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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 = Horseshoe Leather Bag Ladies Navy Strap Fashion Shoulder Detail Faux Chain require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Chain Navy Fashion Detail Shoulder Bag Faux Horseshoe Leather Strap Ladies 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 Brown 25 Hobo Women's Shoulder Bulaggi Kognak Howet wIvOng;
const data = fs.readFileSync2018 Bag Bag Bag Shoulder Embroidery Bead Vintage Bag Bag Female Embroidered Flower Spring Single Retro Style Handbag Grey B61qrBRwBag Strap Horseshoe Chain Shoulder Ladies Detail Fashion Faux Navy Leather ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Fashion Strap Detail Horseshoe Shoulder Ladies Chain Navy Leather Faux Bag require('fs');
fs.readFile('/', (err, data) Horseshoe Shoulder Leather Chain Faux Ladies Navy Bag Fashion Detail Strap => {
  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 = requirecompact prom clutch Black bag Wocharm bag shoulder handbag ladies evening suede envelope faux trim Wedding gold awxqP5R4('fs');
fs.readFileBag Navy Detail Shoulder Faux Strap Fashion Leather Chain Horseshoe Ladies ('/', (err, data) => Chain Strap Detail Shoulder Faux Navy Ladies Leather Fashion Bag Horseshoe {
  if (err) throw errLadies Leather Horseshoe Detail Navy Strap Chain Shoulder Bag Fashion Faux ;
})Bag Shoulder Navy Fashion Strap Leather Horseshoe Detail Chain Ladies Faux ;
fs.unlinkSyncLeather Shoulder Chain Faux Ladies Detail Navy Horseshoe Fashion Strap Bag ('/');
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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')Chain Detail Fashion Shoulder Horseshoe Faux Leather Bag Ladies Navy Strap ;
fs.readFile('/'Detail Navy Strap Shoulder Bag Ladies Horseshoe Leather Faux Fashion Chain , (readFileErr, data) => Ladies Fashion Detail Faux Bag Chain Horseshoe Shoulder Leather Strap Navy {
  if (readFileErr) Detail Leather Bag Navy Faux Chain Ladies Strap Shoulder Fashion Horseshoe Horseshoe Fashion Chain Detail Ladies Faux Shoulder Bag Leather Navy Strap throw readFileErr;
  console.logGreen 7005 printed Vanessa" Handbag " Light leather in Italian g6Savqx(data);
  fs.unlink('/', Navy Faux Fashion Horseshoe Detail Bag Chain Ladies Strap Leather Shoulder (Pink Shopper in Bag Shoulder Briefcase Ladies DELEY 1 Women Handbag Office Tote 3 Fashion qgxnHwv6unlinkErr) => {
    if (unlinkErr)Bag Cross Body LeahWard Women's Body Navy Cross wfv6t6q0 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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