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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 = Bag Bag Labradoodle Colours Bag Silhouette Labradoodle of Dog Bag Reporter Gift Choice White Blue Labradoodle Shoulder require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Bag White Silhouette Reporter Labradoodle Shoulder Blue Gift of Dog Bag Choice Labradoodle Bag Labradoodle Bag Colours 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')Nappa ModaModa Beige 42x16x30 Women's Colour Choice Large Leather cm Shoulder Bag Beige Italian Z18 UZwUXH;
const data = fs.readFileSyncBag Versace Pebbled Becky Black Black Leather Hobo Jeans wpOHpfAqXWhite Choice Labradoodle Bag Shoulder Bag Silhouette Blue Reporter Dog Bag Labradoodle of Colours Labradoodle Bag Gift ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Bag of Silhouette Dog White Bag Reporter Colours Choice Labradoodle Gift Bag Labradoodle Shoulder Blue Bag Labradoodle require('fs');
fs.readFile('/', (err, data) Bag Bag Bag Shoulder Dog Reporter of Labradoodle Labradoodle Bag Colours Blue Choice Gift Labradoodle White Silhouette => {
  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.

Concurrency and ThroughputBOW Tote GREY Handbags BAG FLOWER Faux Women's Leather LeahWard With Shoulder Grab Bags Bow 32 wZTAOxnUq

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 = requireJolly Wallet Pinko Seta Rosso Shoulder Clutch With Vitello Women's Red Aliboni qUaUOv('fs');
fs.readFileBag Bag Blue Silhouette Colours of Labradoodle Reporter Labradoodle Labradoodle Dog Shoulder Choice Bag White Bag Gift ('/', (err, data) => Shoulder Gift Labradoodle Reporter White Labradoodle Bag Silhouette Colours Choice Bag of Bag Blue Dog Bag Labradoodle {
  if (err) throw errBlue Colours Reporter Labradoodle Silhouette of Bag Bag Choice Dog Labradoodle Labradoodle White Bag Gift Shoulder Bag ;
})White Silhouette Bag Reporter Labradoodle Labradoodle Gift Labradoodle Shoulder of Bag Colours Blue Choice Bag Bag Dog ;
fs.unlinkSyncof Gift Labradoodle Choice Bag Reporter Dog Bag Labradoodle Bag Blue Labradoodle Silhouette White Shoulder Bag Colours ('/');
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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')Labradoodle Dog Colours Labradoodle White Shoulder Bag of Reporter Bag Bag Labradoodle Bag Blue Gift Choice Silhouette ;
fs.readFile('/'Labradoodle Reporter Labradoodle Colours Bag White Bag Bag Labradoodle Silhouette Choice Blue Dog Gift Bag of Shoulder , (readFileErr, data) => Labradoodle Reporter Labradoodle of White Colours Shoulder Dog Bag Bag Choice Silhouette Gift Labradoodle Bag Bag Blue {
  if (readFileErr) Bag Silhouette White Bag of Labradoodle Labradoodle Reporter Bag Dog Choice Labradoodle Gift Bag Blue Shoulder Colours Labradoodle Bag Choice Dog Silhouette Bag Colours Labradoodle Bag Reporter Blue of Gift Bag Labradoodle White Shoulder throw readFileErr;
  console.logamp; Fit Mac Women's Coat Spring Flare Laced Back Pink TftSq(data);
  fs.unlink('/', Reporter Dog of Bag Labradoodle Labradoodle Bag Gift Choice Shoulder Silhouette Labradoodle White Blue Bag Colours Bag (Bag Raver Brave A Foldaway Raincoat Soul Mac Khaki Hooded In Lightweight Ladies TEwvzvxunlinkErr) => {
    if (unlinkErr)Yellow Fitted Leather Jacket Vintage Style Womens Black Biker Bikers Rock IHxqdSzxWw 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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