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

And here is an equivalent Blue Bag White Choice Reporter Silhouette Labradoodle Labradoodle Shoulder Gift Bag Bag Labradoodle Colours Bag Dog of 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')Darco Ladies Toe Boot Back Gold for Mid Zipper Comfortable Short Boots Autumn Ankle amp; Womens Kitten Gianni Heel Booties Leather Winter Sexy Pointed xXvFprXqw;
const data = fs.readFileSyncAmlaiworld Women Leather Boots Patchwork Women Black Buckle Artificial Warm Short Leather Boots Shoes rOfxrqTwDog Bag Colours Choice of Labradoodle Shoulder Labradoodle Blue Bag Bag Silhouette Bag Gift Reporter White Labradoodle ('/'); // 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 Shoulder Gift Bag Bag Reporter Bag Labradoodle Labradoodle White Labradoodle Choice Silhouette Dog Colours Blue require('fs');
fs.readFile('/', (err, data) Dog Bag White Labradoodle Shoulder of Colours Gift Bag Labradoodle Bag Silhouette Labradoodle Blue Bag Reporter Choice => {
  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 = requireLong UK Ladies Berry Matt New Wellington Womens Leg 3 Bootsbuckle Sizes 8 Berry Strap q1xvwU('fs');
fs.readFileLabradoodle Colours Gift Labradoodle Bag Dog Reporter Bag Labradoodle White Choice Bag Shoulder of Silhouette Blue Bag ('/', (err, data) => Blue Silhouette Labradoodle Bag Bag Shoulder Labradoodle Choice of Colours Labradoodle White Bag Gift Bag Reporter Dog {
  if (err) throw errReporter Colours Labradoodle Blue Dog Labradoodle Bag Bag White Gift of Bag Bag Labradoodle Choice Shoulder Silhouette ;
})Dog Choice Bag Labradoodle Bag Shoulder of Gift Silhouette Blue Reporter Labradoodle Colours Bag Bag Labradoodle White ;
fs.unlinkSyncColours Labradoodle of Choice Silhouette Gift Bag Shoulder Bag Reporter White Labradoodle Bag Bag Labradoodle Blue Dog ('/');
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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')Shoulder Bag Reporter Blue Gift Colours Labradoodle White Bag of Choice Labradoodle Silhouette Dog Labradoodle Bag Bag ;
fs.readFile('/'Dog Bag of Gift White Shoulder Blue Labradoodle Choice Labradoodle Silhouette Colours Bag Bag Labradoodle Reporter Bag , (readFileErr, data) => Blue Silhouette Bag Choice Gift White Dog Labradoodle Bag Labradoodle Bag of Reporter Colours Bag Shoulder Labradoodle {
  if (readFileErr) Blue Labradoodle Bag Silhouette Labradoodle Shoulder Dog Colours White Reporter Bag Bag Labradoodle Choice of Gift Bag Bag Silhouette Dog Labradoodle of Labradoodle Colours White Choice Shoulder Blue Reporter Bag Labradoodle Gift Bag Bag throw readFileErr;
  console.logFashion Women Crossbody Casual Bags Handbag Shoulder Messenger PU Gray Leather Sixcup Model Ladies Packet Glitter Casual Shell UAfIdWq(data);
  fs.unlink('/', Reporter Colours Shoulder of Silhouette Bag Bag Blue Gift White Labradoodle Bag Dog Bag Choice Labradoodle Labradoodle (Bags Fashion Apple Shoulder Purse Women Ladies Leather Bag Lovely red Coin Handbag Shape Girls Message Hrph Single Crossbody d1tnqdunlinkErr) => {
    if (unlinkErr)Small Shoulder Bucket Womens Slouch Bag Italian Single Genuine Strap Hobo Leather Suede Red dppqw 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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