Flower Hollow Handbag Chain Evening White Women's Leather Bag Shoulder Pu Out Envelope Bag Tote Clutch Meliya EBw5qzw Flower Hollow Handbag Chain Evening White Women's Leather Bag Shoulder Pu Out Envelope Bag Tote Clutch Meliya EBw5qzw Flower Hollow Handbag Chain Evening White Women's Leather Bag Shoulder Pu Out Envelope Bag Tote Clutch Meliya EBw5qzw Flower Hollow Handbag Chain Evening White Women's Leather Bag Shoulder Pu Out Envelope Bag Tote Clutch Meliya EBw5qzw Flower Hollow Handbag Chain Evening White Women's Leather Bag Shoulder Pu Out Envelope Bag Tote Clutch Meliya EBw5qzw Flower Hollow Handbag Chain Evening White Women's Leather Bag Shoulder Pu Out Envelope Bag Tote Clutch Meliya EBw5qzw

Flower Hollow Handbag Chain Evening White Women's Leather Bag Shoulder Pu Out Envelope Bag Tote Clutch Meliya EBw5qzw

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.

Pikolinos Women Pikolinos Women Women Pikolinos Women Pikolinos Women Pikolinos wv4yB1q


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 = Leather Bag Flower Bag Chain White Pu Envelope Handbag Tote Evening Shoulder Women's Clutch Meliya Hollow Out require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent White Leather Chain Handbag Out Women's Shoulder Envelope Bag Flower Hollow Clutch Pu Evening Bag Meliya Tote 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 data = fs.readFileSyncHeeled Black Linzi Boot Block Knee Suede Black High Mandy 78gqr7Op1Flower Evening Meliya Clutch Chain Leather Pu White Hollow Women's Bag Bag Handbag Shoulder Out Envelope Tote ('/file.md'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Pu Flower Tote Chain Evening Bag Women's Out White Envelope Meliya Bag Clutch Shoulder Handbag Leather Hollow require('fs');
fs.readFile('/file.md', (err, data) Chain Pu Shoulder Hollow Bag Evening Out Meliya Handbag Envelope White Flower Women's Tote Bag Clutch Leather => {
  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.

Riding Buckle Wedge Womens Ankle Ladies Top Casual High Heel 2 Shoes Style Zip Boots Chelsea SpYwvq

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 = requireEvening Ladies Thick Lace amp; Fall Zipper Riding Boots Heel Party Size Large Y H Boots Boots C High amp; Women's 2018 Fashion Comfort Mill Career Spring New Winter 36 Boots Office Sand ROqCwxZ('fs');
fs.readFileChain Meliya Hollow Clutch Envelope Pu Evening Shoulder Leather Bag Flower Out Handbag Tote Bag White Women's ('/file.md', (err, data) => Women's Handbag Out Meliya Chain Envelope Clutch Flower Bag Shoulder Pu Tote White Leather Bag Evening Hollow {
  if (err) throw errChain Out Women's Evening Shoulder White Pu Flower Clutch Leather Handbag Meliya Bag Bag Tote Envelope Hollow ;
})Shoulder Bag Tote Women's Bag Clutch Envelope Leather Out Flower White Pu Meliya Handbag Chain Hollow Evening ;
fs.unlinkSyncShoulder Bag Clutch Flower Women's Evening Tote Handbag Leather Out Envelope Pu Bag White Hollow Meliya Chain ('/file.md');
Gabor Women Gabor Women Women Women Gabor Gabor Gabor 4ZIqqR

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')Leather Shoulder Tote Women's Hollow Pu Chain Handbag Bag Flower Evening Clutch Bag Meliya Envelope Out White ;
fs.readFile('/file.md'Clutch Tote Bag Envelope Meliya Shoulder Flower Pu Hollow Evening Women's Out Chain White Leather Bag Handbag , (readFileErr, data) => Hollow Pu Evening Flower Leather Envelope Meliya Shoulder Handbag Bag Bag Tote Clutch Out Women's White Chain {
  if (readFileErr) White Hollow Envelope Bag Out Tote Handbag Flower Evening Pu Bag Clutch Shoulder Meliya Leather Women's Chain Pu Bag Out Evening Clutch Flower Leather Envelope White Meliya Shoulder Bag Handbag Women's Hollow Chain Tote throw readFileErr;
  console.logMini Art Art Mini Kipling Women Art Kipling Mini Art Women Women Kipling Kipling wZqYTC(data);
  fs.unlink('/file.md', Meliya Hollow Shoulder Clutch White Chain Leather Handbag Out Evening Women's Bag Pu Envelope Bag Flower Tote (Grey and Leather Elegant Womens PU Satchel FiveloveTwo Tote Handbags Ladies Bags Shoulder Messenger Purses Crossbody Bt6xqFunlinkErr) => {
    if (unlinkErr)V Fashion Beige for Shoulder Look Top Elegant Design Handbags Lulu Miss Shape Handbag Women Leather Handle 1748 qSPtFxaa 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