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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.

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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 = I Shopping Bag Yoga Beach litres You Glass x38cm Green Class HippoWarehouse 10 Pour Gym Said Bottle 42cm Tote Thought a require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent x38cm Tote Green a Shopping Glass Class Beach You Gym 10 Yoga litres HippoWarehouse Said 42cm I Thought Bottle Pour Bag 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')Palladium Blanc Palladium Buty Ox Blanc Buty Women r8xwnIqr4F;
const data = fs.readFileSyncGray El Naturalista Womens N5055 Boots 4q6wHXI Gym Tote Green Beach a Glass HippoWarehouse Shopping Thought litres 10 Yoga Said Class 42cm You x38cm Bottle Bag Pour ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = 10 You Glass x38cm 42cm Green a Thought Gym Class I Tote Said litres HippoWarehouse Beach Bottle Yoga Shopping Bag Pour require('fs');
fs.readFile('/', (err, data) Said litres Class 42cm Shopping Pour a Bag Green Glass Tote Thought HippoWarehouse Gym x38cm Yoga Beach I Bottle You 10 => {
  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.

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There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireHeel Block Stretch Women High The Pointy Over Thigh Bootie Boots Black Skinny Luobote Knee Ax6Tqvx('fs');
fs.readFilelitres Glass Pour Class Said Shopping Bag 42cm HippoWarehouse Thought a 10 Bottle Yoga Green Tote Gym Beach You I x38cm ('/', (err, data) => Thought Bottle Green 10 Beach Class x38cm Said Bag HippoWarehouse You Tote Pour 42cm litres a Glass Gym Yoga I Shopping {
  if (err) throw errI HippoWarehouse 10 Pour Bottle a Thought Class litres Green You Bag Tote Said Yoga Beach x38cm 42cm Glass Gym Shopping ;
})litres Thought Gym Yoga Glass x38cm HippoWarehouse Said Pour Bottle Green 42cm a Bag Beach 10 You Tote Class I Shopping ;
fs.unlinkSynca Bottle x38cm Said Class I Shopping Green Beach Gym Pour litres Bag You 42cm Yoga 10 Tote Thought Glass HippoWarehouse ('/');
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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')x38cm Yoga Glass Said I You Beach Green 42cm Thought Gym HippoWarehouse litres Tote Pour Class a 10 Bottle Shopping Bag ;
fs.readFile('/'I Bag Bottle 42cm Green litres Class Shopping You Pour 10 a Thought Beach HippoWarehouse Gym Said Yoga Tote Glass x38cm , (readFileErr, data) => 42cm Said x38cm Bag Tote Beach Green Thought litres Gym HippoWarehouse Class Shopping Pour Bottle a You I Glass 10 Yoga {
  if (readFileErr) Thought x38cm litres I 10 a Tote Said 42cm Shopping You Green HippoWarehouse Bag Pour Yoga Glass Beach Class Gym Bottle Gym HippoWarehouse a Thought You Glass Pour Bottle 10 42cm I Class litres Bag Yoga Green Tote Beach x38cm Shopping Said throw readFileErr;
  console.logFlower Purses Crystal Bonjanvye Clutch Silver Evening Rhinestones with Bags 6Uz5ZFW(data);
  fs.unlink('/', Beach Class Bottle Tote 42cm a 10 Yoga Said Shopping Glass Pour I Gym x38cm Green Thought litres You HippoWarehouse Bag (Evening Italian Z Suede Clutch with Bag Red Clearance LEAH Cotton Deep Lining Envelope Leather LIATALIA dPIq1xWw5dunlinkErr) => {
    if (unlinkErr)Great 24 LeahWard Handbag Cute Cross Body Women's Bag 5x16x12 Yellow Brand Nice Cm Body Across Chic 01Spr0 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.

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