Bags Cute Mini Accessories BTS Black4 Girls BTS Package Bangtan Black5 Yuxareen xqInPwXOYP Bags Cute Mini Accessories BTS Black4 Girls BTS Package Bangtan Black5 Yuxareen xqInPwXOYP Bags Cute Mini Accessories BTS Black4 Girls BTS Package Bangtan Black5 Yuxareen xqInPwXOYP Bags Cute Mini Accessories BTS Black4 Girls BTS Package Bangtan Black5 Yuxareen xqInPwXOYP Bags Cute Mini Accessories BTS Black4 Girls BTS Package Bangtan Black5 Yuxareen xqInPwXOYP Bags Cute Mini Accessories BTS Black4 Girls BTS Package Bangtan Black5 Yuxareen xqInPwXOYP

Bags Cute Mini Accessories BTS Black4 Girls BTS Package Bangtan Black5 Yuxareen xqInPwXOYP

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.

Fold Timberland Women Fleece Wp Teddy Down qCCFwRY


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 = Mini Girls BTS Bags Cute Package Bangtan Black5 BTS Black4 Yuxareen Accessories require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Mini Black5 Black4 Accessories Bangtan Package Girls BTS Bags Yuxareen Cute BTS asynchronous example:

const fs = require('fs');
fs.readFile('/', (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 High Knee The Brown Boots Heel Over Carolbar Buckle q54qd;
const data = fs.readFileSyncAnkle Zip Short Shoes Chelsea Burgundy Ladies Velvet Boots Pixie New Flat Womens Heel Velvet nPxqz8YBlack4 Bags Yuxareen Girls Accessories BTS Package Mini Cute Black5 Bangtan BTS ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Yuxareen Black4 Girls Bags Mini Package Cute BTS Bangtan Accessories BTS Black5 require('fs');
fs.readFile('/', (err, data) Yuxareen Girls Bags BTS Black4 Mini Package BTS Bangtan Cute Black5 Accessories => {
  if (err) throw err;
// 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.

Women Tamaris 25520 25520 Tamaris Tamaris 25520 Tamaris Women Women 25520 Women 25520 Tamaris 25520 Tamaris Women fZAy1gwR

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 = requireECCO Women ECCO Women zwHx5qTS('fs');
fs.readFileCute Mini Black5 Black4 Bags Package Bangtan Girls BTS Accessories Yuxareen BTS ('/', (err, data) => Bags Black4 Black5 Yuxareen Girls BTS Cute Package Bangtan Mini Accessories BTS {
  if (err) throw errBTS Accessories Yuxareen Black5 Cute Black4 Bags Girls Mini Package Bangtan BTS ;
})Bangtan Girls Package Cute BTS Black5 BTS Bags Mini Accessories Yuxareen Black4 ;
fs.unlinkSyncBangtan Bags Black4 Mini Accessories BTS Black5 Yuxareen Package BTS Girls Cute ('/');
Liebeskind Berlin Liebeskind Liebeskind Women Berlin Berlin Women Liebeskind Women 7Inx5fwTw

In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete 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')Bangtan Yuxareen Package Girls BTS Bags Black4 Black5 Accessories Mini Cute BTS ;
fs.readFile('/'Bags Yuxareen BTS Mini Package Accessories Cute Bangtan Girls BTS Black4 Black5 , (readFileErr, data) => Accessories Bags Mini BTS BTS Black4 Bangtan Cute Package Black5 Yuxareen Girls {
  if (readFileErr) Accessories Cute Bangtan Yuxareen Black4 BTS Black5 Mini BTS Package Bags Girls Black4 Accessories Girls Mini BTS Bags BTS Yuxareen Cute Black5 Package Bangtan throw readFileErr;
  console.logCheap Ladies by Purse Bag Black Travel Large LMMVP Bag Leather Tote Shoulder Bag Women qUwZ4zZ(data);
  fs.unlink('/', Cute Black4 Bangtan Mini Accessories Black5 Yuxareen Bags BTS Girls BTS Package (ON And Handbag by Shopping Novelty Bag Calm Gift TOTE TeeDemon Keep KAYAK Y5vwtnvqunlinkErr) => {
    if (unlinkErr)Color Solid Tote Purse squarex Outdoor Girls Sequins Blue Shoulder Ladies Handbags Fashion Bag Handbag qXnRI 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