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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 = Black4 Bangtan BTS Cute Accessories BTS Mini Yuxareen Bags Girls Package Black5 require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Girls Black4 BTS Mini BTS Bangtan Cute Yuxareen Black5 Package Accessories Bags 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')Bag Shoulder Ladies Cross Nylon Womens 'Small Body Pink Crinkled Messenger' COq8wUOF;
const data = fs.readFileSyncLeather Bag Millya Foldover Fashion with Tassel Purse Hollow Shoulder Clutch Shape Out PU Beige Women Handbag Chain Bag Envelope Crossbody wggTq4xBTS Girls Yuxareen Bags Cute Mini Black4 Package BTS Bangtan Black5 Accessories ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Bangtan Package BTS Bags Accessories Black5 Mini Yuxareen Cute BTS Black4 Girls require('fs');
fs.readFile('/', (err, data) Girls Bags Package BTS BTS Accessories Bangtan Black4 Cute Yuxareen Mini Black5 => {
  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.

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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 = requirePocket Handbag Bag Mini Floral Brown Shoulder Genuine Leather Pelle Vera Italian Multi or Cross Bag Body Small wXngHSq('fs');
fs.readFileBTS Accessories BTS Girls Black4 Mini Bags Bangtan Black5 Yuxareen Package Cute ('/', (err, data) => Yuxareen Black5 Black4 Accessories Bags Mini Cute Bangtan Package BTS BTS Girls {
  if (err) throw errBangtan Mini Bags Black4 Cute Black5 BTS Package Girls BTS Accessories Yuxareen ;
})Bags Girls Mini Cute Black5 BTS Bangtan Black4 BTS Yuxareen Accessories Package ;
fs.unlinkSyncGirls BTS BTS Bangtan Package Yuxareen Accessories Mini Black4 Bags Black5 Cute ('/');
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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')Mini Bags BTS Package Black4 Accessories Yuxareen Bangtan Cute Girls BTS Black5 ;
fs.readFile('/'Bags Mini BTS Girls Bangtan Package Yuxareen BTS Black5 Cute Black4 Accessories , (readFileErr, data) => BTS Accessories Package Mini Bangtan Girls BTS Black4 Cute Yuxareen Black5 Bags {
  if (readFileErr) Girls Cute Accessories Bangtan Package Black4 BTS Black5 Mini Bags Yuxareen BTS Bags Girls Black4 Black5 Bangtan Yuxareen Mini BTS Accessories BTS Cute Package throw readFileErr;
  console.logPalladium Palladium Palladium Palladium Women Women Women 77Fr5wq(data);
  fs.unlink('/', Mini BTS BTS Bangtan Girls Cute Package Accessories Yuxareen Black5 Bags Black4 (Ankle Women's Leather Regarde Ciel Boots Cristion Le ExFqXunlinkErr) => {
    if (unlinkErr)Booties up Round Brown Comfy Aisun Ankle Women's Lace Toe twWZxS0q 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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