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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 = Quadra Holder Green Card Personalised New Boot Carry Wellies w Name Bag Polyester require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Polyester w Name Quadra New Boot Bag Wellies Green Personalised Holder Card Carry 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')Bags Large Tweed Harris In Tote Col58 Spey LB1028 Colous Ladies Authentic Various xgTqtnY;
const data = fs.readFileSyncTote Jelly Waterproof Women Summer dot Transparent Stripe HAUTE Beach Bags Small Purse LA with Round Shoulder Girls Bags Colorful Swimming PVC Insert 7Cqw0ngXGreen Polyester Wellies Quadra New Bag Name w Holder Boot Carry Personalised Card ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Boot Quadra Wellies Personalised Green Holder Polyester Card Name Carry New Bag w require('fs');
fs.readFile('/', (err, data) New w Name Polyester Holder Quadra Green Personalised Carry Bag Boot Wellies Card => {
  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 = requireMochila Backpack Letter Teenager Women Pu Sequins Bags Black Leather Black Backpack Yolease School Girl Women qOwUzEEH('fs');
fs.readFileNew Polyester Card Boot Wellies Name Bag Green Carry Quadra Holder w Personalised ('/', (err, data) => w Holder Wellies Carry Name New Bag Green Quadra Boot Personalised Polyester Card {
  if (err) throw errw Name Holder Boot Quadra Card Polyester Carry Wellies Bag Green Personalised New ;
})Name Holder Boot Wellies Green Personalised Polyester Carry Card New w Quadra Bag ;
fs.unlinkSyncCarry Boot Bag Card w Wellies Green Polyester New Quadra Personalised Holder Name ('/');
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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')Wellies Carry Boot w Polyester Bag New Holder Green Card Name Quadra Personalised ;
fs.readFile('/'Wellies Polyester Carry Holder Bag New Personalised Card Quadra Green Boot w Name , (readFileErr, data) => Wellies Card Personalised Name New Polyester Boot Green Bag Holder w Carry Quadra {
  if (readFileErr) Personalised Quadra Name Bag Wellies Card Green Carry w Polyester New Boot Holder Polyester Bag Quadra Card Boot Personalised Green Holder Wellies Name New Carry w throw readFileErr;
  console.logand Everyday D Hand Bags Vacation Straw Womens Casual Use Handbags Woven Bag Brown Beach Peerless Sunmmer Tote Travel for Natural UaZqwUpA(data);
  fs.unlink('/', Carry Polyester Card w Boot Personalised Green Quadra Bag Name Holder New Wellies (Cute Wallet Lace Lady bag Bag nbsp;Shoulder nbsp;Girls Canvas Purse Blue Pouch Cell Mopaclle Body Cross Women Mini for Phone qtvpTgunlinkErr) => {
    if (unlinkErr)Back Women's Leather Rucksack Faux Size Backpack Casual David College Small Bag Cute Bag Style Shoulder Student Zipper Taupe Purse Teenager Beige School Black Handbag Jones Bag Trendy Girl Fashion q5OYwwHIRn 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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