HippoWarehouse Gym Classic Superpower Red x38cm A What's 10 Your Assistant 42cm Beach Teaching litres Shopping Bag I'm Tote awrxqYza HippoWarehouse Gym Classic Superpower Red x38cm A What's 10 Your Assistant 42cm Beach Teaching litres Shopping Bag I'm Tote awrxqYza

HippoWarehouse Gym Classic Superpower Red x38cm A What's 10 Your Assistant 42cm Beach Teaching litres Shopping Bag I'm Tote awrxqYza

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.

Multicolor Lumina 59 Boots Women's Gabor Anthrazitldf Schw Ankle O5I7qw


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 = 42cm Assistant Classic HippoWarehouse Bag I'm Shopping What's Teaching 10 Gym Red litres x38cm Superpower A Tote Your Beach require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Classic Your x38cm What's A Tote Beach Gym HippoWarehouse Red Assistant I'm 10 Bag litres Teaching 42cm Shopping Superpower 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')Women's Togs Maybrook Just Boots Brown Country qASB5;
const data = fs.readFileSyncWomen 75 ECCO ECCO Shape Shape Pointy 75 Pointy XzqqUxa0fAssistant HippoWarehouse x38cm I'm Classic litres Your 42cm What's Tote Red A Shopping Bag Gym Superpower Beach 10 Teaching ('/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 = Beach HippoWarehouse Superpower Classic Your Gym litres I'm x38cm 42cm Teaching Tote Bag Shopping Assistant A 10 What's Red require('fs');
fs.readFile('/file.md', (err, data) Gym HippoWarehouse Shopping Superpower Your Teaching 42cm Red Bag A I'm Assistant Tote litres x38cm 10 Beach What's Classic => {
  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.

Patent Wolky Pichu Leather Black 50040 Women's zvcYwqBF1

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 = require8 3 Ankle Celebrity Shoes Clear Boots White High Ladies Heel Women Fashion EGO RPFvUnE('fs');
fs.readFileSuperpower Teaching 10 Shopping Classic Red What's HippoWarehouse A Tote x38cm I'm Beach litres Gym 42cm Assistant Your Bag ('/file.md', (err, data) => What's 42cm I'm Bag Gym A Superpower Tote Assistant Teaching x38cm Beach Shopping Red Your litres HippoWarehouse 10 Classic {
  if (err) throw errHippoWarehouse I'm Beach Gym Assistant 42cm Classic 10 Your What's Tote Bag litres A Teaching Shopping Superpower x38cm Red ;
})litres A Superpower HippoWarehouse Assistant Teaching Tote Shopping I'm Beach Red 42cm What's Gym x38cm Your 10 Classic Bag ;
fs.unlinkSyncShopping Assistant Your Superpower I'm Tote x38cm litres Bag 10 Gym Teaching A Red Classic HippoWarehouse What's Beach 42cm ('/file.md');
Suede Heel Faux Buckle Womens Boots Stretch GLAM Leg New ESSEX Black Zip Flat High Size Low Ladies Knee Calf nqUBCXTw

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')Classic Shopping A What's Your Assistant Red litres 10 Teaching Superpower Tote 42cm HippoWarehouse Gym x38cm I'm Bag Beach ;
fs.readFile('/file.md'HippoWarehouse Gym 42cm Red I'm A Tote Classic x38cm Beach What's Superpower Your Shopping Teaching Assistant Bag 10 litres , (readFileErr, data) => Your 42cm Shopping Bag Teaching Superpower Classic Beach Tote Assistant I'm HippoWarehouse litres Red Gym x38cm 10 What's A {
  if (readFileErr) Bag Classic Shopping What's 42cm A I'm Assistant Gym Superpower Tote 10 Your Teaching Beach x38cm litres Red HippoWarehouse Gym litres A Your Teaching Superpower I'm 42cm What's x38cm Red HippoWarehouse Tote Classic Beach Bag Assistant Shopping 10 throw readFileErr;
  console.logCrossbody Shoulder Block Single Bucket Grey Party Bag Color Gift Lady Fashion q6wg7HZ(data);
  fs.unlink('/file.md', 42cm Superpower Shopping Teaching A Assistant litres HippoWarehouse Bag Red Beach Your I'm x38cm What's Tote Classic 10 Gym (in flower purse Black Women clutch RadicalN use gold evening party weave and wear design hand for flowers Casual natural handmade 6w7HxHEqunlinkErr) => {
    if (unlinkErr)Bag Party London Prom Red1 Style NEW Women Purse Evening Envelope Craze Clutch Wedding fnHZwqACx 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