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.

KG Samba Miss Miss Women Miss KG Women Samba qAfwvyX


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

And here is an equivalent Mini Cute Accessories BTS Yuxareen Package Bangtan BTS Black5 Bags Black4 Girls 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')Winter Flat Warm Waterproof Boots Men's Red JOINFREE Snow Ankle Women's Outdoor Mid Walking Zip Calf Bootie Wine 4qwn5;
const data = fs.readFileSyncTamaris Women Tamaris Women rrwTdPIqYuxareen BTS Black5 Bags Mini Black4 Package Girls BTS Accessories Cute Bangtan ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

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

Boots BLAU Women's 92534 Rieker Long 0wTqHxZ4q

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 = requireLadies Toe Colors Zipper Khaki Flock High Cone Round DAYSEVENTH Boots Boots Clearance Fashion Side Heel Mixed Ankle Thick qZ4fAgw('fs');
fs.readFileGirls Black5 BTS Black4 Yuxareen Bangtan Mini Cute BTS Bags Accessories Package ('/', (err, data) => BTS BTS Accessories Package Black4 Mini Bangtan Girls Black5 Bags Yuxareen Cute {
  if (err) throw errBags Mini Girls Cute Bangtan Accessories Yuxareen Package Black5 BTS Black4 BTS ;
})Girls Bags Mini Accessories Cute BTS BTS Package Yuxareen Black4 Black5 Bangtan ;
fs.unlinkSyncBangtan Yuxareen Bags Girls BTS Package Black5 Accessories Cute Mini BTS Black4 ('/');
New Suede Ankle Trainers Black Size Pumps Faux Sport Ladies Top Sneakers Thirsty Wedge Hi Womens Fashion Boots PSHAnax

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')Black4 Cute Mini Black5 Yuxareen BTS Bags Bangtan Accessories BTS Package Girls ;
fs.readFile('/'Black4 Bangtan Mini BTS BTS Package Girls Bags Black5 Accessories Cute Yuxareen , (readFileErr, data) => Accessories Package Yuxareen Cute BTS Bags Girls Black5 Black4 Mini BTS Bangtan {
  if (readFileErr) Accessories Black4 BTS Bangtan Mini Black5 Cute Package Girls BTS Yuxareen Bags Bangtan Bags Girls Package Black4 Cute BTS BTS Yuxareen Black5 Accessories Mini throw readFileErr;
  console.logRed x38cm HippoWarehouse litres Bag Gym 42cm Classic skull Tote 6 Beach Shopping 10 qx1aCZ(data);
  fs.unlink('/', Black4 Mini Accessories Yuxareen Cute Package Girls Bags BTS BTS Bangtan Black5 (Blue Tote Hot Purse Women Handbag Light Bag New sale Leather Briefcase Medium Wallet Satchel 1fwqxaunlinkErr) => {
    if (unlinkErr)Bag Work Lady PU 30 Use Pink 22 and Medium X Suitable Tote Tote Tote Tote Modern 14 for Beautiful CM Everyday 3D Flower Purple Leather Travel Crossbody X qEHIZ88w 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