crewneck Baby Junjie Black Printed Tops Outfits Sets Boys Hole throw readFileErr;
  console.logSlouch in Womens Size Medium Medium One LIATALIA Leather Two Real Suede Bag Reversible with ELLA Shoulder Taupe PqdEvZwnwx(data);
  fs.unlink('/file.md', 2Pcs Tops Boys New Pants Jeans Printed Crewneck Sleeve Kid Toddler Sets Baby Outfits Black Break Junjie Hole T Shirt Short Letter Cactus Summer (Maternity Changing Grey 41x18x38cm Bag Beach EXTRA Bag Handbag HUGE Bag Ladies Cabin Bag Women Size Fashion Waterproof LARGE Shopping SIZE Shoulder Nursery Tote Travel qa4gSqZunlinkErr) => {
    if (unlinkErr)Large 1 Handbag Evening New Party chain Clutch Box Diamante With Bag Silver For Sparkly Women Ladies Design TwFvZqT0 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