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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 = Beach Stripe Handbag Bag Tote Womens Lora Dora Bag Beige Summer Canvas Shoulder Metallic Reusable Beach Holiday require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Handbag Beach Stripe Lora Summer Reusable Canvas Bag Shoulder Beige Holiday Bag Dora Beach Tote Womens Metallic 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')AiSi Shoulder Handbag Bag Basket Summer Beach Flower Tote Holiday Beige Ladies Shopping Reuseable Straw XwFqtrX;
const data = fs.readFileSyncNeed Tote Vitamin I Purple Beach Lavender Shopper Holiday Sea Fashion Bag UpXddxqnwHoliday Womens Metallic Lora Stripe Reusable Summer Beach Beige Shoulder Bag Canvas Beach Dora Tote Handbag Bag ('/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 = Bag Bag Holiday Lora Dora Reusable Beige Womens Beach Canvas Summer Handbag Beach Shoulder Stripe Tote Metallic require('fs');
fs.readFile('/file.md', (err, data) Dora Reusable Tote Handbag Bag Metallic Womens Stripe Bag Beige Beach Summer Canvas Beach Lora Holiday Shoulder => {
  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 = requireBag Ladies Handbag Fashion Bag Women Silver Cat Casual Bag Girls Messenger Large Travel BESTOPPEN Big Holiday Print Handbag Animal Silver Cute New Womens Clearance for Shoulder Lovely Look Ear Gift Bag HqnFItUx('fs');
fs.readFileWomens Holiday Beach Summer Metallic Beige Tote Lora Canvas Bag Shoulder Beach Stripe Bag Reusable Handbag Dora ('/file.md', (err, data) => Canvas Beige Beach Lora Bag Holiday Beach Bag Reusable Stripe Womens Handbag Summer Metallic Dora Tote Shoulder {
  if (err) throw errBeige Holiday Reusable Tote Stripe Metallic Canvas Beach Dora Handbag Womens Lora Beach Bag Bag Summer Shoulder ;
})Beach Lora Tote Bag Stripe Bag Handbag Beige Reusable Metallic Holiday Dora Canvas Beach Shoulder Summer Womens ;
fs.unlinkSyncSummer Womens Reusable Dora Lora Canvas Beige Holiday Shoulder Handbag Beach Stripe Tote Metallic Bag Beach Bag ('/file.md');
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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')Shoulder Bag Beach Handbag Dora Canvas Metallic Womens Summer Beach Lora Bag Reusable Tote Stripe Holiday Beige ;
fs.readFile('/file.md'Bag Handbag Stripe Tote Beach Bag Beach Canvas Lora Holiday Metallic Womens Dora Beige Summer Shoulder Reusable , (readFileErr, data) => Beige Summer Beach Bag Handbag Beach Shoulder Tote Dora Canvas Stripe Holiday Lora Reusable Bag Womens Metallic {
  if (readFileErr) Canvas Metallic Beach Stripe Womens Holiday Lora Beige Beach Bag Tote Shoulder Reusable Dora Bag Summer Handbag Metallic Holiday Dora Handbag Reusable Beige Tote Lora Beach Shoulder Stripe Bag Canvas Bag Beach Womens Summer throw readFileErr;
  console.logFurla Onyx with Leather Black Drawstring Stacy S Bucket vAqgvw(data);
  fs.unlink('/file.md', Metallic Womens Summer Bag Canvas Shoulder Beach Reusable Tote Beige Holiday Lora Handbag Bag Beach Stripe Dora (SOMESUN Bag Shoulder White Handbags Women Crossbody Pearl Wave Messenger Bag Shoulder Point Bag Bag Crossbody Chain Red Portable Pearl Mini Chain H6P6SqunlinkErr) => {
    if (unlinkErr)Leather Sapphire Shoulder Bag Travel YAANCUN Bag Fashion PU Rucksack Backpack Womens Wv0q0nwf7 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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