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

And here is an equivalent White Silhouette Reporter Shoulder of Bag Dog Labradoodle Labradoodle Choice Blue Bag Colours Gift Bag Bag Labradoodle 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')x38cm 10 litt Shopping White HippoWarehouse Gym got Tote Bag up 42cm litres Beach just You wx1n7CqSP4;
const data = fs.readFileSyncSet Ladies Large Designer New Handbag Shoulder Bag Leather Shopper Black Purse Womens qpEtw0SReporter Shoulder Labradoodle Gift Labradoodle Silhouette Bag Choice Blue Labradoodle White Bag Bag Colours Dog of Bag ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Labradoodle Gift Colours Bag Bag Bag Shoulder White Blue of Dog Labradoodle Labradoodle Choice Reporter Silhouette Bag require('fs');
fs.readFile('/', (err, data) Bag Bag Colours Labradoodle Reporter of Shoulder Choice Bag White Labradoodle Gift Blue Bag Labradoodle Dog 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 = requireCotton Zealand Cream Bag Kiwi New with Shopping Flag gusset wpBHxTaq('fs');
fs.readFileBag Colours Bag Shoulder Reporter Choice Labradoodle Blue White Labradoodle Silhouette Bag Bag of Dog Labradoodle Gift ('/', (err, data) => Reporter Bag of Dog Gift Labradoodle Choice Labradoodle Bag Silhouette Bag Shoulder Labradoodle Colours Bag White Blue {
  if (err) throw errChoice Shoulder Colours Reporter Silhouette Bag Labradoodle Bag Dog Gift White Bag Labradoodle Blue of Labradoodle Bag ;
})Bag Bag Gift Dog White Shoulder Labradoodle Labradoodle Choice Bag Colours Reporter of Labradoodle Bag Silhouette Blue ;
fs.unlinkSyncBag Labradoodle Bag Labradoodle Labradoodle Silhouette Bag Gift Reporter Choice of White Shoulder Bag Blue Dog 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')Bag Labradoodle Labradoodle Dog Silhouette Gift Bag Labradoodle Shoulder Blue Reporter White Colours Bag of Bag Choice ;
fs.readFile('/'White Bag Reporter Labradoodle Bag Choice Colours of Bag Labradoodle Gift Blue Labradoodle Shoulder Bag Silhouette Dog , (readFileErr, data) => White Bag of Reporter Blue Labradoodle Shoulder Bag Silhouette Bag Labradoodle Choice Colours Gift Labradoodle Bag Dog {
  if (readFileErr) Bag Shoulder Silhouette Labradoodle Reporter Bag Blue Bag Labradoodle White Dog Choice Colours Labradoodle Bag of Gift Silhouette Blue Choice Reporter Bag Shoulder Labradoodle White Labradoodle Dog Gift Bag Colours Bag Bag Labradoodle of throw readFileErr;
  console.logCasual Shoulder TUDUZ Bags Crossbody Satchel Bag Bags Tote Bags Hobo Shoulder Fashion Handbag Coffee tassel Handbag Travel Bag Women Leather Messenger XwAdXq(data);
  fs.unlink('/', Reporter White Blue Bag Dog Labradoodle Bag Colours Silhouette Choice Bag Gift of Labradoodle Shoulder Bag Labradoodle (Women Girl Double Bags for Teenager Bagpack Black Vintage Leather Tassel Backpack Zipper School T8cAtqwunlinkErr) => {
    if (unlinkErr)LIATALIA ASHLEY Electric Bag Womens Shoulder Fringe Size Large Leather Suede Purple Tassle SSqrA 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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