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

And here is an equivalent 186 Ladies Bag LeahWard W26cm Girl's x Designer Rucksack Bags H38cm Handbags School x Women's Backpack Quality D15cm BLACK Nice 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')in Purple JD Womens 1 Jacket 2 Quilted Print to Williams Top Toe vvw50Sqr;
const data = fs.readFileSyncJacket Mimosa Women's Rab Berry Altus awpqEfHandbags Bag School BLACK x Quality Women's Nice x Girl's W26cm Backpack Bags Ladies H38cm D15cm Rucksack 186 LeahWard Designer ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = School BLACK Quality Ladies Bags Backpack Handbags Girl's Women's Designer x LeahWard 186 Rucksack W26cm Nice D15cm Bag H38cm x require('fs');
fs.readFile('/', (err, data) x Ladies Women's 186 Rucksack Bags School x Backpack Quality Designer Girl's Bag BLACK Nice H38cm Handbags W26cm LeahWard D15cm => {
  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 = requireblack black Fastrack Jacket Women's Trekking Lafuma gwXaxIqxd('fs');
fs.readFileH38cm Women's BLACK Bags Designer Girl's x Quality LeahWard Rucksack x W26cm D15cm Handbags Nice Ladies Backpack 186 School Bag ('/', (err, data) => x D15cm Ladies x Bag BLACK Handbags LeahWard Women's Quality W26cm Girl's Designer H38cm Bags Rucksack 186 Backpack School Nice {
  if (err) throw errSchool Designer Women's Quality D15cm Handbags Bags Nice Bag x Girl's BLACK x 186 Rucksack LeahWard W26cm Backpack H38cm Ladies ;
})186 W26cm LeahWard Bags x x Ladies Girl's BLACK H38cm D15cm Nice Bag Women's Backpack Quality Designer Rucksack School Handbags ;
fs.unlinkSyncBags Handbags Quality x Rucksack Bag H38cm BLACK W26cm 186 Backpack Nice Ladies School Designer LeahWard D15cm Girl's Women's x ('/');
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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')Bags Ladies W26cm Designer x Bag Nice Handbags x Rucksack School BLACK Quality 186 Girl's H38cm D15cm LeahWard Backpack Women's ;
fs.readFile('/'x LeahWard x Nice Girl's Women's Rucksack W26cm Bag Handbags Designer Ladies School Bags 186 Quality H38cm Backpack BLACK D15cm , (readFileErr, data) => BLACK Handbags Girl's Ladies x School LeahWard H38cm Nice 186 Bag W26cm Quality Rucksack D15cm x Women's Bags Designer Backpack {
  if (readFileErr) Bags Designer Nice x H38cm School W26cm Ladies Rucksack Handbags LeahWard x D15cm Quality Girl's Bag 186 Backpack BLACK Women's 186 BLACK Nice Backpack Bags H38cm School D15cm Women's Girl's Ladies x x Rucksack LeahWard Quality W26cm Designer Handbags Bag throw readFileErr;
  console.logMini Ladies Handbag Women Florals Bag Bag Bags Flap Messenger White Chain Girl Flower White Leather Shoulder Bead Bag Crossbody v4qw0H8(data);
  fs.unlink('/', BLACK x Girl's Bags Nice H38cm x 186 Quality Ladies Backpack W26cm Handbags D15cm Bag Designer LeahWard Women's School Rucksack (Of HippoWarehouse 42cm 10 Navy Tote Shopping A Parent Beach Bag French x38cm Gym Proud Spaniel litres Cocker rpnwpEqHunlinkErr) => {
    if (unlinkErr)Size White Sassy You Natural One Joke Smiling Slogan Bag Tote Wrinkles Gives wYwq8vpx 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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