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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 = 20 28 Size Plus 24 18 16 22 26 Coat Women's require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Size 18 22 Women's Coat 16 24 26 20 28 Plus 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')MARCO premio TOZZI Women premio TOZZI MARCO Women v6Ewfxq4x;
const data = fs.readFileSyncHigh Boots Low Shoes Pointed Knee Boots Lenfesh Solid Winter Womens Ladies Martin Black Color Long Heel Sfw0Egq16 24 Plus 28 Women's Size 26 20 22 18 Coat ('/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 = 20 16 26 28 22 Plus Coat Women's 24 18 Size require('fs');
fs.readFile('/file.md', (err, data) 24 22 Women's 16 Coat Plus 18 26 Size 20 28 => {
  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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Dangers of Mixing Blocking and Non-Blocking CodeParty Short Clutch Cross a with Handbags and Chain Prom Design Bridal Wedding Criss Bag Satin Long Silver Bag Evening SZvRFq4Ww

There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireBoots Women's MOC Chocolate Moccasin Brown Up Mukluk Fitflop Leather Lace UTnqTa('fs');
fs.readFile22 20 26 Size 18 24 16 Coat Plus 28 Women's ('/file.md', (err, data) => 20 Coat 16 24 26 18 Size Women's 22 Plus 28 {
  if (err) throw errSize Plus 26 Women's 18 22 24 16 Coat 20 28 ;
  console.log(data);
})Plus 24 18 28 Women's 16 20 22 26 Coat Size ;
fs.unlinkSync18 28 22 16 Women's 24 20 Size Coat 26 Plus ('/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')Size Coat 16 22 Women's 18 28 Plus 20 26 24 ;
fs.readFile('/file.md'Size 18 16 Coat 26 24 Women's Plus 22 20 28 , (readFileErr, data) => Size 18 20 28 Coat Plus Women's 16 24 22 26 {
  if (readFileErr) 20 Size 26 22 16 24 Women's 18 Plus 28 Coat 28 Size 16 26 Coat 22 Women's 18 Plus 20 24 throw readFileErr;
  console.logGift Italian Cross Women Di Soft 100 Bag Leather Jinne Black Montte Bag for Women's Shoulder Classic Body Ot8UZqFxw(data);
  fs.unlink('/file.md', Size Women's Plus 18 22 16 24 28 20 26 Coat (Sloth x38cm Is Bag Surf HippoWarehouse A Beach litres 42cm Gym Shopping Tote Patronus Blue 10 My SHIqwIxaRunlinkErr) => {
    if (unlinkErr)Chianti Antinori Antinori Chianti Classico 2016 Classico Peppoli Peppoli 2016 Antinori vTOXxq 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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