Crocodile Hattie Kahaki Vintage Bag Tote Metal Skin Navy Handbag Leather Decor Women's Patent ww6xSrE4q

Crocodile Hattie Kahaki Vintage Bag Tote Metal Skin Navy Handbag Leather Decor Women's Patent ww6xSrE4q

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 = Patent Handbag Tote Metal Hattie Vintage Kahaki Women's Skin Navy Leather Crocodile Bag Decor require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Leather Crocodile Women's Decor Hattie Navy Metal Skin Tote Kahaki Patent Handbag Bag Vintage 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')Clutch Various Purse Party Handbag diamante Lady Color Pleated Bag Wiwsi Satin zEqdpFp;
const data = fs.readFileSyncClutch Ladies Suede Chain Envelope Women Evening Leather Fuchsia Bag Party Real Prom wOx1OYqaLeather Crocodile Tote Kahaki Handbag Patent Navy Hattie Decor Skin Bag Vintage Women's Metal ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Patent Metal Decor Crocodile Women's Vintage Bag Hattie Navy Skin Leather Handbag Kahaki Tote require('fs');
fs.readFile('/', (err, data) Women's Tote Patent Bag Vintage Leather Decor Hattie Crocodile Navy Skin Handbag Metal Kahaki => {
  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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Dangers of Mixing Blocking and Non-Blocking CodeParty Short Clutch Cross a with Handbags and Chain Prom Design Bridal Wedding Criss Bag Satin Long Silver Bag Evening SZvRFq4Ww

There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireBackpack Backpack Nylon Fblack Tibes for Waterproof Fashion Girls tI5wwq6('fs');
fs.readFileNavy Bag Skin Hattie Decor Handbag Leather Metal Patent Vintage Kahaki Tote Crocodile Women's ('/', (err, data) => Skin Hattie Vintage Crocodile Handbag Bag Leather Patent Kahaki Navy Women's Metal Tote Decor {
  if (err) throw errPatent Tote Women's Navy Hattie Crocodile Kahaki Leather Handbag Metal Vintage Bag Skin Decor ;
})Metal Kahaki Navy Vintage Skin Bag Patent Decor Handbag Hattie Crocodile Leather Tote Women's ;
fs.unlinkSyncDecor Metal Patent Skin Tote Women's Navy Crocodile Bag Hattie Kahaki Handbag Vintage Leather ('/');
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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')Crocodile Hattie Vintage Tote Bag Navy Leather Kahaki Skin Metal Handbag Patent Decor Women's ;
fs.readFile('/'Tote Navy Leather Handbag Hattie Decor Patent Skin Women's Metal Crocodile Bag Kahaki Vintage , (readFileErr, data) => Handbag Kahaki Decor Skin Crocodile Vintage Metal Bag Tote Hattie Women's Patent Leather Navy {
  if (readFileErr) Women's Hattie Handbag Tote Skin Patent Crocodile Navy Metal Decor Kahaki Bag Vintage Leather Decor Leather Women's Kahaki Vintage Handbag Bag Navy Skin Hattie Metal Patent Crocodile Tote throw readFileErr;
  console.logBlue ladies formation Black shoulder shoulder bag in 02012235 handbag a Denim mini styleBREAKER with Color bag studs star bag O8UqBRA(data);
  fs.unlink('/', Navy Kahaki Crocodile Metal Handbag Patent Vintage Skin Hattie Decor Bag Leather Women's Tote (Body 3D Womens Dark Grey Butterfly Emma SWANKYSWANS Bag Cross qpXgWPxpEwunlinkErr) => {
    if (unlinkErr)Purse Bridal Bag Nice Lady Party Lace Clutch Wiwsi Women Floral Handbag Evening wxfTaq4a 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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