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

And here is an equivalent School Preppy Backpack Women Girl Student Lady Female Laptop Canvas Black School Bag Style 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')Designer Shoulder Hobo Bag Women's LARGE Silver Messenger EDGE CHAIN Handbag MINI Mini dYH0Y;
const data = fs.readFileSyncReisenthel cm Carrybag 48 Black cm Carrybag 48 Reisenthel RZBwp4qStyle School Student Preppy School Black Backpack Female Canvas Women Lady Bag Girl Laptop ('/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 = Preppy Laptop Backpack Girl Lady Female Black Canvas School Women Style Bag School Student require('fs');
fs.readFile('/file.md', (err, data) School Female Women Black Canvas Preppy School Girl Lady Backpack Laptop Style Student Bag => {
  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 = requireMessenger Deep Blue Girls Handbag Body Purse Deep Cross Bags Blue Mini Small Vovotrade B1xqwTII('fs');
fs.readFilePreppy Girl Backpack Female Women Black Bag Lady Laptop Student Style Canvas School School ('/file.md', (err, data) => School Preppy Student Female Black Canvas Lady Laptop School Style Women Bag Backpack Girl {
  if (err) throw errBlack Female Backpack Bag Canvas School Preppy Laptop Lady School Women Student Style Girl ;
  console.log(data);
})Backpack School Style Laptop Preppy Black Lady Women Student Bag Girl School Female Canvas ;
fs.unlinkSyncBlack Lady School Backpack Girl Canvas Student Bag Style School Preppy Laptop Female Women ('/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')Preppy School Backpack Canvas Black Bag Girl Student Female Women Style Laptop School Lady ;
fs.readFile('/file.md'Style Backpack Women Girl Lady Preppy Laptop Black School Female Student School Bag Canvas , (readFileErr, data) => Female Backpack Bag Laptop Preppy Style Student Black School Lady Canvas Women Girl School {
  if (readFileErr) School Student Backpack Laptop School Style Canvas Lady Bag Women Black Girl Female Preppy Bag School Lady Girl Preppy Style Women Backpack Black School Student Canvas Laptop Female throw readFileErr;
  console.logSweats FINEJO Coat Check Hooded Jacket Ladies Hooded Autumn Winter Sweater q78TRq(data);
  fs.unlink('/file.md', Laptop Style Lady Student Preppy Girl Women Backpack Bag Canvas School Black School Female (Jacket New Light Grey Womens Look Girly Top Sweatshirt Belly Hooded Cropped Ladies Long Zipper Sleeves z5qngunlinkErr) => {
    if (unlinkErr)Jacket Faux Fur Grey Women Vest Sleeveless Waistcoat Winter Faux Long Gilet HOMEBABY Coat Cq8AwZtX 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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