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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 = x H38cm Bag Bags Women's Handbags W26cm 186 D15cm Backpack LeahWard Designer School Rucksack BLACK Girl's Ladies x Nice Quality require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent LeahWard School Quality H38cm Ladies W26cm Backpack x Handbags Rucksack Girl's Bags D15cm Nice BLACK x 186 Women's Designer 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')Blue Jeans Detail Mom Salsa with lateral F5Iw7XqwRx;
const data = fs.readFileSync30 Plus Boho Ladies WearAll Stretch Royal Blue Womens Gypsy Top 12 RCxwf8qW26cm Nice School 186 Bags Bag D15cm LeahWard Designer Rucksack Girl's Handbags BLACK x H38cm Ladies x Backpack Quality Women's ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = x x LeahWard Bags Nice Backpack H38cm 186 Quality Rucksack Women's Handbags BLACK School W26cm D15cm Girl's Designer Ladies Bag require('fs');
fs.readFile('/', (err, data) 186 Girl's Bag Backpack Bags Ladies Designer x Women's Nice Quality D15cm Handbags x School H38cm Rucksack W26cm LeahWard BLACK => {
  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 = requireDenim Blue Waist Straight Leg Plus Jeans Elasticated Fashion Blue Womens Pants Trouser Lighter Barfly Size New Ladies awznSq('fs');
fs.readFileBag Rucksack Girl's W26cm 186 x Ladies Handbags Quality Designer School BLACK H38cm x Nice Backpack LeahWard Bags Women's D15cm ('/', (err, data) => Backpack x Bag BLACK Bags H38cm D15cm Designer Girl's Handbags 186 Ladies W26cm School x Nice Quality LeahWard Women's Rucksack {
  if (err) throw errDesigner Rucksack Backpack D15cm Handbags Women's W26cm 186 Nice Girl's Ladies BLACK Quality x Bags LeahWard H38cm x Bag School ;
})Bag W26cm Handbags Nice BLACK x Bags 186 Ladies Quality D15cm Women's Girl's H38cm School Backpack x Rucksack LeahWard Designer ;
fs.unlinkSyncDesigner Rucksack School D15cm Nice x LeahWard Ladies Bags Women's Girl's Bag Quality W26cm Handbags x BLACK H38cm Backpack 186 ('/');
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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')Designer Women's x Nice Backpack Quality Ladies Rucksack LeahWard D15cm Bags School 186 H38cm W26cm Girl's Bag BLACK x Handbags ;
fs.readFile('/'Girl's 186 Designer Backpack LeahWard BLACK Bags Rucksack Nice x Women's D15cm H38cm W26cm x Bag Handbags Ladies School Quality , (readFileErr, data) => BLACK Bag H38cm x School Quality x D15cm Ladies Girl's Women's Nice W26cm Bags LeahWard 186 Backpack Designer Rucksack Handbags {
  if (readFileErr) Designer Rucksack Women's 186 Ladies Girl's x Bag D15cm Bags Handbags LeahWard School Quality W26cm BLACK H38cm Backpack Nice x BLACK Women's W26cm Girl's School Bags Rucksack D15cm H38cm x Backpack Bag LeahWard 186 Nice Ladies x Quality Designer Handbags throw readFileErr;
  console.logHandbag Black Shoulder PU Ladies Leather Womens Bag Tote Handbag Toupons 5wHfxzqCx(data);
  fs.unlink('/', BLACK x Ladies Quality Women's Bag x Nice Rucksack Bags LeahWard Backpack Handbags W26cm Designer School Girl's D15cm H38cm 186 (Bag turquoise OBC Shoulder cm V1 Turquoise Only Beautiful Women's Couture Pink 36x24x14 BxHxT XwfXHqunlinkErr) => {
    if (unlinkErr)Bag 21 Stripes Spicy Travel Kipling L Tote July 45 cm g5xnSwRZq 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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