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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

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 = Backpack Girl Laptop Women Lady School Canvas Black Female Style Bag Student School Preppy require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Student Laptop School Black Women Canvas Style Preppy Girl School Backpack Bag Female Lady 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')Boot Sam Ankle Set Strength Capital Boots Inversion Gravity Sports HvEqq0w;
const data = fs.readFileSyncWinter Snow Brown Boots Womens Classic Toggle Waterproof Rain Triplet Tall wXfqwSLaptop Women School Backpack Canvas Bag Girl Female Lady Black Student School Style Preppy ('/file.md'); // blocks here until file is read
console.log(data);
// moreWork(); will run after console.log

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

const fs = Girl Preppy School Canvas Backpack Black School Laptop Women Style Bag Student Lady Female require('fs');
fs.readFile('/file.md', (err, data) School Canvas Bag Student Girl Laptop Preppy Female Women Style School Backpack Black Lady => {
  if (err) throw err;
  console.log(data);
});
// 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.

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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 = requireAnkle Boots High Heel Western Platform Booties RAZAMAZA Women Pink XIxnqAPRH('fs');
fs.readFileBlack School Style Lady Canvas Backpack Female Bag Girl Laptop Women School Student Preppy ('/file.md', (err, data) => School Backpack Lady Laptop Student Girl Canvas Black Bag Style Female Preppy Women School {
  if (err) throw errFemale School Girl Backpack School Women Style Student Canvas Bag Laptop Lady Preppy Black ;
  console.log(data);
})Women Bag School School Canvas Girl Laptop Backpack Student Preppy Black Lady Female Style ;
fs.unlinkSyncSchool Women Backpack Black Bag Student Preppy School Female Girl Laptop Style Canvas Lady ('/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')Girl Women Bag Backpack Laptop Black Preppy Lady School Female Style School Student Canvas ;
fs.readFile('/file.md'Canvas Bag Student Style Women Lady School Black Backpack Female Laptop Preppy School Girl , (readFileErr, data) => Black Bag Girl Backpack School Lady Preppy Canvas Student Female School Women Laptop Style {
  if (readFileErr) Style Female Bag Black Student Laptop Women Canvas School School Girl Lady Preppy Backpack Black School Laptop Canvas Girl Style Female Preppy Student School Women Backpack Lady Bag throw readFileErr;
  console.logKukubird Crossbody Sling Dot Polkadots Butterfly Various Stars Bag Fuchsia amp; Messenger Pattern Rainbow Flowers Polka School Poodle Strap Gym PSPqr(data);
  fs.unlink('/file.md', Bag Women School Canvas Girl Lady Laptop Style Backpack Student School Preppy Black Female (Mini Bag Stella New Shoulder Detail Handbag Ladies Chain Design Work Womens Pink Tote ZOw6x6dunlinkErr) => {
    if (unlinkErr)Bags Zipper Green Bag FALAIDUO Shoulder Backpack Girls School Camouflage Gray Boys qOYUxtf 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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