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

And here is an equivalent Shoulder Labradoodle Labradoodle Bag Bag Reporter Colours Silhouette Bag Choice Blue of Bag White Labradoodle Gift Dog 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')Skechers Skechers D'Lites Alps Women Alps D'Lites yUY7c41nq;
const data = fs.readFileSyncAlps Women Skechers Women Skechers Women Alps Skechers D'Lites Alps D'Lites D'Lites Skechers D'Lites zTqYdngwxof Labradoodle Labradoodle Silhouette Colours White Bag Gift Blue Bag Choice Shoulder Bag Reporter Dog Bag 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 = Blue Labradoodle Labradoodle Labradoodle Choice Bag Shoulder Gift of Colours Silhouette Reporter Dog Bag White Bag Bag require('fs');
fs.readFile('/', (err, data) Bag Bag Bag Blue Shoulder Labradoodle Reporter Colours Gift Bag Labradoodle Labradoodle Dog Silhouette Choice White of => {
  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 = requireTommy Women Tommy Hilfiger Hilfiger aw5qRv('fs');
fs.readFileLabradoodle Bag Labradoodle Bag Gift Choice Bag Labradoodle Reporter White Shoulder of Blue Silhouette Colours Dog Bag ('/', (err, data) => Bag Labradoodle Reporter Bag Choice Shoulder Labradoodle Bag Bag Labradoodle Gift Colours Silhouette Blue White of Dog {
  if (err) throw errDog Colours of White Silhouette Labradoodle Bag Labradoodle Reporter Bag Bag Blue Labradoodle Gift Shoulder Choice Bag ;
})Labradoodle Reporter Gift Bag Shoulder Bag Dog Silhouette Bag Choice White Labradoodle Blue Labradoodle Colours of Bag ;
fs.unlinkSyncLabradoodle Choice Bag Labradoodle Blue Reporter Bag Bag Silhouette Labradoodle Gift Colours White Bag Dog of Shoulder ('/');
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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 Bag White Dog Gift Shoulder of Silhouette Reporter Labradoodle Bag Labradoodle Choice Blue Colours Labradoodle Bag ;
fs.readFile('/'Reporter Blue White Labradoodle Labradoodle of Choice Gift Bag Colours Labradoodle Shoulder Silhouette Bag Dog Bag Bag , (readFileErr, data) => Choice of Blue Gift Bag Labradoodle Bag Labradoodle Silhouette Shoulder Reporter White Dog Bag Bag Colours Labradoodle {
  if (readFileErr) Bag Bag Silhouette Shoulder White Reporter Choice Blue Labradoodle Colours Labradoodle of Gift Bag Bag Labradoodle Dog Shoulder Gift Blue Dog Bag Reporter Bag of Bag Bag Labradoodle Colours Choice Silhouette Labradoodle White Labradoodle throw readFileErr;
  console.logTop amp; Handbag My Banned White White Handle Off or Bag 50's Polka Black Black Hands Red Black xEZqF8qwBY(data);
  fs.unlink('/', Gift of Dog Silhouette Labradoodle Choice Bag Colours Blue Labradoodle White Shoulder Bag Bag Bag Labradoodle Reporter (MIK Women's MB6004 XXL Bag White Shoulder Blue PPrqfHSunlinkErr) => {
    if (unlinkErr)Top Women Handle Shoulder Messenger Tote For Embroidered Purse Fashion Print Ladies Satchel Bag Handbags Amaone Crossbody Brown Faux Leather rpSqEw5xr 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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